Vortex induced vibrations of wind turbines: From single blade to full-rotor simulations

Description

Vortex induced vibrations (VIV) of wind turbines in standstill are a complex problem. They can currently only be simulated using computationally expensive methods involving computational fluid dynamics (CFD) solvers.

To reduce the complexity, initial studies have focused mainly on rigidly clamped single blades. These studies can either be performed using fully-coupled fluid structure interaction (FSI) methods or forced motion, where vibrations in given mode shapes, typically the first edgewise mode, are prescribed in a CFD solver.

Even these single blade studies are expensive, due to the large parameter space formed by the inclination angle, which determines the spanwise flow component, the pitch angle, which determines angle of attack, and the wind speed. In addition to this, the existence of multiple limit cycle amplitudes (Grinderslev et al., 2022) requires forced motion simulations at a large range of amplitudes or FSI simulations with varying initial excitations.

Due to limited experience with wind turbine VIV it is difficult to determine if the blade-only analysis of these vibrations gives a conservative estimate of the VIV risk. Therefore, full-rotor simulations are necessary which increase the necessary cost considerably.

In a full-rotor configuration with blades pitched by roughly 90 degrees, three structural modes become relevant instead of the first edgewise blade mode: the first yaw, tilt and symmetric mode of the rotor. These modes will have different frequencies, which means that the most critical wind speeds for each mode may be distinct for each mode according to simple Strouhal number considerations. Further, the tip amplitudes of a given blade vary with azimuth angle for the different modes, especially the asymmetric yaw and tilt modes. This also means that the energy dissipated by structural damping for a given blade tip amplitude will vary considerably with azimuth angle and vibration mode.

The talk aims to illustrate the increasing complexity and computational expenses when moving from single blade to full rotor VIV simulations. Initial results for the IEA 10 MW reference turbine will be shown, see Figure 1. A case will be presented where the power injection begins on one blade, exciting a full rotor mode. At a given amplitude, another blade starts injecting power as well, ultimately leading to a critical limit cycle amplitude of 17 meters.

Period	23 May 2023
Event title	Wind Energy Science Conference
Event type	Conference
Conference number	4
Location	Glasgow, United KingdomShow on map
Degree of Recognition	International