CFD study of heat transfer with unstructured codes.

S. Rolfo

1st Year PhD Report, The University of Manchester, (2007).



LES or even DNS computation on complex geometries need to use unstructured mesh, in order to keep the mesh size reasonable. This type of grids raises some numerical issues that are addressed herein. Two simple cases, the Taylor-Green vortices and the laminar channel flow, are used in order to test conservation properties of the numerical method on meshes containing local refinements. These local refinements look like "hanging nodes", but more accurately described as "non-conforming" in the Finite Volume context. The refinement ratio between coarse region ("master") and refined region ("slave") varied between 1 and 0.5. Different mesh smoothing corrections were also taken into consideration in order to smooth the interface between master and slave parts. In order to have conservation properties similar to a Cartesian mesh, the refinement ratio has to be greater than 0.75. If the refinement ratio is, instead, smaller than the previous mentioned value a "sub-pattern" in the refinement is necessary. Surprisingly the mesh smoothing corrections did not produce the expected results, giving, instead, a worse performance with respect to the original mesh. The second part focused on turbulence model by considering the flow inside a vertical pipe flow in presence of a constant heat flux. Several RANS turbulence models were tested using the commercial package Star-CD and the industrial code Code_Saturne. This case is a quite common configuration in the core of a nuclear power plant. It was found that the best performances were obtained with the linear $k-\epsilon$ and the $ \overline{v^{2}}-f $ model. In particular this last model gives good results also in the prediction of the turbulent quantities.

Topic revision: r7 - 2009-10-26 - 15:22:34 - AlastairWest

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Topic revision: r7 - 2009-10-26 - 15:22:34 - AlastairWest
 

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