Pulsed Laser Device Development Program. Volume III. ABEL II (Air-Breathing Electric Laser II) Small-Scale Flow Test Report

摘要

A series of experiments were performed to study laser beam degradation for propagation normal to two-dimensional turbulent mixing layers. An analytical expression for beam degradation, derived earlier, is verified by the experiments. The expression shows that beam degradation is proportional to the square of the mixing layer thickness, L, and the square of the refractive index difference, delta n, between the two fluids across the layer. For a fixed delta n, degradation is thus dependent on L. In these experiments, the effects of free-stream characteristics and velocity shear on the mixing layer growth rate were studied. For layers with gas streams issuing from an orifice plate, the growth rate is high due to large turbulence and large non-uniformities in the free streams. A fine mesh screen positioned over the flow plate is quite effective in reducing the mixing layer growth. Mixing layer bending, which is most severe in the jetting region near the orifice plate, is also minimized by screening.