Method for Increasing Wind Tunnel Mach Number for Large-Scale Inlet Testing

Abstract

FOR the development of efficient mixed-compression inlet systems for supersonic aircraft, relatively large'-scale models are required to accurately assess the inlet performance. Large-scale models are particularly essential as design inlet Mach numbers increase above about M°° = 2.2. That is, for mixed-compression inlets the throat contraction ratio increases, resulting in small throat heights and large negative pressure gradients; under these conditions accurate measurements of the inlet performance parameters would be difficult without large models. For inlet tests above Mach numbers of 3.5, few wind tunnels exist that are suitable for large-scale (50 cm capture diam) models. To overcome this problem, a simple and inexpensive technique to extend the Mach number range in existing wind tunnels for large-scale inlet testing has been investigated. The technique involves an expansion plate, wherein flow is expanded two-dimensionally around the sharp leading-edge to create a local uniform flow field with higher than freestream Mach numbers. Previously, flow expansion plates have been used to vary the Mach number in supersonic tunnels,' and occasionally they have been used to achieve special test conditions. The objective of the investigation was to show the feasibility of using locally expanded flow to extend the Mach number range of supersonic wind tunnels for large-scale inlet testing. The primary considerations of this investigation were to determine the Mach number increase possible and to define the uniformity of the expanded flowfield.