Boundary-layer Stability and Transition in Subsonic and Supersonic Flow

SUMMARY: Accurate prediction of the aerodynamic characteristics of a body at high flight speeds requires knowledge of the characteristics of the existing boundary layer. Although the effects of compressibility on the characteristics of both laminar and turbulent boundary layers are known rather well, little information has been available as to how the transition between the two types of flow is affected by compressibility and other factors. Even in subsonic flow, the effects of such variables as free-stream turbulence, surface curvature, pressure gradient, surface roughness, and surface temperature are known only qualitatively. While the theory of laminar boundary-layer stability yields the conditions necessary for instability, it does not permit prediction of the transition point. This theory is, however, useful in indicating possible effects of the several variables on transition. It has recently been extended to compressible flows with heat transfer and with pressure gradient. This paper presents the available data in subsonic and supersonic flow for the effects of free-stream turbulence, surface curvature, pressure gradient, surface roughness, surface temperature, and Mach Number on the transition position. New supersonic data are included from rocket flights, firing-range tests, and wind-tunnel tests. The effects of the several variables on transition are compared with the theoretically predicted effects on boundary-layer stability. In general, the experimental data for transition confirm the trends indicated by the stability theory. Perhaps the most significant deviation from the trend expected from the stability theory is the relative insensitivity of supersonic transition to surface-temperature variation. The transition Reynolds Number range appears to increase with increasing Mach Number in both firing-range tests, where relatively low surface temperatures occur, and in wind-tunnel tests, where relatively high surface temperatures occur.