Yacine Addad

Large eddy simulation of a forward–backward facing step for acoustic source identification

The feasibility of using a commercial CFD code for large eddy simulation (LES) is investigated. A first test on homogeneous turbulence decay allows a fine-tuning of the eddy viscosity with respect to the numerical features of the code. Then, a flow over forward–backward facing step at Reynolds number Reh ¼ 1:7 � 10 5 is computed. The results found show good agreement with the new LDA data of Leclercq et al. [Forward backward facing step pair: aerodynamic flow, wall pressure and acoustic characterization. AIAA-2001-2249]. The acoustic source term, recorded from the LES and to be fed into a following acoustic propagation simulation, is found to be largest in the separation from the forward step. The source terms structures are similar to the vortical structures generated at the front edge of the obstacle and advected downstream. Structures generated from the backward step rapidly break down into smaller scale structures due to the background turbulence. 2003 Published by Elsevier Science Inc.

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.

Free convection in an inclined square cavity with partial partitions on a wavy hot wall

Free convection in an inclined square cavity with partial partitions has been investigated, by numerically solving the partial differential balance equations of mass, momentum, and energy. The analysed configuration has three undulations and three partitions. Investigations have been performed for different inclination angles, partition lengths and Rayleigh numbers, while keeping the Prandtl number constant. The results show that the hot wall geometry with partitions affects the heat transfer rate in the cavity. It has been found in the present numerical study that the mean Nusselt number decreases notably compared with the heat transfer in the square undulated cavity without partitions.

Controlled Fabrication of Nanoporous Oxide Layers on Zircaloy by Anodization

We have presented a mechanism to explain why the resulting oxide morphology becomes a porous or a tubular nanostructure when a zircaloy is electrochemically anodized. A porous zirconium oxide nanostructure is always formed at an initial anodization stage, but the degree of interpore dissolution determines whether the final morphology is nanoporous or nanotubular. The interpore dissolution rate can be tuned by changing the anodization parameters such as anodization time and water content in an electrolyte. Consequently, porous or tubular oxide nanostructures can be selectively fabricated on a zircaloy surface by controlling the parameters. Based on this mechanism, zirconium oxide layers with completely nanoporous, completely nanotubular, and intermediate morphologies between a nanoporous and a nanotubular structure were controllably fabricated.

LES and URANS Predictions of Thermal Load in Piping Systems: T-Junction

The present numerical study focuses on the predictions of thermal mixing in a T-junction using two types of approaches; the Large Eddy Simulation (LES) and the Unsteady RANS technique. The numerical predictions are compared to the experimental reference data of Westin et al. (2008). Beforehand, the LES using the commercial code Star-CD are across validated with the open source Code_Saturne in a simple academic channel flow case at Re=395. For this case, both codes predictions are found in a satisfactory agreement with the DNS data which provides sufficient evidence, from a numerical dissipation related issues point of view, that any of these codes can be used for the LES runs of the more complex T-Junction test case. For the later, in agreement with previous findings reported in the open literature the LES approach is found capable to mimic correctly the flow behavior and to provide valuable instantaneous data needed for the thermal stress fatigue analysis for instance. The URANS technique on the other hand, even with an advanced non-linear eddy-viscosity model, is not only incapable of predicting correctly the mean variables, but also largely dumping the flow turbulence.

Comparison of large eddy simulation and experimental results of the flow around a forward-backward facing step

SNCF (Societe Nationale des Chemins de Fer francais), PSA (Peugeot Citroen), EDF (Electricite de France) and ECL (Ecole Centrale de Lyon) are involved in a common project whose issue is the potential evaluation of numerical simulation in aeroacoustics for transport applications. One of the methods chosen in the project consists in the calculation of the aeroacoustic source term through a Large Eddy Simulation (LES) and its implementation into a code based on the Linearized Euler’s Equations for the acoustic propagation (LEE). This paper presents a comparison of LES and experimental results of the aerodynamic field around a forward-backward facing step and the principle of the chain-up of LES results and LEE calculations.© 2002 ASME

LARGE EDDY SIMULATION OF A FORWARD-BACKWARD FACING STEP FOR ACOUSTIC SOURCE IDENTIFICATION

The feasibility of using a commercial CFD code for large eddy simulation (LES) is investigated. A first test on homogeneous turbulence decay allows a fine-tuning of the eddy viscosity with respect to the numerical features of the code. Then, a flow over forward-backward facing step at Reynolds-number Reh= 1.7×105 is computed. The results found show good agreement with the new LDA data of Leclercq et al. (2001). The acoustic source term, recorded from the LES and to be fed into a following acoustic propagation simulation, is found to be largest in the separation from the forward step. The source terms structures are similar to the vortical structures generated at the front edge of the obstacle and advected downstream. Structures generated from the backward step rapidly break down into smaller scale structures due to the background turbulence.

Fabrication of Uniform Nanoporous Oxide Layers on Long Cylindrical Zircaloy Tubes by Anodization Using Multi-Counter Electrodes

We have presented a method to prepare a uniform anodic nanoporous oxide film on the surface of a cylindrical zircaloy (Zr) tube. The distribution of the electric field around the Zr tube determines the distribution of the thickness of the anodic nanoporous oxide film. The electric field generated when a cylindrical Zr tube is electrochemically anodized was simulated by using commercial code COMSOL. When four Pt wires were used as counter electrodes, a uniform electric field was achieved with minimal use of Pt. Based on the simulation results, a cylindrical Zr tube was anodized and the distribution of the thickness of the anodic nanoporous oxide layer was measured by FESEM. Also, mass production of uniform nanoporous anodic oxide films was possible by symmetrically arranging the zircaloy tubes and Pt wires.

Numerical Investigation of a Centrifugal Compressor for Supercritical CO2 as a Working Fluid

The supercritical carbon dioxide (S-CO2) Brayton cycle is considered as a strong candidate for power conversion systems. This includes concentrated solar power, coal power, bottoming cycle to fuel cells, and the next generation nuclear systems. In the previous studies, it was identified that the compressor consumes very small compressing work as operating condition approaches to the critical point. Thus, smaller amount of input work contributes to the enhancement of overall cycle efficiency. To achieve an efficient S-CO2 cycle, one of the major technical challenges exists in the compressor design. At KAIST, a research team is conducting a S-CO2 compressor tests to obtain fundamental data for advanced compressor design and to measure the performance of the compressor near the critical point. The measurements reveal the S-CO2 fluid to have properties of gases and liquids at the same time, but in regards to compressibility and density variation, its behavior is much closer to the liquid rather than gas near the critical point. In this paper, a CFD analysis of S-CO2 centrifugal compressor with the full geometry including diffuser and volute is presented. The numerical results are compared to the experimental data from KAIST SCO2 Pressurizing Experiment facility. A 3D grid was generated starting from the model of the compressor full geometry provided by the manufacturer. Furthermore, a property table of CO2 was generated by an in-house code and implemented to the CFD code. Then the performance characteristic of S-CO2 compressor is investigated in terms of compressor efficiency and pressure ratio. Additional flow variables inside the compressor such as velocity, pressure and viscosity are also investigated to help understanding the main reason behind the relatively higher compressor efficiency near the critical point compared to other flow conditions far from this region. In general acceptable results in comparison to the experiment are obtained (order of error from 0.5 to 7% for the compressor efficiency). Hence, the current CFD results should be able to provide additional and detailed information to be used for design enhancements of the compressor for S-CO2 Brayton power cycle.Copyright