Aero-Optic Performance of an Aircraft Forward-Facing Optical Turret

Abstract

The results of three-dimensional calculations are presented for aero-optic analysis of an aircraft forward-facing optical turret. Computational-fluid-dynamics calculations, using a compressibility corrected k-e turbulence model, defined the mean flow properties. Turbulence characteristics were obtained from a three-dimensional g law equation that yielded the variance of index-of-refraction fluctuations. The characteristic scale length of the correlation function of the index-of-refraction fluctuations was calculated. Strehl ratios and wave front distortion were computed for the mean flow and turbulence aberrations for four azimuth angles, 0, 45, 90, and 120 deg. For 90 and 120 deg azimuth, a portion of the beam passes through an open cavity with large-scale turbulence and nonuniform mean flow. For the mean flow, the Strehl ratio decreased to 0.001 at 90-deg azimuth after tip, tilt, and best focus were removed for a wavelength of 1.06 μm. The loss of Strehl as a result of turbulence was not as severe. Wind-tunnel measurements of optical path differences scale favorably to this work. In addition, instantaneous far-field intensity plots for instantaneous realizations of turbulence were constructed. A deformable mirror adaptive optics system might improve the Strehl ratio for the mean flow, but probably not for turbulence.