Refractive index matching in large-scale stratified experiments

The measurement of dilution in jets and plumes by planar laser-induced fluorescence in large-scale stratified environments with refractive index matching is discussed. The density and refractive index of various salt (NaCl) and ethanol solutions were measured to high precision, and it is shown that linear stratifications with density differences at least up to 20 sigma (t) with acceptably small refractive index variations can be achieved at reasonable cost. Equations are presented to facilitate the design of experiments with arbitrary density differences. The presence of ethanol significantly increases the laser attenuation, however. The rate of attenuation was measured for various solute concentrations and equations presented to predict the attenuation for arbitrary salt and ethanol concentrations. The rapid attenuation due to ethanol may limit the maximum density differences attainable with long laser traverse distances. Examples are given of discharges into linearly stratified fluids including a jet in a stationary environment, and a buoyant jet in a cross flow.