Genesis of longitudinal vortices in near-wall turbulence

Using direct numerical simulations of turbulent channel flow, we present new 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-wall turbulence production, drag, and heat transfer. The instability requires sufficiently 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 stretching, rather than Kelvin-Helmholtz rollup, of instability-generated ω-x$/ sheets. In turn, direct stretching results from the positive ∂u/∂x (i.e. positive VISA) associated with streak waviness in the (x, z) plane, generated upon finite-amplitude evolution of the sinuous instability mode. Significantly, the three-dimensional features of the (instantaneous) instability-generated vortices agree well with the coherent structures educed (i.e. ensemble averaged) from fully turbulent flow, suggesting the prevalence of this instability mechanism. These results suggest promising new drag reduction strategies, involving large-scale (hence more durable) control of near-wall flow and requiring no wall sensors of feedback logic.