Reynolds-averaged and wall-modelled large-eddy simulations of impinging jets with embedded azimuthal vortices

We have performed simulations of an impinging jet with embedded azimuthal vortices, a model of the wake of a helicopter hovering near the ground. This problem has considerable practical importance since, when the landing area is covered with sand or snow, the interaction between the helicopter wake, the rotor-tip vortices and the solid particles on the ground can result in the lift up of a cloud of sediment that limits the pilot's vision, causing accidents and potentially loss of life. We compare the results of well-resolved large-eddy simulations (LES) at laboratory scale, with solutions of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with three turbulence models, and also with LES in which the wall layer is modelled (WMLES), using the Delayed Detached Eddy Simulation (DDES) approach, or employing approximate boundary conditions. The URANS solutions do not yield a reliable prediction of the development of the azimuthal vortices (a well-known shortcoming of most eddy-viscosity turbulence models in this type of configuration), while the WMLES and DDES predict vortex decay in good agreement with the resolved LES data. Simulations at a Reynolds number higher by a factor of 20 (intermediate between the laboratory scale and the real configuration) were also carried out. All the simulation methods predict very little effect of the Reynolds number: the flow appears to be driven by the evolution of the large-scale embedded vortices. The trends are the same observed at laboratory-scale: the RANS turbulence models predict a much faster decay of the embedded vortices compared to the WMLES and DDES. The need for reliable experimental data is highlighted.