VELOCITY MIXTURE FRACTION STATISTICS OF ROUND, SELF-PRESERVING, BUOYANT TURBULENT PLUMES

An experimental study of the structure of round buoyant turbulent plum es was carried out limited to conditions in the self-preserving portio n of the flow. Plume conditions were simulated using dense gas sources (carbon dioxide and sulfur hexafluoride) in a still and unstratified air environment. Velocity/mixture-fraction statistics, and other highe r-order turbulence quantities, were measured using laser velocimetry a nd laser-induced fluorescence. Similar to earlier observations of thes e plumes, self-preserving behavior of all properties was observed for the present test range, which involved streamwise distances of 87-151 source diameters and 12-43 Morton length scales from the source. Strea mwise turbulent fluxes of mass and momentum exhibited countergradient diffusion near the edge of the flow, although the much more significan t radial fluxes of these properties satisfied gradient diffusion in th e normal manner. The turbulent Prandtl/Schmidt number, the ratio of ti me scales characterizing velocity and mixture function fluctuations an d the coefficient of the radial gradient diffusion approximation for R eynolds stress, all exhibited significant variations across the flow r ather than remaining constant as prescribed by simple turbulent models . Fourth moments of velocity and velocity/mixture fraction fluctuation s generally satisfied the quasi-Gaussian approximation. Consideration of budgets of turbulence quantities provided information about kinetic energy and scalar variance dissipation rates, and also indicated that the source of large mixture fraction fluctuations near the axis of th ese flows involves interactions between large streamwise turbulent mas s fluxes and the rapid decay of mean mixture fractions in the streamwi se direction.