Micro-Mixing Modelling of Scalar Fluctuations for Plumes in Homogeneous Turbulence

Abstract

We use the interaction by exchange with the mean (IEM) and interaction by exchange with the conditional mean (IECM) mixing models coupled to a Langevin model for particle trajectories to make detailed predictions of statistics of the concentration field downstream of line sources in grid turbulence and compare these predictions with suitable laboratory data. A critical part of the model is the prescription of a mixing time scale. We show that the IECM model gives a good representation of data for a single line source for a range of source conditions and also reproduces very well the variations with both travel time and source separation of the correlation between the concentrations from the two sources. We show that in the limit of zero mixing time scale the IECM model is equivalent to a meandering plume model and thus has a sound physical basis in the near-field where meandering dominates the concentration fluctuations. In contrast, the IEM model ignores the effect of plume meandering and underestimates the centreline intensity of fluctuations in the near and mid fields.