Experiment study on vortex evolution process and vorticity distribution in wave boundary layer flow over a rippled bed

Vortex over rippled bed acts as the main driving force for sediment transport under wave dynamics. Hydrodynamic experiments are carried out under matching conditions of wave dynamics and bed ripples, to reveal the vortex evolution process and vorticity distribution. The results indicate that the vortex body around the ripples experiences the evolution process of clockwise vortex formation, clockwise vortex detachment and dissipation, counterclockwise vortex formation, and counterclockwise vortex detachment and dissipation. Moreover, the vorticity at the ripple crest is proportional to U_w,rms/λ and η/λ, where U_w,rms represents the bottom velocity under wave action, λ represents the ripple length and η represents the ripple height. The vertical distribution of dimensionless vorticity depends on η. As η grows, the vorticity increases in the upper part and the vertical distribution of dimensionless vorticity becomes uniform. The circulation of the vortices is proportional to U_w,rms and η. The proposed expression for the vorticity at ripple crest, dimensionless vertical distribution of vorticity and circulation of the vortices all agrees well with the measured values. These findings lay the foundation for the study of the bottom sediment concentration.