Injection into Supersonic Boundary Layers

Abstract

A method for injection of gas into the boundary layer on a slender body in supersonic flow while minimizing perturbation to the mean flow is examined. Injection of gas is equivalent to a sudden increase in the displacement thickness of the boundary layer, which produces an oblique shock that propagates into the inviscid region of the flow. It is found that modification of the geometry of the body can compensate for the increased displacement thickness created by injection and minimize the production of oblique waves. However, the resulting near-wall injection layer is observed to be unstable and a turbulent boundary layer develops downstream of the injection region. The instability of the flow is examined experimentally using high-speed schlieren visualization and numerically using linear stability analysis of velocity profiles from a compressible Navier–Stokes computation. At the present postshock Mach number of about 3.8, both first- and second-mode instabilities are active, though computations predict that the first mode is primarily responsible for transition downstream of the injector.