DIFFERENTIAL DIFFUSION OF PASSIVE SCALARS IN ISOTROPIC TURBULENCE

The differential diffusion of passive scalars of different molecular diffusivities is studied by performing direct numerical simulations of scalar fields in statistically stationary isotropic turbulence at Taylor-scale Reynolds number 38. Starting from identical initial conditions, each scalar field eventually becomes self-similar, with constant length scale and with its variance decaying exponentially with time. This decay rate depends weakly on the diffusivity. The correlation coefficient between two scalars initially decreases rapidly, but subsequently evolves much more slowly. The scalars ultimately become completely decorrelated at all scales. The scale dependency of correlation is studied through the coherency spectrum, which is affected only indirectly by the diffusivity difference. A Fourier-spectral approach emphasizes the importance of interscale spectral transfer of multiple scalars due to turbulent advection. Small-time behavior is compared with an analysis based on the diffusion equation.