Tech Briefs Magazine - October 2021 - 28a

DESIGN OPTIMIZATION
FIGURE 4 
  
  
 
  
  
FIGURE 3      
     
      
To model the turbulent
flow around the turbine
blade, he tried out different
computational methods for
fluid flow. The standard k-
model was not well-suited for
the problem and did not lead
to an optimal power output.
The so-called SST model
combines the k model in
the free stream and the k
model close to the walls,
which led to good results,
but made the model take too
long to converge. Therefore,
the k- model suited both the
needs of the project balanced
with the computational
resources.
FIGURE 5       
» GO WITH THE FLOW: TURBINE BLADES
SELF-OPTIMIZE THEIR ANGLE OF ATTACK
For Water2Energy's tidal power turbine, the most significant
design decisions involved optimization of the rotor's vertical
blades (Figure 4). By testing and refining both the design
of the blades and a mechanism to adjust their angle, Van
Halewijn tackled two technical challenges. First, a Darrieus
rotor is not always self-starting, even in an environment where
water flows continuously. The second challenge? A spinning
turbine installed in a contained passage, such as a sluice
through a dam, is subject to more turbulence than one that
spins freely in open air or water.
Both of these challenges can be met by continuously
adjusting the turbine blades' angle of attack. With the
correct orientation toward water flow, a Darrieus rotor's
blade will start moving even at very low water speeds. The
problem is that the optimal angle for starting the blade will
be inefficient once the turbine is already moving. Similarly, a
blade's angle can be optimized to move smoothly past the wall
of its enclosure, but that angle will be inefficient when the
28a COMSOL NEWS
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blade is at other points of its
rotation. Van Halewijn used
the COMSOL Multiphysics®
software to model the effects
of different blade positions
on performance.
" Up to now, there has
been no optimal design
solution for this application.
Water2Energy had some ideas
for how to do it, and together
with our simulation, we came
to an even better solution, "
says Van Halewijn. " We were
able to test several ideas in
the software to demonstrate
an optimal approach. "
To do so, Van Halewijn
modeled just one blade of
the turbine in order to find
the optimal angle of attack.
After modeling the
turbulent flow around the
turbine blade, Van Halewijn
was able to find an optimal
blade profile for the project.
" I always explain to customers
that simulation software is a
decision tool for research and
development. I am not selling
mathematics. With simulation,
I am able to move a project in
the right direction, without as
much trial and error. Really,
what I am providing is better
decisions based on the sound
principles of physics, " says Van
Halewijn.
" Once we had modeled
a profile for the blade, we
could run simulations of its
motion past the walls of the
channel (Figures 5, 6). This
meant we had to adjust the
mesh of the blade's surface to
account for every step of its
360 degrees of rotation, " Van
Halewijn explains. " I was able
to add a special node in the
software to maximize energy
generation in the design
phase. And of course, we
had to simulate the passage
of fish through the turbine,
to convince people that sea
life would not be harmed,
not even by our prototype
testing. "
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Tech Briefs Magazine - October 2021

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