Sofiane Benhamadouche

LES, coarse LES, and transient RANS comparisons on the flow across a tube bundle

Abstract The cross-flow in a staggered tube bundle is computed with an LES and a transient Reynolds stress transport model (RSTM) in 2D and 3D, with two levels of grid refinement. The numerical method is based on a finite volume approach on unstructured grids using a collocated arrangement for all the unknowns. It is shown that the LES results on the fine mesh are comparable to a DNS and experiments and reasonable agreement is still achieved with a coarse mesh. The RSTM also produced satisfactory results in 3D but showed no advantage over the LES when the grid was coarsened. The 2D RSTM, which produced strong vortex shedding, was found to be physically unreasonable.

Large eddy simulation of the flow around single and two side-by-side cylinders at subcritical Reynolds numbers

The flow around single and two side-by-side infinite cylinders is numerically modelled using dynamic Smagorinsky large eddy simulation (LES). For the single cylinder, the Reynolds number based on the diameter and the free stream velocity is 3900. A complete sensitivity study was conducted based on the extrusion length in the span-wise direction, the grid refinement at the wall, the convection scheme, and the sub-grid scale (SGS) model. It was found that the mean solution is not influenced by the extrusion length beyond 4 diameters or by 1% up-winding. However, coarsening the mesh in the wall normal direction or switching off the sub-grid scale model led to drastic effects on the recirculation length and on the underlying velocity field. The two side-by-side cylinders were tested for a range of pitch to diameter ratios (T/D=1.0,1.25≤T/D≤5.0) at a Reynolds number of 3000. For the intermediate pitch to diameter ratios (1.25≤T/D≤1.75), multiple shedding frequencies were detected with a biased wake flow deflec...

Presentation of a Numerical 3D Approach to Tackle Thermal Striping in a PWR Nuclear T-Junction

This paper presents a numerical study to tackle thermal striping phenomena occuring in piping systems. It is here applied to the Residual Heat Removal (RHR) bypass system. A large Eddy Simulation (L.E.S.) approach is used to model the turbulent flow in a T-junction. The thermal coupling between the Finite Volume CFD Code_Saturne and the Finite Element thermal code Syrthes, gives access to the instantaneous field inside the fluid and the solid. By using the instantaneous solid thermal fields, mechanical computations (as presented in (Stephan et al 2002)) are performed to yield the instantaneous mechanical stresses seen by the pipework T-junction and elbow.Copyright

Assessment of advanced RANS models against large eddy simulation and experimental data in the investigation of ribbed passages with passive heat transfer

ABSTRACT Rough surfaces are widely used to enhance convective heat transfer by the promotion of higher turbulence levels. The present article reports simulations of the flow and heat transfer in a 2-D rib-roughened passage using a number of advanced Reynolds-averaged Navier-Stokes (RANS) turbulence models including eddy-viscosity models (EVM) and a Reynolds stress model (RSM). Large eddy simulation (LES) is also conducted and results are compared against experimental measurements. In addition, the effects of rib thermal boundary condition on heat transfer are also investigated. In the present work, the blockage ratio of the transversely mounted rectangular ribs is 10% and the rib pitch-to-height ratio of 9 is selected. The Reynolds number, based on the channel bulk velocity and hydraulic diameter, is 30,000. The RANS-based turbulence models investigated here are the k-ω-SST, the v2-f, the ϕ-f, and the elliptic blending RSM. All computations are undertaken using the commercial and industrial CFD codes STAR-CD and Code_Saturne, respectively. Of all the models, the LES predictions were found to be in the best agreement with the experimental data, while the k-ω-SST and EB-RSM returned the least accurate results.

Accumulation and transport of microbial-size particles in a pressure protected model burn unit: CFD simulations and experimental evidence

BackgroundControlling airborne contamination is of major importance in burn units because of the high susceptibility of burned patients to infections and the unique environmental conditions that can accentuate the infection risk. In particular the required elevated temperatures in the patient room can create thermal convection flows which can transport airborne contaminates throughout the unit. In order to estimate this risk and optimize the design of an intensive care room intended to host severely burned patients, we have relied on a computational fluid dynamic methodology (CFD).MethodsThe study was carried out in 4 steps: i) patient room design, ii) CFD simulations of patient room design to model air flows throughout the patient room, adjacent anterooms and the corridor, iii) construction of a prototype room and subsequent experimental studies to characterize its performance iv) qualitative comparison of the tendencies between CFD prediction and experimental results. The Electricité De France (EDF) open-source software Code_Saturne® (http://www.code-saturne.org) was used and CFD simulations were conducted with an hexahedral mesh containing about 300 000 computational cells. The computational domain included the treatment room and two anterooms including equipment, staff and patient. Experiments with inert aerosol particles followed by time-resolved particle counting were conducted in the prototype room for comparison with the CFD observations.ResultsWe found that thermal convection can create contaminated zones near the ceiling of the room, which can subsequently lead to contaminate transfer in adjacent rooms. Experimental confirmation of these phenomena agreed well with CFD predictions and showed that particles greater than one micron (i.e. bacterial or fungal spore sizes) can be influenced by these thermally induced flows. When the temperature difference between rooms was 7°C, a significant contamination transfer was observed to enter into the positive pressure room when the access door was opened, while 2°C had little effect. Based on these findings the constructed burn unit was outfitted with supplemental air exhaust ducts over the doors to compensate for the thermal convective flows.ConclusionsCFD simulations proved to be a particularly useful tool for the design and optimization of a burn unit treatment room. Our results, which have been confirmed qualitatively by experimental investigation, stressed that airborne transfer of microbial size particles via thermal convection flows are able to bypass the protective overpressure in the patient room, which can represent a potential risk of cross contamination between rooms in protected environments.

