An Investigation of Scalar Dispersion in Grid Turbulence

The structure of scalar dispersion from a continuous release point was investigated for moderate Reynolds number flow in grid turbulence. Using laser-induced fluorescence techniques, laser-volume scanning, a custom-designed fast-readout CCD focal plane array, and high-speed digital-imaging/-acquisition/-storage techniques, the instantaneous three-dimensional structure of a passive scalar was investigated in flow in water (high Schmidt number). Laser Doppler and scalar-correlation velocimetry were employed to measure th flow speed entering the test section and in the interrogated volume. Such scalar-dispersion structure away from the release point is typically modeled assuming a Gaussian profile. This provides a good description for the mean scalar profile as a function of the transverse distance from a line parallel with the flow and downstream of the release point, as also confirmed by experiment (Yamamoto & Sato 1979, Gad-el-Hak & Morton 1979, Nakamura et al. 1987, Sawford 2001). The instantaneous three-dimensional structure, however, reveals a rich topology of scalar structures that was found to persist in the volume interrogated, spanning a distance from the grid and release point between 22 and 30 grid mesh lengths, which is in the self-similar grid-turbulence regime where the present three-dimensional scalar-field measurements were conducted