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Juan Uribe - Nicolas Jarrin. Hybrid RANS-LES modelling, single code & grid. University of Manchester, PO Box 88, Manchester M60 1QD, UK. Introduction. Main problem with LES is the near wall restictions, where the size of the cell must decrease in all directions. Ways to avoid that:
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Juan Uribe - Nicolas Jarrin Hybrid RANS-LES modelling, single code & grid University of Manchester, PO Box 88, Manchester M60 1QD, UK
Introduction • Main problem with LES is the near wall restictions, where the size of the cell must decrease in all directions. Ways to avoid that: • Wall functions. • - Bridge the viscous sublayer. • - Not universal. • RANS in one domain, LES in the other. • - Interface freely prescribed. • - Treatment of the interface is very impor tant. Need to promote turbulence ( synthetic, rescaled ...) • RANS-LES in the same domain. • - How to couple two different velocity fields? • - Requires smooth transition.
Hybrid model The problem: • RANS velocity field is statistically averaged. LES field is instantaneousy filtered.-> not the same properties (subgrid/turbulent viscosity/energy) • LES sub-grid model assumes inertial range isotropic motions. • As the wall is approached, mean shear introduces anisotropy → cell size must be reduced. The model must take into account sub-grid contributions to mean shear and isotropic dissipation effects.
Hybrid model • The velocity field can be decomposed as follows: • The subgrid stress tensor can be splited into two parts. One with the instantaneous contribution and one with the averaged. is the subgrid viscosity is the RANS viscosity, based on the mean velocity
Hybrid model • The blending function fb is needed to avoid double counting of the stress and to ensure that as the mesh is refined, the RANS contribution diminishes. The turbulent lenghscale is predicted by an elliptic relaxation model (v2-f) to accurately predict the near-wall anisotropy is the filter width
Trailing edge flow: Case specifications • Re=2.5 106 based on chord, Re=2760 (lower side) and Re=3380 (upper side) based on the momentum thickness • Only rear-most 38% of domain computed. • Inlet conditions based on RANS calculation of full aerofoil plus synthetic method. • Fine LES used for comparison used 1536 x 96 x 48 • Actual grid used for computations: 512 x 64 x 24 • Noise prediction important, therefore unsteady resolution required. • Two simulations: DES +F1 shield function and Hybrid Model.
