Aero-optical and fluidic studies of a forced subsonic compressible shear layer

Abstract

Predicting aero-optical distortions induced by subsonic compressible shear layers is essential for developing effective feed-forward adaptive optic compensation systems for directed energy platforms using common beam director geometries. While the established ”Weakly Compressible Model” (WCM) presented in Ref. 1 reasonably predicts path-integrated wavefront aberrations, recent computational fluid dynamics (CFD) simulations reveal that it fails to capture the sharp, localized stagnation temperature and density gradients that form along the slip lines of vortical structures. This study presents a continuation of the experimental investigation in Ref. 2 to validate the thermo-fluid mechanisms proposed in Ref. 3. Experiments were conducted in a compressible shear layer wind tunnel at a convective Mach number of Mc ≈ 0.23, regularized via acoustic voice-coil excitation at a fundamental frequency of 750 Hz. The fluctuating pressure and temperature fields were mapped using an array of wall-normal unsteady pressure transducers paired with a custom frequency-compensated cold-wire anemometer (τw ≈ 1.8 ms). To characterize the optical effects of the flow environment, spanwise and cross-stream digital holographic wavefront sensing (DHWFS) was coupled with narrow depth-of-field self-aligned focusing schlieren (SAFS) imaging to examine the shear layer on a two-dimensional and path-integrated basis. Phase-averaged cross-stream DHWFS results reveal that spanwise non-uniformities are non-negligible, providing experimental evidence that path-integrated effects may smooth out the intense, small-scale localized thermodynamic gradients when viewed in the spanwise direction, offering an explanation for why lower-fidelity two-dimensional models can inadvertently mimic path-integrated optical data.