PASSIVE SCALAR DISPERSION AND MIXING IN A TURBULENT JET

The dispersion and mixing of passive scalar (temperature) fluctuations is studied in a turbulent jet. The temperature fluctuations were prod uced by heated fine wire rings placed axisymmetrically in the flow. Ty pically the ring diameters were of the same order as the jet, D-j, and they were placed in the self-similar region. However, other initial c onditions were studied, including a very small diameter ring used to a pproximate a point source. Using a single ring to study dispersion (wh ich is analogous to placing a line source in a planar flow such as gri d turbulence), it was found that the intense local thermal field close to the ring disperses and fills the whole jet in approximately 1.5 ed dy turnover times. Thereafter the thermal field evolves in the same wa y as for the traditional heated jet experiments. Two heated rings were used to study the mixing of two independently introduced scalar field s. Here an inference method (invoking the principle of superposition) was used to determine the evolution of the cross-correlation coefficie nt, rho, and the segregation parameter, alpha, as well as the coherenc e and co-spectrum. While initially strongly dependent on ring location s and spacing, rho and alpha reached asymptotic values of 1 and 0.04, respectively, also in about 1.5 eddy turnover times. These results are contrasted with mixing and dispersion in grid turbulence where the ev olution is slower. Measurements in the far field of the jet (where rho = 1) of the square of the scalar derivative conditioned on the scalar fluctuation itself, as well as other conditional statistics, showed s trong dependence on the measurement location, as well as the direction in which the derivative was determined. The crosscorrelation between the square of the scalar derivative and the signal showed a clear Reyn olds-number trend, decreasing as the jet Reynolds number was varied fr om 2800 to 18000. The far-field measurements, using the heated rings, were corroborated by new heated jet experiments.