Physics and Measurement of Aero-Optical Effects: Past and Present

Abstract

The field of aero-optics is devoted to the study of the effects of turbulent flow fields on laser beams projected from airborne laser systems. This article reviews the early and present periods of research in aero-optics. Both periods generated impressive amounts of research activity; however, the types and amount of data differ greatly in accuracy, quality, and type owing to the development of new types of instrumentation available to collect and analyze the aberrated wave fronts of otherwise collimated laser beams projected through turbulent compressible flow fields of the type that form over beam directors. This review traces the activities and developments associated with both periods but particularly focuses on the development of modern high-bandwidth wave-front sensors used in the present research period. We describe how these modern wave-front data are collected and analyzed and the fluid mechanic information that can be gleaned from them; the use of these data in the fundamental study of turbulence is emphasized.

Studies of Density Fields in Non-Adiabatic Boundary Layers Using Wavefront Sensors

“…The peak location near the wall starts at a Strouhal number of Stδ = 3 and is reduced towards Stδ = 0.9 for locations progressively farther away from the wall. The peak location is indicative of the dominant structure size at that height [2,3]. Hence, near the wall optical distortions are dominated by small structures and are weaker in magnitude.…”

“…fluctuations are due to velocity fluctuations only, a model of density fluctuations was developed [4] for the boundary layer. This model was shown to successfully explain levels of aero-optical distortions and related convective speeds for a wide range of Mach numbers between 0.4 and 6.0 [3,6,7].…”

“…The aero-optic effect [1,2,3] is a direct result of the relationship between index-of-refraction, n, and density of air, ρ, via the Gladstone-Dale constant, KGD (which is approximately 2.27×10 -4 m 3 /kg in air for visible wavelengths of light) by, (1) where both index-of-refraction and density are functions of space and time. Unsteady turbulent structures in a flow will vary the index-of-refraction, scatter the focused laser, and decrease the intensity of the beam at distances far away from the emitting device.…”

Measurement of Flow Perturbation Spectra in Mach 4.5 Corner Separation Zone

“…To achieve the high sampling rate of 531 kHz, only a small portion of the image (128 × 64 pixels) was acquired for the full duration of the wind tunnel run. Centroids of dots' locations were extracted and converted into the local deflection angles using in-house software; more details are presented in [20,34,35].…”

“…Also, any corrupting effects from mechanically related vibrations imposed on the laser beam are easily removed from SH wavefront data [29]; these corrupting effects are still present in the data for LDI. Therefore, it is the most adequate wavefront sensor to measure aerooptical distortions [35].…”