Coherent Structures Responsible for Aero-Optical Distortions in Attached and Detached Flows

Aero-optical effects occur when a collimated optical beam propagates through a nonuniform, fluctuating flowfield, causing density fluctuations that disrupt beam propagation. While the relationship between flow structures and optical distortions has been investigated, the specific coherent structures responsible for the unsteady distortions are not fully characterized. This study investigates these unsteady structures using a proper orthogonal decomposition (POD) framework adapted to maximize phase variance rather than conventional kinetic energy. Two flow configurations are analyzed: a turbulent boundary layer over a flat plate and the separated flow around a cylindrical turret. The objective is to characterize and compare the structures contributing to optical distortions in both attached and detached flows. For the attached boundary layer, the identified structures share morphological features with hairpin vortices and hairpin packets and differ from the streaky structures maximizing kinetic energy. For the detached flow, the dominant modes present characteristics similar to Kelvin–Helmholtz instabilities for all three norms. We also find that POD eigenvalues associated with phase variance decrease faster with mode order than density variance POD modes due to the anisotropic nature of the aero-optic norm. These findings suggest that aero-optical distortions are linked to specific coherent flow structures that can be identified through a phase variance-based analysis.