In this paper, we have studied polymer extrusion, an important process in plastics manufacturing. Often the final material’s surface is rough with semi regular grooves resembling the skin of a shark. This phenomenon, known as sharkskin instability, has intrigued the scientific and industrial communities for over 60 years. We have reported the results of a simple physical model which explains the underlying mechanism that causes the sharkskin instability. We simulated the flow of polymer melts being squeezed through a slit and observed that the stretched polymer chains are stretched toward the exit, then recoiled as they moved further away, causing a swell and forming waves. These waves became the familiar sharkskin grooves on the surface of the melt.
Phys. Rev. Lett. 127, 088001
Vol. 127, Iss. 8 — 20 August 2021
DOI: 10.1103/PhysRevLett.127.088001
Published By: American Physical Society
Impact Factor: 9.161
The abstract and the recording of the lecture are available at: http://pcwww.liv.ac.uk/~robpoole/jnnfm/jnnfm_seminar.htm
The recording is also available below:
]]>An open HighPerformanceComputing ecosystem for the ecological transformation and the advancement of the Plastics Industry in the Regions of West and Central Greece.
The fundamental idea of the easyHPC@eco.plastics.industry.WCG Hub is the adjustment of infrastructure, high scientific and technological training, advanced knowledge, and pioneering algorithms developed by leading Greek Research Laboratories, both in the field of Multiprocessing and Process Simulation at the Molecular, Mesoscopic, and Macroscopic level, as well as in the area of highperformance computing and educational software, in a managementfriendly, growth and innovation environment. The aim is to bridge the gap between the daily necessity and practice in the production of plastics, a sector which is the most active not only in Western and Central Greece, but also the entire Country, and the high level of knowledge and techniques used at a computational level, as well as to promote the harmonization of the local industry with the current European environmental protection policy by advancing the use of new biodegradable materials, which unfortunately cannot be easily manufactured using the standard forming processes
By far, the Consortium owns the most complete and highend computer equipment infrastructure in Western and Central Greece, considerably alleviating the significant disadvantage of the region regarding the lack of usable computing resources, which are related to decreased competitiveness and, subsequently, higher unemployment of highly skilled people.
In consistency with the European Research Policy and the principles for the development of European Hubs, easyHPC@eco.plastics.industry.WCG aims to develop of an open, human and technologydriven ecosystem that will operate in a coordinated manner towards the advancement of scientific knowledge, its dissemination in an affordable and straightforward way, while focusing on the training of company executives and engineers, and on the implementation of innovative solutions for SMEs in the production of plastics based on synthetic and biodegradable polymers
The activities of the Hub will start from the design stage to the final production stage, with the primary goal being the design of improved products, with a view to enhancing their competitiveness. The Hub is based on the interdisciplinary approach to this goal and involves Laboratories and Research Teams from different Scientific Fields and Departments: The Department of Chemical Engineering, the Department of Computer Engineering and Informatics, the ECEDU, the Department of Mechanical and Aeronautical Engineering as well as the Association of Materials Production & Packaging Industries (https://www.pac.gr/). The Hub also includes experimental laboratories of international scope, to utilize, during simulations, the real properties of the raw materials used by each industrial unit. All participants have international distinctions and constant interaction with European Companies in the Area.
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Grant agreement ID: 955605  Grant agreement signed 
Start date 1 January 2021 
End date 31 December 2024 
Overall budget € 3 204 390,24 
Materials like toothpaste, mayonnaise, cement and mud are not commonly thought of as fluids by most of us. However, in the realm of materials science, these materials are called ‘yieldstress fluids’: they ‘flow’ only if a large enough shear stress is exerted on them, causing them to yield to it and start moving rather than simply deforming elastically. Understanding yieldstress fluids is important in conventional sectors, such as food processing and construction, and is growing as related to new technologies, such as 3D printing. However, despite their ubiquity and importance, the current theories describing their flow characteristics hold true only under very basic conditions and their experimental study is still quite challenging. YIELDGAP is establishing an innovative training network uniting academic and industrial experts with bright young researchers to advance models and experimental methods in the field of yieldstress fluids.
Industries based on yieldstress fluids are rapidly expanding in Europe, including key traditional sectors (food, building, paper) and key emerging technologies (advanced materials, advanced manufacturing). The European Parliament has called for investment on these sectors, but so far complex fluid processes in these industries are far from understood, as the research has generally been small scale and/or technologically flawed. In particular: the poor understanding of yieldstress fluid rheology and linking of the measured parameters to the yield properties and insufficient models, means that applications are designed without the understanding of even the basic flow properties (noflow zones, pressure needed to initiate flow), which introduces uncertainties and errors; there is a serious shortage of research trained professionals who combine expertise on complex fluid research, technology and industrial needs. In conjunction with ESR the largest network in Europe in rheology, this ITN will: develop a European multidisciplinary, intersectoral educational research framework for Europe, to comprehensively train 12 early stage researchers (ESRs) to PhD level; provide the research evidence to show how modern measurement and modelling methods may improve the cost and resourceefficiency of industrial processes; provide the European workforce with a much needed new generation of excellent research trained professionals. The scientific objectives of YIELDGAP covered by the 4 research workpackages are to: establish advanced and highly improved numerical modelling tools of complex fluids including additives; validate models by lab and field measurements; integrate measured fluid properties with model parameters; build and disseminate best practices for industry and academia. Using advanced methods where the YIELDGAP partners are world leading, the work of this ITN will culminate in international consensus guidelines for yieldstress fluid flow measurement and prediction.
H2020EU.1.3.1. – Fostering new skills by means of excellent initial training of researchers
MSCAITN2020 – Innovative Training Networks
Date: June 29, 2020, Time: 17:00
Committee: J. Tsamopoulos (supervisor), Y. Dimakopoulos, V. Mavrantzas, A. Beris, E. Mitsoulis, G. Karapetsas, G. Georgiou
]]>Date: December 19, 2019, Time: 10:00
Committee: J. Tsamopoulos (supervisor), Y. Dimakopoulos, V. Mavrantzas, G. Karapetsas, S. Yantsios, V. Bontozoglou, D. Papageorgiou
]]>Links
https://www.journals.elsevier.com/journalofpetroleumscienceandengineering
]]>Marco De Corato, Yannis Dimakopoulos, and John Tsamopoulos “The rising velocity of a slowly pulsating bubble in a shearthinning fluid”, published in Physics of Fluids 31, 083103 (2019)
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We report experimental microfluidic measurements and theoretical modeling of elastoviscoplastic materials under steady, planar elongation. Employing a theory that allows the solid state to deform, we predict the yielding and flow dynamics of such complex materials in pure extensional flows. We find a significant deviation of the ratio of the elongational to the shear yield stress from the standard value predicted by ideal viscoplastic theory, which is attributed to the normal stresses that develop in the solid state prior to yielding.
Toothpaste, face creams, hair gel, mayonnaise and ketchup are household items that most people don’t think twice about, but in terms of their flow behavior, they have unusual properties. They’re all elastoviscoplastic (EVP) materials, which behave like solids when at rest, but can yield to flow like liquids when placed under enough stress. Despite their ubiquity, the ability to model and predict their behavior relies on a theory that has only been shown to work under certain conditions.
Scientists from the Micro/Bio/Nanofluidics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) and the Laboratory of Fluid Mechanics and Rheology at the University of Patras have revealed insights about these materials by combining experiments with simulations. Their research, published in PNAS, suggests that the materials’ elasticity in its solidlike state is a key property that should be included in future models.
“Over the last decade, advances in microfluidics experiments have revealed many unexpected phenomena in the flow of EVP materials,” said Professor John Tsamopoulos, from the University of Patras. “Examples include the cusped shapes of bubbles in the gels and the loss of symmetry in the flow. These and other observations hinted that something was missing from the existing theory. Previous research in our lab suggested that elasticity, the ability of the material’s microstructure to deform before yielding, was the missing part of the puzzle.”
Professor Amy Shen, who leads the OIST Unit, said, “Even when basic household items are set aside, having a fundamental understanding of how EVP materials flow is very useful, especially in biomedical science and geophysics.” For example, she explained, blood is an EVP material—it behaves like a solid at rest, but flows like a liquid in arteries. What’s more, she added, some 3Dprinted tissues and scaffolds can have EVP properties, and, on the geophysics side, volcanic lava behaves like an EVP material albeit on a much larger scale.
Previous experimental research on EVP materials has measured their behavior under shear flow, obtained when layers of fluid slide past each other. But, when it comes to the industrial processing and uses of these materials, such as fiberspinning and circuitboard printing, it’s often the extensional flow—when the fluid is stretched—that’s more important.
The study of purely extensional flows is a great challenge in experimental fluid dynamics, and the extensional flow of EVP materials has never previously been successfully measured in experiments. To achieve this for the first time, Dr. Simon Haward, the group leader from the Micro/Bio/Nanofluidics Unit, used a novel microfluidic apparatus known as a crossslot geometry. The apparatus comprised four channels that were all at right angles to each other.
“Inside the crossslot geometry, we used a Pluronic solution, a wellknown EVP material,” said Dr. Haward. “When we put pressure on the two inbound channels, which were located opposite to each other, the solution was pushed towards the center point and it came out of the other two channels. The resulting flow has a point at the center where the velocity goes to zero. In the two outbound channels, we generated an extensional flow where the fluid was stretched.”
Meanwhile, Professor Yannis Dimakopoulos and researchers at the University of Patras created a theoretical model and simulated the flow of two EVP materials—the Pluronic solution and another material called Carbopol. They showed that complex patterns arose in the flow, which included the presence of solidified regions surrounded by the liquidstate. Their findings matched the experiments performed at OIST.
“This model can describe simple EVP materials in shear, extensional and mixed flows. Although we only focused on two materials, it could be used on a wide variety with varying levels of elasticity, plasticity, viscosity, and other properties,” said Stelios Varchanis, a Ph.D. candidate at the University of Patras and first author of the paper. “This makes the model appropriate for simulating flows during the design and optimization of various industrial processes.”
This research suggests that the existing theory needs to be overhauled to include the elasticity of the material. “Depending on the amount of deformation that the EVP material can sustain before yielding, it will either behave in a way close to what is predicted by the existing theory or will behave more like a flowing elasticsolid,” said Stelios.
“The experiments at OIST complimented the simulations,” said Dr. Cameron Hopkins, from the OIST Micro/Bio/Nanofluidics Unit. “Even though the Pluronic solution that we studied only exhibits weak elastic effects, a small amount of asymmetry was observed in the flow indicating a deviation from purely fluidlike behavior, so the elasticity cannot be neglected. Our experiments provided strong support for the proposed modification of the theory.”
Professor Ken Walters FRS
Ken Walters completed his PhD at the University of Swansea in 1959 under J.G. Oldroyd, thereafter moving to the university of Aberystwyth, where he became Professor in 1973. He is currently a distinguished Research Professor in the Institute of Mathematics and Physics.
His many honours include Gold medal of the British Society of Rheology, Fellow of the Royal Society, Foreign Associate of the US National Academy of Engineering, Weissenberg Award of the European Society of Rheology, President of the European Society of Rheology, Chairman of the International Committee on Rheology. In 2014, Professor Walters was elected as a member of the International Academy of Engineering.
Professor Walters has written 5 books and over 150 research papers. He was Founding and Executive Editor of the Journal of NonNewtonian Fluid Mechanics from its launch in 1976 until the publication of Vol. 100 in 200.
Pettas, D., Karapetsas, G., Dimakopoulos, Y., Tsamopoulos, J., “Viscoelastic film flows over an inclined substrate with topography. I. Steady state”, published in Physical Review Fluids (Phys. Rev. Fluids 4, 083303).
Pettas, D., Karapetsas, G., Dimakopoulos, Y., Tsamopoulos, J., “Viscoelastic film flows over an inclined substrate with topography. II. Linear Stability Analysis”, published in Physical Review Fluids (Phys. Rev. Fluids 4, 083304).
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Date: July 30, 2018, Time: 10:30
Committee: J. Tsamopoulos, V. Mavrantzas, Y. Dimakopoulos (supervisor)
A. Syrakos, S. Varchanis, Y. Dimakopoulos, A. Goulas, J. Tsamopoulos
Finite volume methods (FVMs) constitute a popular class of methods for the numerical simulation of fluid flows. Among the various components of these methods, the discretisation of the gradient operator has received less attention despite its fundamental importance with regards to the accuracy of the FVM. The most popular gradient schemes are the divergence theorem (DT) (or GreenGauss) scheme and the leastsquares (LS) scheme. Both are widely believed to be secondorder accurate, but the present study shows that in fact the common variant of the DT gradient is secondorder accurate only on structured meshes whereas it is zerothorder accurate on general unstructured meshes, and the LS gradient is secondorder and firstorder accurate, respectively. This is explained through a theoretical analysis and is confirmed by numerical tests. The schemes are then used within a FVM to solve a simple diffusion equation on unstructured grids generated by several methods; the results reveal that the zerothorder accuracy of the DT gradient is inherited by the FVM as a whole, and the discretisation error does not decrease with grid refinement. On the other hand, use of the LS gradient leads to secondorder accurate results, as does the use of alternative, consistent, DT gradient schemes, including a new iterative scheme that makes the common DT gradient consistent at almost no extra cost. The numerical tests are performed using both an inhouse code and the popular public domain partial differential equation solver OpenFOAM.
