Experimental Measurement of the Density Fluctuation PDF for small Atwood, high Schmidt number, Rayleigh-Taylor mixing

The experimental measurement of the probability density function (pdf) of density fluctuations in a Rayleigh-Taylor small Atwood water channel facility with high Schmidt number is reported. In the experiments, molecular mixing is measured by a phenolphthalein chemical indicator that reacts, turning from transparent to pink, when the heavy (salty \& acidic) and light (fresh \& alkali) water streams mix together. The degree of molecular mixing is determined from the relationship between amount of the chemical reaction formed and the density variance $\bar{\rho'^2}$. By measuring the concentration of the reaction product by backlit optical technique for various initial pH differentials, a detailed pdf of the density fluctuation has been obtained. The shape of the density fluctuation pdf as well as future research will be discussed.