The autonomous near-wall turbulent region

Abstract

The near-wall region is the only place in zero-pressure-gradient boundary layer where the production of turbulent energy exceeds dissipation. The excess energy helps maintain turbulence in the core region, where the opposite is true. It is shown that it is possible to maintain turbulence in the region below y^+≈ 60 without any input from the outer flow. In the numerical experiment all the fluctuations in a plane channel are artificially damped by increasing viscosity with height, and the outer flow is laminar above that level. The near-wall region nevertheless survives indefinitely, suggesting that wall turbulence can be studied in terms of modular units, with the near-wall and the logarithmic and outer layers as interacting but distinct phenomena. The cycle responsible for maintaining near-wall turbulence is shown to involve low-velocity streaks and streamwise vortices, but essentially no hairpins. The intensity of the near-wall longitudinal velocity fluctuations agrees well with those in fully developed flows, but the wall-normal fluctuations are weaker, in agreement with the Reynolds number behaviour found experimentally for those quantities. The reason is explored using higher Reynolds number simulations.