Numerical investigation of optical distortions by turbulent wake and shock wave in the transonic flow

Large eddy simulation with the explicit fifth-order weighted compact nonlinear schemes is performed to investigate the aero-optical distortion caused by a transonic flow over a cylinder at Reynolds numbers of Re=1x10(5), Re=2x10(5), and Re=4x10(5). Proper orthogonal decomposition identifies two dominant modes: the antisymmetric "shifting" mode and the symmetric "breathing" mode, both peaking at the non-dimensionalized Strouhal number St(D)=0.18. Optical distortion refers to the phenomenon where a beam passes through a non-uniform and fluctuating flow field, resulting in defocus, jitter, and significant energy reduction. Optical calculations for 12 cases demonstrate that local shock waves and turbulent wakes notably exacerbate optical distortion. In terms of temporal results, the peak frequency of the beam aligns with St(D)=0.18 when passing through the cylinder shockwave and falls within St(D)=0.16-0.42 as it crosses the shear layer. Significant fluctuations are observed in the turbulent wake and local shock wave, with frequency peaks ranging from St(D)=0.12-0.72. Additionally, streamwise flow structures are found to primarily impact optical distortion. Comparative analysis across the three Reynolds numbers indicates that optical distortion is insensitive to Reynolds number variations within the same order of magnitude. The employed grid sufficiently resolves key flow structures impacting beam transmission.