Bursts and the law of the wall in turbulent boundary layers

Abstract

The bursting mechanism in two different high-Reynolds-number boundary layers has been analysed by means of conditional sampling. One boundary layer develops on a smooth, flat plate in zero pressure gradient; the other, also in zero pressure gradient, is perturbed by a rough-to-smooth change in surface roughness and the new internal layer has not yet recovered to the local equilibrium condition at the measurement station. Sampling on the instantaneousuvsignal in the logarithmic region confirms the presence of two related structures, ‘ejections’ and ‘sweeps’ which, in the smooth-wall layer, appear to be responsible for most of the turbulent energy production, and to effect virtually all that part of the spectral energy transfer that is universal. Ejections show features similar to those of Falco's ‘typical eddies’ while sweeps appear to be inverted ejections moving down towards the wall. The inertial structures associated with ejections show attributes of the true universal motion (Townsend's ‘attached’ eddies) of the inner layer and these are therefore identified as ‘bursts’. In the outer layer, these become ‘detached’ from the wall. The large-scale structures associated with sweeps also appear to be ‘detached’ eddies (‘splats’), but these induce low-wave-number inactive motion near the wall and this is not universal even though the sweep itself is. Neither ejections nor sweeps detected in the rough-to-smooth layer are near a condition of energy equilibrium. The relation of ejections and sweeps to the law of the wall and other accepted laws is discussed.