Environmental Engineering Reference
In-Depth Information
Response to Wind Shear
Wind shear loadings, both vertical and horizontal, can impact the design of blades when
the hub is hingeless. However, the use of a teetered hub reduces wind shear effects to an
insignificant level. The teeter angle amplitudes caused by wind shear are usually only a
few degrees, and present no design problems such as lack of clearance between the blade
and the tower. On a stall-controlled rotor with a hingeless hub, wind shear in high winds
can be an aggravation when one blade is stalled and the other is not. The seriousness of
this problem is reduced considerably if the stall-controlled rotor is also teetered.
Response to a Tower Shadow
Tower shadow (
i.e.
the wake behind the tower structure in which wind speed is reduced
and turbulence is increased) is a particularly severe excitation source for a downwind
HAWT rotor. In a VAWT, the excitation of the downwind blade is much less severe
because of the central column is usually much smaller in diameter than a HAWT shell
tower and the column-to-blade distance at the rotor equator is a relatively large number of
column diameters. Like wind shear, tower shadow responses are high in a hingeless HAWT
rotor, but are not design drivers when the rotor is teetered. However, tower shadow loads
may still design the outboard shell structure of the blade airfoil, even with teetering. As
discussed in Chapter 7, the sudden change in air loading on a blade as it enters and leaves
the tower shadow can create an acoustic noise problem that is severe enough to warrant
abandonment of the downwind rotor location entirely.
Rotor blades operating upwind of a cylindrical tower experience a
bow wave
effect not
unlike a tower shadow, but much less severe.
Potential flow theory
is adequate for
simulating the wind speed disturbance ahead of the tower.
Response to Gusts
Once a turbine design has been optimized to the point of reducing or eliminating large
dynamic responses such as those from tower shadow, gust responses become the design
drivers. If a control system is designed to “clip” the effects of a gust on peak power, it will
at best sacrifice energy capture and may worsen transient thrust loads. It is much more
desirable to avoid a control response to a gust and substitute a rotor speed response instead.
If this is done, the largest gust load responses will still occur, but there will be many fewer
of them in the fatigue load spectrum. It should not be a significant burden to design for
long fatigue life under the action of gusts, if all the opportunities for passive attenuation of
the machine's response to air load transients are grasped.
In accordance with Equation (5-13), a wind turbine rotor slows the wind as it extracts
power, by a fractional amount called the
axial induction factor.
At the turbine design point,
the wind may be retarded by one-third of its free-stream velocity, and at higher wind speeds
the induction factor may approach unity (see Fig. 5-11). Retardation by the rotor, however,
takes time to develop and as such should be viewed as a
quasi-steady state phenomenon
that is appropriate for predicting power performance. For a transient control or load
simulation, however, retardation is usually neglected on the theory that there is insufficient
time for it to develop during the period of the gust. The blades are assumed to feel the full,
unretarded wind speed transient.
Failure to eliminate retardation effects on wind transients
in a dynamic simulation can be a large factor in underestimating gust loads.
Search WWH ::
Custom Search