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TWiki> Main Web>TWikiUsers>StefanoRolfo>StefanoRolfoThesis (2010-05-28, StefanoRolfo)

TWiki> Main Web>TWikiUsers>StefanoRolfo>StefanoRolfoThesis (2010-05-28, StefanoRolfo)

LES and Hybrid RANS/LES Turbulence Modelling

in Unstructured Finite Volume Code and

Applications to Nuclear Reactor Fuel Bundles

This page is a "TWiki vesion" of my PhD thesis. Every

Abstract

Rod bundle is a typical constitutive element of a very wide range of nuclear reactor design. This thesis describes the investigation of such geometry with wall resolved Large Eddy Simulation (LES). In order to alleviate the mesh constrain, imposed by the near wall resolution, the use of embedded refinements and polyhedral meshes is analyzed firstly with a inviscid laminar case (Taylor Green vortices) and secondly with a fully turbulent case (channel flow). The inviscid test case shows that the addition of the embedded refinements decreases the conservation properties of the code. Indeed the order of accuracy decreases from second in a structured conformal mesh, to something in between first and second order depending of the quality of the unstructured mesh. Better results are obtained when the interface between refined and coarse areas presents a more regular and more structured pattern, reducing the generation of skewed and stretched cells. The channel flow simulation shows that the Reynolds stresses of some embedded refined meshes are affected by spurious oscillations. Surprisingly this effect is present in the unstructured meshes with the best orthogonal properties. Indeed analysis of Reynolds stress budgets shows that terms where the gradient in the wall normal direction is dominant have a largely oscillatory behaviour. The cause of the problem has to be search in the convective term and in particular in the method used for the gradient reconstruction.

As a consequence of these contradictory signs between the inviscid and the fully turbulent cases, the rod bundle test case is analyzed using a conventional body fitted multiblock mesh. Two different Reynolds number are investigated reporting Reynolds stresses and budgets. The flow is characterized by an energetic and almost periodic azimuthal flow pulsation in the gap region between adjacent sub-channels, which makes turbulent quantities largely different from those in plane channel and pipes and enhances mixing. Experiments found that a constant Strouhal number, with the variation of the Reynolds number, characterizes the phenomenon. The frequency analysis finds that presents simulations are distinguished by three dominant frequencies, the first in agreement with the experimental value and two higher ones, which might due to the correlation of the azimuthal velocity in the streamwise direction. Several passive temperature fields are added at the simulations in order to study the effects of the variation of the Prandtl number and the change in boundary conditions (Neumann and Diriclet). A simplified case where an imbalance of the scalar between adjacent sub-channels is also investigated in order to evaluate the variation of the heat fluxes with respect to the homogeneous case.

An alternative solution to reduce the mesh constraint imposed by the wall is to hybridize LES with RANS. The main achievement of this work is to integrate the heat transfer modelling to the already existing model for the dynamic part. Further investigation of the blending function used to merge the two velocity fields are carried out in conjunction with a study of the model dependency on the mesh resolution. The validation is performed on a fully developed channel flow at different Reynolds number and with constant wall heat flux. On coarse meshes the model shows an improvement of the results for both thermal and hydraulic parts with respect to a standard LES. On refined meshes, suitable for well resolved LES, the model suffer from a problem of double counting of modelled Reynolds stresses and heat fluxes because the RANS contribution does not naturally disappear as the mesh resolution increases.

Unstructured meshes

Fuel Rod Bundle

Hybrid RANS/LES

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Topic revision: r2 - 2010-05-28 - 09:35:12 - StefanoRolfo

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