Subsonic to Supersonic Wake Stabilization Using Discrete Porosity

Abstract

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.