Earth Science & Engineering, Imperial College London
Doi
Abstract
This research aimed to investigate whether the unique numerical methods
available within CFD model software Fluidity could progress the state–
of–the–art in various aspects of modelling effluent dispersion within the
marine environment. Fluidity contains a large library of models and numerical
methods that enable modelling of flow processes at a wide range
of scales. It has been proven to perform well when used for massively–
parallel simulations (i.e. it scales well), and it has the un–common facility
of unstructured mesh adaptivity, which has the prospect of significantly
increasing the efficiency of CFD simulations when guided skillfully.
This research also forms part of a longer–term project to create a coupled
(or even single) model of effluent dispersion that represents influencing
factors from a wide range of scales (from tidal currents down to turbulent
eddies) entirely using CFD techniques. As such, one aspect of the
research was to validate the numerical methods available within Fluidity
for use in modelling effluent dispersion. To facilitate this validation, some
of the model studies investigate aspects of effluent dispersion modelling
from a hypothetical outfall site off the North–East coast of the United
Kingdom.
Studies were performed in a series of stages in which key aspects of effluent
dispersion modelling were addressed. CFD models were created
of near–field jet dispersion, tidal motion, and far–field plume dispersion.
Idealised test cases were also performed to investigate the performance of
advection–diffusion solver methods. At each stage the aim was to investigate
the benefit of novel numerical modelling techniques and compare
their accuracy and efficiency to existing methods.
A set of near–field buoyant jet dispersion CFD models were created, one
representing conditions associated with power, and combined power and
desalination plants, and one representing conditions typically associated
with desalination discharge. These CFD models utilised a mesh adaptivity
algorithm to optimise mesh resolution during the course of the
simulation. Model predictions were compared with published laboratory
data and the predictions from validated integral models. An assessment
was made of when CFD offers a benefit over other modelling options, and
when it might be sufficient to use cheaper tools. There was also a discussion of the effectiveness of mesh adaptivity in increasing model efficiency,
together with advice for how and when it is best to use mesh adaptivity
when modelling buoyant jet dispersion. Model results showed that with
modest parallel computing resources and expertise, high–resolution simulations
of jet dynamics can be achieved with reasonable accuracy using
CFD modelling.
A model was created of tidal flow within the European continental shelf
and results were compared to a large database of tide gauge measurements.
This model took advantage of recently published methods for
ocean model meshing and coastline resolution reduction. The purpose of
this study was to confirm that these methods offered a benefit to model
accuracy and efficient, and also that Fluidity could be used to accurately
generate the tidal forcing boundary conditions for a far–field model of
effluent dispersion at a hypothetical outfall site.
The predictions of M2 tide amplitude in the vicinity of the outfall site
had an average error of 10.1% compared with tide gauge measurements.
The predictions of S2 tide amplitude in the vicinity of the outfall site
were even closer to tide gauge measurements, with an average error of
3.7%. The speed of the model solve showed a vast improvement over
a previous comparison model study, with 37 days of tidal motion being
simulated in 15.2 hours (58.4 seconds of simulation for each second of
solving), compared to the comparison simulation with a similar level of
accuracy, which simulated 2 seconds of tidal motion for every second of
solver time.
A series of simplified test cases were run to assess a commonly–used
advection–diffusion solution method from the library of those available
within Fluidity. This work was intended to give general confidence that
the numerical methods available within Fluidity are suitable for modelling
coastal processes and so give confidence in later multi–scale results.
The test cases chosen were relevant to coastal dispersion, including those
testing tracer advection, diffusion, point sources and stratification. The
method compared well with results published using world–leading free
surface modelling software, Open TELEMAC.
A model was created of the dispersion of neutrally–buoyant dissolved
pollutant from a hypothetical outfall. The assumed effluent is typical of
that released from a manufacturing plant. The aim of this modelling was
to validate the use of Fluidity for modelling effluent dispersion within
the coastal zone, and also investigate the benefit of using 2–d horizontal
mesh adaptivity to optimise model mesh resolution during the course of the simulation. It was shown that the use of mesh adaptivity improved
model efficiency, significantly lowering the effect of numerical diffusion.
Finally, a short outline was given of a prospective strategy for producing
a coupled–model of effluent dispersion, using as a basis the techniques
developed within this thesis. The proposed coupled model of effluent
dispersion would include a near–field jet model two–way (i.e. “fully–
coupled”) to a far–field plume model. Tidal forcing would be provided
by a one–way coupled tidal model. Fluidity is capable of modelling all
of these processes and so third party coupling software would be unnecessary.Open Acces