The countergradient heat flux in turbulent stratified flows

Countergradient heat flux (CGHF) transports heat from low to high temperature regions. The reasons for this paradoxial behaviour are explained in this paper. We report on three examples for which CGHF has been observed either experimentally or in direct numerical simulations. The examples concern vertical heat flux in convective or stably stratified layers with or without shear. It is shown that CGHF arises in stably stratified parts of the flow if the dissipation of temperature fluctuations is too small to balance source terms for such fluctuations. In such cases, CGHF converts potential energy into kinetic energy. By means of an eigenanalysis of a second order model we support this explanation and show that CGHF appears in particular for high Prandtl number flows. (orig.)