TURBULENT COMPRESSIBLE CONVECTION

Numerical simulations with high spatial resolution (up to 96(3) gridpoints) are used to study three-dimensional, compressible convection. A sequence of four models with decreasing viscous dissipation is considered in studying the changes in the flow structure and transport properties as the convection becomes turbulent. Near the upper boundary the motions form a network pattern of connected downflows and isolated upflows. As the viscosity decreases, the time evolution of the network pattern becomes more vigorous and small filamentary features appear. In the turbulent regime, the convective motions away from the boundaries display a complex mixture of long-range organization and small-scale disorder. The flow structure consists of strong, coherent, long-lived downflows surrounded by disorganized weaker motions. The disorganized ingredient is characterized by a low degree of spatial correlation of the vertical velocity with the fluctuations in the thermodynamic quantities and, thus, a low level of energy transport. In contrast, the stronger downflows are highly correlated with the thermodynamic fluctuations and consequently contribute significantly to the net vertical enthalpy flux. In the limit of small dissipation the strong downflows appear to transport enthalpy upward at nearly the same rate that they transport kinetic energy downward, thereby reorganizing the various energetic ingredients within the convection without actually contributing to the net energy flux. Possible applications of these results to mixing length ideas are discussed.