Whole-Field Measurements of Turbulent Flows for the Study of Aero-Optical Effects

Abstract

Planar laser-Rayleigh scattering has been employed to simultaneously image the index-of-refraction field of a turbulent jet of ethylene into nitrogen and the optical degradation of a laser sheet caused by this turbulent-flow field. The optical degradation occurs in t he turbulent-jet region and manifests itself as phase-front tilts that result in a measurable spatial amplitude modulation (streaks) in the emerging pulsed-laser sheet. The experiments were conducted at elevated pressure, increasing the index-of-refract ion gradient s and improving the signal-to-noise ratio over measurements conducted at atmospheric pressure. The high index-of-refraction gradients in these experiments placed the optical far field within the field of view and allowed us to capture caustic formation in the distorted emerging laser sheet, simulating the aero-optics effects expected at large distances from the smaller index-of-refract ion fluctuations one would more typically encounter.