Subsonic diffusers designed integrally with vortex generators

Abstract

A short subsonic diffuser has been developed incorporating vortex generators as an integral design feature. The principle of operation is that line vortices, when suitably arranged, mutually interact to drive each other towards an adjacent plane wall. The wall may then be pulled away from the vortices at such a rate that the vortices remain a constant distance from it. The ideal arrangement, having the vortex lines running along the edge of the boundary layer, can thus be obtained by design. A simple, two-dimensional diffuser was developed at Lockheed using these principles. It was compared experimentally with a conventional high-performance trumpet-shaped diffuser. Two significant results were observed in this test series. One was that both pressure recovery losses and distortion were reduced by about 40% by the new design. The other was that vortex generator design mismatches carry larger penalties with the integrally designed diffuser than with the conventional type. The subsonic diffuser of an inlet for a Mach 2.7 supersonic transport airplane was shortened and redesigned in two stages, using the integrated vortex generator approach. First, vortex generators were tailored to a short subsonic diffuser of conventional design, and then a new inlet was tested which included a subsonic diffuser designed on the integral basis. The successive changes improved both pressure recovery and flow uniformity. Thus, the basic concept was validated in a practical application.