Structural dynamics

Bladed utilizes a completely self-consistent and rigorous multibody formulation of a wind turbine’s structural dynamics.

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Bladed utilizes a completely self-consistent and rigorous multibody formulation of a wind turbine’s structural dynamics. This provides consistently reliable and accurate results and forms a solid foundation from which to extend the structural model with many new features in the ongoing development programme.

Multibody dynamics
The Bladed structural dynamics code is based on a flexible multibody dynamics approach, similar to the approach defined by Shabana [1].

The multibody system allows various flexible and rigid bodies to be connected in an arbitrary tree-like structure.

This flexible and powerful approach enables you to easily determine many pre-defined turbine configurations in Bladed. The dynamic response of novel systems can be confidently predicted, as the behaviour of each component and the coupling between them is thoroughly validated.

Modal analysis and Campbell diagram
Bladed includes structural flexibility for wind turbine blades, support structure and drive train. Mode shapes and frequencies are calculated for each flexible body using the Craig-Bampton method [2]. 

Modal analysis is regularly performed for the tower, whereas the blade can use vibration modes or direct integration of the finite element degrees of freedom. Modal reduction facilitates improved simulation speed without significant loss of accuracy.

The modes from each component are coupled through the multibody code. Bladed can calculate the coupled modes in the steady state for the whole structure. These coupled frequencies can be compared to the rotor rotational frequency in a Campbell diagram which checks for possible resonance issues. This calculation also derives the coupled modes damping and the contributions of the blade and tower modes to each coupled mode.

Non-linear blade dynamics
Each flexible body in Bladed is a linear finite element body. Very flexible wind turbine blades can be split into multiple flexible bodies to achieve a geometrically non-linear model of blade deflection. This includes outer parts which can undergo large rotations relative to the blade root. This “multi-part” blade approach is key to analyse the stability and dynamic response of large modern wind turbine blades.

The model has been recently validated against full-scale measurements from the GE 6MW Haliade turbine [link to DNVGL paper]

Blade stability analysis
Flexible modern wind turbine blades may be susceptible to instability at high rotor speeds. Bladed’s blade stability tool [link to module page] creates a linearized model at a large range of input conditions for a rotor in power production or parked configurations.

During power production, the damping of the blade modes with increasing wind speed is evaluated, either at a fixed TSR or in a free-spin scenario. The rotor speed where the instability occurs and the vibration modes that contribute to the instability can be identified, providing key insight into the cause of the instability.

Offshore structural models
Bladed includes well-developed structural dynamics models for modelling offshore wind turbines, including support for:

  • Integrated jacket modelling.
  • Jacket super-element import.
  • Floating turbines with explicit and simplified mooring line models.

For further details see the offshore modelling page [link to offshore page]

Pitch and yaw actuator models
Physical devices, such as pitch drives, are also modelled using Bladed’s multibody dynamics framework. Bladed comes pre-loaded with an array of detailed and versatile models of pitch and yaw actuation.

Pitch actuators can be modelled using a characteristic response time or by defining gains for implementing a torque feedback loop. Models are provided for linear or rotary actuators, and various other devices, such as limit switches and end stops.

Bladed also supports pitch actuator modelling through an external DLL interface, as part of the Advanced Pitch Actuator module. [link to module page]

Drive train modelling
Geared and direct drive turbines can be modelled as a 1 degree of freedom system. These can be enhanced by LSS and HSS flexibility as necessary and a slipping clutch, which is also directly included in the multibody structural dynamics framework.

For more detailed drive train modelling, Bladed can be coupled to an external drive train model via a DLL interface. This approach can allow the complex behaviour of a detailed drive train model to be coupled to the dynamics of the rest of the turbine.

The DLL interface supports non-linear and time-varying, models and supports discontinuities, for example, frictional stick-slip and backlash. The gearbox DLL interface can be activated with torsional degrees of freedom only or with a full 6 degrees of freedom giving a complete dynamic model.

The gearbox DLL functionality is available in the Bladed Advanced Transmission Interface module. [link to module page]

References for structural section: 
1. Shabana, A “Dynamics of multibody systems”
2. Craig, R. “Coupling of substructures for dynamic analyses: an overview”, AIAA-2000-1573