Dynamic modal analysis of transonic Airborne Aero-Optics Laboratory conformal window flight-test aero-optics

Abstract

We discuss spatial-temporal characterizations of recent in-flight Airborne Aero-Optics Laboratory wavefront measurements at transonic speeds (Mach 0.65) with a conformal window turret as a function of turret pointing angle. Using both proper orthogonal decomposition and dynamic mode decomposition modal analysis methods, the flow dynamics are characterized. The conformal window wavefronts show shock formation between 85 deg and 90 deg and shear layer formation at a considerably lower turret aft pointing angle than would be expected at subsonic speeds without shock. At larger aft pointing angles, shear layer vortex roll-up dynamics dominate the aero-optical disturbances. In particular, the spatially and temporally periodic vortices grow in width and magnitude while the corresponding oscillation frequency drops with increasing look-back angle, thus maintaining a near constant vortex convection speed equal to about 0.6 times the free-stream velocity. From these results, a modified form of the aero-optics frequency scaling relation is proposed that yields a Strouhal number independent of turret look-back angle in the portion of the flow dominated by such Kelvin-Helmholtz shear layer vortices.