Fluctuations and transport in a stirred fluid with a mean gradient

The effective thermal diffusivity ${\mathit{D}}^{\mathrm{*}}$ 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 ${\mathit{D}}^{\mathrm{*}}$ as a function of Reynolds number R. Above the transitions, the distribution is strongly non-Gaussian and approaches an exponential exp(-‖δT‖/βξ), where β is the local temperature gradient and ξ the correlation length.