Stefano Rolfo

Optimal Unstructured Meshing for Large Eddy Simulations

An attempt is made to provide a criterion for optimal unstructured meshing for LES from the knowledge of different turbulence lengthscales. In particular, the performance of a grid based on the Taylor microscales for turbulent channel flow, is investigated, with the final view of facilitating an a priori determination of the mesh resolution required for LES. The grid dictated by the Taylor microscales is more cubical in the centre of the domain than the typical empirical LES grids. Furthermore, it is as fine in the spanwise direction as it is in the wall normal direction. Empirical LES grids, currently widely used, have a very fine (approximately four times finer) wall normal resolution and a coarse (about twice as course) streamwise resolution as compared to a grid based on the Taylor microscales. A remarkable feature is that the mean velocity and streamwise component of fluctuating velocity (classically over-predicted in coarse grid LES) and the wall normal fluctuating velocity are well reproduced on the new grid. The attempt of building an unstructured LES grid based on the Taylor microscale has been found very successful. However, as the Reynolds number is increased this sort of requirement might be excessive and eventualy a criterion such as one tenth of the integral lengthscale could be sufficient.

RANS AND LES INVESTIGATIONS OF VERTICAL FLOWS IN THE FUEL PASSAGES OF GAS-COOLED NUCLEAR REACTORS

Coolant flows in the cores of current gas-cooled nuclear reactors consist of ascending vertical flows in a large number of parallel passages. Under post-trip conditions such heated turbulent flows may be significantly modified from the forced convection condition by the action of buoyancy, and the thermal-hydraulic regime is no longer one of pure forced convection. These modifications are primarily associated with changes to the turbulence structure, and indeed flow laminarization may occur. In the laminarization situation heat transfer rates may be as low as 40% of those in the corresponding forced convection case. The heat transfer performance of such ‘mixed’ convection flows is investigated here using a range of refined ReynoldsAveraged-Navier-Stokes (RANS) turbulence models. While all belong to the broad class of Eddy Viscosity Models (EVMs), the various RANS closures have different physical parameterizations and might therefore be expected to show different responses to externally-imposed conditions. Comparison is made against experimental and Direct Numerical Simulation (DNS) data. In addition, Large Eddy Simulation (LES) results have been generated as part of the study. Three different CFD codes have been employed in the work: ‘CONVERT’, ‘STAR-CD’, and ‘ Code_Saturne ’, which are respectively in-house, commercial, and industrial packages. It is found that the early EVM scheme of Launder and Sharma [1] is in the closest agreement with consistentlynormalized DNS results for the ratio of mixed-to-forced convection Nusselt number ( Nu/Nu 0). However, in relation to DNS and experimental data for forced convection Nusselt number, other models perform better than the LaunderSharma scheme. The present investigation has revealed discrepancies between direct-simulation, experimental, and the current LES studies.

LES and Hybrid RANS/LES of Turbulent Flow in Fuel Rod Bundle Arranged with a Triangular Array

Turbulent flow parallel to fuel rod bundles arranged in a triangular array are computed using LES and hybrid RANS/LES. Inner-channel flow pulsations are captured and the dominant frequency is in agreement with the experiments. The hybrid method shows a lack of accuracy in the near wall region probably due to the formulation of the blending function.

LES of heated fuel bundle arranged into triangular array

A rod bundle is a key constitutive element of a wide range of nuclear reactor designs. It is composed by a set of long thin rods containing the nuclear fuel and fluid-flow between them and generally parallel to the rods. As experiments in a such densely packed geometries are difficult, thermal-hydraulics simulations are valuable to study heat transfer, fluid-forces, homogeneity of temperatures and flow rates and their fluctuations for current or and future reactor designs.

Development of a Hybrid RANS/LES Model for Heat Transfer Applications

This work presents a scalar flux model in the framework of a hybrid RANS-LES modelling. The model is tested on a heated channel flow at different Prandtl numbers and on a T-junction. Results show a good agreement with both DNS and experimental data.