| Large-eddy simulation (LES) is a promising prediction technique for turbulent flows where only the largest scales of turbulence are simulated, while the effects of the small scales are modeled. However, the application of LES in engineering flows is limited largely by difficulties in LES modeling near walls; this is commonly known as the LES 'wall-modeling' problem. To address the wall-modeling problem, a new approach for treating boundaries in the context of LES was developed. In this approach the wall is treated by 'filtering through it', so that in the LES representation, the wall is not a sharp boundary. A homogeneous (or nearly homogeneous) spatial filter is applied to an extended domain, in which the wall is embedded. By treating the wall in this way, the issue of highly inhomogeneous filtering near a wall in LES can be avoided. When the filter is applied to the Navier-Stokes equations, and the filtered boundaries are accounted for, a boundary term appears in the equations. The boundary term depends on the unfiltered wall stress, which needs to be modeled. Thus two modeling problems need to be addressed for LES of wall-bounded flows in this context: the usual subgrid-scale modeling and boundary-stress modeling. We propose to use optimal LES subgrid models, and a wall-stress model formulated to minimize the 'leakage' of energy and momentum through the filtered wall. This modeling approach was applied to turbulent channel flow at Retau = 590, using a wall-normal filter width deltay+ = 36, which effectively filters out the near-wall dynamics. Nonetheless, the LES yields remarkably good results for both mean and RMS velocities. These promising results suggest that this modeling approach may provide a solution to the well-known LES wall-modeling problem. |
