2011 Volume 6 Issue 1 Pages 56-72
The present work deals with a computational strategy coupling near-wall, eddyviscosity-based RANS models with LES within a zonal Hybrid LES/RANS (HLR) framework. Key questions concerning the coupling of both methods, the inherently steady RANS method and highly-unsteady LES method, are closely connected to the treatment at the interface separating both sub-regions. Large attention was paid to this problem and following three issues were highlighted: (1) the exchange of the variables across the LES/RANS interface was adjusted by implicit imposition of the condition of equality of the modelled turbulent viscosities (by assuming the continuity of their resolved contributions across the interface), enabling a smooth transition from the near-wall RANS layer to the off-wall LES sub-region; (2) utilisation of a dynamic, flow-dependent interface position in the course of the simulation. The control parameter k* representing the ratio of the modelled (SGS) to the total turbulent kinetic energy in the LES region, averaged over all grid cells at the interface on the LES side, is adopted; (3) the third issue, the present work is focussing on, addresses the usage of a special forcing technique, which compensates the loss of information due to strong damping caused by the presence of the RANS region (the typical outcome of such a circumstance is the so-called velocity mismatch in the region of interface) by creation of artificial and correlated fluctuations using a method originating from a digital-filter-based generation of inflow data for spatially developing DNS and LES due to Klein et al. (2003). Herewith, the recovery of the fluctuations on the LES side of the interface is accelerated and the afore-mentioned velocity bump is eliminated to a largest extent. The performances of the model are illustrated against the available DNS and fine-grid LES of periodic flows in a plane channel and over a 2-D smoothly contoured hill respectively.