Transmission of Thin Light Beams Through Turbulent Mixing Layers
Abstract
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 Helium and low-speed Nitrogen) and equal density (high-speed N2 and low-speed He-Ar) 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 (about 1 microsec) 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 Helium-Neon 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 spanwise direction which are closely associated with the streamwise streaks in plan-view shadowgraphs.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 06, 1991
- Accession Number
- ADA243470
Entities
People
- John B. Wissler
Organizations
- Air Force Institute of Technology