Physics and Computation of Aero-Optics

Abstract

This article provides a critical review of aero-optics with an emphasis on recent developments in computational predictions and the physical mechanisms of flow-induced optical distortions. Following a brief introduction of the fundamental theory and key concepts, computational techniques for aberrating flow fields and optical propagation are discussed along with a brief survey of wave-front sensors used in experimental measurements. New physical understanding generated through numerical and experimental investigations is highlighted for a number of important aero-optical flows, including turbulent boundary layers, separated shear layers, and flow over optical turrets. Approaches for mitigating aero-optical effects are briefly discussed.

Experimental studies of aero-optical properties of subsonic turbulent boundary layers

“…This constant depends on the gas mixture and the laser wavelength (Gardiner, Hidaka & Tanzawa 1980); for air over the visible wavelength and into the infrared range K GD is approximately 2.27 × 10 −4 m 3 kg −1 . †Email address for correspondence: sgordeye@nd.edu These optical aberrations caused by either density fluctuations present in the atmosphere, known as the atmospheric-propagation problem (Tatarski 1961), or inside a relatively-thin region of turbulent flow, composed of compressible shear layers, wakes and turbulent boundary layers around an airborne platform, known as the aero-optic problem (Gilbert & Otten 1982;Jumper & Fitzgerald 2001;Fitzgerald & Jumper 2004;Wang, Mani & Gordeyev 2012), can severely degrade the performance of an airborne laser system, be it free-space communication, interrogation, targeting or a direct energy application. The impact of these degrading effects can be quantified in different ways; however, one of the most common is to quantify it in terms of a time-averaged Strehl ratio, SR, defined as time-averaged ratio of the actual on-axis intensity at the target,Ī, to the distortion-free, diffraction-limited intensity, I 0 , SR =Ī/I 0 , after tip and tilt in the wavefront has been removed.…”

Receding-horizon adaptive control of aero-optical wavefronts

“…18 The original aero-optical wavefronts were produced by turbulence over a flat-windowed turret 18,[27][28][29] during a flight test in which a continuous-wave laser was transmitted between two planes flying in a constant formation at an altitude of 4570 m. The planes were separated by approximately 50 m to ensure aero-optical turbulence was the primary source of wavefront aberrations. Other significant parameters for the flight test are give in Table 1.…”

Subsonic Boundary-Layer Wavefront Spectra for a Range of Reynolds Numbers

“…Small disturbances to optical wavefronts in the near-field can result in significant reductions in time-averaged and instantaneous on-target intensity at points very far away from the source aircraft [1,2]. A reduction in on-target intensity poses a significant problem for the performance of airborne optical systems in directed energy, imaging, and free-space communications applications.…”