Aero-Optical and Flow Measurements Over a Flat-Windowed Turret

Abstract

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. HEN an otherwise-collimated laser beam passes through a variable-index-of-refraction turbulent flow, its wave front becomes dynamically aberrated (unsteady). 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 airflow 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 atmosphericpropagation 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 wave front measurements for propagation through a relevant aero-optic flowfield were made [3]; before 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 were available treated the aero-optic problem as a stochastic problem and reduced the measurements to very unspecific measures of optical degradation such as OPD rms . Such measures, although 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 laser-beam apertures. The lack of such characterizations made it impossible to either infer the far-field degradation in the point-spread function or address the requirements for adaptive-optic mitigation schemes.The ability to collect copious spatial and temporal wave front information through relevant aero-optical-type flowfields changed abruptly with the invention by Malley et al.[4] of a new approach to interrogating these fields with a direct optical method that used a single, small-aperture laser beam at a single location in the larger ...

Aero-Optical Measurements in a Heated, Subsonic, Turbulent Boundary Layer

“…The reader is directed to [10] for further information and a discussion of the Malley probe setup, operation, and data analysis. Wyckham and Smits 17 used a two-dimensional Shack-Hartmann sensor to study aero-optical properties of subsonic and supersonic boundary layers and developed a scaling law for these aberrations as a function of the freestream Mach number, the local skin friction coefficient, c f , and the ratio of the freestream static temperature, T ∞ , to the wall temperature, T w ,…”

“…In 1992, Malley et al 5 , introduced a pioneering new instrument to measure wavefront aberrations using a small-aperture beam which would be deflected by the turbulent flow. Gordeyev et al 10 advanced the described instrument by adding a closely spaced beam in the streamwise flow direction in order to extract convective velocity using Taylor's frozen flow assumption. The resulting Malley probe gives accurate, highly time-resolved information (exceeding 100 kHz) on the deflection of the small-aperture beam at a single location.…”

Fluidic Control of a Turret Wake, Part II: Aero-Optical Effects

“…URRETS provide convenient ways of pointing and tracking laser beams from airborne platforms; however, the turret creates a separated turbulent region of the flow, which, even at relatively-low subsonic speeds, starts to distort an otherwise planar emerging laser beam [2][3][4]. This, in turn, leads to the laser beam's unsteady defocus and jitter at the target [5].…”

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

“…The test section flow parameters are given in Table 3, while the model parameters are given in Table 4. Plots of the data sets discussed below, as well as interpretation of the data, are presented in considerable detail in Reference [4]. …”