COMPARISON OF MEASURED AND COMPUTED STREHL RATIOS FOR LIGHT PROPAGATED THROUGH A CHANNEL FLOW OF A HE-N-2 MIXING LAYER AT HIGH REYNOLDS-NUMBERS

A lateral shearing interferometer was used to measure the slope of per turbed wave fronts after they propagated through a He-N-2 mixing layer in a rectangular channel. Slope measurements were used to reconstruct the phase of the turbulence-corrupted wave front. The random phase fl uctuations induced by the mixing layer were captured in a large ensemb le of wave-front measurements. Phase structure functions, computed fro m the reconstructed phase surfaces, were stationary in first increment s. A five-thirds power law is shown to fit streamwise and cross-stream slices of the structure function, analogous to the Kolmogorov model f or isotropic turbulence, which describes the structure function with a single parameter. Strehl ratios were computed from the phase structur e functions and compared with a measured experiment obtained from simu ltaneous point-spread function measurements. Two additional Strehl rat ios were calculated by using classical estimates that assume statistic al isotropy throughout the flow. The isotropic models are a reasonable estimate of the optical degradation only within a few centimeters of the initial mixing, where the Reynolds number is low. At higher Reynol ds numbers, Strehl ratios calculated from the structure functions matc h the experiment much better than Strehl ratio calculations that assum e isotropic flow.