Structural Dynamic Behavior of Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 639

Figure 11-8. Finite-element model of the 2-m VAWT. [Carne et al. 1982]
Non-Rotating System Modes
Table 11-1 lists the frequencies of the first ten modes of vibration on the non-rotating
( i.e. , parked) 2-m VAWT, with its rotor brake on. The first column gives the mode
number, usually assigned in increasing order of frequency. Mode identification terminology
is given in the second column. The corresponding mode shapes, in three views, are
sketched in Carne et al. [1982]. The third column lists the frequencies measured with the
rotor parked, and the fourth column gives the frequencies calculated using initial estimates
of the support mass and stiffness properties. Again, these estimates are the data in
parenthesis in Figure 11-8. In the fifth column of Table 11-1 are the “tuned” calculations
of the modal frequencies for the non-operating condition. The last column in the table
gives the relative changes in frequency resulting from tuning the model.
Tuning the Finite-Element Model
The differences between the initial and tuned frequencies, ranging from -20 to +8 per-
cent, indicate the scale of the errors that can be present when (1) the support system and
the resulting theoretical boundary conditions are complicated, and (2) some of the auxiliary
components in the structural system are not modeled in detail. For practical reasons, the
latter is usually the case. As a result, a finite-element model must be calibrated against a
basic set of measured responses, on the same or a similar structure, to establish confidence
in predictions made with it of natural frequencies under operating conditions. Modal
analysis is clearly recognized to be a combination of theoretical and experimental methods.