Publications

Authors Fraggedakis D., Kouris C., Dimakopoulos Y., Tsamopoulos J. Abstract We study the flow of two immiscible, Newtonian fluids in a periodically constricted tube driven by a constant pressure gradient. Our volume-of-fluid algorithm is used to solve the governing equations. First, the code is validated by comparing its predictions to […]

Authors Papaioannou J., Giannousakis A., Dimakopoulos Y., Tsamopoulos J. Abstract 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 […]

Authors Karapetsas G., Tsamopoulos J. Abstract 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 […]

Authors Papaioannou J., Karapetsas G., Dimakopoulos Y., Tsamopoulos J. Abstract 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 time-dependent problem and […]

Authors Karapetsas G., Tsamopoulos J. Abstract The steady planar and cylindrical stick-slip flows for a viscoelastic fluid are computed using the Phan-Thien and Tanner (PTT) constitutive model. The mixed finite element method is used in combination with the elastic-viscous stress-splitting technique and the streamline upwind Petrov-Galerkin discretization for the constitutive […]

Authors Tsamopoulos J., Dimakopoulos Y., Chatzidai N., Karapetsas G., Pavlidis M. Abstract We examine the buoyancy-driven 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 […]

Authors Lac E., Barthès-Biesel D., Pelekasis N.A., Tsamopoulos J. Abstract The dynamic response of an initially spherical capsule subject to different externally imposed flows is examined. The neo-Hookean and Skalak et al. (Biophys. J., vol. 13 (1973), pp. 245-264) constitutive laws are used for the description of the membrane mechanics, […]

Authors Dimakopoulos Y., Tsamopoulos J. Abstract We examine the displacement by pressurized air of a liquid, which only partially occupies straight or complex tubes, according to the Gas-Assisted Injection Molding (GAIM) process. The process involves the formation and continuous elongation of a gaseous finger, which sets in motion the liquid, […]

Authors Foteinopoulou K., Mavrantzas V.G., Dimakopoulos Y., Tsamopoulos J. Abstract Our recent finite element-based study of the deformation of a single bubble in a Newtonian or viscoelastic filament undergoing stretching is extended here to the case of multiple bubbles simultaneously growing in the stretched medium. The filament, having initially the […]

Authors Pelekasis N.A., Gaki A., Doinikov A., Tsamopoulos J.A. Abstract The translational velocities of two spherical gas bubbles oscillating in water, which is irradiated by a high-intensity acoustic wave field, are calculated. The two bubbles are assumed to be located far enough apart so that shape oscillations can be neglected. […]

Authors Kouris C., Tsamopoulos J. Abstract The nonlinear dynamics of the concentric, two-phase flow of two immiscible fluids in a circular tube is studied when the viscosity ratio of the fluid in the annulus to that in the core of the tube, μ, is larger than or equal to unity. […]

Authors Dimakopoulos Y., Tsamopoulos J. Abstract We study the displacement of a viscous fluid by highly pressurized air in a straight or a suddenly constricted cylindrical tube of finite length. In contrast to previous efforts, the transient situation is examined. A long, narrower than the tube and round-ended bubble is […]

Authors Smyrnaios D.N., Pelekasis N.A., Tsamopoulos J.A. Abstract The steady two-dimensional laminar flow of a stream of saturated vapor flowing over a tube that is kept at a uniform temperature, below the saturation temperature, is examined. Owing to the temperature difference between the vapor stream and the solid surface a […]

Authors Kouris C., Tsamopoulos J. Abstract Nonlinear dynamics of the concentric, two-phase flow of two immiscible fluids in a circular tube of variable cross-section is studied for parameter values where the steady core-annular flow (CAF) is linearly unstable. The simulations are based on a pseudo-spectral numerical method. They are carried […]