]]>S. Varchanis, Y. Dimakopoulos, J. Tsamopoulos
Film flow along an inclined, solid substrate featuring periodic rectangular trenches may either completely wet the trench floor (Wenzel state) orgetpinnedon the entrance and exit corners of the trench (Cassie state) or assume other configurationsin between these two extremes. Such intermediate configurations are examined in the present study. They are boundedby a second gasliquid interface inside the trench, which adheres to itswalls forming two threephase contact lines, and encloses a different amount of air under different physical conditions. The Galerkin finite element method is used to solve the NavierStokes equations in a physical domain, which is adaptively remeshed. Multiple steady solutions, connected by turning points and transcritical bifurcationsas well as isolated solution branches, are revealed by pseudo arclength continuation. Twopossible configurationsof a single air inclusion inside the trench are examined: the inclusion either surrounds the upstream convex corner or is attached to the upstream trench wall. The penetration of the liquid inside the trench is enhanced primarily by increasing either the wettability of the substrate orcapillaryover viscous forcesor by decreasing the flow rate. Flow hysteresis may occur when the liquid wetting of the upstream wall decreases abruptly leading to drastically different flow patternsfor the same parameter values. The interplay of inertia,viscous, gravity and capillary forces along with substrate wettability determines the volume of the air encapsulated in the trench and the extent of deformation of the outer free surface.
]]>E. Martino, G. Koilias, M. Athanasiou, A. Katsaounis, Y. Dimakopoulos, J. Tsamopoulos, C.G. Vayenas
The triode operation of humidified PEM fuel cells has been investigated both with pure H_{2} and with CO poisoned H_{2} feed over commercial Vulcan supported Pt(30%)Ru(15%) anodes. It was found that triode operation, which involves the use of a third, auxiliary, electrode, leads to up to 400% power output increase with the same CO poisoned H_{2} gas feed. At low current densities, the power increase is accompanied by an increase in overall thermodynamic efficiency. A mathematical model, based on Kirchhoff’s laws, has been developed which is in reasonably good agreement with the experimental results. In order to gain some additional insight into the mechanism of triode operation, the model has been also extended to describe the potential distribution inside the Nafion membrane via the numerical solution of the NernstPlanck equation. Both model and experiment have shown the critical role of minimizing the auxiliaryanode or auxiliarycathode resistance, and this has led to improved combshaped anode or cathode electrode geometries.
Nafion membrane, PEM fuel cell, Triode operation, CO poisoning, NernstPlanck equation
]]>D.Fraggedakis, J.Papaioannou, Y.Dimakopoulos, J.Tsamopoulos
A new boundaryfitted technique to describe free surface and moving boundary problems is presented. We have extended the 2D elliptic grid generator developed by Dimakopoulos and Tsamopoulos (2003) [19] and further advanced by Chatzidai et al. (2009) [18] to 3D geometries. The set of equations arises from the fulfillment of the variational principles established by Brackbill and Saltzman (1982) [21], and refined by Christodoulou and Scriven (1992) [22]. These account for both smoothness and orthogonality of the grid lines of tessellated physical domains. The ellipticgrid equations are accompanied by new boundary constraints and conditions which are based either on the equidistribution of the nodes on boundary surfaces or on the existing 2D quasielliptic grid methodologies. The capabilities of the proposed algorithm are first demonstrated in tests with analytically described complex surfaces. The sequence in which these tests are presented is chosen to help the reader build up experience on the best choice of the elliptic grid parameters. Subsequently, the mesh equations are coupled with the Navier–Stokes equations, in order to reveal the full potential of the proposed methodology in free surface flows. More specifically, the problem of gas assisted injection in ducts of circular and square crosssections is examined, where the fluid domain experiences extreme deformations. Finally, the flowmesh solver is used to calculate the equilibrium shapes of staticmenisci in capillary tubes.
Moving boundary problems, Mesh generation, Freesurface flows, Ellipticgrid generation, Moving contact line, Contact angle models
]]>A. Syrakos, S. Varchanis, Y. Dimakopoulos, A. Goulas, J. Tsamopoulos
The divergence theorem (or GreenGauss) gradient scheme is among the most popular methodsfor discretising the gradient operator in secondorder accurate finite volume methods, with a longhistory of successful application on structured grids. This together with the ease of applicationof the scheme on unstructured grids has led to its widespread use in unstructured finite volumemethods (FVMs). However, the present study shows both theoretically and through numericaltests that the common variant of this scheme is zerothorder accurate (it does not converge to theexact gradient) on grids of arbitrary skewness, such as typically produced by unstructured gridgeneration algorithms. Moreover, we use the scheme in the FVM solution of a diffusion (Poisson)equation problem, with both an inhouse code and the popular opensource solver OpenFOAM,and observe that the zerothorder accuracy of the gradient operator is inherited by the FVM solveras a whole. However, a simple iterative procedure that exploits the outer iterations of the FVMsolver is shown to effect firstorder accuracy to the gradient and secondorder accuracy to the FVMat almost no extra cost compared to the original scheme. Secondorder accurate results are alsoobtained if a leastsquares gradient operator is used instead.
Finite volume method; GreenGauss gradient; divergence theorem gradient; diffusionequation; OpenFOAM
]]>D.Pettas, G.Karapetsas, Y.Dimakopoulos, J.Tsamopoulos
Liquid film flow along an inclined plane featuring a slit, normal to the main directionof flow, creates a second gas–liquid interface connecting the two side walls of theslit. This inner interface forms two threephase contact lines and supports a widelyvarying amount of liquid under different physical and geometrical conditions. Theexact liquid configuration is determined by employing the Galerkin/finite elementmethod to solve the twodimensional Navier–Stokes equations at steady state. Theinterplay of inertia, viscous, gravity and capillary forces along with the substratewettability and orientation with respect to gravity and the width of the slit determinethe extent of liquid penetration and freesurface deformation. Finite wetting lengthsare predicted in hydrophilic and hydrophobic substrates for inclination angles moreor less than the vertical, respectively. Multiple steady solutions, connected by turningpoints forming a hysteresis loop, are revealed by pseudoarclength continuation. Underthese conditions, small changes in certain parameter values leads to an abrupt changein the wetting length and the deformation amplitude of the outer film surface. Inhydrophilic substrates the wetting lengths exhibit a local minimum for small valuesof the Reynolds number and a very small range of Bond numbers; when inertiaincreases, they exhibit the hysteresis loop with the second limit point in a very shortrange of Weber numbers. Simple force balances determine the proper rescaling ineach case, so that critical points in families of solutions for different liquids or contactangles collapse. The flow inside the slit is quite slow in general because of viscousdissipation and includes counterrotating vortices often resembling those reported byMoffatt (J. Fluid Mech., vol. 18, 1964, pp. 1–18). In hydrophobic substrates, thewetting lengths decrease monotonically until the first limit point of the hysteresisloop, which occurs in a limited range of Bond numbers when the Kapitza number isless than 300 and in a limited range of Weber numbers otherwise. Here additionalsolution families are possible as well, where one or both contact points (Cassie state)coincide with the slit corners.
coating, microfluidics, thin films
]]>Evan Mitsoulis, John Tsamopoulos
Viscoplasticity is characterized by a yield stress, below which the materials will not deform and above which they will deform and flow according to different constitutive relations. Viscoplastic models include the Bingham plastic, the HerschelBulkley model and the Casson model. All of these ideal models are discontinuous. Analytical solutions exist for such models in simple flows. For general flow fields, it is necessary to develop numerical techniques to track down yielded/unyielded regions. This can be avoided by introducing into the models a regularization parameter, which facilitates the solution process and produces virtually the same results as the ideal models by the right choice of its value. This work reviews several benchmark problems of viscoplastic flows, such as entry and exit flows from dies, flows around a sphere and a bubble and squeeze flows. Examples are also given for typical processing flows of viscoplastic materials, where the extent and shape of the yielded/unyielded regions are clearly shown. The abovementioned viscoplastic models leave undetermined the stress and elastic deformation in the solid region. Moreover, deviations have been reported between predictions with these models and experiments for flows around particles using Carbopol, one of the very often used and heretofore widely accepted as a simple “viscoplastic” fluid. These have been partially remedied in very recent studies using the elastoviscoplastic models proposed by Saramito.
Viscoplastic fluids, Bingham plastics, HerschelBulkley fluids, Viscoplastic models, Simulations, Yield stress, Yielded/unyielded regions, Elastoviscoplastic fluids
]]>G. Karapetsas, N. K. Lampropoulos, Y. Dimakopoulos, J. Tsamopoulos
We examine the transient film flow under the action of gravity over solid substrates with threedimensional topographical features. Our focus is placed on the coating of a periodic array of rectangular cuboid trenches. The Navier–Stokes equations are solved using the volumeoffluid method, fully taking into account the flow in both the liquid and gas phases. Using this scheme, we are able to determine the different wetting patterns that may arise depending on parameters such as the various geometrical characteristics of the trench, the lateral distance between them, the substrate wettability and the liquid viscosity. We present flow maps that describe the conditions under which the liquid film may successfully coat the patterned substrate, resulting in the socalled Wenzel state, or air may become entrapped inside the topography of the substrate. In the latter case, we describe in detail the position and shape of the air inclusions, how they are formed and the conditions under which coating can approach the ideal Cassie–Baxter state. We investigate in detail the effect of the sidewalls, typically ignored when considering the case of ideal 2D trenches (i.e., trenches extending to infinity in the lateral direction), through the enhancement of the viscous resistance inside the trench and the effect of capillarity in the case of narrow trenches. We also examine the coating behavior for a wide range of liquids and show that successful coating is favored for liquids with moderate viscosities. Finally, we perform simulations for the coating of two successive trenches in the flow direction and show that in the case of 3D trenches, the differences between the coating of the first and subsequent trenches are not significant.
Thinfilm flow, Flow over topography, Coating flows, Air entrapment, Cassie and Wenzel states
]]>ABSTRACT
In this work, we examine the flow of elastoviscoplastic materials in the optimizedshape crossslot extensional rheometer (OSCER). The shape of this geometry has been optimized numerically in order to generate a wide region of homogeneous elongational flow. Haward et al. demonstrated experimentally using PEO solutions, that such an ideal flow can remain steady for a wide range of extension rates in the OSCER. Based on their results for viscoelastic materials in the OSCER, we study the dynamics of elastoviscoplastic materials in this purely extensional flow field. Such an investigation of yieldstress materials in elongational flows is necessary as indicated by recent experiments of Zhang et al. In their work, noticeable differences have been revealed with respect to the extensional behavior of complex yieldstress materials (e.g. the ratio of the extension yieldstress to the shear yieldstress is larger, by a factor of 1.5, than expected from the standard theory). In order to investigate the dynamics of elastoviscoplastic materials in strong extension, we have simulated the flow of a 0.08% Carbopol® 940 aqueous solution using the Saramito/HerschelBulkley constitutive equation^{ [3]} in the OSCER geometry. Performing a widerange parametric analysis of the dynamics of the flow, we determine when steadystate or oscillatory solutions arise. Surprisingly, we find that even at high extension rates the system can reach a steady state at which strong extension dominates a wide region around the stagnation point. However, when the flow rate is furtherly increased, we identify a critical value of the flow rate that drives the system to a periodic state.
]]>Place: Small Seminar Room, 1st Floor, Main Building of Chemical Engineering Department
Date: July 17, 2018, Time: 11:00
Organizer: The fluids Lab
Place: XM1, 1st Floor, Main Building of Chemical Engineering Department
Date: July 6, 2018, Time: 09:00
Committee: J. Tsamopoulos, K. Tsitsilianis, Y. Dimakopoulos (supervisor)
ο Τμήμα Χημικών Μηχανικών του Αριστοτέλειου Πανεπιστημίου Θεσσαλονίκης έχει την τιμητική ευθύνη της διοργάνωσης του 11ου Πανελλήνιου Επιστημονικού Συνεδρίου Χημικής Μηχανικής (11 ΠΕΣΧΜ), το οποίο θα διεξαχθεί στη Θεσσαλονίκη, από τις 25 έως τις 27 Μαΐου 2017, στο Συνεδριακό Κέντρο του Μεγάρου Μουσικής Θεσσαλονίκης.
The 6th International Conference on Biomedical Engineering and Biotechnology (ICBEB 2017) will take place on 1720 October 2017 in Guangzhou, China
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23th Congress of the European Society of Biomechanics will take place on 25 July 2017 in Seville, Spain
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The current work investigates the effect of blood viscoelasticity on the stress, velocity and haematocrit fields as well as the formation of the celldepletionlayer for haemodynamics in microvessels lined with a glycocalyx layer. To this end, we advance the inhomogeneous constitutive model proposed by Moyers GonzalezOwensFang (2015), treating properly the shear induced RBC migration mechanism and incorporating the effect of plasma viscoelasticity. The enhanced model (enmodel) can accurately reproduce the experimental data by DamianoLongSmith (2004) and provide reasonable predictions for the RBC distribution and the shearstress along the luminal surface.
It is shown that blood viscoelasticity expressed in terms of the Deborah number is quite small (O(0.3)), but plays a crucial role in the formation of the celldepletionlayer. In particular, there is a critical value of Deborah number above which the formation of the celldepletionlayer occurs.
]]>This force is attractive, if the forcing frequency lies outside the range of eigenfrequencies for volume oscillation of the two bubbles. Here we study the nonlinear interaction of two deformable bubbles set in oscillation in water by a step change in the ambient pressure, by solving the Navier–Stokes equations numerically.
As in typical experiments, the bubble radii are in the range 1–1000 μm. We find that the smaller bubbles (῀5 μm) deform only slightly, especially when they are close to each other initially. Increasing the bubble size decreases the capillary force and increases bubble acceleration towards each other, leading to oblate or spherical cap or even globally deformed shapes.
These deformations may develop primarily in the rear side of the bubbles because of a combination of their translation and harmonic or subharmonic resonance between the breathing mode and the surface harmonics. Bubble deformation is also promoted when they are further apart or when the disturbance amplitude decreases.
