The effects of swell on turbulence over wavy walls

Three-dimensional direct numerical simulations (DNS) are applied to the turbulent air boundary layer over a wavy wall, which imitates the air–water interface consisting of both wind waves and swells, and the effects of the swells with various directions on the turbulence structure and drag over/on the wavy wall are investigated. The results show that parallel swell with the same direction as the wind increases the turbulence intensity and the Reynolds stress over the wavy wall. The swell also increases the pressure drag and decreases the friction drag on the wavy wall, and consequently increases the total drag because of remarkable increase of pressure drag. As the inclination angle of the swell against the wind increases from parallel to perpendicular, the swell effect on the drag becomes weak and finally vanishes. The reduction of friction drag due to swell supports our previous measurements that show reduction of the mass transfer velocity across the wind-driven wavy air–water interface due to the parallel swell.