Abstract
The pile-oscillation effect on foundation local scour cannot be neglected in the large-scale offshore wind turbine. To reveal the fluid dynamics and its influence on the bed shear stress of an oscillating pile foundation, the VOF two-phase flow model is used to study the effect of the transverse and vertical oscillation of pile foundations employed in offshore wind turbines. The changes in the flow field, horseshoe vortices, and bed shear stress are analyzed under different oscillation frequencies and amplitudes are monitored by two symmetry planes ahead of the pile and at pile side respectively. The results show that, at the longitudinal oscillation condition, the backflow in front of the pile is produced and is strengthened as increasing the frequency and amplitude of oscillation; the size of horseshoe vortex ahead of the pile, and the bed shear stress reach the maximum at the moment of T/2 (T is the oscillation cycle). At the transverse oscillation, horseshoe vortices are extended to the pile side, their sizes are increased as the oscillation frequency and amplitude increase, and the maximum size is generated at the moment of T/2; while the bed shear stress is the smallest at T/2, and the maximum is created at T. Both transverse and longitudinal oscillations increase the time-averaged bed shear stresses (TBSS), the longitudinal oscillation generates larger TBSS in the symmetry plane in front of the pile, conversely transverse oscillations generate larger TBSS in the side symmetry plane of the pile.