Studies of turbulent diffusion

Turbulent diffusion was studied both theoretically and experimentally. The purposes of this study were (1) to develop and test a new statistical model for turbulent diffusion which was reasonable in both theory and implementation, (2) to include the effect of shear stress on diffusion, (3) to extend the new model to a particulate system and (4) to obtain experimental data for particle dispersion to test the proposed model, A Langevin equation model was developed by considering the fluctuating velocity as a stochastic process. The same model was also derived from a one dimensional Lagrangian Navier-Stokes equation. This model was physically realistic. The present model was implemented on a hybrid computer. The simulated results of turbulent diffusion were compared with the theoretical predictions for a homogeneous flow and with experimental concentration profiles in a boundary layer and in the atmosphere. Good agreement was found in all cases. A technique to generate two random processes which are correlated with each other to any degree was developed. This method was used to investigate the shear effect on turbulent diffusion. In the presence of both mean velocity gradient and shear, diffusion was shown to be independent of shear for long and short diffusion times but to be strongly dependent on shear for intermediate times. [...]