Stability of helical vortex structures shed from flexible rotors

Abstract

The presented investigation is motivated by the need to uncover connections between underlying rotor fluid–structure interactions and vortex dynamics to fatigue performance and characterization of flexible rotor blades, their hub, and their supporting superstructure. Towards this effort, temporal stability characteristics of tip vortices shed from flexible rotor blades are investigated numerically. An aeroelastic free-vortex wake method is employed to simulate the helical tip vortices and the associated velocity field. A linear eigenvalue stability analysis is employed to quantify stability trends (growth rate v. perturbation wavenumber) and growth-rate temporal evolution of tip vortices. Simulations of a canonical rotor with rigid blades and its generation of tip vortices are first conducted to validate the stability analysis employed herein. Next, a stationary wind turbine is emulated using the National Renewable Energy Laboratory (NREL) 5-MW reference wind turbine base design to investigate the impact rotor aeroelasticity has on tip-vortex stability evolution in time. Blade flexibility is shown to reduce the sensitivity of tip vortex destabilization to low wavenumber perturbations, also blade-pitch reduces growth-rate magnitude and alters the growth-rate peak dependence on perturbation wavenumber, all of which have in the past not been reported in the rotorcraft literature. The presented investigation aims to develop insight into the tip vortex kinematics and stability of the NREL 5-MW reference wind turbine. However, the frameworks presented herein can be applied to generalized rotor designs to work towards identifying the impact tip-vortex kinematics and stability have on fatigue loading and adverse blade–vortex interaction effects, such as excessive noise emission and rotor vibrations.