The attractive force depends on the Ohnesorge number and the ambient pressure to capillary forces ratio, linearly on the radius of each bubble and inversely on the square of their separation. Additional damping either because of liquid compressibility or heat transfer in the bubble is also examined.
]]>The Laboratory of Fluid Mechanics and Rheology (The Fluids Lab) primary research areas are Bubble Dynamics, Multiphase / Biological Flows, Wetting Flows, Coating Process, Complex Flows & Interfacial Instabilities, Pressure Sensitive Adhesives.
]]>Our Lab is equipped with 5 computer clusters with a total of 35 nodes and a total computational power of 214 cores. The total RAM of these clusters is 1.8 TB and the total hard disk capacity 16 TB. It also includes 11 single processor workstations having in total 49 cores, 620 GB RAM and 7 TB total hard disk capacity. 4 of these workstations are equipped with CUDAenabled graphic adapters.
Furthermore the Lab is equipped with a Network Accessible Storage (NAS) with a total capacity of 20TB used as backup server.
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Advanced grid generation algorithms have been implemented for the simulation of freesurface and multiphase flows that typically arise in chemical engineering processes.
Our research is focused on both structured and unstructured grid generation techniques that can optimally adjust the location of nodes according to the flow field.
Sets of elliptic or hyperbolic differential equations are solved for the description of mesh motion along curves, on surfaces, and in volumes.
Flow of highly viscous liquid (in red) through a low viscosity, lubricating liquid (in blue) in a constricted and periodic tube with increasing flow rates (from top to bottom).
]]>For diverse applications such as polymer devolatilization, fermentation, composites processing, plastic foam processing, gas absorption and others it is of interest to know the rise velocity (hence the residence time) and shape of bubbles in polymeric solutions. It is known that single bubbles rising in quiescent viscoelastic liquids may exhibit a sudden (discontinuous) 310 fold increase in their rise velocity, once their volume exceeds a critical value. The abrupt change of the bubble rise velocity is accompanied by a change in the bubble shape from a convex to a cusped or “teardrop” shape. (see below).
Predictions of the effect of fluid elasticity on the shape of a deformable air bubble in very good agreement with experiments.
The motion of air bubbles in complex fluids has drawn our attention because of its numerous applications. Characteristic examples of such applications include: a) bubble removal from structural materials (i.e. cement) through vibration, b) entrapment of air bubbles in many foods to improve their flavor and texture (i.e. chocolate, ketchup), c) prevention of large bubble formation in drilling mud, which may cause dangerous explosions, inhibit production and potentially inflict huge burden on the ecosystem and the finances of oildrilling companies.
]]>Topleft view: 163 Cavitating bubbles from the experiment by the French collaborating group. Bottomleft view: Cavitating bubbles from our simulations. Rightview: Detail of the cavitating bubbles
Cross sections of the mid plane of a cylindrical sample of PSA that contains an arbitrary number of deformable cavities, for different nominal elongational strains (ε) .
]]>During fabrication of microelectronic components, polymer solutions are used to produce thin films over surfaces with uneven topography. The patterning of the substrates may require using multilayer coatings, where one of the purposes of the bottom layer is to planarize the topography, i.e. to smooth out the uneven topography. A nonuniform coating leads to low quality products or to manufacturing failures.
The problem of liquid advancing over orthogonal 2D and 3D substrates with variable depth, width and length under the influence of the gravitational force resulting in different physical phenomena is addressed.
Firstly, we can achieve continuous coating by which a thin film of fluid is developed throughout the trench while a steady flow of fluid is established upstream and downstream the topography which is welcome by coating of microelectronic devices.
Secondly, we may result in poor coating, namely the entrainment of air between the trenches of the substrate and the coating liquid inducing less fluid friction, therefore desirable in producing superhydrophobic surfaces for microfluidic applications.
The software tool used is the application interFoam comprised in the OpenFOAM open source CFD software package for simulation of twophase flow of two immiscible fluids, namely air and a viscous liquid. All calculations were implemented on the laboratory HPC clusters using Message Passing Interface (MPI).
Snapshots of the the covering process (red color: liquid, blue color: water).
Sharkskin instability in an extruded polymer melt.
Spatial form of the eigenvector for a cylindrical die.
]]>APIVITA SA (Project Coordinator)
University of Patras (Scientific Coordinator)
University of Athens
Anna Lotan Ltd
Technion
Project Meetings Project Results
Mud is one of the most often used natural materials for preventive, healing and cosmetic reasons. Although mud has been used since the antiquity, little light has been shed on its physical, chemical and biological properties. Besides the lack of information concerning its chemical composition, mud demonstrates significant physicochemical stability issues. The main problem is the phase separation between aqueous and solid phase, which makes its use difficult a short while after storage. This fact limits the application of mud in the areas nearby its origin and its incorporation in high added value formulations. Furthermore, the biological activity of mud, although well accepted, has never been scientifically proven. The main goals of this coordinated research effort are:
a) Characterization in terms of chemical composition of widely used muds from different areas of Greece and Israel,
b) Stabilization of these muds over time and simultaneously theoretical examination of some of the open fundamental questions related to particle(s) motion in a viscoelastoplastic fluid,
c) Proof and promotion of their biological properties, and
d) Formulation of selected muds into face masks and scaling up the process.
In this way, mud will improve its potential to reach a wide range of people in need of its beneficial properties through its incorporation into cosmetic product forms.
Dead Sea mud from Israel is a betterknown and established material. On the contrary Greek mud, although it has been used since antiquity and is currently being used for mud baths at SPAs in different areas of Greece, it has remained unexplored scientifically and technologically. The unique advantage of Greek muds is their variable chemical composition, which is responsible for a variety of beneficiary activities. Therefore it is a challenge to investigate the composition of mud from different areas of Greece. The mutual interest of both companies in the project lies on the physicochemical instability of mud independent from its origin, a fact that makes its formulation and further exploitation rather difficult. The collaboration with the Chemical Engineering Department in the University of Patras is of utmost importance due to the unknown structure, physicochemical complexity and stability problems of the material, while the collaboration with the Medical school of the University of Athens is equally important because of the need to characterize its most relevant biological properties. Although muds from Israel and Greece are expected to differ in composition, the transfer of knowledge between all partners (Universities and Industry from Greece and Israel) is of crucial importance for the successful completion of the project.
APIVITA is a leading cosmetic company in Greece and demonstrates very significant sales worldwide in that area. As Greek mud is almost completely unexploited, it is certain that developing face masks with specific properties and incorporating mud as active ingredient will improve the competitiveness of APIVITA SA in the field. APIVITA face masks exhibited in 2012 an annual turnover of approximately 3 million euros, a fact that justifies the cost effectiveness of the investment even for the sustainability, if not the growth, of the company in the field. The market share of APIVITA combined with the fact of the rapid expansion of APIVITA in hotels & SPAs as well as the opening of new APIVITA standalone stores around the world strengthens the need for investment in new mud applications. It is certain that upon successful completion of the project, i.e. in early 2016, APIVITA will have at least 2 new face masks in the global market containing Greek mud.
Project Meetings Project Results
Contours of the velocity magnitude on the surface of an evolving air enclosure and on the solid substrate, via a slip condition.
Contours of the magnitude of the rate of strain tensor on the same surfaces.
]]>Two Dimensional FibreReinforced Polymer are characterized by a laminated structure in which the fibres are only aligned along the plane in xdirection and ydirection of the material. This means that no fibres are aligned in the through thickness or the zdirection, this lack of alignment in the through thickness can create a disadvantage in cost and processing. Costs and labour increase because conventional processing techniques used to fabricate composites, such as wet hand layup, autoclave and resin transfer moulding, require a high amount of skilled labor to cut, stack and consolidate into a preformed component.
When the raw material contains reinforcing fibres, a compression molded part qualifies as a fibrereinforced plastic. More typically the plastic preform used in compression molding does not contain reinforcing fibres. In compression molding, a “preform” or “charge”, of SMC, BMC is placed into mould cavity. The mould is closed and the material is formed & cured inside by pressure and heat. Compression moulding offers excellent detailing for geometric shapes ranging from pattern and relief detailing to complex curves and creative forms, to precision engineering all within a maximum curing time of 20 minutes.
The current effort deals with the numerical study of a polymer melt reinforced by a network of two dimensional fibers under compression. An advanced Lagrangian Finite Element Algorithm has been developed which assumes the Fibers as a stiff NeoHookean material and the melty matrix as a multimode viscoelastic fluid.
]]>Extrusion damper configuration
Viscous dampers dissipate mechanical energy into heat through the action of viscous stresses in a fluid. The damper response to external stimulus (force or displacement) depends on the properties of the enclosed fluid. NonNewtonian fluids have widely varying properties, enabling a range of possibilities for the damper response. We use numerical models to simulate the operation of dampers containing nonNewtonian fluids, such as viscoplastic fluids.
Flow field snapshot: Pressure contours, streamlines, and unyielded areas (white).
Rate of viscous dissipation of energy.
]]>1. Advanced constitutive modeling of the thixotropic elastoviscoplastic behavior of blood: Description of the model and rheological predictions Journal Article
In: Materials , 13(18) , pp. 4184, 2020.
2. Dynamics of viscoplastic filament stretching Journal Article
In: Journal of NonNewtonian Fluid Mechanics, 284 (104371), 2020.
3. PEGAFEMV: A new petrovgalerkin finite element method for free surface viscoelastic flows Journal Article
In: Journal of NonNewtonian Fluid Mechanics, 284 , pp. 104365, 2020.
4. Transition between solid and liquid state of yieldstress fluids under purely extensional deformations Journal Article
In: PNAS (Proceedings of the National Academy of Sciences of the United States of America), 117 (23) , pp. 1261112617, 2020.
5. Asymmetric flows of complex fluids past confined cylinders: A comprehensive numerical study with experimental validation Journal Article
In: Physics of Fluids, 32(5) (0008783), 2020.
6. Steady flow of a viscoelastic film over an inclined plane featuring periodic slits Journal Article
In: Journal of NonNewtonian Fluid Mechanics, pp. 104243, 2020, ISSN: 03770257.
7. Electroosmotic flow of electrolyte solutions of PEO in microfluidic channels Journal Article
In: Journal of Colloid and Interface Science, 563 , pp. 381393, 2020, ISSN: 00219797, (cited By 0).
8. A finite volume method for the simulation of elastoviscoplastic flows and its application to the liddriven cavity case Journal Article
In: Journal of NonNewtonian Fluid Mechanics, 275 , 2020, ISSN: 03770257, (cited By 0).
9. Viscoelastic film flows over an inclined substrate with sinusoidal topography. II. Linear stability analysis Journal Article
In: Physical Review Fluids, 4 (8), 2019, ISSN: 2469990X, (cited By 1).
10. Viscoelastic film flows over an inclined substrate with sinusoidal topography. I. Steady state Journal Article
In: Physical Review Fluids, 4 (8), 2019, ISSN: 2469990X, (cited By 1).
11. The rising velocity of a slowly pulsating bubble in a shearthinning fluid Journal Article
In: Physics of Fluids, 31 (8), 2019, ISSN: 10706631, (cited By 1).
12. Modeling the rheology of thixotropic elastoviscoplastic materials Journal Article
In: Journal of Rheology, 63 (4), pp. 609639, 2019, ISSN: 01486055, (cited By 1).
13. Oscillations of small bubbles and medium yielding in elastoviscoplastic fluids Journal Article
In: Physical Review Fluids, 4 (7), 2019, ISSN: 2469990X, (cited By 1).
14. A new finite element formulation for viscoelastic flows: Circumventing simultaneously the LBB condition and the highWeissenberg number problem Journal Article
In: Journal of NonNewtonian Fluid Mechanics, 267 , pp. 7897, 2019, ISSN: 03770257, (cited By 1).
15. Dynamics and motion of a gas bubble in a viscoplastic medium under acoustic excitation Journal Article
In: Journal of Fluid Mechanics, 865 , pp. 381413, 2019, ISSN: 00221120, (cited By 2).
16. The PAL (Penalized Augmented Lagrangian) method for computing viscoplastic flows: A new fast converging scheme Journal Article
In: Journal of NonNewtonian Fluid Mechanics, 256 , pp. 2341, 2018, ISSN: 03770257, (cited By 13).
17. Theoretical study of the flow in a fluid damper containing high viscosity silicone oil: Effects of shearthinning and viscoelasticity Journal Article
In: Physics of Fluids, 30 (3), 2018, ISSN: 10706631, (cited By 8).
18. Evaluation of tube models for linear entangled polymers in simple and complex flows Journal Article
In: Journal of Rheology, 62 (1), pp. 2547, 2018, ISSN: 01486055, (cited By 9).
19. How viscoelastic is human blood plasma? Journal Article
In: Soft Matter, 14 (21), pp. 42384251, 2018, ISSN: 1744683X, (cited By 14).
20. Steady film flow over a substrate with rectangular trenches forming air inclusions Journal Article
In: Physical Review Fluids, 2 (12), 2017, ISSN: 2469990X, (cited By 7).
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Project Overview Project Results
Photos from the visit of Greekside Project Coordinator, Dr. Konstantinos Gardikis (APIVITA SA) to the premises of the Israeli partner Anna Lotan Ltd. at Ceseare on 2930 November 2015.
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Project Overview Project Results
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Project Overview Project Meetings
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Professor John Tsamopoulos has been named a Fellow of the American Physical Society “For outstanding contributions, via insightful computations and analyses, to the fundamental understanding of flows of twophase materials and viscoplastic fluids” as stated in the official assessment. The nomination ceremony will be held on November 2016 during the 69th Annual Meeting of the APS Division of Fluid Dynamics and will be included in APS News magazine (December edition).
The American Physical Society (APS) is a nonprofit membership organization working to advance and diffuse the knowledge of physics through its outstanding research journals, scientific meetings, and education, outreach, advocacy, and international activities. APS represents over 51,000 members, including physicists in academia, national laboratories, and industry in the United States and throughout the world.
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The award is given biennially and recognises the contribution of members of the viscoplastic fluids community.
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University of Patras, Greece  