Authors Kouris C., Tsamopoulos J. Abstract Nonlinear dynamics of the concentric, two-phase flow of two immiscible fluids in a circular tube is studied. The viscosity of the fluid around the axis of symmetry of the tube is larger than the viscosity of the fluid that surrounds it and gravity acts […]

Authors Kouris C., Tsamopoulos J. Abstract The concentric, two-phase flow of two immiscible fluids in a tube of sinusoidally varying cross-section is studied. This geometry is often used as a model to study the onset of different flow regimes in packed beds. Neglecting gravitational effects, the model equations depend on […]

Authors Pelekasis N.A., Tsamopoulos J.A. Abstract The flow of a gas stream past a flat plate under the influence of rainfall is investigated. As raindrops sediment on the flat plate, they coalesce to form a water film that flows under the action of shear from the surrounding gas stream. In […]

Authors Pelekasis N.A., Economou K., Tsamopoulos J.A. Abstract Small amplitude oscillations of viscous, capillary bridges are studied in the presence of an electric dc field. The electric field is proposed as a means to maintain bridges longer than their perimeter and of uniform cylindrical shape. This is desired in the […]

Authors Housiadas K., Tsamopoulos J. Abstract The unsteady extrusion of a viscoelastic film from an annular and axisymmetric die is examined. This moving boundary problem is solved by mapping the inner and outer liquid/air interfaces of the extruded film onto fixed ones, and by transforming the governing equations accordingly. The […]

Authors Housiadas K., Tsamopoulos J. Abstract The unsteady extrusion of a viscoelastic film from an annular and axisymmetric die is examined, when the gravitational and the capillary forces in the film are small relative to the viscous forces. The Oldroyd-B constitutive equation is employed. This moving boundary problem is solved […]

Authors Housiadas K., Tsamopoulos J. Abstract The unsteady extrusion of a viscoelastic film from an annular and axisymmetric die is examined. External, elastic, viscous and inertia forces deform the film, which is simultaneously cooled via forced convection to the ambient air. This moving boundary problem is solved by mapping the […]

Authors Housiadas K., Georgiou G., Tsamopoulos J. Abstract The steady extrusion of a Newtonian liquid through an annular die and its development outside and away from the die are studied under the influence of gravitational and surface tension forces. The finite element method (FEM) is used for the simulations. The […]

Authors Kouris C., Tsamopoulos J. Abstract The concentric, two-phase flow of two immiscible fluids in a tube of sinusoidally varying cross section is studied. Assuming that the tube radius is much smaller than the period of the constriction, the Navier-Stokes equations in each phase are simplified accordingly. This geometry is […]

Authors Smyrnaios D.N., Pelekasis N.A., Tsamopoulos J.A. Abstract The steady two-dimensional laminar flow of an air stream, flowing past a solid surface at high Reynolds number, is examined in the presence of rainfall. As raindrops sediment on the surface they coalesce and form a continuous water film that flows due […]

Authors Kouris Ch., Neophytides St., Vayenas C.G., Tsamopoulos J. Abstract A dynamic model is developed to described two parallel reactions between one gaseous and two non-volatile components on partially wetted catalysts with a time-dependent wetting efficiency. This simulates the performance of catalytic particles in a trickle-bed reactor operating in the […]

Authors Housiadas K., Tsamopoulos J. Abstract The unsteady flow of an annular and axisymmetric film under gravity is examined. This moving boundary problem is solved by mapping the inner and the outer interface of the film in the radial direction onto fixed ones and by transforming the governing equations accordingly. […]

Authors Borkar A.V., Tsamopoulos J.A., Gupta S.A., Gupta R.K. Abstract Spin coating of two commercially used polymer solutions is studied both theoretically and experimentally. Physical and rheological characterization of these solutions indicates that under the spinning conditions currently used they behave as nonvolatile, viscoelastic fluids with constant viscosity and elasticity. […]

