Feature

From CAD User Mechanical Magazine  Vol 22 No 10 - OCTOBER/NOVEMBER 2009

Joel Thakker of IDAC Ltd. examines aerodynamic and structural simulation for the wind industry

It is estimated that by 2020, wind
farm capacity will have increased by
nearly five-fold for new installations
within the UK itself. While such a
forecast does look promising for the
Renewable industry, it is crucial that the
planning stages of such installations
are thoroughly validated via testing and
prototyping. On prior installations Wind
Park Effects, blade interference and
overestimated power yields have only
come to light after the masts have
already been put into the ground or sea
bed. Needless to say, such
occurrences are not only embarrassing
for the designer, but also to the
industry as a whole, as it reduces
public confidence in alternate energy.

Computational Fluid Dynamics (CFD)
and Finite Element Analysis (FEA) are
two engineering solutions that could be
used within the wind industry to predict
a wind farm's performance before it
went into production. The technology is
far greener than experimental testing
and in the long run also turns out to be
more cost effective. Combining today's
hardware capabilities with available
software prowess allows an engineer to
assess multiple scenarios in parallel, all
of which can all be evaluated usually
within a week or even less.

In the early stages of development
itself, CFD can be used to predict
aerodynamic characteristics and
pressure loadings existing on a single
blade or solitary turbine. The results
from the above CFD simulation can
then be fed to an FEA solver within the
structures department to determine Von
Mises stress and total deformations.
Figure 1a-b shows the aerodynamic
pressures and resultant deformations
on a generic airfoil that may be used on
a wind turbine. Extending this example
to the case of a complete wind turbine,
it is easy to see the benefits of having
seamless data transfer between an
aerodynamicist and structural analyst,
both of whom would be feeding off the
same CAD model and software
platform.

Once the blade or turbine design has
been locked in place for a single
turbine, interference that occurs
between the blades on the same
turbine and those downstream in the
farm can also be studied. Similar to the
wind industry, the aerospace market in
particular has paid a lot of attention to
modelling and validating wake effects
and blade interference. Multistage
engine compressors or twin blade
helicopters are the most obvious examples that come to mind. The problem with conducting
such a simulation however is finding the expertise to do it.
On one hand, the analysis can be labour intensive and
requires someone who knows turbulence modelling, while on
the other, the amount of data obtained from such an analysis
almost always surpasses practical experimental testing.
The graphics show a simple wind farm design that was
studied for blade interference under the effect of a south
easterly wind. The results got from this CFD analysis are
again useful to not only the aerodynamicist, but also to the
farm layout designer and structural analyst.

Figure 3 shows the predominant vortex frequencies that are
being experienced by the second row of turbines, as a
consequence of upstream turbulence. An ideal situation
would be where these aerodynamic frequencies do not
match the natural structural resonance frequencies of the
turbine in question. In the event that they do, a user has the
option to reposition the turbine first and rerun the analysis,
before going back and changing the structural properties of
the turbine itself. Once the aerodynamicists and wind farm
designers have run the CFD that optimizes power
generation, the same model can then be given back to the
FE analysts to guarantee fatigue life as would be seen under
cyclic operating conditions.

So far we've only touched on the potential uses of fluid and
structural analysis for windfarm development and
environmental planning. The truth is that simulation
technology can be taken further to account for noise
prediction, radar cross section or even impact damage such
as bird strikes or falling hailstones. The technology exists as
does the expertise for delivering a complete engineering
solution.

Feature

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