Stability of High-Speed, Three-Dimensional Boundary Layers

Abstract

Boundary-layer experiments are performed in the low-disturbance, Mach 6 Quiet Tunnel (M6QT) at Texas A&M University. The experiments are focused specifically on investigating the physics of two three-dimensional phenomena in hypersonic boundary-layer stability and transition: the breakdown of second-mode waves and the growth and breakdown of crossflow waves.

In order to enable these experiments, a new, three-dimensional probe traversing mechanism was designed and constructed. In order to investigate the breakdown of second-mode waves, experiments are conducted on a flared cone with a 5° half angle at the tip at zero angle of attack. Experiments were typically performed at unit Reynolds number Re’ ≈ 10 × 10^6/m with a slightly hot wall, T/Taw ≈ 1.05. A new, durable method of roughness element application is discussed for the purpose of exciting the unstable waves. Hot-wire measurements were made of the boundary layer and it is shown that even with roughness elements, transition to turbulence does not occur on the model. Therefore, the expected Λ vortices are not observed.

The crossflow instability in a hypersonic boundary layer is studied on a 7° right circular cone at 5.6° angle of incidence. Experiments were performed at Re’ ≈ 10 × 10^6/m with an adiabatic wall. Hot wire measurements are made at a series of axial locations to generate contours of streamwise mass flux. The stationary vortex structure is shown through its saturation. Traveling waves are observed in the expected frequency range, 10 kHz to 60 kHz, predicted by computations and are located generally in the high-speed troughs in the vortex structure. Secondary instability is observed between 80 kHz and 130 kHz. Frequency scaling and location is shown to agree with low-speed experiments and good preliminary agreement with hypersonic computations is obtained. Transition does not naturally occur on the model. Distributed roughness is applied to the tip in order to excite crossflow and cause transition. Transition is shown to occur with the rough tip, but is not likely a result of crossflow.