Flow topology and small-scale dynamics in turbulent Rayleigh-Bénard convection
Without fluid turbulence, life might have rather different look. The atmosphere and oceans could nearly maintain a much larger temperature differences resulting in ultimate heating or cooling to the earth surface. The water and air flow could rather run much faster at rates of the speed of sound. Tu...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2017 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/458520 |
| Acceso en línea: | http://hdl.handle.net/10803/458520 https://dx.doi.org/10.5821/dissertation-2117-112427 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria mecànica 004 531/534 55 |
| Sumario: | Without fluid turbulence, life might have rather different look. The atmosphere and oceans could nearly maintain a much larger temperature differences resulting in ultimate heating or cooling to the earth surface. The water and air flow could rather run much faster at rates of the speed of sound. Turbulence is a highly active nature of chaotic, random and three-dimensionality of swirling fluid. Its nonlinear convective property transports the momentum and energy in a helical mechanism leading eventually to an enriched fluid mixing and generating of small scale motions. These scales chiefly rule the hairpin vorticity dynamics, the strain production and the cascade of kinetic energy mechanisms. Hence, the key feature in turbulence is around disclosing the small scale motions. Studying the fine-scale dynamics gives us fundamental perspectives of flow topology and thus, improves our knowledge of turbulence physics. The turbulence dynamo becomes more complex when the active thermal gradient constitutes into the pure generator of turbulence. This particularly happens in the so-called buoyancy-driven Rayleigh-Bénard convection (RBC), when an infinite/bounded lying fluid is heated from below and cooled from above in the field of gravity. The main goal of this thesis is investigating the flow topology and small-scale dynamics in turbulent RBC, in order to better understanding its thermal turbulence mechanism and improve/validate the turbulence modeling for the foreseeable Computational-Fluid-Dynamics future. To do so, a complete direct numerical simulation (DNS) of turbulent RBC in a rectangular air-filled cavity of aspect ratio unity and pi spanwise open-ended distance, has been presented at Rayleigh numbers Ra={1e8, 1e10}, in chapter 1. A global kinetic energy conservation is inherited using a fourth-order symmetry-preserving scheme for the spatial discretization, and the flow dynamics is explored by analysis of kinetic and thermal energy power spectra, probability density function (PDF) of viscous and thermal dissipation rates, and identification of the wind in RBC. In chapter 2, the DNS dataset is used to investigate several universal small-scale features observed in various turbulent flows and recaptured here in turbulent RBC through the bulk. For instance, the inclined "teardrop" shape of joint PDF velocity gradient tensor invariants (Q,R), the preferential alignment of vorticity with the intermediate eigenstrain vector, and the spiraling degenerated behavior of the average rates invariants (<DQ/Dt>,<DR/Dt>). It is found that a self-amplification of viscous straining -Qs results at Ra=1e10, helps in contracting the vorticity worms and enhances slightly the linear contributions of the vortex stretching mechanism. On the other hand, the evolution of relevant small-scale thermals has been addressed by investigating the average rate of invariants pertained to the traceless part of velocity-times-temperature gradient tensor i.e., (<DQt/Dt>,<DRt/Dt>). The new invariants are shown to follow correctly the evolution and lifetime of thermal plumes in RBC and hence disclose interactions of buoyant production and viscous dissipation. In chapter 3, the DNS dataset is employed to understand the underlying physics of the subgrid-scale (SGS) motions in turbulent RBC in the spirit of Large-eddy simulation (LES) turbulence modeling. To do so, the key ingredients of eddy-viscosity, eddy-diffusivity and turbulent Prandtl number, are calculated a priori and investigated in a topological point-of-view. As a result, it has been suggested the restricted application of the hypothesis of a constant turbulent Prandtl number only in the large-scale strain-dominated areas. More arguments have been attained through a priori investigation of the alignment trends imposed by existing parameterizations for the SGS heat flux. Finally, a new tensorial approach of modeling the SGS of thermal turbulence is sought, that opens new research trends in the future. |
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