University of Patras, Greece  

National Technical University of Athens (NTUA), Greece  

university of Nottingham, UK  

University of Cambridge, UK  

Baker Hughes, UK  

University of Berkeley, USA  

Texas A&M, USA  

Massachusetts Institute of Technology, USA  

École polytechnique fédérale de Lausanne (EPFL), Switzerland  

Charles University in Prague, Czech Republic  

Managing Partner at Leonardo Group in Vietnam  



Stanford University, USA.  

Lecturer & Research Associate at ETH Zürich  

Public sector, Greece  

TEI of Athens, Greece  

Director of Manufacturing at Passion Fashion, Greece  



Research Engineer, Praxair Technology, USA.  

Senior Plastics Engineer, TAIWAN.  

Senior Engineer, National Starch & Chemical, USA  

Dep. of Mechanical Engineering, Univ. of Thessaly, Greece  

Senior Engineer, TAIWAN  

Kavala oil, Greece  

Engineer, ENIA Co., Greece.  

Dep. of Mathematics, University of the Aegean, Greece.  

Senior Engineer, Heracles Group of Companies, Greece.  

Engineer, ENIA Co., Greece.  

University of Pireaus, Greece.  

Secondary Education, Greece.  

Post Doctoral Fellow, Institute for Optoelectronics and Microsystems (ISOM), Universidad Politécnica de Madrid, Spain.  

Engineer, Unilever Co., Greece.  

Public sector, Greece.  

New York University, USA.  

Forschungszentrum Jülich GmbH, Germany.  

RWTH Aachen University, Germany  

University of Patras, Greece  

National Technical University of Athens (NTUA), Greece.  

University of Patras, Greece  

UT Austin, USA  

Senior Engineer, General Electric – Plastics, USA.  

Consultant, NeuStar/Accenture, USA.  

Private Business, Greece. 
Konstantina Kalkasina
Email: konakalk [at] hotmail.com 

Dimitris Zabelis
Email: dimitriszambelis[at]gmail.com 

Panagiota Kollia
Email: kogiota1[at]hotmail.com 

Alexandros Spyridakis
Email: up1055897 [at] upnet.gr 
Place: Main Seminar Room, 1st Floor, Main Building of Chemical Engineering DepartmentDate: July 6, 2016 , Time: 12:00
Committee: J. Tsamopoulos (supervisor), V. Mavrantzas, Y. Dimakopoulos
Asst. Prof. George Karapetsas email: gkarapetsas [at] gmail.com


Lecturer Alexandros Syrakos email: alexandros.syrakos [at] gmail.com


Dr. Dionysios Photeinos email: photeinos [at] gmail.com 

Dr. Dionisis Pettas email: d.pettas [at] chemeng.upatras.gr 

Stelios Varchanis email: varchanis [at] hotmail.com 
Konstantinos Giannokostas email: giannoko [at] hotmail.com 

