Final Report 4.1

UPWIND
same as in state 1, changes in geometry of the inflow occur. On the one hand the resulting
inflow shows a lower magnitude which is negligible over a wide range of operational conditions,
especially in the outer part of the rotor blades. On the other hand the inflow angle is decreased
resulting in a decreased angle of attack. Decreasing the angle of attack results in changes of
the aerodynamic coefficients as shown in Figure 2.10. In normal operation conditions a
decreased angle of attack correlates with decreased lift coefficients and therefore with
decreased lift forces. The decreased inflow angle tends to increase the sectional thrust force as
a portion of the lift force, but the influence of the decreased sectional lift force is generally larger
due to relatively small inflow angles. Of course this is only valid for the outer part of the blades,
but the influence from the inner parts of the blades is much smaller due to the much smaller
inflow velocity c. This reduction of the total thrust force Ft can be considered as an additional
force superimposed to the reference thrust force (from state 1) acting against the direction of the
tower top movement and therefore having a damping effect.
It should also be noted that the circumferential (= tangential) force dFc decreases due to the
change in the lift coefficient resulting in a lower overall torque and therefore in a lower power
output.
In state 3 the RNA shows the maximum downwind deflection but a zero velocity from the
support structure movement. The instantaneous aerodynamic inflow conditions and forces
correspond to those given in state 1 as shown in Figure 2.9. Differences in the deformed
configurations of state 1 and state 3 due to the different orientation of the rotor plane with
respect to the undeflected rotor plane are neglected.
In state 4 the tower top shows no displacement relative to the undeflected configuration while
the velocity �V against the wind direction is at the maximum. Again, the wind speed
experienced by the rotor is changed due to the tower top movement and the relative wind speed
results from the superposition of the wind speed V2 and the speed of the RNA �V. The relative
wind speed therefore is Vrel = V2 + �V increasing the inflow angle and the angle of attack as
shown in Figure 2.11. By the increased angle of attack the corresponding lift coefficient also
increases resulting in a larger sectional lift force. Although the increased inflow angle � tends to
lower the sectional thrust force, which is a portion of the sectional lift force, the resulting
sectional thrust force increases since the influence of the increased lift coefficient is generally
larger due to relatively small inflow angles (compare state 2). This leads to an increase in the
total thrust force Ft which can be considered as an additional force superimposed to the mean
thrust force (from state 1 or 3). The additional force is acting against the direction of the tower
top movement and therefore has a damping effect.
Figure 2.11: Aerodynamic condition at the reference blade section in
state 4 (upper) and resulting angle of attack and aerodynamic
coefficients (lower) [5]
Page 27 of 146