Effective drift velocity from turbulent transport by vorticity

We highlight the differing roles of vorticity and strain in the transport of coarse-grained scalars at length scales larger than by smaller scale (subscale) turbulence. We use the first term in a multiscale gradient expansion due to Eyink [J. Fluid Mech. 549, 159 (2006)], which exhibits excellent correlation with the exact subscale physics when the partitioning length is any scale smaller than that of the spectral peak. We show that unlike subscale strain, which acts as an anisotropic diffusion/antidiffusion tensor, subscale vorticity's contribution is solely a conservative advection of coarse-grained quantities by an eddy-induced nondivergent velocity, v*, that is proportional to the curl of vorticity. Therefore, material (Lagrangian) advection of coarse-grained quantities is accomplished not by the coarse-grained flow velocity, ut, but by the effective velocity, ut + v*, the physics of which may improve commonly used LES models.