Flow organization near shear layers in turbulent wall-bounded flows

Abstract

Coherent vortex clusters are extracted from a boundary layer direct numerical simulation database by means of conditional averages associated with the local occurrence of strong shear layers, extending the work of Klewicki and Hirschi (J. C. Klewicki and C. R. Hirschi, Flow field properties local to near-wall shear layers in a low Reynolds number turbulent boundary layer, Phys. Fluids 16 (2004), p. 4163). The results support strong association between shear layers and vortex tubes, and suggest that latter are primarily produced upon roll-up of vortex sheets. Four configurations, which are characterized in terms of the vorticity field geometry and their contribution to the turbulent shear stress and skin friction, are recovered. The vortex modes associated with streamwise and wall-normal aligned shear layers resemble symmetric and one-legged hairpin vortices, and suggest that inner–outer layers interactions may be important in stimulating the eruption of near-wall shear layers. The modes associated with quasi-longitudinal shear layers closely recall the staggered vortices arrangement proposed in models of wall turbulence self-sustainment (J. Jeong, F. Hussain, W. Schoppa, and J. Kim, Coherent structures near the wall in a turbulent channel flow, J. Fluid Mech. 332 (1997), p. 185), and also highlight a mechanism of generation of secondary vorticity through a rubbing effect (P. Orlandi, Vortex dipole rebound from a wall, Phys. Fluids A 2 (1990), p. 1429).