Ordered motion in the turbulent boundary layer over wind waves

Abstract

The turbulent boundary layer over young wind waves (C/u* ∼ 1, where C is the phase speed of wind waves and u* is the friction velocity) has been investigated in a laboratory tank. Ordered motions have been found, and their structures studied in detail. Visualization of the outer boundary layer (0.4δ–1δ, where δ is the boundary-layer thickness) by paraffin mist has demonstrated the existence of a train of large-scale ordered motions having a horizontal lengthscale that corresponds to the wavelength of the underlying wind waves. Hot-wire measurements combined with the visualization have shown that the passage of the outer boundary-layer bulge is related to the occurrence of a low-speed air mass, usually accompanied by an upward velocity to produce large Reynolds stress. In the vicinity of the wave surface (0–0.15δ), flow separation occurs over these wind waves. Instantaneous velocity shear measurements, using two hot wires 0.15 cm apart vertically, have detected a high-shear layer at the edge of the separation bubbles. This high-shear layer, the potential site for generating much turbulence, reattaches on the windward side of the preceding wind waves. A pressure rise and a shear-stress spike, expected near the reattachment region, could be the mechanisms for supplying energy to the wind waves.The bursting phenomena over wind waves have been examined in detail in the logarithmic boundary layer (0.15δ–0.3δ). The bursting phenomena are a major mechanism for producing Reynolds stress and have a specific relationship with the phase of the wind wave. To explain the bursting phenomena, two mechanisms (not present in the boundary layer over a flat plate) are proposed, involving air-flow separation and the large-scale ordered motions, respectively. The two mechanisms are a ‘big burst’ related to the discharge of a whole separation bubble, and a ‘small burst’ which is the upward bursting of a low-speed air mass from the unstable separated shear layer into the ordered motions passing over a separation bubble.