Performance Evaluation of an S-Duct Diffuser of a Flight-Vehicle Inlet in High-Subsonic Flow

Abstract

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.

Reynolds-averaged Navier–Stokes modelling in transonic S-ducts with passive flow control

“…The Transient Transonic Wind-tunnel is an indraft tunnel; ambient air is drawn through the bell mouth and duct, the outlet of which is connected to a valve and vacuum tank. The reader is referred to Asghar et al 3 for more details on the experimental methodology.…”

“…The pressure inlet had a radius of 3D, which was found by computations on similar ducts by Asghar et al 3 to be sufficiently large to not affect the solution. Similarly, the duct outlet was extruded for a further 1D down stream to avoid interactions with the pressure outlet boundary condition affecting the AIP.…”

“…They can lead to fan and compressor blade fatigue, and reduce the surge margin of the compressor. 12 The descriptors used in papers by Asghar et al 1,3,15,16 are used here for the sake of easy comparison with experimental data. For all the distortion coefficients discussed in this report, a high value corresponds to a more severely distorted AIP.…”

“…9 More complex coefficients include the circumferential distortion coefficient, ÁPC=P ð Þ avg , and the radial distortion coefficient, ÁPR=P ð Þ max . 3,9,15 Vortex generators and boundary layer fences…”

“…2 An engine face Mach number of 0.4-0.6 is usually targeted. 3 S-ducts are often used in unmanned aerial vehicles (UAVs), older transport aircraft, combat aircraft, cruise missiles 3,4 and boundary layer ingesting propulsion on blended wing/body designs. 5 To avoid a degradation in compressor surge margin and propulsion system efficiency, an ideal S-duct would provide a uniform axial velocity and pressure distribution at the engine face, 2 but the behaviour of the boundary layer in the hostile environment of an adverse pressure gradient and duct curvature causes secondary flows and separations which complicate intake development.…”