Environmental Engineering Reference
In-Depth Information
time t, the following logic has been assumed to take the decision about the optimal
status (''on'' or ''off'') to be set for the dampers in that instant:
if
j
r
ðÞj
\r
lim
!
i
ðÞ¼
i
max
ð
13
:
41
Þ
if
j
r
ðÞj
r
lim
and x
ðÞ
j
j
\x
lim
!
i
ðÞ¼
0
ð
13
:
42
Þ
j
x
lim
and x
ðÞ
X
ðÞ
[ 0
!
i
ðÞ¼
i
max
if
j
r
ðÞj
r
lim
and x
ðÞ
j
ð
13
:
43
Þ
j
x
lim
and x
ðÞ
X
ðÞ
0
!
i
ðÞ¼
0
:
if
j
r
ðÞj
r
lim
and x
ðÞ
j
ð
13
:
44
Þ
In other words, the controller keeps ''stiffer'' the base restraint until the stress
does not exceed the limit value (Eq.
13.41
), whereas ''relaxes'' it (switching off
the MR dampers) when this limit is passed and the displacement falls within the
limits (Eq.
13.42
). When both stress and displacement are beyond the respective
threshold values, the controller switches on the dampers if the displacement is
going in the direction of a further increase (so trying to invert or at least to damp
this trend; Eq.
13.43
), otherwise it switches off the MR devices to make sure they
do not hinder the ongoing reduction of displacement (Eq.
13.44
).
Figure
13.15
graphically describes this algorithm, in a schematic way, showing
what is the decision of the controller (switch on or switch off) depending on the
occurrence of each of the four above-mentioned possible combination about the
state of base stress and top displacement.
It is worth noting that the practical use of such control algorithm require a
preliminary calibration through properly setting the three involved parameters, i.e.
i
max
, r
lim
and x
lim
.
13.6 Experimental Activity and Results
Two load cases were considered:
• an extreme operating gust (EOG), i.e., a sharp increase, then decrease in wind
speed within a short period of time;
• a high velocity wind buffeting, i.e., a load case (called ''parking'', PRK) that
typically concerns a wind turbine when ''parked'' (with a controlled shut-
down) due to the high-velocity wind.
Chen and Georgakis [
12
,
13
], for both load cases, have defined an equivalent
base acceleration time history (Fig.
13.16
), that is the base input that would give
the same top mass response of the real, fixed base structure subjected to the wind
action. This kind of analysis has been made using the wind turbine aeroelastic code
HAWC2 (Horizontal Axis Wind turbine simulation Code, second generation),
realized at the DTU (Denmark) for calculating wind turbine response in time
