On hypersonic boundary-layer stability

Abstract

This paper reviews experimental hypersonic boundarylayer stability results obtained using hot-wire anemometry techniques. Data are obtained at a freestream Mach number of 8 on water-cooled and uncooled 7-degree half angle cones and on a water-cooled cylinder. A limited amount of cone data were obtained at M, = 6 . It is shown that one can not just extend subsonic and supersonic stability concepts and transition data to hypersonic Mach numbers. Hypersonic boundary-layer transition phenomena have several unique features and the topics must be treated independently. In low speed boundary layers one is accustomed to thinking of the vorticity instability mode which produces low frequency, low amplitude velocity fluctuations. A unique feature of a hypersonic boundary layer is the presence of the higher instability modes, the Mack modes. These instabilities v produce high frequency, large amplitude density fluctuations which can dominate the transition process. Some hypersonic trends are different from lower Mach number trends. Surface temperature effect is a good example. Cooling the surface stabilizes low Mach number boundary layers, but can destabilize a hypersonic boundary layer. Many of the parametric effects are very sensitive to Mach number. For example, it is shown that a small nosetip bluntness can completely dominate the stability of a hypersonic boundary layer, resulting in very large critical Reynolds numbers. This paper reviews general hypersonic stability characteristics, comparisons with theory, several parametric effects, and cone versus planar boundary-layer stability.