Turbulent transport of a passive scaler in a round jet discharging into a co-flowing stream

The mass transport properties of a round turbulent jet of water discharging into a low velocity co-flowing water stream, confined in a square channel, is investigated experimentally. The measurement region is the self-similar range from x/d = 70 to x/d = 140. Combined laser-induced fluorescence and 2D laser Doppler velocimetry are used in order to measure simultaneously, instantaneously and in the same probe volume, the molecular concentration of a passive scalar and two components of the velocity. This technique allows the determination of moments involving correlations of both velocity and concentration fields, which are necessary to validate the second-order modelling schemes. Both transport equations of Reynolds shear stress (uv) over bar and turbulent mass flux (vc) over bar have been considered. In both cases, advection, production and diffusion terms have been determined experimentally. The pressure-strain correlation and the pressure scrambling term are inferred with the help of the budget of Reynolds shear stress and mass turbulent transport equations. Second order closure models are evaluated in the light of the experimental data. The turbulent Schmidt number is found to be almost constant and equal to 0.62 in the center region and decreases strongly to zero in the mixing layer of the jet. The effects of the co-flow on the turbulent mixing process are also highlighted. (C) 2001 Editions scientifiques et medicales Elsevier SAS.