  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 5MW 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 current investigation aims to develop insight into
rotorcraft tip vortex kinematics and stability to work towards quantifying
fatigue loading and its effects, minimizing adverse blade-vortex interaction
effects, such as excessive noise emission and large blade vibrations also
affecting fatigue performance.
