Physics-Based Modeling of Viscous Ground Effect for Rotorcraft Applications

In this paper, a physics-based flow solver was used to address the challenging rotor wake and interference problems for rotorcraft operating in ground effect (IGE). The flow solver combines a state-of-the-art viscous vortex particle method (VPM) with a viscous ground model. VPM is based on the incompressible Navier-Stokes equations and adopts a grid-free Lagrangian formulation, which makes it well suited for aerodynamic interaction simulations to address complex rotor/ground interaction problems. The viscous ground model is applied to effectively enforce the no-slip unsteady boundary condition in VPM in a very efficient manner. With the enhancement for the viscous ground model, VPM accurately handles the physical no-slip boundary condition for both moving and static ground surfaces, which is missing in most image-based ground effect models. This paper discusses the detailed formulation and implementation of the viscous ground model in VPM. The flow solver thus developed was used to simulate various challenging aspects in the modeling of the rotor ground effect, which include the rotor tip vortex dynamics, the ground vortex structure, the rotor performance in hover and forward flight, the moving/static ground effect, and the rotor outwash and flow field. In addition, the impact of the interference between the main/tail rotors and the ground vortex on the tail rotor performance was also studied. Simulation results were compared with available measurements and excellent correlations were obtained from the current physics-based modeling technology.