Acoustic-wave/blade-row interactions establish boundary conditions for unsteady inlet flows

Reliable computation of unsteady flows in high-speed airbreathing inlets re quires that the outflow boundary condition applied at the compressor face p rovide a reasonable simulation of the compressor behavior. Traditional boun dary conditions, for example, constant pressure or constant velocity, disag ree with experimental observations. A one-dimensional, linearized theory is offerred to predict transients induced by downstream- moving acoustic wave s arriving at a single row of stationary or moving blades. The results incl ude acoustic reflection and transmission coefficients and also a coefficien t describing the strength of the induced vorticity wave. The knowledge of t he acoustic reflection coefficient allows formulation of the outflow bounda ry condition at the compressor face for a time-accurate analysis of the inl et flow. The analysis considers the effects of inflow Mach number, flow ang les upstream/downstream of the blade row, and the blade height change acros s the blade row. The coefficients are given as simple algebraic expressions applicable to both stationary and rotating rows. The reflective behavior o f a fixed geometry blade row is illustrated throughout its performance map. The reflection coefficients vary between wide limits, suggesting that the use of correct outflow conditions is essential to the realistic, time-accur ate prediction of unsteady inlet flows.