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 ...

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Aero-optical measurements of a zero-pressure-gradient, supersonic boundary layer along the test-section wall at M 2:0 were performed using a Malley probe. The Malley probe captured both the amplitude of optical distortions and the convective speed. The convective speed of the optically active structures inside the supersonic boundary layer was found to be 0.84 of the freestream speed. The deflection-angle spectra were found to collapse with the local displacement thickness. The streamwise correlation function for the supersonic boundary layer revealed the presence of a pseudoperiodic structure with the typical size of 1.5 of the local boundary-layer thickness. A new model was developed to describe aero-optical effects of both the subsonic and the supersonic boundary layers. Finally, this new model and several other theoretical scalings were tested in the attempt to collapse both subsonic and supersonic boundary-layer aero-optical results.

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.