Wakes of rotorcraft in advancing flight: A large-eddy simulation study

Caprace, Denis-Gabriel [UCL] Chatelain, Philippe [UCL] Winckelmans, Grégoire [UCL]

We perform the large-eddy simulation of the flow past a helicopter rotor to support the investigation of rotorcraft wake characteristics and decay mechanisms. A hybrid Lagrangian-Eulerian Vortex Particle-Mesh method is employed to simulate the wake development, with the blades modeled using immersed lifting lines. The validity of the numerical approach is first evaluated through a comparison of the rotor trim parameters with experimental results. Then, the rotor wake at low, medium and high advance ratios is simulated over up to 30 rotor diameters. The wake generation and roll-up are described (i) qualitatively using rotor polar plots and three-dimensional (3-D) vortex dynamics visualizations; (ii) quantitatively using classical integral diagnostics in cross-sections. The highly 3-D unsteady near wake transitions to a system dominated by two parallel vortices, over a distance that depends on the advance ratio. This process is accelerated by the multiple interactions between successive tip vortices, supporting the generation of self-induced turbulence, and uncovering a mechanism of vorticity alignment along the streamwise axis. The vortices in the far wake are compared to typical aircraft ones, and exhibit less compact cores and faster decaying energy. Finally, we illustrate the loss of time periodicity in the far wake using the power spectral density of the kinetic energy, and the backscattering of energy from high rotor harmonics to lower frequencies, as complementary evidence of the intense vortex interaction activity.