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S-Ducts have wide application on air vehicles with embedded engines. The complex geometry is known to lead to separation downstream of curved profiles. This paper reports the influences on that flow of passive flow control geometries. In these experiments, stream-wise tubercles were applied in an effort to improve the internal performance of S-duct diffusers, parameters including pressure recovery, distortion and swirl. The test articles were tested with the high subsonic (Ma = 0.8) flow and were manufactured using 3D printing. Stream-wise static pressure and exit-plane total pressure were measured in a test rig using surface pressure taps and a 5-probe rotating rake, respectively; the baseline and variant S-ducts were simulated through computational fluid dynamics. The experiments showed that some subtle improvements to the S-Duct distortion could be achieved through careful selection of tubercle geometry. Generally, the recovered flow downstream of the inner radius of the second bend of the S-duct deteriorated, but overall pressure recovery improved. The simulations were useful in characterizing swirl, whereas experiments were not so equipped. Adjustments to the numerical approaches resulted in reasonable agreement with the experiments.

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The characteristic aerodynamics of inlets in a fuselage-embedded propulsion system of an air-vehicle vary from one configuration to other, making it necessary to document the performance of each and every type of inlet in various flight conditions. This paper focuses on the internal performance evaluation of a baseline S-duct diffuser for a future parametric investigation of a generic S-duct inlet. The generic baseline was a rectangular-entrance, transitioning S-duct diffuser in high subsonic (Mach number > 0.8) flow. The test section was manufactured using rapid prototyping for facilitating a future parametric investigation of geometry. Streamwise static pressure and exit-plane total pressure were measured in a test-rig using surface pressure taps and a 5-probe rotating rake, respectively and was simulated through computational fluid dynamics. The investigation indicated the presence of streamwise and circumferential pressure gradients leading to three dimensional flow in the S-duct diffuser and distortion at the exit plane. Total pressure losses and circumferential and radial distortions at the exit plane were higher than that of the podded nacelle type of inlet. The work represents the beginning of the development of a database for the performance of a particular type of generic inlet. This database will be useful for predicting the performance of aero-engines and air vehicles in high subsonic flight.

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