Genesis of longitudinal vortices in near-wall turbulence

Using direct numerical simulations of turbulent channel flow, we present ne w insight into the formation mechanism of near-wall longitudinal vortices. Instability of lifted, vortex-free low-speed streaks is shown to generate, upon nonlinear saturation, new streamwise vortices, which dominate near-wal l turbulence production, drag, and heat transfer The instability requires s ufficiently strong streaks (the wall-normal circulation on either side of a streak exceeding 7.6) and is inviscid in nature, despite the proximity of the no-slip wall. Streamwise vortex formation (collapse) is dominated by st retching, rather than Kelvin-Helmholtz rollup, of instability generated ome ga(x) sheets. In turn, direct stretching results from the positive partial derivative u/partial derivative x (i.e. positive VISA) associated with stre ak waviness in the (x, z) plane, generated upon finite-amplitude evolution of the sinuous instability mode. Significantly, the three-dimensional featu res of the (instantaneous) instability-generated vortices agree well with t he coherent structures educed (i.e. ensemble averaged) from fully turbulent flow, suggesting the prevalence of this instability mechanism. These resul ts suggest promising new drag reduction strategies, involving large-scale ( hence more durable) control of near-wall flow and requiring no wall sensors or feedback logic.