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Control of Separating Flow Over Aerodynamic Turrets

Active, passive, and hybrid flow control concepts are applied to the separated flow over a turret – a surface-mounted hemisphere-on-cylinder.  The main motivation for this work is the objective of suppressing optical aberrations within the separated flow over a conformal optical aperture embedded in the hemispherical cap of the turret.  Two major sources of optical aberrations are the shear layer that bounds the separated region and the two large-scale ‘horn’ vortices that form off the aft turret surface.  Active flow control is effected by a number of high-frequency actuators (synthetic jets / fluidic oscillating jets) distributed in multiple arrays upstream from the aperture.  Active flow control is further assisted by global flow alterations induced by a passive forward partition plate, and, when combined, constitute a hybrid flow control.  It is shown that the hybrid flow control combines the positive effects of its component control elements to yield superior results in cumulative aero-optics and aerodynamic aspects of the separated flow.  A signature favorable effect of passive control in the flow separation delay and substantial suppression of the turret wake extent is further enhanced by the active dissipative effect of small-scale (‘high frequency’) excitation imposed by the control jets.  Overall, it is shown that the flow control reduces the separated flow domain, suppresses the ‘large-scale’ velocity and density fluctuations and thereby mitigates the optical aberrations.

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Supported by AFRL