Conditional scalar mixing statistics in homogeneous isotropic turbulence

We use simple scalar mixing models coupled to a Lagrangian stochastic trajectory model to calculate a range of conditional scalar statistics for three different source configurations, a uniform gradient source, a line plume and a scalar mixing layer. Comparison with appropriate direct numerical simulation results and experimental data shows that the interaction by exchange with the conditional mean mixing model gives good results for statistics conditioned on the scalar concentration ( mean velocity, mean scalar diffusion and mean scalar dissipation rate) for the uniform gradient source and the line plume and for the mean velocity conditioned on the concentration in the mixing layer. The simpler interaction by exchange with the mean model does not perform as well and in particular does not capture the non-linearity of the conditional diffusion for the line plume and the mixing layer. Our results suggest that some of the approximations used for these conditional statistics may not be appropriate.