Coherent structure dynamics in near-wall turbulence

The regeneration and dynamics of near-wall longitudinal vortices - which do minate turbulence production, drag, and heat transfer - are analyzed using direct numerical simulation of turbulent channel flow. These dominant strea mwise vortices are shown to result from nonlinear saturation of an instabil ity of lifted low-speed streaks near a single wall, free from any initial v ortex. The newly-found instability mechanism initiates streak waviness in t he (x,z) plane, generating streamwise vorticity sheets. Streak waviness in turn induces positive partial derivative u/partial derivative x (i.e. posit ive VISA), which causes these sheets' vorticity to then concentrate via str etching (rather than roll up) into new streamwise vortices. The instability requires sufficiently strong streaks (y circulation per unit x > 7.6 wall units) and is inviscid in nature, despite the close proximity of the no-sli p wall. We find that self-annihilation of streaks due to viscous cross-diff usion of opposite-signed wall normal vorticity across each streak causes th e instability amplification to scale in wall units, suggesting the relevanc e of our results to high Re as well. Significantly, the strongly 3D vortice s generated by streak instability agree well with the CS educed from fully turbulent flow, suggesting the prevalence of this streak instability-based vortex formation mechanism. Simultaneous to vortex generation, an internal shear layer is generated across the streak from each streamwise vortex by s tretching of spanwise vorticity.-Such a shear layer eventually rolls up at the downstream end of a streamwise vortex, and the two link up and propagat e outward to form a spanwise "arch" vortex. The newly generated streamwise vortices act to sustain/strengthen the preexisting streak which spawned the m through localized lifting of near-wall fluid by induction. We develop a n ew spatiotemporal vortex generation mechanism, in which vortices leave behi nd "vortex-less" streaks, whose instability, due to the mechanism explained herein, initiates streamwise vortex formation. (C) 2000 The Japan Society of Fluid Mechanics and Elsevier Science B.V. All rights reserved.