Rotor unsteady aerodynamics model using an efficient free-vortex method

Purpose ‐ The purpose of this paper is to present and validate an efficient time-marching free-vortex method for rotor wake analysis and study the rotor wake dynamics in transient and maneuvering flight conditions. Design/methodology/approach ‐ The rotor wake is represented by vortex filament elements. The equations governing the convection, strain and viscous diffusion of the vortex elements are derived from incompressible Navier-Stokes equations based on the viscous splitting algorithm. The initial core size of the blade tip vortices is directly computed by a vortex sheet roll-up model. Then, a second-order time-marching algorithm is developed for solving the governing equations. The algorithm is formulated in explicit form to improve computing efficiency. To avoid the numerical instability, a high order variable artificial dissipative term is directly introduced into the algorithm. Finally, the developed method is applied to examine rotor wake geometries in steady-state and maneuvering flight conditions. Comparisons between predictions and experimental results are made for rotor wake geometries, induced inflow distributions and rotor transient responses, to help validate the new method. Findings ‐ The algorithm is found to be numerically stable and efficient. The predicted rotor responses have good agreement with experimental data. The transient behavior of the wake dominates the rotor responses following rapid control inputs in hover. The wake curvature effect induced by rotor pitching or rolling rate significantly changes the rotor off-axis response. Research limitations/implications ‐ This method should be further validated using experimental measurements of full-scale helicopter rotors. Originality/value ‐ The paper presents a new time-marching free-vortex wake method, which is suitable for application in helicopter flight simulation.