Environmental Engineering Reference
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for the baseline condition of zero wind and rotor speed. There is a set amount of
rotor “overhang” (OH 1 ) and the blades are positioned within a vertical rotor plane.
Sketch /B/ depicts an extreme load case for the swept surface of the rotor plane under
operational wind conditions. Clearly this scenario is not acceptable, as the blades
will interfere with the tower. In an attempt to gain clearance by moving the rotor
forward, sketch /C/ shows that even with signifi cant increase of the overhang dimen-
sion OH 2 , the minimum clearance is still not acceptable. Increasing overhang is very
costly as it requires signifi cantly more mass in the machine head (MH) structure to
support the rotor mass and operational loads due to the longer lever arm.
Sketch /D/ shows the same starting assumption as /A/ with the rotor in a vertical
plane. Various types and amounts of “zero wind” geometry are depicted for
sketches /E/ through /H/:
￿
/E/ - Blade prebend, an amount of forward reaching curvature characterized by
the axial dimension measured from the blade tip vertical plane to the rotor cen-
tre vertical plane. Typical values are 1
3 m (larger values are anticipated as
rotor diameters continue to increase).
/F/ - Blade cone, an amount of forward reaching cone characterized by the cone
￿
angle measured between the normal vector at the blade root attachment and the
rotor centre vertical plane. Typical values are 2
6°.
/G/ - As typically the case use a combination of blade prebend and cone.
￿
￿
/H/ - Tilt the rotor and the drivetrain by a small angle relative to the horizontal
plane. Typical values are 2
6°.
Using rotor /H/, sketch /I/ depicts a return of the rotor overhang to the more
economical OH 1 value. Extreme loadings for this arrangement result in sketch /J/.
Since the blade-tower minimum clearance exceeds 1/3 the static clearance, this
combination of design parameters provides a solution to the OH 1 constraint. All
new turbine concepts must go through this type of optimization process, and be
revisited throughout the structural design phases to ensure loads and defl ections
are accommodated.
4.2.4 WT system modelling for determining design loads
There are a number of WT structural calculation programs currently being used by
WT designers. These include readily available models using commercial fi nite ele-
ment analysis (FEA) programs such as ADAMS, ANSYS and NASTRAN. Some
of the most widely used programs specifi cally developed within the WT design
community are BLADED and FLEX5. FLEX5 has the advantage of wide industry
support and open source code, making it the preferred platform for many OEMs.
One drawback for FLEX5 from the conceptual design perspective (where rapid
solution times are needed to explore a large design space) is the relatively long run
times. Usually 1
2 h are required to run a turbine confi guration through a reduced
suite of load cases using a dual-processor workstation. This can be improved with
ever-faster hardware, but the preferred method for performing value analyses sets
up a large number of cases using the method of design of experiments (DOEs).
These results are used to populate the design space with solutions that can later be
used to interpolate conceptual designs in seconds instead of hours. This “DOE
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