A Study of Turbulent Natural Convection Boundary Layers Using Large-Eddy Simulation

Doctoral thesis, 2007

The structure of turbulent natural convection boundary layers in different geometries and the effect of the buoyancy on a mixed convection boundary layer are investigated. These geometries comprise a vertical finite cylinder, a vertical infinite channel, a cavity and a vertical finite channel. In the three cases of vertical cylinder, cavity and finite channel, the boundary layer is in the state of development whereas in the case of the infinite channel, the boundary layer has a fully developed condition. In the vertical cylinder the natural convection is the dominant phenomenon although a small air-flow enters the geometry to reduce flow recirculations. In the case of the vertical infinite channel and cavity there exist only a pure natural convection boundary layer. In the vertical finite channel, however, the boundary layer is of mixed convection type and radiation heat transfer affects its development owing to high wall temperatures. The Grashof numbers based on the cylinder height, channel width, cavity and finite channel widths reach to Gr=5*10^11, Gr=9.6*10^5, Gr=3.9*10^8 and Gr=9.4*10^7, respectively. The boundary layers are studied using two powerful numerical methods namely Direct Numerical Simulation or DNS and Large Eddy Simulation or LES. Due to high Reynolds numbers, DNS is only used in the case of the vertical infinite channel. Three different sub-grid scale models are used in the case of the cavity and it is shown that the dynamic SGS model is the only model that is capable of predicting the location of the transition from laminar to turbulent flow correctly. Mean flow parameters as well as turbulence parameters in all cases are studied and the results are compared with the existing experimental results. Comparisons are also made between the results of the vertical cylinder and experimental results obtained from a vertical flat plate.