Abstract
Mixing refers to the homogenization of concentrations of passive scalars in fluids. On small scales it is dominated by diffusion and on large scales it is assisted by stirring. In the presence of scalar sources and sinks the concentration field remains inhomogeneous, but the combined effect of stirring and dissipation may lead to a statistically stationary state. One measure of the quality of mixing is then the standard deviation of the scalar concentration from the mean, and the effectiveness of a stirring velocity field can be gauged by comparing the concentration fluctuations in the presence of stirring to those in its absence. It was recently noted that the maximum possible effectiveness of any stirring depends on the detailed structure of the sources and sinks. We present results from particle-based simulations that confirm this strong source–sink dependence of the mixing enhancement by stirring.