Backwards and forwards dispersion of a scalar in turbulent wall flows

A direct numerical simulation of turbulent flow in an infinite channel is used in conjunction with the tracking of scalar markers to investigate turbulent dispersion. The study is focused on four regions of turbulent wall bounded flows, namely the viscous layer, the transition region, the logarithmic layer, and the outer region of the flow at the center of the channel. The velocities of scalar markers captured in each of these regions are correlated both forwards and backwards in time. It is found that backwards and forwards dispersion proceeds at different rates at different regions of the flow. A change in the rates of backwards and forwards dispersion is also seen with a change in Prandtl number. However, for Prandtl number six and higher, this change is negligible. The observed differences are due to the competing effects of convective and molecular dispersion at different distances from the wall, and for different Prandtl number fluids.