Authors Tsamopoulos J., Borkar A. Abstract The transient flow of an Oldroyd-B fluid over an infinite disk set in rotation impulsively is studied under the similarity assumption. The unsteady velocity and stress field is calculated exactly for short times by a power series expansion in time. The order of magnitude […]

Authors Mollot D.J., Ashgriz N., Tsamopoulos J., Chen T.-Y. Abstract An experimental investigation of forced and free oscillations of liquid bridges positioned between two rods of equal diameter is presented. Both the resonance frequencies and damping rates for different aspect ratios of the bridge are reported. The damping rate data […]

Authors Pelekasis N.A., Tsamopoulos J.A. Abstract It is well known from experiments in acoustic cavitation that two bubbles pulsating in a liquid may attract or repel each other depending on whether they oscillate in or out of phase, respectively. The forces responsible for this phenomenon are called ‘Bjerknes’ forces. When […]

Authors Chen T.-Y., Tsamopoulos J. Abstract Finite-amplitude, forced and free oscillations of capillary bridges are studied. They are characterized by a resonant frequency and a damping rate which, in turn, depend on fluid properties, dimensions of the bridge, gravitational force relative to surface tension and amplitude of the external disturbance. […]

Authors Pelekasis N.A., Tsamopoulos J.A. Abstract The motion of two gas bubbles in response to an oscillatory disturbance in the ambient pressure is studied. It is shown that the relative motion of bubbles of unequal size depends on the frequency of the disturbance. If this frequency is between the two […]

Authors Pelekasis N.A., Tsamopoulos J.A., Manolis G.D. Abstract Different formulations of free-surface inviscid flows lead to Fredholm integral equations of the first or second kind. In the present study, these formulations are compared in terms of efficiency and accuracy when different time and space discretization schemes are employed in studying […]

Authors Chen T.-Y., Tsamopoulos J.A., Good R.J. Abstract Equilibrium shapes and the stability of capillary bridges between parallel and nonparallel solid surfaces are determined. Asymptotic and computer-aided techniques from bifurcation theory are used to determine the limits of stability in terms of the minimum volume of fluid in the bridge. […]

Authors Tsamopoulos J., Chen T.-Y., Borkar A. Abstract Small-amplitude oscillations of viscous, capillary bridges are characterized by their frequency and rate of damping. In turn, these depend on the surface tension and viscosity of the liquid, the dimensions of the bridge, the axial and azimuthal wavenumbers of each excited mode […]

Authors Poslinski A.J., Orlicki D., Tsamopoulos J.A. Abstract Inflation of an annular meniscus within a mold cavity is an important fluid mechanical problem in blow molding and glass blowing operations. Deformation of the meniscus surfaces directly affects the thickness distribution of the finished product. This paper analyzes axisymmetric inflation for […]

Authors Pelekasis N.A., Tsamopoulos J.A., Manolis G.D. Abstract It has been shown experimentally (Lee et al. 1982) that water drops with injected air bubbles inside them may be forced dynamically to assume the spherosymmetric shape. Linear analysis is unable to predict a centring mechanism, but provides two distinct modes of […]

Authors Poslinski A.J., Tsamopoulos J.A. Abstract Axisymmetric inflation of fluid annular menisci under an imposed pressure gradient is analyzed by solving the Navier-Stokes equations for velocities in the meniscus and the kinematic and dynamic conditions for interface deformation subject to surface tension forces. Numerical calculations combine Galerkin/finite-element discretization with a […]

Authors Borkar A., Tsamopoulos J. Abstract Small-amplitude oscillations of capillary bridges are examined in the limit of large modified Reynolds number. The contact line between the free surface of the bridge and the upper and lower supporting walls is allowed to undergo a restrained motion by taking its velocity to […]

Authors Vincett M.E., Tsamopoulos J.A., Lund C.R.F. Abstract A mathematical model for the steam gasification of carbon using group VIII transition metal catalysts has been developed. The reaction mechanism is assumed to involve the dissolution of carbon into the metal catalyst at the front half of the spherical particle followed […]

