Contribution to the study of the mechanisms of turbulent diffusion and of related matters

Turbulent diffusion mechanisms and the related problems are studied. The diffusion mechanisms considered are those which concern a passive scalar contaminant not subject to molecular effects. This latter character makes it possible to isolate in the diffusion phenomenon that which is inherent in the turbulent dispersion. The system of natural coordinates for following the movement of a particle and for describing the dispersion is that of Lagrange. This description must be statistical, hence, the specific properties of the probability densities of the velocities and movements in this type of coordinate are examined. The expressions of the probability densities of the concentration and turbulent flow of a contaminant are sought. These expressions make it possible to formulate the diffusion laws linking the mean flow and concentration to the statistical characteristics of the velocity field.