Optical Turret Aerodynamics- a Preliminary Study

Sluder, Robin; Gris, Laurie; Katz, Joseph

doi:10.2514/6.2008-429

Cited by 6 publications

(2 citation statements)

References 3 publications

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“…The pressure coefficient contours for the baseline turret shown in Fig. 9 suggest a three-dimensional separation as expected and match well with [14,15]. The minor discrepancy in the separation angle prediction could be due to the use of unsteady RANS turbulence modeling at the wall, the wall boundary condition used on the bottom surface of the computational domain (which shifts the stagnation point downstream), geometric differences between the experimental and CFD models, and experimental uncertainties.…”

Section: Necklace Vortexsupporting

confidence: 66%

Subsonic to Supersonic Wake Stabilization Using Discrete Porosity

Patil²

2014

Journal of Aircraft

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The feasibility of using discrete suction to improve the laser optics environment associated with a hemisphere-andcylinder turret was investigated using computational fluid dynamics. Simulations were conducted at Mach numbers of 0.5,0.7, and 1.5. The discrete suction control generates and positions a large-scale counter-rotating vortex pair that induces a strong downwash over the hemispheric beam director. Flow attachment is maintained regardless of the position of the optical window and whether a shock is present. The density fluctuation in the wake is greatly reduced. Consequently, an improvement in the optical environment over the baseline can be expected for the Mach number range studied. I. IntroductionW HEN a laser plafform is placed on an aircraft, the two main causes of beam degradation are the thin-layer and immediate air flow around the aircraft, referred to as the aero-optic problem [ 1 ], as well as the intervening, orders-of-magnitude-longer propagation path through the atmosphere to the target, referred to as the atmospheric-propagation problem. Because atmospheric effects operate at relatively low frequencies, they can be mitigated relatively easily with modem beam-control, adaptive-optic methods when a low-bandwidth deformable mirror compensates these slow aberrations in real time. Aero-optical effects, on the other hand, are consequences of fast-changing turbulent flow around an aircraft with typical bandwidth on the order of kilohertz, which place it well outside the capabilities of these traditional approaches [2], although the recent development of high-bandwidth wave-front sensors combined with an open-loop control strategy offer some promises [3],The two primary aero-optic aberration-generating phenomena that limit system effectiveness and the lethal fleld of regard, in particular, are the separated shear layer and the shock that occurs in transonic flow [1][2][3], The effect that separated shear layers of the kind that occur over the exit apertures of an airborne directed-energy and freespace communication systems have on the "focusability" of a laser beam are now well known. In particular. Jumper and Fitzgerald [2] and Fitzgerald and Jumper [4] have shown through experiment and simulations that the unsteady aberrated optical wave fronts imposed on an otherwise collimated laser beam are caused mainly by the naturally occurring, large-scale, coherent, two-dimensional (crossspan) vortical structures in a separated free shear layer. These structures form as a result of a natural instability of the shear layer, with the most unstable and thus first to form being the twodimensionai structures. Because both the two-and three-dimensional instabilities are relatively broadbanded, the development of the vortical structures can vary considerably over time and space. This stochastic characteristic poses a severe challenge for both sensing the aberrations and ultimately controlling the devastating effects of the flow.

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Section: Necklace Vortexsupporting

confidence: 66%

Subsonic to Supersonic Wake Stabilization Using Discrete Porosity

Patil²

2014

Journal of Aircraft

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“…Typically, optical turrets have AR less than 6, and von Kármán vortex shedding does not develop. Limited work has been performed to characterize the temporal nature of the turret wake, as most research has focused on obtaining information regarding the mean aerodynamic loading [5,6], surface-flow visualization [4,7,8], or frequency content of the shear layer that develops in the aero-optic region as a result of separation from the cap geometry [8,9]. Pattenden et al present surface-flow visualization and particle image velocimetry (PIV) measurements in the wake of a finite cylinder with a flat-cap geometry and AR 1.0, in addition to providing a nice review of past work studying wall-mounted cylinders with a free end [4].…”

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confidence: 99%

Closed-Loop Active Flow Control of a Three-Dimensional Turret Wake

Shea

Glauser

2014

AIAA Journal

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An experimental study has been performed to investigate the wake of a three-dimensional turret with aspect ratio of 1.17 at a diameter-based Reynolds number of 5.5 × 10 5 . Stereoscopic particle image velocimetry was used to evaluate the spatial structure of the wake, and dynamic surface-pressure measurements were used to study the time-dependent behavior. The uncontrolled wake was seen to be strongly three-dimensional with two main structures on opposing sides of the center plane. Cross correlations from the surface pressure indicated that opposite sides of the wake had nearly zero correlation in time. With the application of active flow control, both the spatial structure and timedependent behavior of the wake were significantly altered. The downwash velocity along the center plane was seen to increase significantly, causing the wake to spread more rapidly near the support plate, and the spectra from the pressure sensors contained multiple peaks at different frequencies throughout the wake that were not seen in the baseline case. Closed-loop control achieved similar control authority as steady open-loop forcing, but with a 45% reduction in the mean jet-momentum coefficient. Nomenclaturereduced forcing frequency H = turret height Re D = diameter-based Reynolds number Sr D = diameter-based Strouhal number U = streamwise velocity component U J = suction-jet velocity U ∞ = freestream velocity V = wall-normal velocity component W = cross-stream velocity component x = streamwise coordinate y = wall-normal coordinate z = cross-stream coordinate ρ J = suction-jet density ρ ∞ = freestream density

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