Vlassis Mitsoulas email: blazzee1 [at] gmail.com 

Pantelis Moschopoulos
email: pmoschopoulos[at]outlook.com 

Thanos Kordalis
email: tkordalis8[at]gmail.com 

Konstantina Psaraki email: psaraki.k@gmail.com 

Antonis Marousis email: antonismar96[at]gmail.com 

Christos Psevdos email: c.psevdos[at]gmail.com


Eleni Kouni email: elenikouni [at] gmail.com


Milad Mousavi email: m.mousavi.092 [at] gmail.com


Spyros Plakogiannakis email: splakogiannakis [at] gmail.com

Email: dimako [at] chemeng.upatras.gr
Download Business card (as PDF)
Diploma in Chemical Engineering, University of Patras (1997)
Master in the Simulation, Optimization and Control of Processes, University of Patras (2003)
Doctoral Diploma in Chemical Engineering, University of Patras (2003)
Dimakopoulos, Y., Karapetsas, G., Malamataris, N.A., Mitsoulis, E., “The Free (open) Boundary Condition at Inflow Boundaries”, J. NonNewtonian Fluid Mech., 187188, 1631 (2012) DOI: 10.1016/j.jnnfm.2012.09.001
Dimakopoulos, Y., Bogaerds, A., Anderson, P., Hulsen, M., Baaijens, F.P.T., “Direct numerical simulation of a 2D idealized aortic heart valve at physiological flow rates”, Comp. Meth. Biomech. and Biomed. Engrg, 15 (11), 11571179 (2012) DOI: 10.1080/10255842.2011.581238
Dimakopoulos, Y., “An efficient parallel fully implicit algorithm for the simulation of transient free surface flows of multimode viscoelastic liquids”, J. NonNewtonian Fluid Mech., 165 (78), 409424 (2010) DOI: 10.1016/j.jnnfm.2010.01.017
Dimakopoulos, Y., and Tsamopoulos, J., “A quasielliptic transformation for moving boundary problems with large anisotropic deformations”, J. Comp. Physics, 192, 494522 (2003) DOI: 10.1016/j.jcp.2003.07.027
Tseropoulos, G., Dimakopoulos, Y., Tsamopoulos, J., and Lyberatos, G., “On the flow characteristics of the conical Minoan pipes used in water supply systems, via Computational Fluid Dynamics”, J. Archaeological Sc. 40 (4), 20572068 (2013) DOI: 10.1016/j.jas.2012.11.025
Pavlidis, M., Dimakopoulos, Y., and Tsamopoulos, J., “Analytical and numerical solutions for the flow of an undeformed viscoelastic film down a vertical cylinder”, Rheologica Acta, 48 (9), 10311048(2009)
Dimakopoulos, Y., Bogaerds, A., Anderson, P., and Baaijens, F., “Application of the ficticious domain/Lagrange multiplier method on the simulation of the aortic valve”, GRACM 2011, JuneJuly 2011, Athens, GREECE.
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Email: tsamo [at] chemeng.upatras.gr
Download Business card (as PDF)
Diploma, Chemical Engineering, National Technical University of Athens, 1979.
M.S. Chemical Engineering, Mass. Inst. of Technology, 1981.
Ph.D.,Chemical Engineering, Mass. Inst. of Technology, 1985.
Varchanis, S., Syrakos, Al., Dimakopoulos, Y., and Tsamopoulos, J., “A new finite element formulation for viscoelastic flows: Circumventing simultaneously the LBB conditions and the highWeissenberg number problem”, J. NonNewtonian Fluid Mech., 267, 7897, (2019), doi.org/10.1016/j.jnnfm.2019.04.003
Varchanis, S., Makrigiorgos, G., Moschopoulos, P., Dimakopoulos, Y., and Tsamopoulos, J., “Modelling and Rheology of Thixotropic Elastoviscoplastic Fluids”, J. Rheol. 63, 609639 (2019), https://doi.org/10.1122/1.5049136
De Corato, M., SaintMichel, B., Makrigiorgios, G., Dimakopoulos, Y., Tsamopoulos, J., Garbin, V., “Oscillations of small bubbles and medium yielding in elastoviscoplastic fluids”, Phys. Rev. Fluids, paper 073301 (2019), https://doi.org/10.1103/PhysRevFluids.4.073301 , DOI:10.1103/PhysRevFluids.4.073301
Pettas, D., Karapetsas, G., Dimakopoulos, Y. and Tsamopoulos, J., “Viscoelastic film flows over an inclined substrate with sinusoidal topography, II. Linear stability analysis”, Phys. Rev. Fluids, 4(8), (2019) URL: https://link.aps.org/doi/10.1103/PhysRevFluids.4.083304, Editors’ Suggestion.
Syrakos, Al., Dimakopoulos, Y., and Tsamopoulos, J., “A finite volume method for the simulation of elastoviscoplastic flows and its application to the liddriven cavity case” J. NonNewtonian Fluid Mech., 275, 104216 (2020), https://doi.org/10.1016/j.jnnfm.2019.104216
Moschopoulos, P., Dimakopoulos, Y., and Tsamopoulos, J., “Electroosmotic flow of electrolyte solutions of PEO in microfluidic channels”, J. Colloid Interf. Sci. 563, 381393 (2020), https://doi.org/10.1016/j.jcis.2019.12.052.
Varchanis, S., Cameron C. Hopkins, A. Q. Shen, J. Tsamopoulos, and S. J. Haward, “Asymmetric flows of complex fluids past confined cylinders: A comprehensive numerical study with experimental validation”, Phys. Fluids, 32, 053103 (2020). https://doi.org/10.1063/5.0008783, featured article in the May 2020 issue of the Journal.
Varchanis, S., Haward, S.J., Hopkins C.C., Syrakos, A., Shen, A.Q., Dimakopoulos, Y. and Tsamopoulos, J., Transition between solid and liquid state of yieldstress fluids under purely extensional deformations”, Proceed. National Academy of Sciences, USA (PNAS) (2020). https://doi.org/10.1073/pnas.1922242117
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The Annual European Rheology Conference is coorganized with the 26th Nordic Rheology Conference on 3 – 6 April 2017, in Copenhagen, Denmark.
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Place: Main Seminar Room, 1st Floor, Main Building of Chemical Engineering Department
Date: February 25, 2016 , Time: 12:00
Committee: J. Tsamopoulos (supervisor), Ko Van der Weele, Y. Dimakopoulos
Global Engage’s 2nd Microfluidics Congress will be held on 20 – 21 October 2016 in London UK.
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The American Physical Society – Division of Fluid Dynamics (APSDFD) Meeting will be held in Portland, Oregon 20 – 22 November, 2016. The Meeting is hosted by Portland State University, University of Washington, and Oregon State University.
The Meeting will be held at the Oregon Convention Center. The scientific program will include award lectures, invited talks and minisymposia, many parallel sessions with contributed papers, poster presentations, a poster competition, and the Gallery of Fluid Motion.
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Place: Main Seminar Room, 1st Floor, Main Building of Chemical Engineering Department
Date: December 21, 2015 , Time: 11:00
Committee: J. Tsamopoulos (supervisor), J. Ekaterinaris, Y. Dimakopoulos, V. Mavratzas, S. Pandis, Ch. Paraskeva, N.Pelekasis
Place: Main Seminar Room, 1st Floor, Main Building of Chemical Engineering Department
Date: June 8, 2015 , Time: 9:00
Committee: J. Tsamopoulos (supervisor), V. Mavrantzas, Y. Dimakopoulos
Place: Main Seminar Room, 1st Floor, Main Building of Chemical Engineering Department
Date: June 4, 2015 , Time: 17:00
Committee: J. Tsamopoulos (supervisor), V. Mavrantzas, Y. Dimakopoulos
Fraggedakis D., Dimakopoulos Y., Tsamopoulos J.
The sedimentation of a single particle in materials that exhibit simultaneously elastic, viscous and plastic behavior is examined in an effort to explain phenomena that contradict the nature of purely yieldstress materials. Such phenomena include the loss of the foreandaft symmetry with respect to an isolated settling particle under creeping flow conditions and the appearance of the “negative wake” behind it. Despite the fact that similar observations have been reported in studies involving viscoelastic fluids, researchers conjectured that thixotropy is responsible for these phenomena, as the aging of yieldstress materials is another common feature. By means of transient calculations, we study the effect of elasticity on both the fluidized and the solid phase. The latter is considered to behave as an ideal Hookean solid. The material properties of the model are determined under the isotropic kinematic hardening framework via Large Amplitude Oscillatory Shear (Laos) measurements. In this way, we are able to predict accurately the unusual phenomena observed in experiments with simple yieldstress materials, irrespective of the appearance of slip on the particle surface. Viscoelasticity favors the formation of intense shear and extensional stresses downstream of the particle, significantly changing the entrapment mechanism in comparison to that observed in viscoplastic fluids. Therefore, the critical conditions under which the entrapment of the particle occurs deviate from the wellknown criterion established theoretically by Beris et al. (1985) and verified experimentally by Tabuteau et al. (2007) for similar materials under conditions that elastic effects are negligible. Our predictions are in quantitative agreement with published experimental results by Holenberg et al. (2012) on the loss of the foreaft symmetry and the formation of the negative wake in Carbopol with wellcharacterized rheology. Additionally, we propose simple expressions for the Stokes drag coefficient, as a function of the gravity number, Yg (related to the Bingham number), for different levels of elasticity and for its critical value, under which entrapment of particles occurs. These criteria are in agreement with the results found in the recent work by Ahonguio et al. (2014). Finally, we propose a method to quantify experimentally the elastic effects in viscoplastic particulate systems. © 2016 The Royal Society of Chemistry.
Fraggedakis D., Pavlidis M., Dimakopoulos Y., Tsamopoulos J.
We examine the abrupt increase in the rise velocity of an isolated bubble in a viscoelastic fluid occurring at a critical value of its volume, under creeping flow conditions. This ‘velocity discontinuity’, in most experiments involving shearthinning fluids, has been somehow associated with the change of the shape of the bubble to an inverted teardrop with a tip at its pole and/or the formation of the ‘negative wake’ structure behind it. The interconnection of these phenomena is not fully understood yet, making the mechanism of the ‘velocity jump’ unclear. By means of steadystate analysis, we study the impact of the increase of bubble volume on its steady rise velocity and, with the aid of pseudo arclength continuation, we are able to predict the stationary solutions, even lying in the discontinuous area in the diagrams of velocity versus bubble volume. The critical area of missing experimental results is attributed to a hysteresis loop. The use of a boundaryfitted finite element mesh and the openboundary condition are essential for, respectively, the correct prediction of the sharply deformed bubble shapes caused by the large extensional stresses at the rear pole of the bubble and the accurate application of boundary conditions far from the bubble. The change of shape of the rear pole into a tip favours the formation of an intense shear layer, which facilitates the bubble translation. At a critical volume, the shear strain developed at the front region of the bubble sharply decreases the shear viscosity. This change results in a decrease of the resistance to fluid displacement, allowing the developed shear stresses to act more effectively on bubble motion. These coupled effects are the reason for the abrupt increase of the rise velocity. The flow field for stationary solutions after the velocity jump changes drastically and intense recirculation downstream of the bubble is developed. Our predictions are in quantitative agreement with published experimental results by Pilz & Brenn (J. NonNewtonian Fluid Mech., vol. 145, 2007, pp. 124–138) on the velocity jump in fluids with wellcharacterized rheology. Additionally, we predict shapes of larger bubbles when both inertia and elasticity are present and obtain qualitative agreement with experiments by Astarita & Apuzzo (AIChE J., vol. 11, 1965, pp. 815–820). © 2016 Cambridge University Press
drops and bubbles, interfacial flows (free surface), nonNewtonian flows
]]>Tsouka S., Dimakopoulos Y., Tsamopoulos J.
We consider the two dimensional, steady flow of a dilute polymer solution over a solid substrate with periodic topography under the action of a body force. We examine how the distribution of polymer is affected by flow conditions, physical properties and substrate geometry and how it affects the dynamics of the freesurface. The MavrantzasBeris twofluid Hamiltonian model is used in order to account for polymer migration due to stress gradients. The model is solved via the mixed finite element method combined with a quasielliptic grid generation technique. Results for polymer concentration, stress and velocity fields are presented as a function of the nondimensional parameters of the problem. The basic phenomena that appear in the case of polymer migration in undulating channels (Tsouka et al., 2014), also appear in free surface flows. Increasing the elastic stresses, increases migration of macromolecules towards the free surface, developing a polymerdepleted layer especially over the substrate maxima, which finally gives rise to “apparent” slip. Increasing the cavity steepness also enhances migration and the thickness of the depletion layer and induces strong variation in the stresses away from the substrate wall, especially in low polymer concentrations. All these phenomena cannot be captured by the homogenous OldroydB model. The evaluation of the Stokes number shows that flow resistance decreases as elasticity increases, increases monotonically up to an asymptote with the capillary number, and exhibits a nonmonotonic dependence on the Reynolds number. Moderate inertial effects can give rise to large deformations of the free surface, developing waves with complex shapes. If we use a similarly modified ePTT model, we observe that its inherent shearthinning is enhanced by the stress induced migration and tends to reduce the gradients on both polymeric stresses and polymer concentration along the film.
Film flow over topography, Finite element solution, Stressinduced migration, Viscoelastic fluids
