Compatibility of Characteristic Boundary Conditions with Radial Equilibrium in Turbomachinery Simulations

Abstract

Setting up outlet boundary conditions in configurations that have a strong rotating motion is a crucial issue for turbomachinery simulations. This is usually done using the so-called radial equilibrium assumption, which is used before the simulation and provides an approximate expression for the pressure profile to impose in the outlet plane. This paper shows that recent methods developed for compressible flows, based on characteristic methods, including the effects of transverse terms, can capture the radial equilibrium naturally without having to impose a precomputed pressure profile. In addition, these methods are also designed to control acoustic reflections on boundaries, and the present work suggests that they could replace classical radial equilibrium assumption approximations when nonreflecting boundary conditions are required at the outlet of a turbomachine simulation, for example, in large eddy simulation. This is demonstrated in two cases: 1) a simple annulus flow with a swirl imposed at the inlet and 2) a transonic turbine vane.