PRESSURE TRANSPORT IN DIRECT NUMERICAL SIMULATIONS OF TURBULENT NATURAL-CONVECTION IN HORIZONTAL FLUID LAYERS

Direct numerical simulation (DNS) data of two types of turbulent natur al 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. Wh ile for Rayleigh-Benard convection in air the pressure correlation dom inates the triple correlation, for the convection in an internally hea ted layer the opposite behaviour is observed:the dominance of pressure transport in the Rayleigh-Benard convection and its minor importance in the internally heated laver is explained by the coherent structures and dynamics of the respective how. The coherent structures are inter mittent; they exist only for limited time intervals. Thus, conventiona l closure relations for turbulent diffusive transport, which are used in statistical turbulence models basing on long-time averaged quantiti es, may not be appropriate for the flows under consideration. (C) 1998 Elsevier Science Inc. All rights reserved.