Effects of engine acoustic waves on optical environment around turrets in-flight on AAOL-T

Abstract

Wavefront measurements were performed in-flight for a hemispherical turret on the Airborne Aero-Optics Laboratory-Transonic (AAOL-T). For turret side-looking angles near 90 deg, upstream-propagating optical distortions over the turret aperture were observed. These distortions were contributed to acoustic waves originating from the engine downstream of the turret. The acoustical waves were modeled by combining a potential flow solution for a single-frequency moving acoustic source and a potential flow around a sphere. The model was shown to properly predict the spatial statistics and convective speeds of the experimentally observed wavefronts. The model also correctly explained the viewing-angle dependence of the acoustic-related optical distortions. It was shown that for side-viewing angles between 90 deg and 100 deg, these acoustical distortions can be a significant contributor to the overall aero-optical distortions.