Further Experimental Studies on Grid-Generated Turbulence

Abstract

This thesis describes three experimental investigations of velocity fluctuations and turbulent diffusion and mixing downstream of uniform grids. The first experiment examined the enhancement of turbulent diffusion of a passive scalar with the use of porous obstructions placed downstream of the grid. It was shown that the adopted strategy, which was based on theoretical and empirical arguments, was successful in increasing significantly the spread of a slightly heated plume produced by a line source. The second experiment examined the flow fields behind grids at very small turbulence Reynolds numbers, including values that were lower than any of those in the literature. When Reynolds number was sufficiently large, the grids generated conventional grid turbulence and, when Reynolds number dropped below a certain threshold, which depended on the grid, the flow was essentially steady. For intermediate Reynolds number values, the flow structure depended on the grid geometry and Reynolds number. The Reynolds stress and dissipation anisotropies increased drastically as Reynolds number was decreased within the intermediate range. The Kolmogorov-scaled power spectra differed strongly in this range, but nearly collapsed in their large wavenumber ranges, when normalized by an effective dissipation rate that was fitted to a universal normalized spectrum. The third study examined the effect of mean flow pulsation on grid turbulence and turbulent diffusion. It showed that flow pulsation augmented the turbulent kinetic energy and passive scalar diffusion. The results are discussed in the context of maximizing turbulence and turbulent mixing in diverse engineering applications.