DNS and LES of Turbulent Natural Convection Boundary Layer

Licentiatavhandling, 2004

Turbulent natural convection boundary layers possess some special characteristics compared to the other boundary layers. In order to study this kind of boundary layer, two powerful numerical methods, Direct Numerical Simulation or DNS and Large Eddy Simulation or LES, are used. The former is used to investigate the natural convection boundary layer in a vertical channel for which the Grashof number based on the channel width is Gr_h=9.6*10^5 and the latter is used to study the natural convection in a cylindrical vertical shell and tube. The local Grashof number in the latter case reaches Gr_z=5*10^11. Mean flow parameters as well as turbulence parameters in the both cases are studied and the results are compared with the existing experimental results for natural convection boundary layer on a flat plate and natural convection boundary layer on a vertical slender cylinder. Although many qualitative and quantitative agreements are found between the results, there are still some discrepancies in the turbulence characteristics. A region with negative shear stress in the boundary layer along the vertical cylinder, which is in agreement with many experiments is observed and the effect of it on the boundary layer is studied. However, there is no such region in the boundary layer inside the vertical channel. Also the existence of a linear region for the velocity profile which is much thinner than the ordinary forced convection boundary layers is confirmed.