High subsonic aircraft with fuselage-embedded engines often employ inlet ducts with multiple bends in order to induct ambient air into the propulsion system while also diffusing it to engine-acceptable Mach numbers. Engine performance, stability margin, and safety of the integrated aircraft-engine system can be negatively affected by separated, swirling and distorted flow that often characterizes S-ducts. This paper reports the investigation of a flow control strategy aimed at the improvement of the aerodynamic performance of S-duct diffusers. Passive pressure equalization was employed to reduce the size and intensity of the separated flow downstream of curved duct sections, utilizing naturally occurring pressure differences. Characteristic secondary flows promote instability and contribute to flow separation and losses in the inner radius region of a duct bend. In the present scheme, boundary layer flow upstream of the separation point on the inner radius of the first bend is energized by re-injecting higher momentum air, drawn from the higher pressure region at the outer radius of the same bend. The flow control effectiveness of this passive pressure equalization was evaluated by test-rig measurements of the flow in an S-duct at an inlet Mach number of 0.80. Static surface pressure was measured along the length of the S-duct and the total pressure was measured at the aerodynamic interface plane using a pressure rake with five high performance pressure transducers. It was possible to reveal pressure recovery, total pressure loss, and the general nature of flow distortion at the AIP.

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