]]>Lampropoulos N.K., Dimakopoulos Y., Tsamopoulos J.
We study the transient, twodimensional film flow over solid substrates with variable topography, a flow that has practical applications in microelectronics and microfluidics. The problem we address here is the advancing of a thin liquid film over squareshaped trenches with different depths and widths, under the influence of the gravitational force. We use the volumeoffluid method to obtain completely different wetting patterns depending on the dimensions of the topography, the capillary and Reynolds numbers, and the contact angle. On one hand, we predict continuous coating, i.e., the formation of the Wenzel state, in which a thin liquid film covers the entire trench, while steady flow is established upstream and downstream this topographical feature. This is the desirable pattern, when perfect coating is sought, as in the manufacturing of microelectronic devices. Under different conditions, we predict that the film almost completely bypasses the trench, entrapping air inside it, i.e., forming the Cassie state. The coating quality is clearly poor in this case, but this pattern reduces the drag on the film, and therefore, it is desirable in the operation of superhydrophobic surfaces for microfluidic applications. Between these two extreme configurations, we uncover a large variety of patterns, in which the film partially wets the trench forming an air inclusion all along its bottom or its upstream or downstream inner corners or the film may break up periodically. We produce comprehensive flow maps covering a wide range of relevant parameter values.
Air entrapment, Cassie and Wenzel states, Coating flows, Flow over topography, Thinfilm flow
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Syrakos A., Dimakopoulos Y., Georgiou G.C., Tsamopoulos J.
Numerical simulations of the flow in an extrusion damper are performed using a finite volume method. The damper is assumed to consist of a shaft, with or without a spherical bulge, oscillating axially in a containing cylinder filled with a viscoplastic material of Bingham type. The response of the damper to a forced sinusoidal displacement is studied. In the bulgeless case the configuration is the annular analogue of the wellknown liddriven cavity problem, but with a sinusoidal rather than constant lid velocity. Navier slip is applied to the shaft surface in order to bound the reaction force to finite values. Starting from a base case, several problem parameters are varied in turn in order to study the effects of viscoplasticity, slip, damper geometry and oscillation frequency to the damper response. The results show that, compared to Newtonian flow, viscoplasticity causes the damper force to be less sensitive to the shaft velocity; this is often a desirable damper property. The bulge increases the required force on the damper mainly by generating a pressure difference across itself; the latter is larger the smaller the gap between the bulge and the casing is. At high yield stresses or slip coefficients the amount of energy dissipation that occurs due to sliding friction at the shaftfluid interface is seen to increase significantly. At low frequencies the flow is in quasi steady state, dominated by viscoplastic forces, while at higher frequencies the fluid kinetic energy storage and release also come into the energy balance, introducing hysteresis effects.
Annular cavity, Bingham flow, Finite volume method, Slip, Viscous damper, Viscous dissipation
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Pavlidis M., Karapetsas G., Dimakopoulos Y., Tsamopoulos J.
We examine and compare five constitutive models, which have been recently proposed to describe the behavior of elastoviscoplastic fluids. The comparison is performed in simple rheometric flows, i.e. simpleshear, uniaxial elongation and large amplitude oscillatory tests and in the complex flow generated by a falling spherical particle. The first set of three models do not explicitly include shearthinning. These are the ones proposed by Saramito [21], Park & Liu [27] and Belblidia et al. [28]. The first one has been derived under a thermodynamic framework, while the other two have been based on viscosity regularization methods. When spatial and temporal inhomogeneity are not present in the flow field, the models generally produce acceptable predictions, except for: (a) the BWW in predicting the primary normal stress under small shear rate and under small strain in the LAOStrain test, (b) all models in predicting different parts of the spectra of G′ and G″, although the predictions of the SRM can be corrected when kinematic hardening is accounted for and (c) the P&L model in LAOStress because of nonexistence or multiplicity of solutions. In the complex flow, the predictions of each model are compared with the experimental data of Holenberg et al. [34] under non shearthinning conditions and the predictions of the SRM model are clearly superior. The second set of models have been proposed by Saramito [22] to explicitly account for shear thinning either by extending the Herschel–Bulkley model to include elastic effects, SRMHB, or by introducing a PTTtype term in the constitutive model, SRMPTT. Both these models provide acceptable results in the rheometric tests. In the falling sphere test, their predictions are compared with the experimental results by Putz et al. [15] for the settling of a particle under conditions that the Carbopol solution exhibits shearthinning. Here the SRMHB is found to be superior.
Elastoviscoplastic fluids, Falling sphere in EVP fluids, LAOS tests, Viscometric tests
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Fraggedakis D., Dimakopoulos Y., Tsamopoulos J.
We examine and compare five constitutive models, which have been recently proposed to describe the behavior of elastoviscoplastic fluids. The comparison is performed in simple rheometric flows, i.e. simpleshear, uniaxial elongation and large amplitude oscillatory tests and in the complex flow generated by a falling spherical particle. The first set of three models do not explicitly include shearthinning. These are the ones proposed by Saramito [21], Park & Liu [27] and Belblidia et al. [28]. The first one has been derived under a thermodynamic framework, while the other two have been based on viscosity regularization methods. When spatial and temporal inhomogeneity are not present in the flow field, the models generally produce acceptable predictions, except for: (a) the BWW in predicting the primary normal stress under small shear rate and under small strain in the LAOStrain test, (b) all models in predicting different parts of the spectra of G′ and G″, although the predictions of the SRM can be corrected when kinematic hardening is accounted for and (c) the P&L model in LAOStress because of nonexistence or multiplicity of solutions. In the complex flow, the predictions of each model are compared with the experimental data of Holenberg et al. [34] under non shearthinning conditions and the predictions of the SRM model are clearly superior. The second set of models have been proposed by Saramito [22] to explicitly account for shear thinning either by extending the Herschel–Bulkley model to include elastic effects, SRMHB, or by introducing a PTTtype term in the constitutive model, SRMPTT. Both these models provide acceptable results in the rheometric tests. In the falling sphere test, their predictions are compared with the experimental results by Putz et al. [15] for the settling of a particle under conditions that the Carbopol solution exhibits shearthinning. Here the SRMHB is found to be superior.
Elastoviscoplastic fluids, Falling sphere in EVP fluids, LAOS tests, Viscometric tests
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Dimakopoulos Y., Kelesidis G., Tsouka S., Georgiou G.C., Tsamopoulos J.
BACKGROUND: In microcirculation, the nonNewtonian behavior of blood and the complexity of the microvessel network are responsible for the high flow resistance and the large reduction of the blood pressure. Red blood cell aggregation along with inward radial migration are two significant mechanisms determining the former. Yet, their impact on hemodynamics in nonstraight vessels is not well understood. OBJECTIVE: In this study, the steady state blood flow in stenotic rigid vessels is examined, employing a sophisticated nonhomogeneous constitutive law. The effect of red blood cells migration on the hydrodynamics is quantified and the constitutive model’s accuracy is evaluated. METHODS: A numerical algorithm based on the twodimensional mixed finite element method and the EVSS/SUPG technique for a stable discretization of the mass and momentum conservation equations in addition to the constitutive model is employed. RESULTS: The numerical simulations show that a celldepleted layer develops along the vessel wall with an almost constant thickness for slow flow conditions. This causes the reduction of the drag force and the increase of the pressure gradient as the constriction ratio decreases. CONCLUSIONS: Viscoelastic effects in blood flow were found to be responsible for steeper decreases of tube and discharge hematocrits as decreasing function of constriction ratio. © 2015 – IOS Press and the authors.
blood viscoelasticity, celldepleted layer, FahraeusLindqvist effect, RBC migration, shearinduced migration, stenotic microvessels
]]>Fraggedakis D., Kouris C., Dimakopoulos Y., Tsamopoulos J.
We study the flow of two immiscible, Newtonian fluids in a periodically constricted tube driven by a constant pressure gradient. Our volumeoffluid algorithm is used to solve the governing equations. First, the code is validated by comparing its predictions to previously reported results for stratified and pulsing flow. Then, it is used to capture accurately all the significant topological changes that take place. Initially, the fluids have a coreannular arrangement, which is found to either remain the same or change to a different arrangement depending on the fluid properties, the pressure driving the flow, or the flow geometry. The flowpatterns that appear are the coreannular, segmented, churn, spray, and segregated flow. The predicted scalings near pinching of the core fluid concur with similarity predictions and earlier numerical results [I. Cohen et al., “Two fluid drop snapoff problem: Experiments and theory,” Phys. Rev. Lett. 83, 11471150 (1999)]. Flowpattern maps are constructed in terms of the Reynolds and Weber numbers. Our result provides deeper insights into the mechanism of the pattern transitions and is in agreement with previous studies on coreannular flow [Ch. Kouris and J. Tsamopoulos, “Coreannular flow in a periodically constricted circular tube, I. Steady state, linear stability and energy analysis,” J. Fluid Mech. 432, 3168 (2001) and Ch. Kouris et al., “Comparison of spectral and finite element methods applied to the study of interfacial instabilities of the coreannular flow in an undulating tube,” Int. J. Numer. Methods Fluids 39(1), 4173 (2002)], segmented flow [E. Lac and J. D. Sherwood, “Motion of a drop along the centreline of a capillary in a pressuredriven flow,” J. Fluid Mech. 640, 2754 (2009)], and churn flow [R. Y. Bai et al., “Lubricated pipeliningStability of core annularflow. 5. Experiments and comparison with theory,” J. Fluid Mech. 240, 97132 (1992)]. © 2015 AIP Publishing LLC.
Pettas D., Karapetsas G., Dimakopoulos Y., Tsamopoulos J.
It is wellknown that by increasing the flow rate in polymer extrusion, the flow becomes unstable and the smooth extrudate surface becomes wavy and disordered to an increasing degree. In order to investigate the mechanisms responsible for these instabilities we perform a linear stability analysis of the steady extrusion of a viscoelastic fluid flowing through a planar die under creeping flow conditions. We consider the PhanThienTanner (PTT) model to account for the viscoelasticity of the material. We employ the mixed finite element method combined with an elliptic grid generator to account for the deformable shape of the interface. The generalized eigenvalue problem is solved using Arnoldi’s algorithm. We perform a thorough parametric study in order to determine the effects of all material properties and rheological parameters. We investigate in detail the effect of the interfacial tension and the presence of a deformable interface. It is found that the presence of a finite surface tension destabilizes the flow as compared to the case of the stickslip flow. We recognize two modes, which become unstable beyond a critical value of the Weissenberg number and perform an energy analysis to examine the mechanisms responsible for the destabilization of the flow and compare against the mechanisms that have been suggested in the literature. © 2015 Elsevier B.V.
Extrudate swell, Extrusion instabilities, Polymer melts, Sharkskin, Stability analysis, Surface tension
]]>Papaioannou J., Giannousakis A., Dimakopoulos Y., Tsamopoulos J.
