STATISTICAL ANISOTROPY IN FREE TURBULENCE FOR MIXING LAYERS AT HIGH REYNOLDS-NUMBERS

A lateral shearing interferometer was used to measure the slope of per turbed wave fronts after propagating through free turbulent mixing lay ers. Shearing interferometers provide a two-dimensional flow visualiza tion that is nonintrusive. 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. Experiments were performed on an unboun ded, plane shear mixing layer of helium and nitrogen gas at fixed velo cities and high Reynolds numbers for six locations in the flow develop ment. Statistical autocorrelation functions and structure functions we re computed on the reconstructed phase maps. The autocorrelation funct ion results indicated that the turbulence-induced phase fluctuations w ere not wide-sense stationary. The structure functions exhibited stati stical homogeneity, indicating that the phase fluctuations were statio nary in first increments. However, the turbulence-corrupted phase was not isotropic. A five-thirds power law is shown to fit orthogonal slic es of the structure function, analogous to the Kolmogorov model for is otropic turbulence. Strehl ratios were computed from the phase structu re functions and compared with classical estimates that assume isotrop y. The isotropic models are shown to overestimate the optical degradat ion by nearly 3 orders of magnitude compared with the structure functi on calculations. (C) 1996 Optical Society of America