Numerical Simulation of Aero-Optical Distortions by a Turbulent Boundary Layer and Separated Shear Layer

Compressible large-eddy simulations are carried out to study the aero-optical distortions caused by a Mach 0.5 flat-plate turbulent boundary layer and a separated shear layer over a cylindrical turret with a flat window. The fluctuations of refractive index are calculated from the density field and ray tracing is employed to compute the optical path differences (OPD). In the flat-plate boundary layer, the log layer and wake region contribute most to the optical wavefront distortions. Consistent with previous experimental findings, the distortion magnitude is dependent on the propagation direction due to anisotropy of the boundary-layer vortical structures. In the case of separated shear layer over a cylindrical turret, wavefronts are tilted toward downstream due to the angle between the flat window and separated shear layer, and the low-density region therein. The root-mean-square of OPD (OPDrms) caused by the separated shear layer is found to be five times as large as that caused by the attached boundary layer upstream of the turret. The predicted OPDrms for both flows are in good agreement with experimental values despite much reduced Reynolds numbers in the simulations, suggesting relative insensitivity to the Reynolds number.

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