Spatial and temporal characterization of AAOL flight test data

This paper discusses spatial-temporal characterizations of recent in-flight Airborne Aero-Optics Laboratory (AAOL) wavefront measurements at transonic speeds (Mach 0.61 - 0.65) with both flat and conformal window turrets as a function of turret look-back angle. Using both proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) analysis methods, the flow dynamics were characterized. The conformal window wavefronts showed shock formation between 85 degrees and 90 degrees that prematurely induced separation at a considerably lower turret look-back angle than would be expected at subsonic speeds without shock. Vortex shedding for the flat window is initiated by the leading edge of the window at a slightly lower angle than the shock-induced conformal window separation. Nevertheless, the vortex shedding dynamics of both flat and conformal window cases were found to be similar. In particular, the vortices grew in width and magnitude with increasing look-back angle. Furthermore, the corresponding shedding frequency dropped with increasing look-back angle maintaining a near constant vortex convection speed equal to about a third of the platform (free-stream) velocity. From these results a new form of the aero-optics frequency scaling relation is proposed that yields a Strouhal number independent of turret look-back angle in the vortex shedding portion of the flow.