Pressure losses and distortion in a serpentine diffuser with a cowl inlet are investigated experimentally and numerically over a range of flow rates (Mach numbers at aerodynamic interface plane M_(AIP) of up to 0.6). The present investigations show that in the presence of the cowl inlet the diffuser flow is dominated by two pairs of counter-rotating streamwise vortices that are formed at the sharp edges of the inlet cowl lip. The subsequent evolution and unsteady interactions of these vortices result in significant instantaneous pressure losses that are coupled with high flow distortion at the AIP (remarkably, owing to the flow unsteadiness, the time-averaged distortion is nearly negligible). These losses and flow distortions are strongly mitigated by deliberately drawing ambient air across the cowl surface to form jets that interact with the cowl flow along the its inner surface. The jets are formed through spanwise slots across the cowl's surface by exploiting the pressure difference effected by the cowl flow. The interaction of these jets with the cowl flow alters the formation and evolution of the streamwise vortices and appears to suppress their interactions and thereby significantly diminish the losses and distortion across the full operating range of the diffuser. For example, at Maip = 0.5, the total pressure recovery increases by 8% while the peak circumferential distortion decreases by 35%. The time-averaged circumferential distortion can be further reduced (nearly halved) by using concomitant active flow control at the second diffuser turn.
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