Hybrid LES-RANS-Coupling and Wall Modeling for Complex Flows with Separation

• Applicant: Professor Dr.-Ing. Michael Breuer
• Subject Area: Fluid Mechanics
• Term: from 2004 to 2010
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5470235

The objektive of the present joint French-German research project is the development of urgently required techniques in order to apply large-eddy simulation (LES) and its derivatives to high Reynolds number turbulent flows of practical interest. Two main development strategies will be considered which are closely related and belong together. These are on the one hand appropriate wall models combined with suitable (auto-adaptive) subgrid scale models. The goal is to bridge the near-wall region and therefore allow high Re simulations without resolving the direct vicinity of the wall with extremely fine and CPU-time consuming grids. On the other hand, hybrid LES-RANS approaches will be developed, which try to split up the simulation (ideally not the domain) into a RANS part and an LES part. Typically, RANS is adequate for attached boundary layers requiring reasonable CPU-time and memory, where LES can also be applied but demands extremely large resources. Contrarily, RANS often fails in flows with massive separation and/or with large-scale vortical structures. Here, LES is without doubt the better choice. The basic concept is to combine the advantages of both methods yielding an optimal solution at least for a special class of flows. A real non-zonal hybrid technique is preferred since it avoids the predefinition of RANS and LES regions leading to an approach where the suitable simulation technique is chosen more or less atitomatically. Although the idea of combined LES-RANS methods is not new, a variety of open questions has to be answered. This includes in particular the demand for appropriate coupling techniques between LES and RANS, adaptive control mechanisms, and proper SGS/RANS models. Since approximate wall boundary conditions partially rely on RANS modeling and the classical wall function is a limiting case of a RANS model in the near-wall region, both topics of the present proposal, i.e., wall modeling and LES-RANS coupling, are strongly interconnected. In order to investigate the above mentioned open questions, in the first step it is definitely inevitable to consider some geometrically simple test cases including pressure-induced flow separation and subsequent reattachment, e.g. test configuration T2, the flow over periodic hills. The final objectives are, of course, flow configurations of practical interest including high-Re cases. Because all partners already studied the flow around airfoils at high angles of attack, such a configuration (test case T6) will serve as a second, more complex test case. As a long-term goal a geometrically three-dimensional configuration of practical interest will be investigated. One option is the flow around a simplified car model (Ahmed body, test case T4). Especially for the hybrid methods, the flow configurations chosen should include regions which can be reasonably resolved by RANS and additionally complex flow features such as large-scale flow separation which definitely require LES instead of RANS. These kinds of flows are also extremely challenging for the wall modeling techniques which allows a direct comparison and evaluation of both approaches based on the same test cases. The CFD groups at LIMSI Orsay and LSTM Erlangen have long lasting experience in the development and application of LES and DNS for complex flows. At Ecole Centrale de Nantes, the numerics group has many years of experience in RANS computations for complex geometries. Owing to these complementary fields of main expertise, the cooperation is expected to be particularly fruitful leading to strong synergy effects.

• DFG Programme: Research Units
• Subproject of: FOR 507: Large Eddy Simulation (LES) of complex flows
• International Connection: France
• Participating Person: Dr. Michel Visonneau