Authors Poslinski A.J., Tsamopoulos J.A. Abstract Nonisothermal inflation of a viscous annular parison and its cooling and solidification before and upon wall contact are analyzed by solving the unsteady momentum and energy conservation equations. The algorithm simultaneously determines the flow field and temperature distribution within the parison together with the […]

Authors Gupte S.M., Tsamopoulos J.A. Abstract The process of chemical vapor infiltration for densification of porous ceramic preforms consists of reactant diffusion and decomposition followed by solid product deposition in the porous body. A mathematical model for isothermal and nonisothermal CVI is presented where the transient porous structure is approximated […]

Authors Gupte S.M., Tsamopoulos J.A. Abstract A mathematical model for forced-flow chemical vapor infiltration (CVI) for densification of porous ceramic composites is presented. The process consists of mass transport of reactants by both convection and diffusion into a porous preform, decomposition of reactants, and subsequent deposition of solid product on […]

Authors Pelekasis N.A., Tsamopoulos J.A., Manolis G.D. Abstract It was shown by Rayleigh [Philos. Mag. 14, 184 (1882)] that a conducting spherical drop becomes unstable when the net dimensionless charge on its surface, Qc, exceeds the value of 4√π. More recently, Tsamopoulos et al. [Proc. R. Soc. London Ser. A […]

Authors Tsamopoulos J.A., Dandekar H.W., Scholtz J.H. Abstract The channeling action of metal catalysts during graphite gasification is studied. A comprehensive model is developed using all available data in the literature. The dependence of the channeling velocity on the inverse of the particle diameter under oxidation over Pd is shown […]

Authors Gupte S.M., Tsamopoulos J.A. Abstract A mathematical model that describes the combined diffusion and reaction of trichloromethylsilane in a porous preform under typical chemical vapor infiltration (CVI) conditions is presented. The model utilizes a single pore to demonstrate the importance of the pore geometry, total gaseous pressure, temperature, and […]

Authors Tsamopoulos J.A., Poslinski A.J., Ryan M.E. Abstract Equilibrium shapes and stability of annular fluid menisci held together by surface tension are analysed by applying asymptotic and computer-aided techniques from bifurcation theory. The shapes and locations of the menisci are governed by the Young-Laplace equation. These shapes are grouped together […]

Authors Beris A.N., Tsamopoulos J.A., Armstrong R.C., Brown R.A. Abstract A solid sphere falling through a Bingham plastic moves in a small envelope of fluid with shape that depends on the yield stress. A finite-element/Newton method is presented for solving the free-boundary problem composed of the velocity and pressure fields […]

Authors Tsamopoulos John A., Brown Robert A. Abstract The objective of this paper is to develop a rigorous framework for studying the nonlinear dynamics of drops and bubbles and to apply this theory to several of the outstanding problems. As opposed to numerical solutions, this semi-analytical method is relatively inexpensive, […]

Authors Tsamopoulos J.A., Brown R.A. Abstract Moderate-amplitude axisymmetric oscillations of charged inviscid drops held together by surface tension are calculated by a multiple-timescale expansion. The corrections to the drop shape and velocity caused by mode coupling at second order in amplitude are predicted for two-, three-and four-lobed motions of drops […]

Authors Beris A.N., Tsamopoulos J.A., Armstrong R.C., Brown R.A. Abstract A solid sphere falling through a Bingham plastic moves in a small envelope of fluid with shape that depends on the yield stress. A finite element/Newton method is presented for solving the free boundary problem composed of the velocity and […]

Authors Tsamopoulos J.A., Brown R.A. Abstract Moderate-amplitude axisymmetric oscillations of incompressible inviscid drops and bubbles are studied using a Poincaré-Lindstedt expansion technique. The corrections to the drop shape and velocity potential caused by mode coupling at second order in amplitude are predicted for two-, three-and four-lobed motions. The frequency of […]