IMA Journal of Applied Mathematics Advance Access published online on January 7, 2008
IMA Journal of Applied Mathematics, doi:10.1093/imamat/hxm066
Analysis of a 1D incompressible two-fluid model including artificial diffusion

Scandpower Petroleum Technology, Gåsevikvn. 2-4, 2027 Kjeller, Norway
Institute for Energy Technology, PO Box 40, 2027 Kjeller, Norway
Scandpower Petroleum Technology, Gåsevikvn. 2-4, 2027 Kjeller, Norway
Simula Research Laboratory, PO Box 134, 1325 Lysaker, Norway and Department of Informatics, University of Oslo, PO Box 1080 Blindern, 0316 Oslo, Norway
Hydro Oil and Energy Research Centre, PO Box 2561, 3907 Porsgrunn, Norway
Email: havard.holmas{at}sptgroup.com
Received on June 25, 2007; Accepted on October 16, 2007
This article examines a 1D incompressible two-fluid model including artificial tensor diffusion. The aim is to obtain a formulation that provides convergent numerical solutions for all flow conditions within the stratified and the stratified wavy flow regime. With appropriate simplifications, the two-fluid model reduces to one momentum balance, one mass conservation and two algebraic equations. It has previously been established that a formulation that is well posed in possessing exclusively real characteristics can be obtained by including an axial diffusion term in the momentum balance. In this article, however, we demonstrate that this is not sufficient to obtain a system suitable for numerical simulations. Although the unbounded growth rates of the standard two-fluid model are eliminated, linear stability theory predicts that infinitesimal wavelengths still experience finite growth. This entails that grid refinement always will result in new unstable wavelengths being resolved. On the other hand, if artificial axial diffusion is added to both the mass and the momentum equations as suggested here, a cut-off wavelength is established below which all wavelengths are stable. Thus, a numerically converging model is formed, which retains the long-wavelength properties of the standard two-fluid model. The conclusions of the mathematical analysis are substantiated by numerical simulations of 1D gravity waves.
Keywords: multiphase pipe flow; two-fluid model; diffusion; simulation; stability; well posedness.