AFRL-OSR-VA-TR-2015-0159 Control of Boundary Layers for Aero-optical Applications

This report presents results of systematic experimental studies of various passive mitigation techniques to reduce aero-optical effects caused be turbulent boundary layers. Parametric studies of Large-Eddy BreakUp devices showed that aero-optical distortions can be suppressed by 45% for several boundary layer thicknesses. It was shown that moderate cooling of the wall also reduces aero-optical distortions by 60%. A model to predict effects of non-adiabatic walls, both full and partial, over a range of subsonic and supersonic Mach numbers was developed and shown to correctly predict experimentally-observed reductions. Heating the wall allows extending the use of wavefront sensors into low subsonic speeds. Using this technique, aero-optical effects of boundary layers at low (1,700-8,000) Reynolds numbers, were investigated. Simultaneous velocity-optical measurements revealed that large-scale vortical structures are mostly responsible for aero-optical distortions and also indicated that the pressure fluctuations inside these vortical structures might be significant. Distribution Statement This is block 12 on the SF298 form. Distribution A Approved for Public Release Explanation for Distribution Statement If this is not approved for public release, please provide a short explanation. E.g., contains proprietary information.