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Person# Abdelouahab Dehbi

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Christoph Bosshard, Abdelouahab Dehbi, Michel Deville, Emmanuel Leriche, Alfredo Soldati

In nuclear safety, some severe accident scenarios lead to the presence of fission products in aerosol form in the closed containment atmosphere. It is important to understand the particle depletion process to estimate the risk of a release of radioactivity to the environment should a containment break occur. As a model for the containment, we use the three-dimensional differentially heated cavity problem. The differentially heated cavity is a cubical box with a hot wall and a cold wall on vertical opposite sides. On the other walls of the cube we have adiabatic boundary conditions. For the velocity field the no-slip boundary condition is applied. The flow of the air in the cavity is described by the Boussinesq equations. The method used to simulate the turbulent flow is the large eddy simulation (LES) where the dynamics of the large eddies is resolved by the computational grid and the small eddies are modelled by the introduction of subgrid scale quantities using a filter function. Particle trajectories are computed using the Lagrangian particle tracking method, including the relevant forces (drag, gravity, thermophoresis). Four different sets with each set containing one million particles and diameters of 10 mu m, 15 mu m, 25 mu m and 35 mu m are simulated. Simulation results for the flow field and particle sizes from 15 mu m to 35 mu m are compared to previous results from direct numerical simulation (DNS). The integration time of the LES is three times longer and the smallest particles have been simulated only in the LES. Particle statistics in the LES and the DNS were similar and the settling rates were practically identical. It was found that for this type of flow no model was necessary for the influence of the unresolved flow scales on the particle motions. This can be explained by the dominant nature of gravity settling compared to turbophoresis which is negligible for the particle sizes of the present study. (C) 2013 Elsevier B.V. All rights reserved.

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Large eddy simulations of the turbulent natural convection flow in a differentially heated cavity have been carried out at a Rayleigh number of 10(9) using the spectral element method. To obtain the large eddy simulation equations, a low pass filter given by the numerical space discretisation is applied to the Boussinesq equations. The subgrid tensor in the filtered momentum equation is modelled by a subgrid viscosity computed by the dynamic Smagorinsky model. To model the subgrid heat flux vector in the filtered temperature equation, a subgrid diffusivity is used which is related to the subgrid viscosity by a dynamically computed subgrid Prandtl number. All filtering operations are done in an elementwise defined hierarchical polynomial basis. The test filter for the dynamic procedure is chosen so that the grid filter and the combination of the grid with the test filter are self-similar. An important parameter of the simulation namely the choice of the decomposition of the computational domain into spectral elements is fully discussed. Large eddy simulations for three different grid resolutions are validated and compared with a highly accurate direct numerical simulation. At the end, turbulence structures associated with the maximum of the turbulent kinetic energy production are identified by the lambda(2) criterion. (C) 2013 Elsevier Ltd. All rights reserved.

Abdelouahab Dehbi, Konstantin Mikityuk

Particulate flow in closed space is involved in many engineering applications. In this paper, the prediction of particle removal is investigated in a thermally driven 3D cavity at turbulent Rayleigh number Ra = 1E9 using Coarse Large Eddy Simulation (CLES). The depletion dynamics of SiO2 aerosol with aerodynamic diameters between 1.4 and 14 µm is reported in an Euler/Lagrange framework. The main focus of this work is therefore to assess the effect of the subgrid-scale motions on the prediction of the particulate flow in a buoyancy driven 3D cavity flow when the mesh resolution is coarse and below optimal LES standards. The research is motivated by the feasibility of modeling more complex particulate flows with reduced CPU cost. The cubical cavity of 0.7 m side-length is set to have a temperature difference of 39 K between the two facing cold and hot vertical walls. As a first step, the carrier fluid flow was validated by comparing the first and second-moment statistics against both previous well-resolved LES and experimental databases [Kalilainen (J. Aero Sci. 100:73–87, 2016); Dehbi (J. Aero. Sci. 103:67–82, 2017)]. First moment Eulerian statistics show a very good match with the reference data both qualitatively and quantitatively, whereas higher moments show underprediction due to the lesser spatial resolution. In a second step, six particle swarms spanning a wide range of particle Stokes numbers were computed to predict particle depletion. In particular, predictions of 1.4 and 3.5 µm particles were compared to LES and available experimental data. Particles of low inertia i.e. dp

2022