Aero-Optical Mitigation of Turbulent Boundary Layers Using Large-Eddy Break-Up Devices

Results of recent experimental measurements of the effect of passive boundary layer flow control method using both singleand multiple-element Large-Eddy Break-Up devices (LEBU) on aero-optical distortions in turbulent subsonic boundary layers at a range of subsonic speeds are presented. Measurements at different locations downstream of LEBU devices were performed using a Malley probe to collect instantaneous one-dimensional wavefronts with high temporal resolution. Detailed statistical analysis of optical spectrum of aero-optical distortions are presented in order to evaluate the effect of modified turbulent structures on optical wavefront fluctuations, and to assess the mitigating aero-optical effect of each tested LEBU configuration. It was found that a long, at least 1.6 boundary-layer thicknesses, two-element LEBU device, spaced in the streamwise direction by several boundary-layer thicknesses with individual elements placed at either 0.5 or 0.6 boundarylayer thicknesses away from the wall resulted in a significant, greater than 40%, reduction in overall level of aero-optical distortions. This reduction was contributed to a suppression of the large-scale structure in the boundary layer.