| The optical aberrations induced by mixing layers of dissimilar gases are recorded and analyzed. Laser light was propagated through a mixing layer of helium and nitrogen gas, having velocities of 8.5 m/sec and 1.5 m/sec, respectively. The mixing layer was evaluated in a free turbulent flow and in a channel flow. The optical perturbations induced by the mixing layer were recorded using a lateral shearing interferometer and a point spread function camera. Autocorrelation functions and structure functions were computed from the spatially resolved phase surfaces obtained using the shearing interferometer. For both the free and channel flows, the phase fluctuations were not wide-sense stationary. Consequently, the Strehl ratio predicted by traditional aero-optical models did not agree with experimental measurements except in regions of the flow where the Reynolds number was low. However, the phase fluctuations were locally homogeneous. A two-dimensional power law model was developed, analogous to the one-dimensional Kolmogorov model for isotropic turbulence. This model predicted a relative Strehl ratio which closely matched experiment throughout the flow. In a second series of experiments, the gas velocities were reduced to 4.5 m/s and 1.0 m/s for the helium and nitrogen gas, respectively. Flow orientation was rotated 90 degrees. Simultaneous high-speed measurements of shadowgraphs and wavefront slopes were used to assess the applicability of Taylor's frozen flow hypothesis to the mixing layer. Results indicated that frozen flow is at best limited to time scales on the order of 1 ms. For this flow orientation the phase fluctuations were found to be nonhomogeneous for all regions analyzed. |
