Experimental investigation on turbulent diffusion phenomena of meandering plume in the quasi-isotropic turbulent field (Variation of mean concentration properties for turbulent Reynolds numbers)

Particle diffusion from a continuous point source was experimentally investigated to clarify the mechanism of turbulent diffusion for a wide range of turbulent Reynolds number Rλ. The values of Rλ were systematically changed from 43 to 445 by controlling turbulent intensity and vortex scale, using an active turbulent generator. Particle concentration was measured by a laser system. The mean concentration profiles agree with Gaussian distribution, and streamwise growth rate of the variance became larger as Rλ increased, as predicted by the Taylor's diffusion theory. Lagrangian properties (TL，vL，T0) were determined from the streamwise growth rate of the variance σT2. Diffusion fields shifted from the short-time diffusion dominated by meandering diffusion to the long-time diffusion dominated by relative diffusion as Rλ increased. Then we discussed the ratio, β(=TL/TE), between the Lagrangian and Eulerian time scales. The contributions of the relative and meandering diffusions to the total diffusion were evaluated by the Lee & Stone's model