Motivated by the probe experiment for characterizing the adhesion strength of polymeric materials, we studied the axisymmetric extensional flow of a viscoelastic liquid filament that contains one or three initially spherical bubbles along its axis of symmetry. The filament is confined between two parallel and coaxial disks of the same radius and its initially cylindrical outer surface is surrounded by air. The flow is induced by the axial extension of the upper disk under constant velocity, while the lower disk remains stationary. The rheology of the liquid is described by the exponential PhanThien and Tanner viscoelastic model. A quasielliptic transformation combined with domain decomposition and local mesh refinement is employed to discretize the domain. The mixed finite element Galerkin method for the mass and momentum balances combined with the EVSSG method and SUPG weighting for the constitutive equation are used to obtain their weak forms. All these are necessary for the successful simulation of much larger filament extensions and of liquids with higher elasticity than those reported earlier. The evolution of the filament and the bubble(s)free surfaces depends on the interplay of the viscous, elastic, and capillary forces. It was found that, when the ratio of the elastic and the capillary forces is small compared to the viscous forces, the bubble attains very large deformations. The onset of cusps was observed at the bubble poles for intermediate values of capillarity and elasticity. The force varies with the filament extension (or time) in a way reported in experiments measuring the strength of adhesive materials: Its initial increase up to a maximum is followed by a plateau, and finally it drops to zero when the adhesive tends to fail. Increasing the filament elasticity delays the development of stresses (and the applied force). © 2014 American Chemical Society.
Tseropoulos G., Dimakopoulos Y., Tsamopoulos J., Lyberatos G.
The Minoan Terracotta pipes with their conical shape were widely used in the water distribution system in the ancient Minoan civilization. They remain one of the brightest achievements of the Minoan tribe in water supply technology and raise admiration as well as many questions about the technological advancements of antiquity, that are yet to be understood. The present work aims at answering the following questions: a) what inspired the Minoans to manufacture pipes with such a peculiar shape, that differs greatly not only from later pipe designs of antiquity, but also from contemporary cylindrical pipes and b) why was the design of those pipes abandoned after the fall of the Minoan civilization? It tries to address these questions by investigating the flow physics and dynamics that take place in such pipes, adopting advanced numerical and computational methods. The timeaveraged NavierStokes equations along with the k – e{open} turbulence model are solved for a variety of geometrical parameters, pipe orientations and flow rates, in order to produce a comparative picture of the hydraulic efficiency of the conical Minoan pipes. The flow field is visualized and critical flow parameters, such as the head loss, the velocity magnitude and turbulence intensity, are calculated. These calculations show clearly that the conical Minoan pipes exhibit significantly higher pressure drops along their length compared to an equivalent straight pipe. In their widest part an extended recirculation appears, which could wash out impurities that may be present in the water, which at the same time cannot be deposited on the pipe wall. This evidence proves that the Minoan pipes are energetically expensive to operate and consequently their replacing by cylindrical pipes was inevitable. Therefore, it seems that the main advantage and purpose of the particular geometry was that they could be easily connected on site, forming long straight or slowly bending lines without having to add straight or many different fittings in between. © 2012 Elsevier Ltd.
Computational fluid mechanics, Minoan civilization, Minoan hydraulic pipes, Turbulence modeling
]]>Karapetsas G., Tsamopoulos J.
During extrusion of viscoelastic fluids various flow instabilities may arise resulting in a distorted free surface. In order to investigate the factors generating these instabilities we performed a linear stability analysis at zero Reynolds number around the steady solution of the cylindrical or planar stickslip flow for a viscoelastic fluid following the affine exponential PhanThien Tanner (PTT) model. Stickslip flow is an important special case of the extrudate swell problem, since the latter reduces to it in the limit of infinite surface tension but avoids the complications of a freeboundary flow. The linear stability analysis is performed for various values of the rheological parameters of the PTT model in order to determine the effects of all material properties. It is found that the flow becomes unstable as the Weissenberg number increases above a critical value, due to a Hopf bifurcation suggesting that the flow will become periodic in time. Both the critical value of the Weissenberg number and the frequency of the instability depend strongly on the rheological parameters of the viscoelastic model. The corresponding eigenvectors indicate that the perturbed flow field has a spatially periodic structure, initiated at the rim of the die, extending for up to 57 die gaps downstream, but confined close to the surface of the extrudate, in qualitative agreement with existing experiments. This suggests that instability is generated by the combination of the singularity in the velocity and stress fields at the die lip and the strong extension that the extruded polymer undergoes near its surface. The elasticity alone can be responsible for the appearance of instabilities in the extrusion process of viscoelastic fluids and the often used assumptions of wall slip or compressibility, although they might be present, are not required. Finally, the mechanisms that produce these instabilities are examined through energy analysis of the disturbance flow. © 2013 AIP Publishing LLC.
Dimakopoulos Y., Pavlidis M., Tsamopoulos J.
The steady, buoyancydriven rise of a bubble in a HerschelBulkley fluid is examined assuming axial symmetry. The variation of the rateofstrain tensor around a rising bubble necessitates the coexistence of fluid and solid regions in this fluid. In general, a viscoplastic fluid will not be deforming beyond a finite region around the bubble and, under certain conditions, it will not be deforming either just behind it or around its equatorial plane. The accurate determination of these regions is achieved by introducing a Lagrange multiplier and a quadratic term in the corresponding variational inequality, resulting in the socalled Augmented Lagrangian Method (ALM). Additionally here, the augmentation parameters are determined following a nonlinear conjugate gradient procedure. The new predictions are compared against those obtained by the much simpler Papanastasiou model, which uses a continuous constitutive equation throughout the material, irrespective of its state, but does not determine the boundary between solid and liquid along with the flow field. The flow equations are solved numerically using the mixed finiteelement/Galerkin method on a mesh generated by solving a set of quasielliptic differential equations. The accuracy of solutions is ascertained by mesh refinement and comparison with our earlier and new predictions for a bubble rising in a Newtonian and a Bingham fluid. We determine the bubble shape and velocity and the shape of the yield surfaces for a wide range of material properties, expressed in terms of the Bingham, Bn, Bond, and Archimedes numbers. As Bn increases, the bubble decelerates, the yield surfaces at its equatorial plane and away from it approach each other and eventually merge immobilizing the bubble. For small and moderate Bingham numbers, the predictions using the Papanastasiou model satisfactorily approximate those of the discontinuous HerschelBulkley model for sufficiently large values of the normalization exponent (≥104). On the contrary, as Bn increases and the rateofstrain approaches zero almost throughout the fluidlike region, much larger values of the exponent are required to accurately compute the yield surfaces. Bubble entrapment does not depend on the power law index, i.e. a bubble in a HerschelBulkley fluid is entrapped under the same conditions as in a Bingham fluid. © 2012 Elsevier B.V.
Augmented Lagrangian Method, Free surface flow, HerschelBuckley model, Papanastasiou model, Rising bubble, Viscoplastic material
]]>Papaioannou J.G., Dimakopoulos Y., Tsamopoulos J.A.
Because of the three dimensional nature of many physical problems encountered in industrial processes and applications, we develop a novel 3D elliptic grid generator for the construction and the deformation of structured grids. The specific application we consider is related to the large deformations of an air bubble in a liquid between two parallel plates. The new technique is based on the inverted Laplace equations along with internal constraints, which are considered essential in order to adjust grid spacing. This set of equations is accompanied with appropriate set of boundary conditions that is the imposition of the 1D differential arclength at each of the two coordinates of the boundary surfaces in the computational domain. The new 3D grid generator provides the smoothness of the coordinate lines not only in the bulk, but also on the boundary surfaces. The kinematic condition is invoked for the motion of the 3D surface of the bubble and we also consider the free motion of the 2D contact line (points where the air in the bubble and the surrounding liquid meet the solid boundary). In order to resolve better the flow field and the shape of the bubble free surface we have also developed a local mesh refinement technique. In this way the demand for computational memory is overly reduced. Our method in 3D grid generation follows the Arbitrary Lagrangian Eulerian (A.L.E.) formulation, which when combined with the finite element/Galerkin method provides a flexible and robust tool for simulating free surface flows in three dimensions.
3D simulations, Continuum mechanics, Elliptic mesh generator, Free surface flows
Chatzidai N., Dimakopoulos Y., Tsamopoulos J.
According to linear theory and assuming the liquids to be inviscid and the bubbles to remain spherical, bubbles set in oscillation attract or repel each other with a force that is proportional to the product of their amplitude of volume pulsations and inversely proportional to the square of their distance apart. This force is attractive, if the forcing frequency lies outside the range of eigenfrequencies for volume oscillation of the two bubbles. Here we study the nonlinear interaction of two deformable bubbles set in oscillation in water by a step change in the ambient pressure, by solving the NavierStokes equations numerically. As in typical experiments, the bubble radii are in the range 11000 m. We find that the smaller bubbles (~5 m) deform only slightly, especially when they are close to each other initially. Increasing the bubble size decreases the capillary force and increases bubble acceleration towards each other, leading to oblate or spherical cap or even globally deformed shapes. These deformations may develop primarily in the rear side of the bubbles because of a combination of their translation and harmonic or subharmonic resonance between the breathing mode and the surface harmonics. Bubble deformation is also promoted when they are further apart or when the disturbance amplitude decreases. The attractive force depends on the Ohnesorge number and the ambient pressure to capillary forces ratio, linearly on the radius of each bubble and inversely on the square of their separation. Additional damping either because of liquid compressibility or heat transfer in the bubble is also examined. Copyright © Cambridge University Press 2011.
bubble dynamics, interfacial flows (free surface)
]]>Pavlidis M., Dimakopoulos Y., Tsamopoulos J.
Twodimensional, steady flow of a viscoelastic film over a periodic topography under the action of a body force is studied. The exponential PhanThien and Tanner (ePTT) constitutive model is used. The conservation equations are solved via the usual mixed finite element method combined with a quasielliptic grid generation scheme in order to capture the large deformations of the free surface. The constitutive equation is weighted using the SUPG method and solved via the polymeric stress splitting EVSSG technique. First, the code is validated by verifying that in isolated topographies the periodicity conditions result in fully developed viscoelastic film flow at the inflow/outflow boundaries and that its predictions for Newtonian fluids over 2D topography under creeping flow conditions coincide with those of previous works. Since the lubrication approximation is not invoked here, the topographical features can have wall segments that form any angle with the main flow, but only slight smoothing of the convex corners assists in reducing the stress singularity there. Thus, steadystate solutions are computed accurately up to high Deborah numbers, resulting in large deformations of the free surface. The magnitude of the capillary ridge in the film before the entrance to a step down of the substrate and of the capillary depression before a step up is increased as De increases up to ∼0.7 due to increased fluid elasticity. Above this value they decrease, because increasing De increases also the shear and elongational thinning, which eventually affect them more. Increasing the ratio of solvent to polymer viscosities, β, the elongational parameter, e{open} and the molecular slip parameter, ξ, monotonically increases their magnitudes and especially that of the capillary ridge, but the mechanisms leading to these changes are different as explained in the text. © 2010 Elsevier B.V.
Elliptic mesh generation, Flow over topography, Viscoelastic film flow
]]>Chatzidai N., Giannousakis A., Dimakopoulos Y., Tsamopoulos J.