LES, COARSE LES, AND TRANSIENT RANS COMPARISONS ON THE FLOW ACROSS A TUBE BUNDLE

The cross-flow in a staggered tube bundle is computed with an LES and a transient Reynolds Stress Transport Model (RSTM) in 2D and 3D, with two levels of grid refinement. The numerical method is based on a finite volume approach on unstructured grids using a collocated arrangement for all the unknowns. It is shown that the LES results on the fine mesh are comparable to a DNS and experiments and good agreement is achieved with a coarse mesh. The RSTM also produced satisfactory results in 3D but showed no advantage over the LES when the grid was coarsened. The 2D RSTM, which produced strong vortex shedding, was found to be physically unreasonable.

Computation of Fluid Forces Acting on an Infinite Cylinder Submitted to a Single Phase Cross Flow

In Pressurized Water Reactors (PWR), Steam Generator (SG) tubes constitute one of the three barriers which preserve the environment from radioactivity. Excessive tube vibrations under fluid forces, due to the steam-water mixture flow across the tube bundle, can lead to the failure of some tube. Several methods have been proposed to estimate some upper bounds for these forces. These bounds are applicable at the design stage and are helpful to avoid tube failures. Most of the available methods are based on experimental results that have been obtained on tube bundles installed in scaled test-facilities. Unlike this popular test-based approach, one combines here Computational Fluid Dynamics (CFD) to High Performance Computing (HPC), in order to estimate fluid forces in a simple case by applying the Direct Numerical Simulation (DNS) method to solve the Navier-Stokes equations. In the first paragraph, one summarizes the general standard method which allows one to derive the auto-power spectral density of the displacement response at any point of an SG tube, departing from the cross-power spectral densities of fluid forces between any two points along the tube. In the second paragraph, one recalls the equivalent dimensionless spectrum, which was proposed by Axisa et al. in the early nineties, and which still remains a useful reference in the domain. One then applies DNS to the test case of a single infinite cylinder, which is submitted to a single phase cross-flow in a rectangular channel. The Reynolds number is equal to 3900. One presents the time dependent tensors of fluid pressure and viscous stresses, and uses this tensor to estimate the field of non stationary forces that are applied by the fluid, per unit length, at a set of equidistant locations along the tube. Even if they do still require experimental validations, our computation results are more abundant and detailed than standard experimental results, as well as more flexible to use. They therefore provide an interesting additional source of information. They already allow us to try to get new insights into quantities that would be, in any case, very difficult to obtain experimentally. Lift, drag, and even the forces acting in the direction of the tube axis, are computed, and can be distinguished one from the other. Fluid forces due to viscous stresses can also be compared to the ones caused by pressure. The degree of correlation of the forces along the tube can also be examined.Copyright

Method Used and Highlight Results achieved with the code_Saturne Software at EDF

The numerical method used in EDF’s unstructured finite volume code is described, with an emphasis on boundary conditions. Through close collaboration with FLOMANA partners, UMIST in particular, implementation of SSG, SST, V2F models and scalable wall functions could be finalised. The wing-tip vortex and the 3D hill cases were computed in URANS mode, the latter also with LES using a synthetic turbulence method.

Non Conforming Meshes and RANS/LES Coupling: Two Challenging Aims for a CFD Code

This paper focuses on the geometry/solver interface for the CFD Software Code_Saturne ® which has been developed by EDF R&D since 1997 to replace its FE and BFC solvers. The solver includes various RANS models, as well as LES features and is parallelized through a domain splitting technique. It is based on a cell centered unstructured finite volume scheme, and accepts cells of any shape. This opened the possibility of using non conforming meshes, making it easier to build meshes with well-controlled quality and refinement even for complex geometries. Adjacent boundary faces of non-conforming input meshes may be automatically split into their intersecting subsets so as to build a conforming mesh of polyhedra with an arbitrary number of faces per cell. This also extends to the handling of periodic boundary conditions as a geometrical condition. We will explain how this is handled and illustrate the algorithm’s behavior on different complex grid examples.Copyright