Controlled Dynamics of Streamwise Vorticity Effected in an Offset Diffuser

The flow in offset diffusers of modern propulsion systems is dominated by streamwise vorticity concentrations that advect low-momentum fluid from the flow boundaries into the core flow and thereby give rise to flow distortion and losses. As formation of these vortices is strongly coupled to trapped vorticity concentrations within locally-separated flow over diffuser bends, this coupling is exploited for controlling their streamwise evolution and thereby significantly reduce the flow distortion and losses. The effectiveness of fluidic actuation for distortion suppression is demonstrated at M_throat = 0.64 by implementing actuator arrays at the downstream diffuser turn that leads to over 60% reduction in the average circumferential distortion. Spectral and POD analyses of high-speed, time-resolved total-pressure measurements at the exit plane indicate that these streamwise, counter-rotating vortex pairs are unstable within a narrow frequency band centered about 1 kHz. The actuation alters the spectral content of the pressure fluctuations and leads to broadband suppression that includes the unstable frequency band of the secondary vortices. This is further reflected in suppression of the time-averaged total-pressure distortion and its instantaneous fluctuations.