Effective drag reduction by large-scale manipulation of streamwise vortices in near-wall turbulence

Abstract

Using direct numerical simulations of turbulent channel flow, we present new insight into the regeneration dynamics and control of near-wall longitudinal vortices, which dominate turbulence production, drag, and heat transfer. Initially linear instability of lifted low-speed streaks, free from any initial vortex, is shown to generate these dominant streamwise vortices upon (nonlinear) saturation. 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 caused by the positive du/dx (i.e. positive VISA) associated with streak waviness rather than roll-up of cox sheets. Significantly, the 3D features of the instability-generated vortices are close to those of both instantaneous and ensemble-averaged flows, suggesting that this instability mechanism is prevalent in the (uncontrolled) fully turbulent flow. We develop effective new control approaches for turbulent boundary layers, via large-scale streak manipulation, which exploit this crucial role of streaks in vortex generation and hence turbulence production. Using control flows with no x variation, a spanwise wavelength of 400 wall units, and a (frozen) amplitude of only 5% of the channel centerline velocity, we find a significant sustained drag reduction: 20% for imposed counterrotating streamwise vortices and 50% for colliding spanwise wall jet-like forcing. These results suggest promising new drag reduction strategies, e.g. passive vortex generators and spanwise jets from ^-aligned slots, involving large-scale (hence more durable) actuation and requiring no wall sensors or feedback logic.