ACTIVE BOUNDARY-LAYER CONTROL IN DIFFUSERS

Flow separation in diffusers leads to loss in energy recovery and is often a source of unsteadiness due to stall movements. This paper investigates the use of wall jets as a remedy to these problems. New jet geometries are compared to the conventional annular blowing. Steady wall jets are found to be capable of raising the mean pressure recovery in both conical and rectangular air diffusers, but high injected mass flow rates are required before the flow in rectangular diffusers becomes steady. Active feedback control is applied to reduce this flow unsteadiness. To implement the control, the air supplied through the wall jets is modulated in response to the unsteady pressures within the diffuser. Results are presented for two feedback mechanisms. Both are found to be effective with the level of attenuation well predicted by theory. A combination of steady and unsteady blowing is found to be capable of giving both a good mean pressure recovery and reduced pressure oscillations.