Direct Reduction of Aero-Optical Aberrations by Large Structure Suppression Control in Turbulence

Direct reduction of aero-optical aberrations incurred by a laser propagated through a refractive turbulent flow has been achieved by introducing plasma into the flow at various pulsing frequencies. The plasma is generated by a dielectric barrier discharge device which operates at elevated pressure to force flow near the separation point of a single-stream shear layer that is generated in a variable pressure wind-tunnel testing facility with Re similar to 6 x 106 and M-infinity similar to 0.9. The levels of aero-optical aberrations present in the laser are determined by directly profiling the propagated laser using a high-resolution Shack-Hartmann wave front sensor with similar to 1000 microlenses. Reductions in the ensemble-averaged optical path difference root mean square of up to 27% were achieved. Whole-field shadowgraphs were recorded for various forcing frequencies and indicate that the observed aero-optical effects are the result of modifications in the level of organization of the large structures. Because the dominant contributions to the aberrations in unforced flows are caused by large-scale organized structures, our findings indicate that the mechanism by which the significant reduction is observed in the forced experiments is due to large-scale suppression of the turbulent structures directly effected by the pulsed plasma actuator. Conversely, increases in the aero-optical aberrations, which have also been observed at certain plasma pulsing frequencies, are the result of large-scale regularization of the structures within the turbulent shear layer. The presence of these large-scale suppression and regularization mechanisms is confirmed by the shadowgraphs at forced cases compared with the unforced case.