Pressure transport in DNS of turbulent natural convection in horizontal fluid layers

Direct numerical simulation data of two types of turbulent natural convection in horizontal fluid layers are used to compute turbulent diffusive transport terms for turbulence kinetic energy and vertical turbulent heat flux. For both quantities turbulent diffusive transport is represented by a pressure correlation and a triple correlation. While for Rayleigh-Benard convection in air the pressure correlation dominates the triple correlation, for the convection in an internally heated layer the opposite behaviour is observed. The dominance of pressure transport in the Rayleigh-Benard convection and its minor importance in the internally heated layer is explained by the coherent structures and dynamics of the respective flow. The coherent structures are intermittent and exist only for short time intervals. Thus, conventional closure relations for turbulent diffusive transport, which are used in statistical turbulence models based on long-time averaged quantities, may not be appropriate for the flows under consideration.