Mitigation of Aero-Optic Distortions by Active Flow Control

"Laser beam interactions with free shear turbulent structures in the context of a recessed cavity window will be investigated. The proposed program will investigate the aero-optic degradation of a laser beam, housed within the cavity, and its remediationusing high frequency flow control. Beam wander and displacement statistics, obtained via a Position Sensitive Detector (PSD), will be complemented by non-intrusive flowfield measurements using Particle Image Velocimetry (PIV), Laser Speckle for densityfluctuations, Schlieren, etc. Using a previously generated very high-resolution database of high quality, temporal Large Eddy Simulation data, turbulent length-scales, phase-velocity, turbulent kinetic energy spectrum, cross-correlations, etc. will beextracted for the cavity shear layer. Mechanisms underlying effectiveness of high frequency control will be identified and correlated with growth of vortical, large-scale structures. Analytical predictions of beam displacement will be performed andcompared with measurements. A Low Dimensional Model, based on Proper Orthogonal Decomposition (POD), will be used to examine alternate time-dependent strategies for control of turbulent structures in the cavity flowfield. Ascertaining the ability of flowcontrol to manipulate laser/turbulence interactions, thereby reducing the demand upon any adaptive-optics system, is the underlying goal of the proposed STTR effort. The proposed STTR research will lead to a high frequency actuator design concept, whichcan be directly transitioned to the JSF& UCAV programs. Beyond DE weapons, the flow control technology for reduction of optical degradation by turbulence is of great relevance for reduction of bore-sight-error (BSE) for a tracking system or blurelimination for an imaging system. From the perspective of flow control, the high frequency actuator is closely related to current research in aircraft exhaust noise reduction, exhaust plume infra-red (IR) signature control and weapons bay dynamic loadattenuation."