Characteristic-Based Boundary Conditions for the Two-Fluid Model

摘要

A set of characteristic-based boundary conditions for multiphase flow is suggested. Control functions are used to avoid drifting values for open boundary conditions. Particularly, P- and Pi-control functions are examined. PI-conlrollers have the ability to specify a given variable exactly at the outlet as well as at the inlet, without causing spurious reflections which are amplified. A simplified two-fluid model is used in the boundary analysis and the computations. Due to the averaging procedure, signal speeds are reduced to the order of the flow speed. The inviscid model is shown to possess complex eigenvalues, and still the characteristic boundary conditions give reasonable results. The governing equations are solved with a Runge-Kutta scheme for the time integration. For the spatial discretisation, a finite-volume and a finite-difference method are used. Both implementations give equivalent results. The boundary conditions are tested for a symmetric shock-tube case with perfectly non-reflecting boundary conditions, and in a one-dimensional tube. The transient and steady solution is examined.