Investigation of the Flow within Partially Submerged Scoop Type Air Intakes

Abstract

Design constraints for air intakes of unmanned combat aerial vehicles (UCAVs) or manned combat aircrafts have been tightened and partially changed due to addional requirements in regard to infrared and radar signature. These requirements have a crucial impact on the aerodynamic shape of air intakes and their integration in aircraft systems. Especially, radar signature is the reason why submerged or partially submerged air intakes are preferred solutions for UCAVs. Moreover, in order to further reduce the radar signature and to avoid backscattering it is also necessary that air intake associated ram air channels have to be S-shaped. Unfortunately, those intake and channel designs reveal a severe impact on the aerodynamic properties, in particular, under transonic flight condition: Especially partially submerged air intakes are very sensitive to onflow conditions. For example, in dependence of the thickness of the boundary layer in front of the intake fluid with more or less momentum enters the air intake which, consequently, effects the total pressure recovery. In fact, a thicker boundary layer means less total pressure recovery. Consequently a partially submerged air intake and its integrated ram air S-duct together exhibit the disadvantage that in regard to the pressure distribution the inhomogenity of the onflowing air at the engine throat plane might be critical for the performance of the engine. Moreover, an intake and duct shape design, which might be beneficial for one flight condition, may not be robust enough for those varying flow conditions which have to be faced over a full mission. Experimental and numerical studies are necessary in order to clarify the essential dependencies between boundary layer thickness, the effect of shock boundary layer interaction and the required engine performance coe�cients. This study addresses these problems by comparing experimental and numerical results.