Scalar concentration reduction in a contaminant cloud

The expected mass fraction (EMF) is used to quantify concentration values in a contaminant cloud. Low-ordered moments of the probability density function (PDF) of concentration are integrated and inverted to approximate the EMF. A simple prescription for the distributed moments of the PDF is shown to be related to a set of relationships between normalized moments. These relationships appear to be very robust and approximately universal across measured values from laboratory clouds and plumes in the atmospheric boundary layer over different stability classes. Also, these relationships have the important advantage that they can be validated from isolated fixed-point measurements. In this paper, we verify our results with data from a plume in grid turbulence. The EMF for these experiments is shown to be well approximated by a simple Beta function.