ECN: Bladmode

ECN

The BLADMODE program

The design of large wind turbine rotors deals with aerodynamic, but also with structural dynamic aspects/choices and with operational states of a wind turbine. The latter includes amongst others ‘peak-shaving’ strategy or a rotor speed range to avoid resonant frequencies. The design of large rotor blades is not only driven by aerodynamic performance, but also by e.g. reduction of the induced velocity, positive aerodynamic damping, resonant-free operation. Bladmode is a tool that facilitates design choices and evaluation of either the aerodynamics or structural dynamics, or the combined state of a wind turbine rotor. Bladmode can already be used in the early design phase due to the fact that doesn’t need a controller.

Description of BLADMODE

Bladmode is an eigenmode analysis tool, which can be used to solve the quasi-steady operational state of a wind turbine for a given (series of) wind velocity. This quasi-steady state is in terms of aerodynamic loads (BEM or Vortex-Wake), blade deformation (incl, torsion), rotor-speed and pitch angle, that describe an equilibrium state.

The code contains several ‘flags’ or ‘switches’ by which different aerodynamic conditions can be set (or switched off) and by which one may choose the detail of structural dynamic modeling. With these ‘flags’ Bladmode can be used to calculate eigenmodes and frequencies of a vibrating beam, and to design e.g. the blade tip shape using a vortex-wake model. Using all options, Bladmode analyses the rotor properties up to the aerodynamic damping of each of the rotor eigenmodes for the operational range of wind velocities.

Figure 1: Collective edgewise mode of a large size rotor

Bladmode will analyse a power curve without needing a controller, using a table with the quasi-steady relation of the pitch angle as function of rotor speed. This together with all ‘flags’ for parts of the algorithm makes Bladmode very suitable as tool for design steps/choices in the early design of a wind turbine rotor. By predicting rotor eigenmodes and with the ability to design the ‘peak-shaving’ strategy, Bladmode can be used to provide the wind turbine properties that are needed for controller design.

Figure 2: Campbell diagram with rotor frequencies

Relation with other design tools

Because Bladmode was initially developed as a tool for the prediction of rotor blade eigenmodes it has a very detailed model for structural dynamic deformation. This includes amongst others blade torsion (can be switched ‘off’), pre-bend, non-linear bending, bending-twist coupling terms. In Bladmode the interaction with the turbine dynamics is limited to the tower fore-aft bending deformation, and drive train dynamics in terms of shaft torsional deformation and variable-speed generator characteristics.

Eigenmode analysis of a complete wind turbine can be performed with the more extended tool TURBU. TURBU requires a more detailed description of the wind turbine and applies to a later design phase of a wind turbine. 

The BLADMODE code is also available in the wind turbine design package FOCUS. As module of FOCUS, the input for BLADMODE is obtained from a database which is also used by the other wind turbine analysis tools such as PHATAS and TURBU. The implementation in FOCUS includes 3D visualizations of the eigenmodes. {link FOCUS}

The detailed structural dynamic input properties that can be included in the BLADMODE input can be obtained with the ECN program CROSTAB. CROSTAB has a structural model of a rotor blade modelled as prosmatic beam, of which the input is in terms of layers of orthotropic material (as fiber-reinforced composites). The program CROSTAB also analyses the torsional stiffnesses and bend-twist coupling of anisotropic rotor blades. When using BLADMODE in the design package FOCUS, the detailed structural properties of the blades can be analyzed with the blade modeling tool FAROB.

Workshop on the use of BLADMODE

ECN/EWIS offers a workshop on aeroelastic design of wind turbine rotors. This workshop starts with simple design issues such as Eigen frequencies of a vibrating rotor blade and the aerodynamic analysis of the flow over the blade tip. Next design issues are treated such as the choice of the ‘peak-shaving’ pitch strategy and about resonant frequencies of the rotor. Finally the aeroelastic analyses of rotor vibrations are performed for the entire range of operation, with sensitivity analyses of several aspects.

In this workshop Bladmode is used because it is both fast and can also include a lot of details that can be switched on/off aiming to investigate/demonstrate several effects.

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