The presence of a ground plane significantly alters the rotor aerodynamics and generates a more complex flow-field. To analyze the characteristics of the rotor tip vortices and flow-field in ground effect, a ground aerodynamic model is then proposed based on the vorticity concept and no-slip boundary condition. In this model, the distribution of vorticity vector is solved through the second kind equation of Fredholm, and is diffused to the flow based on the diffusion equations, accounting for the viscous effects of the ground plane. An aerodynamic method of the rotor in ground effect is then established by coupling the ground aerodynamic model with an unsteady panel/viscous vortex particle hybrid method in which the unsteady aerodynamics of rotor and unsteady behavior of the rotor wake are taken into account. After then, the performance and geometry of tip vortex of Lynx tail rotor, vertical and radial velocities of a Maryland scaled rotor and NASA scaled rotor in ground effect are computed by the present method. Compared with the experiments and CFD results, the computed results show that the present method can capture the unsteady behavior of rotor wake and the complex flow-field of rotors in ground effect with high accuracy. Furthermore, the physical phenomena including the radial contraction and expansion of tip vortex, fountain flow, and wall jet of rotors in ground effect are captured by the present method.
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