Optical Diagnostics of Spanwise-Uniform Flows

Abstract

Aerooptical aberrations along the spanwise direction of a canonical subsonic turbulent boundary layer were measured and studied nonintrusively using a Shack–Hartmann wave front sensor. It was demonstrated that in this case some important fluidic statistics in the wall-normal direction, like the mean velocity profiles, the local skin friction, and the spanwise integral scales, can be directly extracted from aerooptical aberrations. To avoid various spectral contamination in optical data at low frequencies, a model function for the deflection angle autospectral density at low frequencies was proposed. The spectral cross-correlation method and the dispersion method were used to extract the local convective velocities, and the dispersion analysis was demonstrated to be most accurate in computing the velocities. It was shown that it is possible to reconstruct the spectra above the Nyquist frequencies through the newly proposed stacking method. Convective velocities in the log region of the turbulent boundary layer were found to agree well with the direct measurements using a single hot wire. From the convective velocities, the local wall shear stress was nonintrusively extracted, using the Clauser method. Using corrected deflection angle spectra and the convective velocity, the local values of aerooptical aberrations were reconstructed. Finally, using the strong Reynold’s analogy, a wall-normal profile of the spanwise density correlation length was estimated and shown to be in good qualitative agreement with velocity-based spanwise length scales observed in the literature.