Three-Dimensional Roll-Up of a Separated Shear Layer into a Persistent Vortex

A separated shear layer in the wake of configurations such as a cargo aircraft, inclined projectiles, high-speed trains, or an Ahmed body, frequently rolls up into a persistent vortex. These vortices create unwanted drag and unsteady forces on the body. They also disrupt the nearfield flow environment, posing challenges for missions such as paratrooper deployment from cargo aircraft. Previous work has shown that, for a given geometry, vortex formation depends on freestream conditions. Furthermore, in some cases where geometry and freestream remain fixed, vortex formation, or lack thereof as a turbulent wake state, depends on the initial condition (hysteresis). This implies the presence of a threshold that determines vortex formation, though its parameters and values are not yet identified. In this study, large-eddy simulations of a canonical geometry are employed to explore the necessary features for the formation of this kind of vortex. Quasi-two-dimensional simulations with an imposed spanwise velocity component fail to produce the vortex, underscoring the inherently three-dimensional nature of the phenomenon. Roll-up additionally requires separation over an edge that is angled from the freestream direction, into a field where the shear layer is accelerated both inward and along the separation line. These findings serve as the foundation for future investigations into vortex suppression by exploiting the roll-up threshold or other unsteady mechanisms important to vortex formation.