Transmission of thin light beams through turbulent mixing layers

Citation

Wissler, John B. (1992) Transmission of thin light beams through turbulent mixing layers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pw33-qc68. https://resolver.caltech.edu/CaltechETD:etd-08162007-133929

Abstract

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Light transmission through incompressible gaseous turbulent mixing layers is investigated with the objective of understanding the effects of large-scale coherent structures and mixing transition on the optical quality of the mixing layer. Experiments are done in a vertically flowing mixing layer which is enclosed inside a pressure tank and fed by two banks of high-pressure gas bottles. The study considers both the unequal density (high-speed [...] and low-speed [...]) and equal density (high-speed [...] and low-speed [...]) cases; the mixing of dissimilar gases is the source of the optical aberrations. Large-scale Reynolds numbers range between 3500 and 80000 over pressures from 2 to 6 bar. Light transmission characteristics are first studied qualitatively using a network of thin sheets of short-exposure ([...]) white light which are aberrated by the mixing layer and then image directly onto photographic film. Light transmission characteristics are then studied quantitatively using a lateral effect detector to dynamically track a thin He-Ne laser beam as it wanders under the influence of the passing coherent structures.

The study finds that the spanwise coherent structures generate systematic deflections of the light beam in the streamwise direction; the greatest deflections occur near the trailing edges of the structures at a formation called the cusp, where the high-speed fluid and low-speed fluid are entrained into the vortex core. The streamwise coherent structures, which form later in the mixing layer's development than the spanwise structures, generate substantial beam deflections in the span-wise direction which are closely associated with the streamwise streaks in plan-view shadowgraphs. The rms fluctuations of the streamwise and spanwise deflection angles increase rapidly during mixing transition, peaking at 380 high-speed-side momentum thicknesses downstream from the splitter plate, then decrease far down-stream to asymptotic values of 0.6 to 0.8 as scaled by the static pressure and the Gladstone-Dale constant shift across the mixing layer. The data suggest that a possible mechanism for the deflections is the interaction of the beam with the thin interfaces which bound relatively uniform bodies of fluid inside the structures.

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