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Modélisation des phénoménes transitoire lents avec la méthode de Boltzmann sur réseau

Abstract : A new three-dimensional CFD solver, LaBS, based on the lattice Boltzmann alogorithms has been developed in a framework of university and industry consortium. In this thesis, this solver is used to simulate thermal flows, with a new thermal boundary condition for immersed solid boundary. The new proposed thermal boundary condition is based on the reconstruction method of the distribution function and is evaluated for immersed solid with coincident and non-coincident wall on the case of diffusion and natural convection phenomena.Renault case study, deals with a vehicle moving at constant speed (highway) that suddently slows down and stops (with or without a cut off contact). In such situation the cooling of the engine compartment first driven by forced convection during taxiing stage, abruptly switches to natural convection in low velocity stages. As natural convection is a slow process, it can take several minutes to remove the accumulated heat in the engine compartment. Such duration could be damaging for some components of the engine compartement which do not tolerate high temperature.In order to anticipate overheating of the engine compartment, where a lot of automotive parts with complex geometry are present and to avoid the above mentioned damages, the phenomenon of natural convection is here studied with the new thermal boundary condition.%The new proposed thermal boundary condition is first tested on academic case studies for validation, and then applied to the case of a real car.The modelling of thermal flows with the lattice Boltzmann method (LBM) can be classified into three categories: the multispeed approach, the hybrid approach and the double-distribution-function (DDF) approach. The multispeed approach, uses only one equation to resolve velocity, density and temperature field, which is solved by the LBM. Whereas the hybrid approach and the DDF approach utilize two sets of equations, one to resolve velocity field and density field and another to resolve temperature field. The hybrid approach solves velocity field and density field by the LBM method and the temperature field by finite-different or finite-volume methods. On the other hand the DDF approach solves the two equations with LBM.The thermal model used in the solver LaBS is based on the coupled DDF approach. In this model, the flow field is solved by a D3Q19 velocity model while the temperature field is solved by a D3Q19 or a D3Q7 velocity model. The coupling between the momentum and the energy transport is made by the boussinesq approximation. The new proposed thermal boundary condition decomposes the distribution function at the boundary node into its equilibrium and non-equilibrium part. The non-equilibrium part is calculated from the theoretical solution based on Chapman-Enskog developement.LaBS thermal model based on the coupled DDF approach is evaluated on a set of cases with increasing complexity. The results obtained with LaBS are compared with analytical solutions or with reference articles and are in a good agreement with the results expected. Results show that the model D3Q19/D3Q7 is qualitatively as good as the model D3Q19/D3Q19 but quantitatively the model D3Q19/D3Q19 remains the best.
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Gayathiri Thandavamoorthy. Modélisation des phénoménes transitoire lents avec la méthode de Boltzmann sur réseau. Mécanique des fluides [physics.class-ph]. Université Pierre et Marie Curie - Paris VI, 2016. Français. ⟨NNT : 2016PA066210⟩. ⟨tel-02982346⟩

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