[Thesis]. Manchester, UK: The University of Manchester; 2010.
Rod bundle is a typical constitutive element of a very wide range of nuclear reactor
designs. This thesis describes the investigation of such geometry with wall-resolved
Large Eddy Simulation (LES). In order to alleviate the mesh constraint, imposed by
the near wall resolution, the usage of embedded refinements and polyhedral meshes
is analysed firstly with a inviscid laminar case (Taylor Green vortices) and secondly
with a fully turbulent case (channel flow only with embedded refinement). The inviscid
test case shows that the addition of embedded refinements decreases the conservation
properties of the code. Indeed the accuracy decreases from second order in a structured
conformal mesh, to something in between first and second order depending on the quality
of the unstructured mesh. Better results are obtained when the interface between refined
and coarse areas presents a more regular and 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 is attributed to 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 analysed using a conventional body fitted multiblock mesh. Two different Reynolds
numbers are investigated reporting Reynolds stresses and budgets. The flow is characterised
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, characterises the phenomenon.
The frequency analysis finds that present simulations are distinguished by three dominant
frequencies, the first in agreement with the experimental value and two higher ones,
which might be 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 Dirichlet). 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 investigations 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 numbers 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 wall-resolved LES, the model suffers 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.