Aero-Optical and Hot-Wire Measurements of the Flow Around the Hemispherical Turret with a Flat Window

Abstract

Extensive investigation of the flow over the semispherical turret with the flat window was performed in order to document optical distortions over the window using 2-dimensional wavefront sensor and the Malley probe, complemented with simultaneous Malley probe -single hot-wire measurements of streamwise component velocity's profiles normal to the window at several points across the window's aperture for different azimuthal angles and a range of Mach numbers. The results provide the levels of unsteady optical aberration across the window's aperture, as well as the local thickness, intensity and a convective speed of the separated flow over the window. Results reveal that the optical distortions grow approximately as a square of the incoming Mach number multiplied by a freestream density, OPD rms ~ ρM 2 . I. Motivation.When an otherwise-collimated laser beam passes through a variable-index-of-refraction turbulent flow its wavefront becomes dynamically (unsteady) aberrated. These aberrations degrade the beam's ability to be focused in the far field, thereby reducing the system utility of the beam that may be used for communication, interrogation and targeting or as a directed-energy weapon. When the laser platform is 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 ; and the intervening, orders-of-magnitude-longer propagation path through the atmosphere to the target, referred to as the atmospheric-propagation problem. Modern beam-control, adaptive-optic methods appear to now be able to mitigate the atmosphericpropagation effects on the beam; however, both the spatial and temporal bandwidths of the aero-optic problem place it well outside the capabilities of these traditional approaches 2 . It has only been a decade since the first time-resolved wavefront measurements for propagation through a relevant aero-optic flow field were made 3 ; prior to that time, aero-optic propagation environments were characterized by limited time-unresolved interferograms and indirectly inferred from hot-wire anemometry techniques 1,2 . In general, the paucity of such characterizations that are available treated the aero-optic problem as a stochastic problem and reduced the measurements to very-unspecific measures of optical degradation as root-meansquare optical path difference, OPD rms . Such measures, while providing an estimation of the degradation that might be expected, provided little in the way of higher-order information about the aberrating environment's aberration coherence length (spatial bandwidth) and temporal bandwidth over relevant laserbeam apertures. The lack of such characterizations makes it impossible to either infer the far-field degradation in the point spread function or address the requirements for adaptive-optic mitigation schemes.

Survey of Optical Environment over Hemisphere-on-Cylinder Turret Using Suite of Wavefront Sensors

“…This paper compliments an earlier paper 5 that examined a similar turret but with a flat window. In the present study the flat window was replaced with a conformal window.…”

Aero-Optics Code Development: Experimental Databases and AVUS Code Improvements

“…A similar flow topology was discovered over the turret with a flat insert and results and related discussion can be found in Ref. [2].…”

CFD-based aero-optical performance prediction of a turret

“…11) and was also observed in wind tunnel test. 8 It should also be noted that the smaller 50cm beam, relative to the 58cm flat segment on the hemisphere, benefits from the area of the reattached flow where the density gradients are smaller. This results in lower OPD.…”