Aero-Optical Distortions of a Turbulent Boundary Layer at Back-Looking Viewing Angles

This paper presents the results of wavefront measurements of aero-optical effects due to a subsonic turbulent boundary layer at various laser beam viewing angles relative to the tunnel wall. Data were taken at Mach numbers of 0.3, 0.4, and 0.5 at back-looking angles ranging from 90 to 170 deg. A multipoint spectral cross-correlation technique was used to minimize the corrupting effects of the mechanical vibrations on the aero-optical spectra. A scaling of the aero-optical spectra as a function of the viewing angle was developed and discussed. Both the apparent optical convective speeds and the overall values of aero-optical distortions were extracted from data. Angular dependence of the aero-optical distortions was computed and found to agree with the isotropic model within a few percent. Two new models, accounting for the presence of the large-scale structures and for the spatial growth of the boundary layer, were proposed and discussed.