Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 498

response to a gust. But it is possible to considerably increase the aerodynamic
damping and to decrease the peak load.
The measuring of infl ow could increase the load alleviation potential of the
concept. Then so-called collocated control [114] is possible, where a local fl ow
sensor directly coupled to a local control surface keeps the local aerodynamics
constant. However, a global control system must be installed too to make sure that
the ultimate goal on the system, reducing the load fl uctuations, is assured. Keeping
the aerodynamic load at certain stations constant does not assure that, because not
all stations can be controlled and because non-aerodynamic loading on the blade
exist, e.g. wave loading on the tower for offshore turbines. Including infl ow sen-
sors will complicate the control system and it can be questioned whether acting
solely on the structural response is not already suffi cient to attain a satisfying level
of load reduction.
To answer this question, it must be researched which part of the load spectrum
is dominant: are the loads dominated by quasi-static components or is the turbu-
lence exciting the dynamics of the blade? MacMartin [114] makes the same
distinction: he calls the mitigation of load fl uctuation due to forced excitation
isolation, whereas the suppression of dynamic modes is called active damping. In
the latter case, basing the control system on the structural response, possibly with
feedforward control on deterministic components of turbulence, will suffi ce. For
issues concerning the control algorithms, please refer to [115].
5.4 Rotating experiments
These control issues are also being addressed in a second series of experiments in
which an actual blades equipped with fl aps is tested on a small turbine. This will
allow for the study of the effect of rotationally induced disturbances, such as yaw
misalignment, as well as the interaction between multiple blades and between the
rotor and the wake. In addition, some design enhancements are made.
Blade design and manufacturing was done similar to the non-rotating experi-
ment, except that the blade was made out of one piece and the dynamic scaling was
performed with respect to the ratio between the rotational and the eigenfrequency,
not the reduced frequency. This was done because from the non-rotating experi-
ment it was concluded that the proximity of natural modes to parts of the distur-
bance spectrum has an infl uence on the dynamics loading of the blade. However, the
reduced frequency for the model is higher than with the reference blade, and thus is
the aerodynamic delay. A photo of the fi nished blades can be seen in Fig. 23 . As you
can see, the blade has twist and tapper, but a straight tip. This is to facilitate the
installation of the actuators.
In this experiment, more attention was given to the dynamic behavior of the
blade. In harmonic analyses, the transfer between fl ap excitation and stresses at
different points on the blade was calculated in order to determine the right place-
ment of sensors. The sensors were placed at locations where the normal stresses
for a given excitation were relatively high and a safe phase margin existed. Both
piezoelectric MFCs as well as strain gages are adhered to the blade. At the center
of the fl aps accelerometers are build in. The accelerometers record both in and out