We present an improved method to generate a sequence of structured meshes even when the physical domain contains deforming inclusions. This method belongs to the class of Arbitrary LagrangianEulerian (ALE) methods for solving moving boundary problems. Its tools are either (a) separate mappings of the domain boundaries and enforcing the node distribution on lines emanating from singular points or (b) domain decomposition and separate mappings of each subdomain using suitable coordinate systems. The latter is shown to be more versatile and general. In both cases a set of elliptic equations is used to generate the grid extending in this way the method advanced by Dimakopoulos and Tsamopoulos [Y. Dimakopoulos, J.A. Tsamopoulos, A quasielliptic transformation for moving boundary problems with large anisotropic deformations, J. Comput. Phys. 192 (2003) 494522]. We shall present examples where this earlier method and all other mesh generating methods which are based on a conformal mapping or solving a quasielliptic set of PDEs fail to produce an acceptable mesh and accurate solutions in such geometries. Furthermore, in contrast to other methods, appropriate boundary conditions and constraints such as, orthogonality of specific mesh lines and prespecified node distributions on them, can be easily implemented along a specific part of the domain or its boundary. Hence, no attractive terms at specific corners or singular points are needed. To increase the mesh resolution around the moving interfaces while keeping low the memory requirements and the computational time, a local mesh refinement technique has been incorporated as well. The method is demonstrated in two challenging examples where no remeshing is required in spite of the large domain deformations. In the first one, the transient growth of two bubbles embedded in a viscoelastic filament undergoing stretching in the axial direction is examined, while in the second one the linear and nonlinear dynamics of two bubbles in a viscous medium are determined in an acoustic field. The large elasticity of the filament in the first case or the large inertia in the second case coupled with the externally induced large deformations of the liquid domain requires the accurate calculation which is achieved by the method we propose herein. The governing equations are solved using the finite element/Galerkin method with appropriate modifications to solve the hyperbolic constitutive equation of a viscoelastic fluid. These are coupled with an implicit Euler method for time integration or with Arnoldi’s algorithm for normal mode analysis. © 2008 Elsevier Inc. All rights reserved.
Elliptic mesh generation, Moving boundary problems, Structured meshes
]]>Dimakopoulos Y., Tsamopoulos J.
We examine the pressuredriven coating of a straight, long tube with a finite amount of viscoelastic liquid. This combines the formation and elongation of an open bubble pushing the liquid downstream with the motion of the advancing liquid front and the deposition of a film along the tube wall. The mixed finite element method is combined with the discontinuous Galerkin method for computing the polymeric contribution to the stress tensor, while a quasielliptic grid generation scheme is used for tessellating the highly deforming domain. Global remeshing is adopted to overcome local distortion and coarsening of the mesh which arises from the attachment of advancing front nodes on the wall. The accuracy of the solution is achieved by selectively refining the mesh along areas where sharp boundary layers in stress arise and in particular at the bubble front. An extensive parametric analysis is performed using the affine, singlemode PhanTien and Tanner constitutive model. The thickness of the remaining film increases as the solvent contribution to viscosity decreases, because of the sharply increased normal viscoelastic stresses along the bubble front. The effect of decreasing the extensional parameter εPTT is similar because this decreases the shear and extensional thinning in the liquid. The transient nature of the process delays the development of the viscoelastic stresses as the Deborah number increases. The streamlines relative to the liquid tip follow the usual ‘fountain flow’ pattern as long as the bubble and the liquid front do not interact. The nonaffine, multimode PTT model with constants obtained from the literature for a PIB solution in C14 predicts a slightly thinner deposited film and a flatter bubble front because of its stronger shear thinning. Moreover, it predicts a higher dimensionless bubble velocity because of the very small viscosity ratio of the solvent and a Vshape in the radial distribution of the first normal stress difference and the birefringence just behind the contact line in qualitative agreement with experiments and theory for planar fountain flow. © 2009 Elsevier B.V. All rights reserved.
Coating flows, Discontinuous Galerkin, Elliptic mesh generation, Exponential PTT, Fountain flow, Injection molding
]]>Pavlidis M., Dimakopoulos Y., Tsamopoulos J.
The onedimensional, gravitydriven film flow of a linear (l) or exponential (e) PhanThien and Tanner (PTT) liquid, flowing either on the outer or on the inner surface of a vertical cylinder or over a planar wall, is analyzed. Numerical solution of the governing equations is generally possible. Analytical solutions are derived only for: (1) lPTT model in cylindrical and planar geometries in the absence of solvent, β ≡ η̃s(η̃s + η̃p) = 0, where η̃p and η̃s are the zeroshear polymer and solvent viscosities, respectively, and the affinity parameter set at ξ = 0; (2) lPTT or ePTT model in a planar geometry when β = 0 and ξ ≠ 0; (3) ePTT model in planar geometry when β = 0 and ξ = 0. The effect of fluid properties, cylinder radius, R̃, and flow rate on the velocity profile, the stress components, and the film thickness, H̃, is determined. On the other hand, the relevant dimensionless numbers, which are the Deborah, De = λ̃Ũ/H̃, and Stokes, St = ρ̃g̃H̃2/ (η̃p + η̃s)Ũ, numbers, depend on H̃ and the average film velocity, Ũ. This makes necessary a trial and error procedure to obtain H̃ a posteriori. We find that increasing De, ξ, or the extensibility parameter ε increases shear thinning resulting in a smaller St. The Stokes number decreases as R̃/H̃ decreases down to zero for a film on the outer cylindrical surface, while it asymptotes to very large values when R̃/H̃ decreases down to unity for a film on the inner surface. When ξ ≠ 0, an upper limit in De exists above which a solution cannot be computed. This critical value increases with ε and decreases with ξ. © SpringerVerlag 2009.
Gravitydriven flow, PTT fluid model, Viscoelastic film flow
]]>Papaioannou J., Karapetsas G., Dimakopoulos Y., Tsamopoulos J.
The injection of a viscoplastic material, driven by a constant pressure drop, inside a pipe or between two parallel coaxial disks under creeping flow conditions is examined. The transient nature of both flow arrangements requires solving a timedependent problem and fully accounting for the advancing liquid/air interface. Material viscoplasticity is described by the Papanastasiou constitutive equation. A quasielliptic grid generation scheme is employed for the construction of the mesh, combined with local mesh refinement near the material front and, periodically, full mesh reconstruction. All equations are solved using the mixed finite element/Galerkin formulation coupled with the implicit Euler method. For a viscoplastic fluid, the flow field changes qualitatively from that of a Newtonian fluid because the material gets detached from the walls. For small Bingham numbers, the contact line moves in the flow direction, so that initially the flow resembles that of a Newtonian fluid, but even in that case detachment eventually occurs. The distance covered by the contact line, before detachment takes place, decreases as the Bingham number increases. For large enough Bingham numbers, the fluid may even detach from the wall without advancing appreciably. In pipe flow, when detachment occurs, unyielded material arises at the front and the flow changes into one under constant flow rate with pressure distribution that does not vary with time. In the flow between disks, it remains decelerating and the material keeps rearranging at its front because of the increased cross section through which it advances. The wall detachment we predict has been observed experimentally by Bates and Bridgwater [Chem. Eng. Sci. 55, 30033012 (2000)] in radial flow of pastes between two disks. © 2009 The Society of Rheology.
Karapetsas G., Tsamopoulos J.
The steady planar and cylindrical stickslip flows for a viscoelastic fluid are computed using the PhanThien and Tanner (PTT) constitutive model. The mixed finite element method is used in combination with the elasticviscous stresssplitting technique and the streamline upwind PetrovGalerkin discretization for the constitutive equation. This combination of methods when applied to the PTT constitutive model allows us to compute steady state solutions up to high Weissenberg numbers; practically without an upper limit. Equally important, the global Jacobian matrix is generated in order to be able to perform a linear stability analysis of the computed steady state. The dependence of the steady solutions on all the problem parameters is examined. In the limit of a Newtonian fluid, the expansion coefficients near the singularity are computed with comparable accuracy to those from previous analytical and numerical studies, which include the singular finite element method. In the case of a viscoelastic liquid, it is shown that the computed solutions converge quadratically with mesh refinement even at the exit plane of the die and also locally very close to the singularity. The form of the converged solution near the singularity is examined as well as its dependence on various rheological parameters. It is shown that the singularity at the die exit is a logarithmic one and always integrable. Under such conditions our calculations can be extended to determine the linear stability of the herein computed steady states. © 2009 American Institute of Physics.
Tsamopoulos J., Dimakopoulos Y., Chatzidai N., Karapetsas G., Pavlidis M.
We examine the buoyancydriven rise of a bubble in a Newtonian or a viscoplastic fluid assuming axial symmetry and steady flow. Bubble pressure and rise velocity are determined, respectively, by requiring that its volume remains constant and its centre of mass remains fixed at the centre of the coordinate system. The continuous constitutive model suggested by Papanastasiou is used to describe the viscoplastic behaviour of the material. The flow equations are solved numerically using the mixed finiteelement/Galerkin method. The nodal points of the computational mesh are determined by solving a set of elliptic differential equations to follow the often large deformations of the bubble surface. The accuracy of solutions is ascertained by mesh refinement and predictions are in very good agreement with previous experimental and theoretical results for Newtonian fluids. We determine the bubble shape and velocity and the shape of the yield surfaces for a wide range of material properties, expressed in terms of the Bingham Bn = tauy*/rho*g*Rb* Bond Bo = rho*g*Rb* 2/γ* and Archimedes Ar = ρ*2g* Rb *3/μo*2 numbers, where ρ* is the density, μ*o the viscosity, γ* the surface tension and τ*y the yield stress of the material, g* the gravitational acceleration and R*b the radius of a spherical bubble of the same volume. If the fluid is viscoplastic, the material will not be deforming outside a finite region around the bubble and, under certain conditions, it will not be deforming either behind it or around its equatorial plane in contact with the bubble. As Bn increases, the yield surfaces at the bubble equatorial plane and away from the bubble merge and the bubble becomes entrapped. When Bo is small and the bubble cannot deform from the spherical shape the critical Bn is 0.143, i.e. it is a factor of 3/2 higher than the critical Bn for the entrapment of a solid sphere in a Bingham fluid, in direct correspondence with the 3/2 higher terminal velocity of a bubble over that of a sphere under the same buoyancy force in Stokes flow. As Bo increases allowing the bubble to squeeze through the material more easily, the critical Bingham number increases as well, but eventually it reaches an asymptotic value. Ar affects the critical Bn value much less. © 2008 Cambridge University Press.
Housiadas K., Tsamopoulos J.
The effect of the air jet, which is used to increase the process stability and cool the polymeric film which is produced by the widelyused Film Blowing Process (FBP) is presented. The air jet is supplied circumferentially and tangentially on the outside surface of the tubular film that is pulled in the same upward direction. The two steady flow fields interact through the outside surface of the film which must be determined as part of the solution. Viscous, elastic, inertial, gravitational and interfacial forces are taken into account for the film, while only viscous and inertial forces determine the airflow. The film viscoelasticity is described using a variety of constitutive equations including the UCM and the affine exponential PPT models. The governing equations are simplified under the thin film approximation and a boundary layer formulation for the air. A similarity solution is obtained for the air jet and the air drag on the film is evaluated in terms of the bubble radius and introduced in the equations governing the film flow which are solved numerically by finite differences. It is found that the aerodynamic force on the film is very important in determining its shape, keeping it much closer to the vertical position for a longer distance from its extrusion level and, thus, stabilizing the process. © 2008 American Institute of Physics.
Film blowing, Free surface flows, Laminar boundary layer, Tangential wall jet, Thin film approximation
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