High-Speed Boundary-Layer Transition Induced by an Isolated Roughness Element

Abstract

Progress on an experimental effort to quantify the instability mechanisms associated with roughness-induced transition in a high-speed boundary layer is reported in this paper. To simulate the low-disturbance environment encountered during high-altitude flight, the experimental study was performed in the NASA-Langley Mach 3.5 Supersonic LowDisturbance Tunnel. A flat plate trip sizing study was performed first to identify the roughness height required to force transition. That study, which included transition onset measurements under both quiet and noisy freestream conditions, confirmed the sensitivity of roughness-induced transition to freestream disturbance levels. Surveys of the laminar boundary layer on a 7• half-angle sharp-tipped cone were performed via hot-wire anemometry and pitot-pressure measurements. The measured mean mass-flux and Mach-number profiles agreed very well with computed mean-flow profiles. Finally, surveys of the boundary layer developing downstream of an isolated roughness element on the cone were performed. The measurements revealed an instability in the far wake of the roughness element that grows exponentially and has peak frequencies in the 150 to 250 kHz range.

Crossflow Instability on a Wedge-Cone at Mach 3.5

“…This is a significant increase in pressure load over the previous efforts 9,10 with this traverse design. Vibration of the probe 9,10 became too large at the higher stagnation pressures in this test to use hot-wires. Noticeable wear was found on the fouling side of the flattened Pitot probe after the test.…”

“…This height would often vary over the length of the scan, primarily due to the traverse support pin not remaining at the same angle throughout the scan as it bears a significant load. The flattened Pitot probe and pressure sensor are the same hardware as was used in the previously referenced experiments by Owens et al 9 and Kegerise et al 10 for boundary layer and meanflow measurements. The focus of their work on the Pitot was to develop a probe that would cause minimum distortion to the boundary layer.…”

“…The traverse is similar to one presented by Owens, Kegerise, and Wilkinson 9 for a cone and is almost identical to that used on a flat plate by Kegerise, Owens and King. 10 A picture of the model with the integral traverse is shown in Fig. 3 (without the extension shelf shown in Fig.…”

Off-Body Boundary-Layer Measurement Techniques Development for Supersonic Low-Disturbance Flows

“…As discussed in Kegerise, Owens, and King, 12 obtaining mean and fluctuating measurements inside thin boundary layers (δ ~ 1 mm) on small-scale models is quite challenging. Accurate measurements require a high spatial resolution in the wall-normal direction and a frequency response in excess of 100kHz; therefore hot-wire anemometry was chosen as the primary instrument.…”

“…8-11, so here we only provide a brief overview of the facility similar to the one presented in Kegerise, Owens, and King. 12 Tests are performed in this tunnel with either a 2-D nozzle or an axisymmetric nozzle. For the cone research described in this paper, the axisymmetric nozzle 11 was used.…”