Measurements of turbulent diffusion in uniformly sheared flow

The diffusion of a plume of dye in uniformly sheared turbulent flow in a water tunnel was investigated using simultaneous stereoscopic particle image velocimetry (SPIV) and planar laser-induced fluorescence (PLIF). Maps of the mean concentration and the turbulent concentration fluxes in planes normal to the plume axis were constructed, from which all components of the second-order turbulent diffusivity tensor were determined for the first time. Good agreement between the corresponding apparent and true diffusivities was observed. The turbulent diffusivity tensor was found to have strong off-diagonal components, whereas the streamwise component appeared to be counter-gradient. The different terms in the advection-diffusion equation were estimated from the measurements and their relative significance was discussed. All observed phenomena were explained by physical arguments and the results were compared to previous ones.