Genesis of Longitudinal Vortices in Near-Wall Turbulence

Abstract

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 (y circulation per unit x > 7.6) and is inviscid in nature, despite the proximity of the no-slip wall. Streamwise vortex formation (collapse) is dominated by stretching, rather than rollup, of instability-generated ωx 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 3D 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) actuation and requiring no wall sensors or feedback logic.KeywordsDirect Numerical SimulationTurbulent Boundary LayerCoherent StructureVortex FormationStreamwise VortexThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.