Quantifying the spatio-temporal effects of optically-active turbulent flowfields on a coherent optical wave.

摘要

A method for measuring the spatial and temporal optical wavefront distortions imposed by Aero-Optical flowfields was developed. The method, referred to as the Small-Aperture Beam Technique (SABT), consisted of a novel algorithm that received as input beam-jitter signals acquired at discrete locations in the optical aperture and then determined, as output, the time-history of the wavefront distortion over the aperture. The SABT-algorithm was developed using a discrete-vortex numerical model of a heated two-dimensional jet flowfield. The accuracy of the SABT was shown to depend on flowfield evolution rate and probe-beam spacing. The performance of the SABT was compared to a Hartmann wavefront sensor and was shown to perform at the same level of accuracy with fewer probe beams. This reduction in the number of required probe beams allowed the SABT to measure optical wavefronts at rates of 100 kHz as opposed to the current state-of-the-art Hartmann wavefront sensor at 2.3 kHz. The SABT development was performed in a general manner so as to be applicable to any Aero-Optical flowfield.; The SABT was applied to the measurement of dynamically-distorting wavefronts due to propagation through an experimental heated two-dimensional jet. The SABT was compared to the Integral-Equation technique, a method of quantifying the statistical optical aberration of an Aero-Optical flowfield through fluid-mechanic measurements. The Integral-Equation technique was found to underpredict the SABT-measured optical distortion by 15%, a finding in agreement with other investigations. The 15% underprediction was attributed to signal attenuation in the temperature signals measured by constant-current anemometry. This result implies that optical-degradation estimates made using fluid-mechanic measurements will err on the non-conservative side. The SABT-measured wavefronts were also compared to wavefronts from a conditionally-sampled temperature-field database and, after compensating for the frequency response of the constant-current anemometer, showed good agreement between the methods. The SABT-acquired optical wavefronts were analyzed by spectral-analysis methods, the results of which showed the larger flowfield-aberrating structures to be moving faster than the smaller flowfield-aberrating structures. An explanation was formulated based on the distribution of vorticity in the evolving shear layer.