The decay of temperature fluctuations and heat flux in grid generated turbulence

Abstract

Previous experimental determinations of the decay of passive scalar fluctuations in grid generated isotropic turbulence show large variations in their decay rate. We simulate some of these measurements and show that for passive temperature fluctuations their decay rate is a function of the initial intensity of the temperature fluctuations. Variation in heat applied to the grid to create the temperature fluctuations changes the thermal length scale and probably induces anisotropy in the temperature field. In order to eliminate the dependence of the decay rate on initial temperature fluctuation intensity we describe a new way of generating temperature fluctuations by means of placing a heated parallel array of fine wires (a mandotine) downstream from the unheated grid. Results of this experiment show that the decay rate of thermal fluctuations is uniquely determined by the scale size of the initial temperature fluctuations. By increasing the heat applied to the mandoline we generate a heat flux and present preliminary results of the decay of heat flux in isotropic decaying turbulence. Full details of the work described here will appear in Warhaft and Lumley (1978) and Newman, Warhaft and Lumley (1978).