Abstract: Local scour at piles poses a significant threat to the safety of offshore wind power and sea-crossing bridges. Addressing the issue of reducing scour depth is an urgent and pivotal challenge that requires immediate attention. The collar is a disk-like structure that reduces the scour depth by weakening the hydrodynamics around the pile. In this study, numerical experiments on the local scour at composite piles with different collars were conducted. The influence of collar layout position, thickness, width, and attack angle on local flow field, bed shear stress, and scour protection effect were discussed. The results show that the position, width, and attack angle of the collar exert a significant influence on local scour, whereas the thickness of the collar has a negligible impact on its effectiveness in scour protection. The larger the collar width and the negative value of the attack angle, the better the scour protection effect. When the collar is set at the edge of the pile cap foundation, with a width ranging from 1～1.5 times the height of the pile cap foundation and an attack angle of -45°, its scour protection effect on the composite pile is the best. The results can provide references for engineering design of pile foundation scour protection.

Key words: composite piles, local scour, numerical simulation, pile cap, collar