A new roughness array for controlling the nonlinear breakdown of second-mode waves at Mach 6
Transition of hypersonic boundary layers can be caused by many different instabilities. The current research focuses on the growth and breakdown of second-mode waves. All experiments were performed in the Boeing/AFOSR Mach-6 Quiet Tunnel at Purdue University. The low freestream noise environment at this facility is ideally suited for studying boundary-layer transition at conditions similar to those in flight. A flared cone model was used to maximize the growth of second-mode waves at a constant frequency. The resulting disturbance amplitudes were large enough to cause transition of the boundary layer. Characteristic hot-cold-hot streaks of heat transfer were observed on a smooth wall model. Maximum second-mode pressure fluctuation magnitudes were measured to be approximately 25% of the mean pressure on the surface of the model. The effect of surface roughness was measured on the 4-inch flared cone model and the Roughness Insert Cone model (with a 4.5-inch base). The first successful interaction of a roughness element with the second-mode instability was measured. Epoxy dots applied to the 4-inch cone changed the pattern of heating when the roughness element height was approximately 25% of the boundary-layer thickness at the point of application. Increasing the epoxy dot heights to nearly 40% of the boundary-layer thickness further changed the heating pattern, but a change in the viscosity of the epoxy prohibited further testing. The Rod Insertion Method roughness insert was then developed and successfully tested on the Roughness Insert Cone. Two RIM inserts were fabricated with 30 brass rods inserted around the circumference every 12°. Each brass rod had a diameter that spanned 3° azimuthally. RIM Insert #1 had roughnesses with a height of 380 microns while RIM Insert # 2 had elements with a height of 250 microns. Both inserts altered the smooth wall heat transfer pattern while still allowing large amplitude second-mode waves to amplify. The ratio of the roughness-element height to the boundary-layer thickness varied between 24% and 46% based on the unit Reynolds number. The elements never acted as a boundary-layer trip. Thus, the RIM roughness is a viable method for future studies of the nonlinear breakdown of large second-mode waves.
Schneider, Purdue University.
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