Turbine data: Blades

Blade count

The number of blades.

Pre-cone angle

The angle the blade $z$-axis makes with the turbine frame xy-plane; a positive pre-cone rotates the blade $z$-axes away from the turbine frame $z$-axis.

Pitch control mode

One of the following values

Initial pitch

The initial angle between the blade's fitting frame, which is fixed relative to the hub, and its root frame. This pitch angle can subsequently be altered by a pitch controller during the dynamics simulation.

For a conventional turbine, with clockwise rotation sense, a positive pitch rotates the root frame anticlockwise about the blade $z$-axis relative to its fitting frame.

For a turbine, with anticlockwise rotation sense, a positive pitch rotates the root frame clockwise about the blade $z$-axis relative to its fitting frame.

For common pitch control mode, the initial pitch is specified for the first blade, and all other blades use that value. For individual pitch control model, the initial pitch is specified for each blade.

Pitch controller

Specifies the external function (if any) that controls the blade pitch in dynamics (by returning the pitch angle and its first two derivatives). If none, then the pitch is constant and remains at the initial pitch throughout the simulation.

A guide to using external functions to model turbine controllers is available at https://github.com/Orcina-Ltd/turbine-controllers. This includes example Python controllers and an example of using external functions to wrap a Bladed style control DLL.

Notes: For a conventional turbine, with clockwise rotation sense, the sign convention for blade pitch, for which a positive value defines an anticlockwise rotation about the blade $z$-axis, looking from root to tip, is contrary to the usual OrcaFlex convention. For a turbine with an anticlockwise rotation sense, the sign convention for blade pitch is consistent with the usual OrcaFlex convention.
If the imposed motion consistent with solver option is checked (implicit integration scheme only), then the pitch controller need only return the pitch angle. The returned derivatives will be ignored and instead will be determined by the solver.

Blade DOFs

Determines whether or not the blade nodes have calculated degrees of freedom (DOFs). If blade DOFs are fixed, each blade is represented by a rigid body. If they are free, then the calculation will include the six DOFs (three translational and three rotational) of each node, thus allowing the blades to deform. To model this deformation, you will need to specify the blade profile stiffness and inertia data.

Rayleigh damping coefficients

A named Rayleigh damping coefficient data set. This value may also be set to '(no damping)', in which case no Rayleigh damping will be applied to the blades.

This item is not available when the blade DOFs are fixed, nor when using the explicit integration scheme.

Mass proportional Rayleigh damping acts to damp out rigid body motions. Therefore, it is inappropriate for a turbine blade and the mass proportional coefficient must be set to zero.

Blade section count

The number of sections used to represent the blade. These are defined sequentially from root to tip, and the following data are given for each section.

Section length

The unstretched length of the section.

Target segment length, number of segments

These determine the segmentation of the section.

If the target segment length is set to '~', then the number of segments in the section is set according to the number of segments.

Otherwise, the segmentation is based on the target segment length, and the number of segments value is not editable: it reports the actual number of segments used, calculated as (section length / target segment length) rounded to the nearest integer.

The properties of each segment are determined from linear interpolation of the blade profile, evaluated at the mid-segment arc length.

Wing type

The wing type used to represent the aerofoil for this blade section. The lift, drag and moment coefficients used in calculating the aerodynamic load on the segment are obtained by interpolating the wing type data.

Cumulative length, cumulative segments

These columns report the cumulative length and cumulative number of segments, counting from the first section. The values are for reporting purposes only and cannot be edited.