The role of microscopic mixing in the description of turbulent diffusion in fluid continuum /

The role of molecular mixing (as opposed to molecular-collision transport) in the description of turbulent diffusion in continuum framework is examined. This is done by comparing a new virtual fluid parcel treatment with the classical fluid particle treatment of the BMDFE (Basic Macroscopically Describable Fluid Element). It is found that the classical fluid particle treatment conceptually excludes molecular mixing between different BMDFEs, due to its postulated constraint that individual BMDFEs maintain their integrities in motion. The new virtual fluid parcel treatment, on the other hand, conceptually incorporates molecular mixing between different BMDFEs, by relaxing this constraint to permit disintegration of individual BMDFEs. The main improvement made by the new virtual fluid parcel treatment lies in the introduction of a feedback mechanism in the form of physically coupled disintegration and integration of the BMDFEs. This improvement suggests that molecular mixing is a controlling agent of the mixing mechanism in every time-step of turbulent diffusion, whose significance would increase cumulatively. By applying the two treatments to the evolution of the diffusion cloud on the level of single time-step diffusion redistribution, it is shown that molecular mixing persistently and cumulatively influences the evolution of the diffusion cloud by reducing the diffusion distribution variance. This indicates that the exclusion of molecular mixing in the classical fluid particle treatment would lead to a potential mathematical-physical inconsistency in the description of turbulent diffusion by exaggerating the diffusion distribution variance. The results of this analysis are qualitatively supported by experiments of passive scalar diffusion in water flow with moderate turbulence intensity. As a preliminary test, a simplified numerical modeling of scalar diffusion based on the virtual fluid parcel treatment is executed in two wind tunnel models. In this case, measu