Environmental Engineering Reference
In-Depth Information
Fig. 13.1 Basic idea of SA
control of a wind turbine via
MR dampers
(a)
(b)
(c)
m
m
m
k , c
T
k , c
k , c
T
T
T
T
T
ct
( )
k
φ
φ
ct
( )
ct
d
( )
k
s
k
s
d
l
l
d
s
(damping constant c
/
, variable in time) externally driven in real time by a control
algorithm. The same concept can be practically implemented by means of two
vertical linear springs (k
s
) installed at a given distance (l
s
) from the hinge and two
vertical variable dampers (c
d
), each placed at the distance l
d
from the hinge
(Fig.
13.1
c).
MR dampers are suggested to be adopted as variable devices within this setup.
At macro scale, MR fluids behave as semi-solid bodies as far as the shear stress
does not exceed a threshold, whose value depends on the intensity of the magnetic
field the fluid is immersed in, the latter depending in turn on the current intensity in
coils placed around the fluid. This behavior is associated to the nature of the fluid,
a suspension of micron-sized magnetizable particles. Upon application of a
magnetic field, particles align and form fibrous aggregates that generate orders-
of-magnitude viscosity changes within characteristic times of milliseconds.
When the base damping is set to a low value, the tower is able to relax, hence to
convert its potential energy into kinetic energy and to reduce the bending moment
at the base. Thus the SA base system implies stress reduction, even at the cost of a
possible increase of top displacement, which has to be bounded within certain
limits to control the top displacements. The springs allow to reset the tower to the
initial position at the end of a severe wind-induced excitation.
This idea has been physically realized and tested at the laboratory of the
DTU (Copenaghen) by using prototype MR dampers provided by Maurer Söhne
(Germany). The results gained from this campaign will be presented and discussed
in the following sections.
13.3 Experimental Setup
A 3 MW wind turbine with horizontal power transmission axle has been assumed
as reference structure for the experimental campaign performed at the DTU lab-
oratory (Fig.
13.2
). The tower is 102.4 m tall, made of steel Q345 (modulus of
