Optical Measurements of Transitional Events in a Mach-6 Boundary Layer

Abstract

Results of aero-optical measurements of naturally occurring transitional structures-turbulent spots and secondmode wave packets-in a hypersonic laminar boundary layer are presented. Optical spectra of turbulent spots were similar to spectra for fully turbulent boundary layers, and the average thickness of the boundary layer during the turbulent spot was approximately 50% of the fully turbulent boundary layer. The convective speed of the leading edge of the spot was measured to be 0.95 of the freestream speed, whereas the trailing edge moves at 0.7 of the freestream speed. The dominant frequency of the wave packet structures was about 70 kHz, consistent with the theoretically predicted second-mode frequency; the average convective speed of the packets was measured to be 0.88 of the freestream speed. Using the frozen field hypothesis, the wave fronts were stitched together to reveal streamwise/ spanwise topology of the packets. The packets were found to be fairly narrow, about 2-3δ in the spanwise direction, extending for 30-60δ in the streamwise direction.

New Insights into Turbulent Spots

“…A strong and broad frequency impulse excited by a spark device rapidly developed into turbulent spots that coexisted with second-Mack mode instability waves, as sketched in Figure 2c and similar to the DNS of Jocksch & Kleiser (2008). Turbulent spots and second-Mack mode instability wave packets were also detected in the BAM6QT by Gordeyev & Juliano (2017). Wave packets and turbulent spots on a 7°half-angle cone with a length of 0.517 m were investigated by Casper et al (2016) in the Sandia Hypersonic Wind Tunnel at M = 5, 8, and 14 using schlieren, surface pressure, and surface temperature measurements under FST levels between 3% and 5%.…”

Experimental investigation into the effects of shock waves on Shack-Hartmann wavefront sensor measurements

“…[1][2][3][4] In a broad sense, this interaction of light with turbulent aerodynamic flows is called the aero-optics problem. [5][6][7][8][9] In the recent past there has been significant research into the aero-optics problem across a wide range of flow regimes including subsonic, [10][11][12][13][14][15] supersonic, 16,17 and hypersonic 18,19 flows.…”