Large-eddy simulation of impinging jets with embedded azimuthal vortices

We have carried out large-eddy simulations of an impinging jet with embedded azimuthal vortices, a model of the wake of a helicopter hovering in ground effect. The azimuthal vortices are generated by sinusoidal forcing of the velocity at the jet exit. They strengthen while they are advected towards the ground; when they are close to the solid surface, a layer of opposite-sign vorticity is formed at the wall, and lifted up to form a secondary vortex that interacts with the primary one. Regions of reversed flow are caused by the strong, localised, adverse pressure gradient. After this interaction, the primary vortices begin to decay, mostly due to the Reynolds shear stresses, which contribute to the turbulent diffusion of vorticity term in the budget of the phase-averaged azimuthal vorticity. This mechanism is extremely robust, and plays the most important role in the vortex decay even if no turbulence is initially present in the jet, or if the no-slip condition is removed. A three-dimensional instability also plays a role: removing it leads to slower decay. Our results also point out some challenges for turbulence models for the unsteady Reynolds-averaged Navier–Stokes equations.