Optical Distortions by Compressible Turbulence

Optical Distortions by Compressible Turbulence

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Optical distortions induced by refractive index fluctuations in turbulent flows are a serious concern in airborne communication and imaging systems. This project focuses on aero-optical flows in which compressible turbulence is the dominant source of optical distortions. These flows include boundary layers, free shear layers, cavity flows, and wakes typically associated with flight conditions. The present study consists of two theoretical analyses and an extensive numerical investigation of optical distortions by separated shear layers and turbulent wakes. We present an analysis of far-field optical statistics in a general aero-optical framework. Based on this analysis, measures of far-field distortion, such as tilt, spread, and loss of focus-depth, are linked to key flow statistics. By employing these measures, we quantify distortion effects through a set of norms that have provable scaling properties with key optical parameters. The second analysis presents a theoretical estimate of the range of optically important flow scales in an arbitrary aero-optical flowfield. We show that in the limit of high Reynolds numbers, the smallest optically important scale does not depend on the Kolmogorov scale. For a given geometry this length scale depends only on the flow Mach number, freestream refractive index, and the optical wavelength. The provided formula can be used to estimate grid resolution requirements for numerical simulations of aero-optical phenomena. A rough estimate indicates that resolution requirements for accurate prediction of aero-optics is not much higher than typical LES requirements. As a model problem, compressible turbulent flows over a circular cylinder is considered to study the fundamental physics of aero-optical effects. Large-eddy simulation with a high-resolution numerical scheme is employed to compute variations of the refractive index field in the separated shear layers and turbulent wakes in a range of flow Mach numbers (0.2--0.85) and Reynolds numbers (3900--10 000). A combination of ray tracing and Fourier optics is used to track the propagation of optical beams and their far-field intensity patterns. Instantaneous and statistical descriptions of optical distortions are obtained for different flow conditions and optical configurations. Dependence of distortion on key parameters such as flow Mach number, Reynolds number, optical wavelength, and aperture size are investigated. For the considered regimes, aero-optical effects are found to dominate the diffraction effects by an order of magnitude. A wide range of flow scales are found to play an important role in causing distortions, including scales of order aperture width down to the smallest scales resolved by a typical LES. Optical effects of transitional flow regimes were found to be sensitive to Reynolds number, while effects of fully turbulent regions where found to be weakly sensitive, or insensitive, to this parameter. For low Mach number regimes the distortion effects where found to scale with the square of the Mach number. To numerically treat the shocks in the transonic flow simulations, a localized artificial bulk viscosity model is used. The proposed improvements to this model are shown to significantly improve LES capability in predicting sound, turbulence, and aero-optics in the transonic regime. Shock turbulence interactions and unsteady shocklets are shown to significantly amplify distortion effects of the flow. Optical effects of flow-radiated sound are analyzed and found to be important in the transonic regime.This project focuses on aero-optical flows in which compressible turbulence is the dominant source of optical distortions.

Title:Optical Distortions by Compressible Turbulence
Author: Ali Mani
Publisher:Proquest, UMI Dissertation Publishing - 2011-09

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