Observations on the Transition Process of Two-Dimensional Supersonic Wakes

Abstract

A detailed study has been made of the flow properties in the two-dimensional supersonic wake behind a heated slender flat plate. This wake exhibits a sequence of well-defined laminar, transitional, and turbulent regimes. The data included the mean (average) properties, the mod ally-resolved fluctuations and their spectra, and the fluctuation scales. The lateral wake profiles first depart from laminar similarity at the virtual origin of turbulence and reach turbulent similarity 500 thicknesses later; a large increase in kinematic viscosity and a small decrease in Prandtl number mark the transition zone. The wide-band fluctuations increase smoothly and the scales decrease greatly through transition towards magnitudes representative of turbulent wakes. However fluctuations as high as 10% of the deficits, as well as distinct scale sizes, were noted in the pretransitional wake. Spectrum analysis suggested that this is because of a laminar oscillation fitting in the predictions of laminar stability theory. Evidence was gathered that this oscillation triggers transition by spectral dispersion and remains visible in the turbulent wake, perhaps as a turbulence instability.