Measurement of Wall-Cooling Effects on Hypersonic Boundary-Layer Transition Using Focused Laser Differential Interferometry

Abstract

The boundary layer on a sharp-tip, partially flared, actively cooled cone with discrete roughness elements has been studied in a quiet Mach 6 wind tunnel using linear array–focused laser differential interferometry (LA-FLDI). The conditions were varied by sweeping the unit Reynolds number of the flow from to and the wall temperature was set to one of two conditions by setting the coolant setpoint temperature to 293.2 or 253.2 K. The spectral content showed the presence of the second-mode instability, which is the dominant instability, and two of its harmonics across the Reynolds number sweep. A novel analysis of FLDI data is presented where turbulent wave packet amplitudes have been analyzed and show how the cooler wall condition transitions to turbulence sooner and it is supported by observations in the spectra. This analysis may also be of benefit for defining a transition location or region from FLDI data in general. The turbulent wave packet analysis results were compared with an intermittency analysis and showed consistency.