The algebraic wall stress models mentioned above all imply the logarithmic ͑ power ͒ law of the wall for the mean velocity, which is not valid in many complex flows. 7 and 8 and the references therein for a review of the various wall stress models. 6 to empirically account for the phase shift between the wall stress and near-wall tangential velocity due to the tilting of near-wall eddies. A number of modifications to Schumann’s model have been made by, for example, Gr ̈ tzbach and Werner and Wengle 5 to eliminate the need for a priori pre- scription of the mean wall shear stress and to simplify computations, and by Piomelli et al. This approach was first employed in a channel flow simulation by Schumann, 3 who assumed that the streamwise and spanwise velocity fluctuations are in phase with the respective surface shear stress components. The wall function provides an algebraic relationship between the local wall stresses and the tangential velocities at the first off-wall velocity nodes. The simplest wall stress models are analogous to the wall functions commonly used in Reynolds-averaged Navier–Stokes ͑ RANS ͒ approaches except that they are applied in the instantaneous sense in time-accurate calculations. Wall models which supply wall stresses to the LES are also called wall stress models. The dynamic effects of the energy-containing eddies in the wall layer ͑ viscous and buffer regions ͒ are determined from a wall model calculation, which provides to the outer flow LES a set of approximate boundary conditions, often in the form of wall shear- stresses. In this approach, LES is conducted on a relatively coarse grid designed to resolve the desired outer flow scales. 1,2 To cir- cumvent the severe near-wall resolution requirement, LES can be combined with a wall-layer model. The Reynolds number scaling of the required number of grid points is nearly the same as for direct numerical simulation. simulation LES of wall-bounded flows becomes prohibitively expensive at high-Reynolds numbers if one attempts to resolve the small but dynamically important vortical structures in the near-wall region.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |