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COUPLING CONDITIONS FOR LES WITH DOWNSTREAM RANS FOR THE PREDICTION OF INCOMPRESSIBLE TURBULENT FLOWS

Dominic von Terzi
Institut für Thermische Strömungsmaschinen, Universität Karlsruhe (TH), Kaiserstr. 12, 76128 Karlsruhe, Germany

Jochen Frohlich
Institute for Technical Chemistry und Polymer Chemistry University of Karlsruhe; Institute of Fluid Mechanics, Dresden University of Technology, George-Bähr-Str. 3c, D-01062, Germany

Abstract

Interfacing Large Eddy Simulation (LES) with a downstream Reynolds-Averaged Navier−Stokes (RANS) zone for incompressible flows is investigated. The mean velocity fields are matched at the predefined interface and velocity fluctuations of the LES zone are allowed to leave the domain by employing a convective boundary condition. For incompressible flows, in addition, it is also necessary to prescribe conditions for the pressure or an equivalent variable. Two situations are considered: (i) The instantaneous pressure is solved globally within the union of the LES and RANS domains and (ii) the pressure fields are completely decoupled. For the latter, several alternatives of handling the issue of mass conservation at the interface were studied. The performance of the different methods is scrutinized for turbulent channel flow and the flow over periodic hills. It was found that for the first test case the pressure coupling was uncritical whereas, for the hill flow, decoupling of the pressure with explicitly enforced mass conservation at the interface yielded the best and, for some situations, the only acceptable results.