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HAWT Blades

Only one blade has to be defined since all blades are identical. You may define a set of section along the blade span in the table shown. The total length of the blade line is calculated with TipRad and HubRad given in Turbine sub-tab: Blade_Length = TipRad HubRad.

Blades properties can be defined in a regular way, where the user inputs the data himself, or in an encrypted mode where the user selects an encrypted blade file to load.

Damping

Structural damping parameters are entered to account for structural damping due to the blades material. Structural damping is based on the Rayleigh viscous damping model, where the achieved damping rate depends on the frequency of the dynamic deformation. Details about the Rayleigh damping formulation used are presented in the Theory section.

  • Coefficient type: Select either "Beta" or "Actual rate Ksi". Ksi refers to the actual damping rate (i.e. damping value over critical damping value ratio). Enter for instance 0.01 rate if you wish to set a damping equal to 1% of the critical damping. The damping coefficient considered in the dynamic analysis is eventually defined through the Beta coefficient which depends on the structural damping rate Ksi and on a given frequency as follows as shown below.

    \[ \beta = \frac{2\xi}{\omega} = \frac{\xi}{\pi f} \]

    \(\xi\) coefficients are automatically switched into \(\beta\) coefficients by considering the wave period (i.e. regular wave period or peak period with irregular waves) since the wave and resulting floater motion is expected to be the main source of motion for the line.

  • Rayleigh: With this option, a general damping coefficient will be applied to any material deformation; tension, bending and torsion.

  • By mechanism: With this option, a damping coefficient will be specified by the user for each type of deformation; tension, bending or torsion.

Added mass: A reference to a variation table defining added mass for blades. Only variation tables with 4 columns are available through the combo box control. An example of such a variation table is shown below.

Number of sections : An edit box for the number of sections to be defined along the length of the blade line element. The default value is 25.

The table in the lower part of the sub-tab requires you to add properties for each of the sections:

  • Length: Used to define the length of each section.

  • Chord: Defines the chord of the airfoil element.

  • Structural twist along blades: Used to define the twist along the length of the blade (principal axis of the stiffness).

  • Aerodynamic pitch: Used to define the pitch along the length of the blade (for angle of attack).

  • Profile: This is the name that is used to refer to the profile.

  • IP & OP prebend: Used to define the in-plane and out-of plane prebend.

  • Dry weight: This is the weight of the blades, per unit length.

  • EI flap & EI edge: Defines the bending stiffness of the blades.

  • GCT: Defines the torsional stiffness of the blades.

  • EA: Defines the axial stiffness of the blades.

  • Inertia flap & Inertia edge: Defines the inertia of the blades.

  • Torsional inertia: Defines the torsional inertia of the blades.

  • Aero center: Defines the position of the aerodynamic center.

  • COG: Defines the local coordinates x & y of the COG for each section.

  • Mass axis orientation: This is the mass axis orientation to consider for the blades.

  • Shear center: This is the shear center position.

  • Flap and Edge shear stiffness: defines the shear stiffness of the blades.

Encryption

In the encrypted mode, the blades properties (described above) will be hidden. This mode is activated by clicking on the Encrypted mode checkbox and by selecting a valid encrypted blade profile file to load.

A second click on the Encrypted mode checkbox will disable the encrypted mode and will create new default sections.

The default mode is the non encrypted mode.

Once an encrypted blade file is loaded and the HAWT edit dialog box is closed by using the OK button, the blades properties will not be accessible from the GUI.

As shown below, the blade line type is accessible any longer.

Simplified Blade Model:

Rigid Blade

Based on development performed for using DLW as a software in the loop to model turbine wind loads on a floater in tank test, simplified approaches are now available with a gain in computational time considering the blades either rigid or responding on their eigenmodes.

A more efficient option than the “rigidification” is to use the new loading “Simplified turbine”, that is defined on the reference point for the turbine’s hub as indicated in Figure "Rigid blade option". When this object is added to an analysis where the turbine is present, the blades are considered rigid in the simulation.


Rigid blade option

Blade Modal Basis

Response of the blades on their modal basis in time domain is also possible. A first step is to create a modal database as shown in Figure "Generation of the blade modal database". By going to tool and selecting “Generate Blade’s modal file” the user can request the creation of a “modal” file.


Generation of the blade modal database

Then the user selects the turbine as well as the number of modes. Clicking on “Generates modes” will create the file. It is also possible to edit the .json file (containing the blade data), the log file (used to generate the mode file) and check the resulting modes. In the mode file, the eigenfrequencies are saved as well as the modal deformation along the blades for the six directions.


Launch the mode generation

The file is initially stored as “Line_Blade_modes.txt “ in the directory “Name_Blade_GenModes” where “Name” is the turbine object name. The file can then be moved anywhere.

After creating the mode file, the user can then define a displacement as for rigid blades but this time with the option “Flexible blades” (see Figure "Flexible blade (modal) option"). Then the path to the file and the file named of the mode file is provided. Finally the total number of modes to use as well as the individual mode number to extract from the database is defined together with the modal damping ratio (0.02 stand for 2%). Note that to help mode selection, the file “modal_ratio.dat” created with the mode file provides information on the direction that responds the most (ratio at 1) per direction and mode. For instance


Flexible blade (modal) option

Note that to help mode selection, the file “modal_ratio.dat” crated with the mode file provides information on the direction that responds the most (ratio at 1) per direction and mode. For instance

DOF Ratio
1 1.00000
2 0.08576
3 0.00000
4 0.00102
5 0.00941
6 0.00000

indicates that mode 1 responds mainly in the X direction (direction with respect to blade root frame without pitch control).