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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.paerosci.2010.06.001 Byline: Stanislav Gordeyev, Eric Jumper Abstract: A summary of research efforts for last several years on fluid-dynamics and aero-optics of hemisphere-on-cylinder turrets with flat and conformal windows is presented. The topology of flow behind turrets and both steady and unsteady sources of optical distortions are discussed. Scaling laws for levels of optical aberrations are proposed and results of several experimental studies are compared and discussed. Effects of passive and active flow control in mitigation of aero-optical environment around turrets, as well as current computational studies of aero-optics of turrets are summarized and discussed. Author Affiliation: Department of Aerospace and Mechanical Engineering, University of Notre Dame, 120 Hessert Center, Notre Dame, IN 46556, USA

This paper uses a discrete-vortex code to examine a shear layer's response to forcing at its origin and to develop a relationship between a shear layer's optical characteristics and the commonly used characteristic growth length, vorticity thickness. The code and its thermodynamic overlay have been used in previous studies to predict the optically aberrating characteristics of relatively high-Mach-number, subsonic shear layers that can be classified as weakly compressible. A weighted-average natural frequency is introduced and used to characterize the unforced shear layer in terms of an optical characteristic length referred to as optical coherence length. It is shown that optical coherence length is related to vorticity thickness by a factor of approximately 3.18. The study also shows that the use of single-frequency forcing produces a regularized shear layer for distances preceding the point at which the unforced shear layer's natural frequency matches the forcing frequency. In the case of the forced shear layer, a greater thickness is produced closer to its point of origin until collapsing onto the unforced shear-layer thickness past the point of regularization. The aberration periodicity is shown to have lower robustness toward the furthest downstream extent of regularization due to uncontrolled pairing. DOI: 10.2514/1.34244

This paper presents the aero-optical environment around a generic conformal-window turret formed from a hemisphere on a short cylindrical base. A suite of optical instruments consisting of a Malley probe, a conventional two-dimensional Shack-Hartmann wave-front sensor, and a new high-bandwidth, lower-resolution Hartmann wave-front sensor were used to measure the aberrations on the wave front of a laser beam emanating from the turret at various angles in both the forward and aft direction in the turret's zenith plane. The measurements were made over a range of Mach numbers from 0.35 to 0.45. Complementary steady- and unsteady-pressure measurements over a slightly larger range of Mach numbers were also made, along with a surface-flow-visualization study of the complex flowfield over and around the turret. The use of the suite of sensors allowed for the recognition and separation of the aberrating optical environment into components associated with stationary disturbances and convecting disturbances at the frequency of the turret's separated wake and at order-of-magnitude-higher frequencies associated with structures that form in the separated shear layers, respectively. The optical data separated in this way are valuable because of the implications for adaptive optics.

An extensive investigation of the aberrating character of flow over a hemisphere-on-cylinder turret with a flat window was performed. Optical distortions over the window were measured using a two-dimensional wave front sensor and a Malley probe. The Malley probe measurements were complemented with simultaneous hot-wire measurements of the velocity field normal to the window at several points across its diameter. The tests were run for a fixed elevation for several azimuthal angles over a range of Mach numbers. The results provide the levels of unsteady optical aberration across the window, as well as the local thickness, intensity, and convective speed of the separated flow over the window. Results reveal that the optical distortions grow approximately with the square of the incoming Mach number multiplied by the freestream density, [OPD.sub.rms] ~[rho][M.sup.2], as well as with the changing character of the aberrating flow with azimuth angle. DOI: 10.2514/1.24468