How to model a wind turbine

Supernodes form the basis of RIFLEX models, defining the start and end points of each structural component. The supernodes given in Table 2 are required to model a wind turbine. Additional supernodes may be required to model e.g. the suport structure and foundation.

Turbine blades, eccentricity lines and the shaft are modelled as lines, where a line represents each structural component, while the properties of each line are described by a line type. Typically, one line type is needed to represent the blade lines, one for the eccentricity lines, and one for the shaft line. Only in case e.g. the blades have different properties, multiple blade line types are needed. Typical properties of each line type are outlined in Table 3.

The line type and supernode connections of each line are given in Table 4.

While location of the supernodes takes the rotor cone and shaft tilt angles into account, the prebend, sweep and twist of the blades are modelled in the line type. Prebend is modelled as Offset Z, sweep is modelled as Offset Y, and twist is modelled as Twist end1 and Twist end2. It is recommended that Offset Y and Offset Z gives the distance from the pitch axis to the reference line used to model the structural properties, while Twist end1 and Twist end2 describes the angle of the chord line used to define the aerodynamic properties, relative to the element coordinate system as defined at the blade root. An example is shown in the figure below, where the dash-dot line is the pitch axis, and the solid line represents the reference line for defining the cross section properties.

Blade mass, stiffness and aerodynamic properties are defined for the segments in the blade line type using cross-sections. The properties defined in the cross-sections are given relative to the reference line described by Offset Y, Offset Z, and Twist end1/Twist end2. Either the general cross section or the double symmetric cross section can be used. In case of the general cross section being used, the principal axis orientation is given relative to the chord line. For the airfoil properties, the coordinates of the reference point for the aerodynamic coefficients must be given. This is defined using Foil origin Y \((Y_{FC})\) and Foil origin Z \((Z_{FC})\).

An example is shown in the figure below, using the general cross section. The origin of the local coordinate system is the reference line defined in the previous section.

The shaft is modelled as a RIFLEX line with two segments, with the generator torque being applied at the interface between the two segments. The shaft segment closest to the rotor is rotating and should contain the equivalent inertia of rotating parts of the shaft and generator. Note that this must contain the inertia of both the low-speed and high-speed drivetrain components, where the inertia of the latter should be scaled by the square of the gearbox ratio. The shaft segment closest to the tower transfers structural loads from the rotor and generator to the support structure.

The hub mass and inertia are modelled using a SIMO body or nodal body attached to fore shaft node.

The nacelle is modelled as a SIMO body using a slender system connection. Any property of a SIMO body can be defined, but the most important are the mass, mass moment of inertia and centre of gravity. Note that the mass moments of inertia are given relative to the local origin of the SIMO body, not the centre of gravity. Coupling terms are therefore expected.