Aero-Optical Measurements of a Mach 8 Boundary Layer

Measurements are presented of the aero-optic distortion produced by a Mach 8 turbulent boundary layer in the Sandia Hypersonic Wind Tunnel. Flat optical windows installed in conformal test section walls enabled a double-pass arrangement of a collimated laser beam. The distortion of this beam was imaged by a high-speed Shack–Hartmann sensor using variable aperture sizes at a sampling rate of up to 1.75 MHz. Analysis is performed using two processing methods to extract the aero-optic distortion from the data: 1) a stitching method is applied to extract wavefronts without bias from a limited aperture size, and 2) a novel de-aliasing algorithm is proposed to extract convective-only deflection angle spectra and is demonstrated to correctly quantify the physical spectra even for relatively low sampling rates. Measurements of speed and size of large-scale convecting aero-optical structures are also presented. Overall levels of aero-optic distortions were estimated, and the results are compared with an existing theoretical model. It is shown that this model underpredicts the measured distortions regardless of the processing method used. Possible explanations for this discrepancy are presented. Finally, levels of the global streamwise jitter were estimated for different aperture sizes and compared with the results for the subsonic boundary layer. The results represent to-date the highest Mach number for which aero-optic boundary-layer distortion measurements are available.