Modal analysis is the process of determining the modal parameters, which are then sufficient for formulating a mathematical dynamic model.
Modal analysis can be accomplished through experimental techniques. It is the most common method for characterization of the dynamic properties of a mechanical system. The free dynamic response of the wind turbine blade can be reduced to a discrete set of modes.
- The modal parameters are:
- The modal frequency
- The modal damping
- The mode shape
Figur 1 Test setup
Within the frequency range of interest the modal parameters of all the modes constitute a complete dynamic description of the wind turbine blade. The modes of vibration represent the inherent dynamic properties of the wind turbine blade. The range of applications for modal data includes:
- Checking the modal parameters.
- Verifying and improving analytical models Making Computer simulations.
The modal analysis is performed as a system analysis where you have an input and an output signal.
Because the wind turbine blade is a large structure (length >20m) it is necessary to treat the blade in cross sections successively. Exciting the blade by a hammer impact at a fixed point during the test performs the modal analysis of the wind turbine blade.
Figure 2 Frequency Response Function
The hammer impact represents the input signal to the system. The excitation energy is concentrated in the frequency band of interest, typically in a range between 0.5 Hz to 25 Hz. The output signal consists of accelerations measured in cross sections along the blade. At every cross section the acceleration is measured by three accelerometers in order to determine the flap- and edgewise properties of the blade. Furthermore the torsion is determined.
Figure 2 shows the frequency response from three accelerometers at one cross section. The peaks indicate the natural frequencies in different directions of the blade. Modal analysis is offered as a service on blades in test at BLAEST, or as an on-site service that can be carried out on blades mounted on turbines.