This paper presents the aero-optical environment around a generic conformal-window turret formed from a hemisphere on a short cylindrical base. A suite of optical instruments consisting of a Malley probe, a conventional two-dimensional Shack-Hartmann wave-front sensor, and a new high-bandwidth, lower-resolution Hartmann wave-front sensor were used to measure the aberrations on the wave front of a laser beam emanating from the turret at various angles in both the forward and aft direction in the turret's zenith plane. The measurements were made over a range of Mach numbers from 0.35 to 0.45. Complementary steady-and unsteady-pressure measurements over a slightly larger range of Mach numbers were also made, along with a surface-flow-visualization study of the complex flowfield over and around the turret. The use of the suite of sensors allowed for the recognition and separation of the aberrating optical environment into components associated with stationary disturbances and convecting disturbances at the frequency of the turret's separated wake and at order-of-magnitude-higher frequencies associated with structures that form in the separated shear layers, respectively. The optical data separated in this way are valuable because of the implications for adaptive optics.

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A computational fluid dynamics (CFD)-based aero-optics validation study was conducted in wind tunnel tests at the US Air Force Academy. A 12 in diameter hemisphere-on-cylinder laser turret was tested in the 3 ft × 3 ft subsonic wind tunnel at flow speeds ranging from mach 0.3 to 0.5. Flow validation was based on mean and rms velocity, mean pressure profile, rms unsteady pressure, and separation point. Optical validation was based on rms phase variance and inflow phase correlation length derived from two-dimensional Hartmann wavefront sensor data, measured over a 5 in beam. The CFD code used a two-equation turbulence model with partially-averaged Navier-Stokes approach. Good agreement was observed between measurements and predictions over line-of-sight angles ranging from 60 to 132 • measured with respect to flow heading.A high-energy laser on a tactical aircraft offers a variety of target opportunities and engagement advantages. Its integration and operation, however, present several challenges. Due to aerodynamics, beam director location and configuration may adversely affect the transmitted beam wavefront error (WFE), depending on line-of-sight (LOS). For the inviscid flow field about an airborne turret, the perturbations vary at a relatively slow rate (few Hertz) and are easily compensated by using adaptive optics. However, the aerodynamicinduced flow closer to the transmitter window will produce a thin turbulent boundary layer, and for LOS beyond flow separation, a free turbulent shear layer that increases in thickness with LOS angle. The separated flow is characterized by high-frequency density fluctuations that translate to optical phase errors. These are difficult to correct with current adaptive optics systems.In the absence of expensive flight or wind tunnel testing, the accuracy of determining requirements and performance benefits of phase compensation are limited by the fidelity of the model used to determine the wavefronts corrupted by the flow. The approach in this study, defining the computational fluid dynamics (CFD)-based aero-optics model, is to use an upgraded CFD code to determine the timedependent flow solution, for the conversion of the resulting three-dimensional density map to an optical index of refraction grid, and a path integration to determine the optical path difference (OPD) or WFE along each LOS through the index field. Until now, no CFD code described in the open literature has been validated on a one-to-one wind tunnel scale at an optical wavelength level. In the past, validation has been limited to scaling optical figures-of-merit (FOMs) and anchoring to flow properties such as pressure, velocity, and vorticity. JAERO385

Mountain-top to mountain-top optical link experiments have been initiated at JPL, in order to perform a systems level evaluation of optical communications. Progress made so far is reported.

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