60230 - Investigation of a Passive Flow Control Device in an S-Duct Inlet at High Subsonic Flow 

In military aircraft with fuselage-embedded engines, one or more inlet ducts with multiple bends are commonly used for inducting ambient air to the propulsion system. For high subsonic aircraft, these ducts also diffuse flow to an engine-acceptable Mach number. The combination of bends and diffusion creates a flow that is prone to separation in regions of high pressure gradients, the occurrence of which can cause a concomitant drop in total pressure, swirl and other flow distortion at the engine face. This affects engine performance, stability margin, and safety of the integrated aircraft-engine system. Avoiding degraded flow has always motivated designers of this type of inlet to apply flow control strategies to reduce boundary layer separation and associated flow distortion and swirling flow. In the open literature, various flow control studies are reported, revealing various levels of success. This paper reports the investigation of a flow control strategy aimed at the improvement of the aerodynamic performance of S-duct diffusers. The method incorporates streamwise tubercles, also named here as ‘dales and fells’, and aims to enhance the performance of S-duct inlets by reducing the size and intensity of separated flow.  Tubercles, bio-inspired from humpback whale flippers’ leading edge protuberances,  have been shown to be effective in increasing post stall coefficients of lift of airfoils.  The mechanism is reported to be the formation of streamwise pairs of counter-rotating vortices that draw higher momentum flow from the near-boundary layer region into the low-momentum flow of the boundary layer, rendering it more resistant to separation when faced with adverse pressure gradients. Any momentum exchange provided has a strong potential to suppress separation.  In S-duct diffusers, the presence of convex curvature next to the separated region provides an ideal location for the installation of a tubercle-like device. The flow control effectiveness was evaluated by test-rig measurements and computational fluid dynamics (CFD) simulations of the flow in an S-duct at high subsonic Mach numbers (Ma = 0.80). The dales and fells for flow control purpose were created in the S-duct diffuser with a height and a pitch determined experimentally from related experiments on the control of separated flow on a symmetric airfoil. The S-duct test articles were rapid prototyped in high-density plastic. Static surface pressure was measured streamwise, while total pressure and velocity vector was recorded using a 5-hole probe at the exit plane to evaluate pressure recovery, total pressure loss, flow distortion and swirl distribution at the S-duct exit. CFD simulations used ANSYS FLUENT with RANS solver closed with SST turbulence model. The CFD simulation compared well with the test-rig data and provided useful information on flow mechanism and for understanding flow features. The performance of the baseline and variant with the flow control device was compared and flow control strategy was evaluated.