Aero-optical distortions along the spanwise direction of several spanwise uniform subsonic turbulent flows were measured and studied using a Shack-Hartmann wavefront sensor. It was demonstrated that in this case, some important fluidic statistics in the wall normal direction, like the local convective speed, fluctuating density profiles, and the spanwise integral scales, can be directly extracted from aero-optical distortions.
A canonical turbulent boundary layer (TBL) was investigated in the subsonic wind tunnel to validate the proposed approach. To avoid the spectral contamination due to finite spatial resolution of the wavefront sensor, a model function for the deflection angle autospectral density was proposed. The dispersion analysis was demonstrated to be most accurate in computing convective velocity. 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 TBL were found to agree well with the direct measurements using a single hotwire. From the convective velocities, the local wall shear stress was non-intrusively extracted, using the Clauser’s Method. Using deflection angle spectra and the convective velocity, the local values of aero-optical distortions were reconstructed. Using the Strong Reynold’s Analogy, a wall-normal profile of the spanwise density correlation length was extracted and shown to be in agreement with spanwise length scales observed in the literature.
Two additional TBLs were also chosen: a TBL with non-adiabatic wall and a TBL externally forced by a shear layer. In the case of the non-adiabatic TBL, the density fluctuation profiles in the wall-normal direction at different wall temperatures were extracted and analysed. Using these profiles, several issues with the Extended Strong Reynold’s Analogy were observed and discussed. The extracted fluctuating density profiles were demonstrated to correctly predict aero-optical distortions in the wall-normal direction. In the case of the TBL externally forced by a shear layer, global wavefronts were found to be significantly affected by aperture effects. A new approach, based on analysis of the local deflection angles only, was proposed to remove contamination from aperture effects. For this forced TBL, the SRA was found to satisfactorily predict the experimentally-measured aero-optical distortions.