On the role of wall-pressure fluctuations in deterministic motions in the turbulent boundary layer

Abstract

The wall-pressure fluctuations beneath a turbulent boundary layer have been conditionally sampled on a basis of the high-frequency activity of the pressure fluctuations themselves, the high-frequency activity of the streamwise velocity fluctuations in the vicinity of the wall, and the excursions in velocity in the vicinity of the wall. This has led to the identification of a characteristic wall-pressure fluctuation pattern which is associated with the burst–sweep cycle of events in the wall region. The pattern has the form of an overpressure over a streamwise extent of about 1·5−2·0δ_*, with a region of underpressure and a pressure minimum to either side of it, the distance between pressure minima being about 3·0−3·5δ_*. This pattern is convected at a velocity 0·67 times the freestream velocity. Its phase relationship with velocity fluctuations close to the wall and the wall shear-stress fluctuations during the burst–sweep cycle have been established. It appears to be produced by the inclined shear layer which forms the upstream surface of the large organized structures in the layer, and calculated pressure patterns support this conclusion.

The phase relationships indicate that fluid involved in the bursting process is subjected to a favourable streamwise pressure gradient by the characteristic wall-pressure pattern at the time that the lift-up of low-speed streaks in the wall region begins. In addition, order-of-magnitude estimates suggest that the adverse pressure gradients associated with the characteristic pressure pattern, even if their phasing with streak lift-up were appropriate, would be insufficient to initiate the lift-up. It is therefore concluded that the streamwise pressure gradients associated with the pressure patterns do not play an active role in the dynamics of the wall flow and are not the direct cause of the bursting process.