Optical diagnostics investigation of wake flow fields behind geometrically modified turrets R. Harris Haynes * , Brian S. Thurow † , and Anwar Ahmed ‡ Department of Aerospace Engineering, Auburn University, 211 Davis Hall, Auburn, Alabama 36849, USA An experimental investigation was conducted on the flow fields behind various hemisphere- on-cylinder turret geometries in subsonic flow. The effects that geometric modifications had on the baseline flow field were characterized to determine which designs, if any, demon- strate potential as passive flow control mechanisms. Qualitative results were obtained for each turret model using a pulse burst laser to perform three-dimensional flow visualiza- tion. Additional, quantitative measurements along the wake centerline for each design were made via planar particle image velocimetry (PIV). The results of each modified flow field are presented with the baseline case for comparison. Suggested future work is also given for those designs showing the most promise as passive flow control devices. I. Introduction Turrets, from an optical point of view, are favorable platforms for optical instruments due to their large fields-of-regard for projecting and receiving directed energy, such as laser beams. 1 Unfortunately, conven- tional turret designs with smooth hemispheres create highly complicated, three-dimensional flow patterns in the wake region that can result in significant aero-optical distortions. In particular, the flat optical window through which directed energy propagates is subject to a complex flow field consisting of large, turbulent, separated flows with steep density gradients as well as noise and eddy structures in the separated shear layer. Consequently, a transmitted beam undergoes wavefront distortions and a loss of intensity due to the changes in the refractive index of the fluid. 2, 3 At any given instant, the flow over a turret is a combination of four different flows that exist around a surface mounted bluff body, namely: 1) the end-wall flow, 2) the flow in the vicinity of the attachment point, 3) the flow due to short aspect ratio, and 4) the wake flow. 4–6 Furthermore, the flow satisfies the classical definition of an absolute instability. 7 The control of flow and/or modification of flow around a turret therefore requires a synergistic strategy that addresses all four aspects previously mentioned. A number of active and passive flow control methods for the modification of bluff body flows are discussed in a review paper, 8 and research work specifically related to turrets has been attempted computationally 9 and experimentally 10 . However, the efficacy of these techniques is limited by the unpredictable trajectory of fluid dynamic scales in the presence of structural vibrations and atmospheric conditions. Because the flow around a turret, or more precisely a hemisphere-on-cylinder turret, has been extensively studied and described in literature, only a brief overview of the most common flow features associated with subsonic flow around a turret is given. In the most general sense, as the flow approaches the turret base, the streamwise pressure continues to rise until a separation of the boundary layer occurs. The vorticity in this separated boundary layer results in the formation of a horseshoe or necklace vortex system consisting of three to five vortices and submerged in the near wall region. This region is marked by a bifurcating streamline that separates the near wall flow from the uniform outer flow. The vortices comprising the horseshoe or necklace vortex system periodically shed and convect downstream into the wake region. 11 The outer flow stagnates near the front attachment point on the turret before accelerating over the hemispherical top where it reaches a maximum velocity. Past the minimum pressure point on the surface of the turret, the flow separates due to the adverse pressure gradient and forms a highly complicated, highly three-dimensional, * Graduate Research Assistant, Department of Aerospace Engineering. Student Member AIAA. † Associate Professor, Department of Aerospace Engineering. Senior Member AIAA. ‡ Professor, Department of Aerospace Engineering. Associate Fellow AIAA. 1 of 16 American Institute of Aeronautics and Astronautics 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 09 - 12 January 2012, Nashville, Tennessee AIAA 2012-0580 Copyright © 2012 by R. Harris Haynes. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.