Influence of inlet and outlet boundary conditions in internal flow computations at low Mach numbers

The influence of inlet and outlet boundary conditions in steady and unsteady internal flow computations at low Mach numbers is shown to be considerable by comparing reflecting and different non-reflecting boundary conditions. With non-reflecting inlet and outlet boundary conditions, the convergence to steady state is drastically accelerated, and the reflections at the artificial boundaries are drastically reduced in unsteady flow simulations. A simple way of implementing non-reflecting boundary conditions is proposed and tested: the ambient flow states adjacent to inlet and outlet are prescibed and fixed in time. With the reconstructed interior states at the boundaries, Riemann problems are defined. If the boundary fluxes are calculated by a Riemann solver, the correct incoming characteristic variables are fixed in time and the outgoing characteristic variables are correctly calculated from the interior.