Simulation of the Aerodynamic Interaction between Rotor and Ground Obstacle Using Vortex Method

The aerodynamic interaction between rotor wake and surrounding obstacles is complex, and generates high compensatory workload for pilots, degradation of the vehicle handling qualities and performance, and unsteady forces on the structure of the obstacles. The interaction also affects the minimum distance between rotorcraft and obstacles to operate safely. A vortex-based approach is here employed to investigate the complex aerodynamic interaction between rotors and ground obstacles and identify the distance where the interaction ends. This is also one of the objectives of the GARTEUR AG22 effort. In this approach, the aerodynamic loads of the rotor blades are described through a panel method, and the unsteady behavior of the rotor wake is modelled using a vortex particle method. The effects of the ground plane and obstacle are accounted for via a viscous boundary model. The method is then applied to "Large" and "Wee" rotor near the ground and obstacle and compared with earlier experiments carried out at the University of Glasgow. The results show that the predicted rotor induced inflow, and flow-field compare reasonably well with the experiments in terms of magnitude and phase, for the peaks of the radial outwash and vertical downwash. Furthermore, at certain conditions the tip vortices are pushed upwards and are re-ingested into the rotor wake due to the effect of the obstacle resulting in a recirculation. Moreover, contrary to cases without the obstacle, the peak and thickness of the radial outwash near the obstacle is lower due to blockage effects, and an up-wash is observed. Additionally, as the rotor closes to the obstacle, the rotor slipstreams impinge directly on the obstacle, and the up-wash near the obstacle is faster, indicating a stronger interaction between the rotor wake and the obstacle. Also, contrary to the case without the obstacle, the fluctuations of the rotor thrust, roll and pitch moments are obviously strengthened. When the distance between the rotor and the obstacle is larger than 3R, the effect of the obstacle is small.