SUMMARY

The flows in offset and serpentine diffusers are dominated by streamwise vorticity concentrations that advect of low-momentum fluid from the flow boundaries into the core flow, giving rise to flow distortion and losses at the engine face. Because the formation of these vortices is strongly coupled to locally separated flow domains over the diffuser bends, the present experimental investigations exploit this coupling for controlling their evolution to reduce flow distortion and losses. Investigations are performed in gradually more challenging geometries, from a single-turn offset diffuser which requires modification to trigger separation, to a two-turn serpentine diffuser with cowl inlet, whose geometry is more aggressive and approximates an aircraft-integrated inlet diffuser. Experimental techniques including pressure sensitive paint (PSP), particle image velocimetry (PIV), and surface oil visualization are used to characterize flow topology and secondary flow dynamics engendered by shocks and separation. Based on each geometry’s unique base flow topology, customized active flow control devices are designed to target specific flow features to alter the evolution and distribution of associated streamwise vortices. The present investigations characterize the fundamental mechanisms by which the actuation methods (fluidic oscillating jets, autonomous bleed) control the flow, showing that these methods can diminish vortices’ strength by injection of vorticity concentrations of a dominantly opposite sense and mitigate the effects of separation and streamwise vorticity, effecting significant reductions in flow distortions and losses.