# Comparison of spectral and finite element methods applied to the study of the core-annular flow in an undulating tube

### Authors

Kouris C., Dimakopoulos Y., Georgiou G., Tsamopoulos J.

### Abstract

A Galerkin/finite element and a pseudo-spectral method, in conjunction with the primitive (velocity-pressure) and streamfunction-vorticity formulations, are tested for solving the two-phase flow in a tube, which has a periodically varying, circular cross section. Two immiscible, incompressible, Newtonian fluids are arranged so that one of them is around the axis of the tube (core fluid) and the other one surrounds it (annular fluid). The physical and flow parameters are such that the interface between the two fluids remains continous and single-valued. This arrangement is usually referred to as Core-Annular flow. A non-orthogonal mapping is used to transform the uneven tube shape and the unknown, time dependent interface to fixed, cylindrical surfaces. With both methods and formulations, steady states are calculated first using the Newton-Raphson method. The most dangerous eigenvalues of the related linear stability problem are calculated using the Arnoldi method, and dynamic simulations are carried out using the implicit Euler method. It is shown that with a smooth tube shape the pseudo-spectral method exhibits exponential convergence, whereas the finite element method exhibits algebraic convergence, albeit of higher order than expected from the relevant theory. Thus the former method, especially when coupled with the streamfunction-vorticity formulation, is much more efficient. The finite element method becomes more advantageous when the tube shape contains a cusp, in which case the convergence rate of the pseudo-spectral method deteriorates exhibiting algebraic convergence with the number of the axial spectral modes, whereas the convergence rate of the finite element method remains unaffected. Copyright © 2002 John Wiley & Sons, Ltd.

### Keywords

Core-annular flow, Finite element method, Pseudo-spectral method, Two-phase flow, Undulating tube