Since the velocity in sink direction does not increase
much for angles that are larger than 35°, it is probably
better to choose a barrier angle between 25° and 35°. This
will still give a relatively high particle velocity in sink direction,
but on the other hand the construction costs and
the overall size of the structure will be smaller.
The study presented in Sub-chapter 3.5 shows that the
barrier will also tilt like a pendulum as a result of current
and waves hitting the barrier. For simplicity the tilting
is neglected in this report, even though it is expected
that the tilting will have a strong effect on the particle
behavior around the boom, and especially on the catch
probability. With relatively little additional computational
effort it would be possible to tilt the barrier statically in
order to investigate the influence this will have on the
catch probability.
Since it is not clear yet how strong the influence of waves
and wind is on the catch probability, this should be investigated
first. This could be done with more sophisticated
simulations that take waves and wind into account, or by
comparing the simulated results to experiments in reality.
Nevertheless, the presented results in this study give a
good estimate on the particle size that will not be catchable
with the barriers.
3.4.2 ANSYS CFX 3D CFD model
3.4.2.1 Model Summary
The numerical approach implemented for both computing
the flow behavior around the boom and the transportation
of the plastic particles along the boom is performed
with ANSYS-CFX. To formulate the model of the boom and
the ocean setup, assumptions and approximations were
made in order to reduce computational efforts, time, and
also to achieve reasonable conclusions from the project
in a short while.
• The Fluid Structure Interaction (FSI) analysis has been
carried out on a scaled-down version of the boom span
with appropriate dimensions instead of taking the
whole structure into account.
• The length of the boom designed in the simulation is
10 m whereas the real booms may be extended over
several kilometers. Hence, our reduced simulation domain
is intended to analyze the flow behavior far away
from the collection station by implementing periodic
boundary conditions.
• The boom is considered a fixed object with no degrees
of freedom. As a consequence, the motion or
deformation of the boom influenced by the flow
behavior is not taken into account.