A new grey area mitigation technique for des. Theory and assessment
- Autores: Arnau Pont Vilchez
- Directores de la Tesis: Francesc Xavier Trias Miquel (dir. tes.), Assensi Oliva Llena (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2020
- Idioma: español
- Materias:
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- Resumen
The use of hybrid RANS-LES methods has become widespread during the last decade, as an interesting approach for covering the gap between RANS and LES turbulence models in terms of both computational resources and degree of modelling. In particular, for those situations where the flow unsteadiness needs to be well-captured or those flow configurations where RANS has demonstrated to be unreliable, such as massive flow separation.
Within the family of hybrid models, Delayed - Detached Eddy Simulation (DDES) outstands due to its user-friendly non-zonal approach and its proved success in several applications.Despite their benefits, these models usually suffer from a slow RANS to LES transition (named Grey Area), resulting in unphysical delays of critical flow instabilities in sensitive regions, such as Kelvin-Helmholtz structures in free shear layers. This delay in the triggering process could significantly affect the flow dynamics downstream of the flow, as well as those kinds of physics that require high quality unsteady turbulent motion, such as fluid structure interaction and computational aeroacoustics. In this regard, the present thesis aims to perform a consistent study of different techniques for mitigating such delay, as well as presenting a promising easy-to-apply new strategy. Due to the lack of publicly available highly reliable data set, a Direct Numerical Simulation (DNS) of a Backward-Facing Step (BFS) at Re ? = 395 and expansion ratio (ER) 2 has been carried out during the first part of this thesis for comparison purposes. In contrast to the rest of reference cases, it provides a detailed view of the triggering and feeding processes of the flow instabilities through the free shear layer. As a result, the thesis provides a highly reliable data set publicly available on internet, as well as a competitive new technique for mitigating the Grey Area shortcoming.
The thesis content is arranged as follows. In the first chapter, a general overview of the different approaches for modelling turbulence is presented, emphasizing the importance of the Hybrid RANS-LES strategies for industrial applications. In second chapter, the DNS of the BFS at Re ? = 395 and ER = 2 is explained in detail. Specialattention is paid on the triggering of the flow instabilities in the free shear layer downstream the step-edge. The third chapter describes the new techniques proposed in this thesis, based on the LES literature, for mitigating the Grey Area shortcoming. These are tested in the fourth chapter, which presents a consistent study of the different methodologies for addressing the unphysical delay of the shear layer instabilities. Finally, the last chapter gathers the main conclusions of the overall thesis and defines possible further work lines.
