Local scour in offshore wind farms affects the safety and stability of wind farms to a certain extent. Currently, the installation of sacrificial piles to reduce local scour is one of the main protective measures. However, the research on the influence of sacrificial piles on the circulation structure is still relatively limited. Based on the decomposition of the Lagrangian and Eulerian mean momentum equations, the effects of different arrangements of sacrificial piles on the change of circulation structure and its mechanism are investigated in an idealized rectangular bay in this study. It is found that the Lagrangian residual velocity(LRV) along the estuary shows a surface inflow and bottom outflow structure in the no-monopile case, while in the monopile and sacrificial piles cases, the inflow becomes dominated near the monopile at the bottom layer. In the monopile case, the LRV in the cross-estuary direction is dominated by offshore flow at the surface, and the bottom is dominated by onshore flow with offshore structure only downstream of the monopile. In the surface layer, the laterally aligned sacrificial piles resulted in multiple areas of high flow, whereas the triangularly aligned sacrificial piles resulted in staggered flow on the north side. It's attributed to the fact that the sacrificial piles result in flow staggering and high magnitude regions for the Lagrangian mean acceleration components and the horizontal nonlinear advection components. The Eulerian residual velocity(ERV) flows overall toward the sea in the no-monopile case. It exhibits a different structure in the monopile and sacrificial piles cases. The ERV along the estuary is dominated by outflow in front of the monopile and inflow on both sides in front of the monopile, with the opposite structure behind the monopile and in front of the monopile. Sacrificial piles induce a structural inversion of the vertical nonlinear advection components downstream of the monopile compared to the monopile case. In the cross-estuary direction, the ERV flows toward the shore on the north and south sides of the monopile, and leaving the shore upstream and downstream of the monopile. It's also found that there is a equilibrium between the vorticity change with time components and the spin of the eddy viscosity components; the horizontal advection components of relative vorticity is in equilibrium with the spin of the barotropic pressure gradient components; and the vertical advection components of relative vorticity and torsion components show the opposite structure and are of comparable magnitude.
 

Sacrificial pile,Residual current,Ideal bay