Computational and Experimental Laser Differential Interferometry for Supersonic Turbulent Boundary-Layer Flow

Abstract

High-fidelity computational laser differential interferometry analysis was performed on flow in a supersonic isolator which had a turbulent equilibrium boundary-layer present on all four walls. A high-order implicit large-eddy simulation was used to generate the numerical flow-field. The beam projection simulations were performed by solving the high-fidelity paraxial beam equation. Several lower-order virtual laser beam models were also solved and compared with the high-fidelity simulations. The high-fidelity computations were performed in parallel with experimental laser differential interferometry measurements for a similar flow. This included replicating the post-measurement signal processing technique to minimize procedural differences between the computations and the experiment. The goals of this work are to better understand and validate the experimental technique for this flow while obtaining validation-quality measurements for the high-order high-fidelity implicit large-eddy simulations. Several parameters about the virtual beam and the accompanying virtual photosensor were varied to better understand their sensitivity to the resultant spectrum. One conclusion from the parametric study showed that the radius of the virtual laser beam can act to filter the flow’s spectral content, especially as the beam’s diameter approaches the size of the boundary-layer thickness. The results from the highfidelity beam study also found that a long wavelength beam can not respond to fine-scale structures in the boundary-layer when the wavelength is much greater then the feature size. This observation was not predicted in the low-order virtual beam models since they do not have a mechanism to account for the beam’s wavelength only its diameter. Overall, laser differential interferometry was seen to represent the spectrum of turbulent density fluctuations fairly accurately, despite averaging the fluctuations over the entire optical path that is inherent in the technique.