(281i) Behavior of Forwards and Backwards Dispersion in Turbulent Channel Flows

This study utilizes a direct numerical simulation of turbulent flow in an infinite channel in conjunction with the tracking of scalar markers to investigate turbulent dispersion. The channel essentially comprises of a computational block, which is 4πh × 2h × 2πh in the x, y, z dimensions (where h is the half channel height and h =150). The four important regions of wall bounded turbulent flows, namely the viscous sub-layer, the transition region, the logarithmic layer, and the outer region of the flow at the center of the channel are considered. Scalar markers captured in these four turbulent flow regions are correlated both forwards and backwards in time. It is found that backwards and forwards dispersion proceed 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, there are negligible differences in the behavior of backwards and forwards dispersion. The observed differences are associated with the form of the probability density function of the velocity fluctuations, and more specifically with the skewness of the probability density function. The velocity autocorrelation coefficients are used to determine the principal direction of heat flow in the channel with time. The gradient of heat flux and the direction of the transfer of heat show a difference for the backwards and forwards dispersion. The Prandtl number also plays a significant role in deciding the amount and the principal direction of heat transfer.