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Theses

Turbulence modelling of mixed and natural convection regimes in the context of the underhood-space of automobiles.

Abstract : The subject of this thesis is the turbulence modeling of buoyancy-driven flows, which emanate through the interaction of the gravitational force with a density difference. The motivation of this investigation comes from the problem faced by the PSA group in simulating natural convection flows in the under hood space of cars.The main goal of the present investigation is to test several models to account for buoyancy and to propose effective improvements which could provide a model applicable to buoyancy-driven flows and in addition to that, can be easily implemented in the software Ansys Fluent for the computation of natural convection flows in the Underhood-space of cars.In the context of this goal, three eddy-viscosity turbulence models are sensitized to the effects of buoyancy. The first approach which offers the better physical framework involves the extension of the constitutive relations for the Reynolds stress and turbulent heat flux in a linear way, to account for the anisotropic influence of buoyancy. This approach is applied to three different models and brings in drastic improvement of the results in reproducing the mean flow and the turbulent quantities and thus it is realized that this approach leads to physically based improvements.Furthermore, it is observed that, using a simple gradient diffusion hypothesis (SGDH) approach to model the buoyancy source terms leads to underestimate the effect of buoyancy on turbulence and the comparison with the DNS data shows that the generalized gradient diffusion hypothesis (GGDH) give improved predictions of the mean flow and temperature field. Another issue addressed in this work involves the sensitiveness to the buoyancy production term in the ε or ω equations and after a detailed analysis, it is realized that the results are very sensitive to this term and the optimal value of the coefficient is linked to the choice of the turbulence model. To avoid this limitation, another expression for the model of the buoyancy source term in the ε or ω equations is applied which considers the flux Richardson number and it is observed that there is an improvement in the prediction of mean flow profiles.Three different regimes of convective flows are studied namely, forced, mixed and natural convection and the more challenging differentially heated vertical channel flow configuration which poses a major challenge to the eddy-viscosity models is considered to develop the buoyancy sensitized model. As an outcome of these studies, the more physical and simplified forms of buoyancy sensitized model are proposed which is considered as the best compromise between the physical accuracy and numerical stability for buoyancy-driven flows.These buoyancy-sensitized models provide an opportunity to investigate other buoyancy-driven flows and paves the way for these models to be applied in the under hood space simulation.
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Submitted on : Tuesday, June 15, 2021 - 1:01:49 AM
Last modification on : Tuesday, June 22, 2021 - 3:53:38 AM

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Syed Mohd Saad Jameel. Turbulence modelling of mixed and natural convection regimes in the context of the underhood-space of automobiles.. Other [cond-mat.other]. Université de Pau et des Pays de l'Adour, 2020. English. ⟨NNT : 2020PAUU3033⟩. ⟨tel-03260430⟩

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