FLUCTUATIONS AND TRANSPORT IN A STIRRED FLUID WITH A MEAN GRADIENT

The effective thermal diffusivity D* and the probability distribution of temperature fluctuations are measured in a stirred fluid across which a temperature gradient is maintained. A distinct mixing transition is observed for D* as a function of Reynolds number R. Above the transition, the distribution is strongly non-Gaussian and approaches an exponential exp(-\delta-T\/beta-xi), where beta is the local temperature gradient and xi the correlation length.