The formation of U‐shaped vortices on vortex tubes impinging on a wall with applications to free surfaces

Numerical simulations of a pair of vortex tubes impinging on a no‐slip wall show the formation of U‐shaped vortices that are wrapped around the primary vortex tubes. The essential stages of U‐vortex formation are as follows: generation of helical vortex sheets due to the onset of an instability on the primary vortex tubes, attachment of the helical vortex sheets to the separated wall boundary layer, wrapping of U‐shaped vortices around the primary vortex tubes, and feeding of boundary‐layer vorticity into the U vortices. The generation of U vortices helps to explain the striations and whirls that form on a clean or contaminated free surface above a pair of trailing‐tip vortices.