For fuselage embedded engines, S-shaped ducts with multiple bends are usually employed to induct oncoming air. The combination of bends and diffusion creates a flow that routinely separates in regions of high-pressure gradient causing a concomitant reduction in static pressure recovery, a drop in total pressure, flow distortion and swirl at the engine face: the aerodynamic interface plane. This affects engine performance, stability margin, and safety of the integrated aircraft-engine system. It has previously been established that for a symmetric S-duct with a rectangular entrance, exit flow showed the presence of counter-clockwise, 10° bulk swirl. It was attributed to the axial transition of cross-section from a rectangular entrance to a circular exit and/or to high entrance Mach number. The present tests are done to investigate the exit flow bulk swirl and S-duct performance at Ma = 0.80 using a circular entrance. In addition, a 0.48 offset-to-length ratio, rectangular-entrance S-duct was tested at Ma = 0.80, 0.60 & 0.40. The S-duct was rapid prototyped in plastic using 3D printing. Stream-wise static surface and exit total pressure were measured using static taps and a 5-hole probe, respectively. The measured parameters were used to determine static pressure recovery, total pressure loss, exit flow swirl and pressure field polar distortion. These experiments provided information on flow mechanism and the evidence of the origin of bulk swirl in a symmetric S-duct with rectangular entrance.

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