The diffusion of heat spots in isotropic turbulence

Abstract

A technique has been developed for the production of small heat spots in the turbulent flow behind bi-plane square-mesh grids, and their distribution downstream of the point of production has been studied using a Wollaston wire-resistance thermometer of response time 0.0002 sec. The transverse spatial distribution function is found to be a Gaussian error function (within the limits of experimental error) at all distances, in agreement with previous results on the temperature wake of a long thin wire. It is possible to determine a mean peak temperature of the spots by measuring the probability that the production of a spot will subsequently raise the temperature at the detector by more than a threshold value as a function of that value. The mean peak temperature is found to decrease with distance downstream more rapidly in turbulent flow than in non-turbulent flow, the observed temperatures being in quantitative agreement with those computed by assuming a spatially uniform rate-of-strain over the volume occupied by the spot. Since the extended spot is too large for this assumption to be literally true, it is concluded that, in the final form, the heat is probably concentrated in a contorted thin layer, and that, following the spot, the shear pattern remains nearly constant in type, decaying aperiodically with the turbulence.