Modeling Rotor Wake Dynamics with Viscous Vortex Particle Method

Rotor-induced-flow modeling and prediction has been one of the central issues for rotorcraft performance, control, stability, loads, and vibration analysis for decades. Traditional singularity-based methods used in most current comprehensive rotorcraft analysis codes are limited by the potential flow assumption and thus have to rely on empirical formulations (e.g., vortex decay factor, vortex core size, etc.) to reach a solution. This paper discusses the development and validation of a viscous vortex particle model for modeling the complicated rotor wake vorticity transportation and diffusion. Instead of solving the viscous vorticity equations through numerical discretization over the flowfield grid, the vortex particle method addresses the solution through a Lagrangian formulation in which there is no artificial numerical dissipation involved. The Lagrangian approach also allows the application of the hierarchical TreeCode and the fast multipole method. These methods can dramatically improve the computational efficiency of the viscous vortex particle simulation, which enables it to be used for practical and comprehensive rotorcraft analysis.