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The Effect of Lip Extrusion on Performance of a
Breakwater Integrated Bent Duct OWC WEC
Damon Howe#1, Jean-Roch Nader#2, Jarrah Orphin #3, Gregor Macfarlane#4
#National Centre for Maritime Engineering & Hydrodynamics, Australian Maritime College, University of Tasmania
Locked Bag 1395, Launceston, Tasmania 7250, Australia
1damon.howe@utas.edu.au
2 JeanRoch.Nader@utas.edu.au
3 jarrah.orphin@utas.edu.au
4gregorm@amc.edu.au
Oscillating Water Column (OWC) Wave Energy Converters
(WECs) are one of the most studied, developed and tested devices
associated with Ocean Renewable Energy (ORE) today.
Variations in concept design and hydrodynamic characteristics
have been researched extensively, however the main issue
associated with ORE is the high Levelised Cost of Energy (LCOE).
One of the most promising applications devised to potentially
decrease the LCOE is maritime structure integration by sharing
the costs associated with construction, and lowering the costs of
maintenance. This paper investigates physical model
experimentation into the effect that lip extrusion of a breakwater
integrated bent duct type OWC has on the performance. The
importance of the investigation has implications regarding tuning
of the device for site-specific breakwater integration and
constructability. It is documented that a bent duct OWC observes
change in its natural resonance frequency through variation in
length of the swept path of the geometrical cross-section.
Subsequently, lip extrusion could present a way of tuning the
device to suit a site-specific sea state, and is a design consideration
that will affect both optimal power absorption and
constructability. It was found that the longer the lip extrusion the
lower the performance and that the no lip extrusion case resulted
in the maximum non-dimensional capture width of 2.2 compared
to the maximum lip extrusion considered in this paper
corresponding to 1.4. This is a positive result for construction
consideration as less materials are required, however, using lip
extrusion to tune OWC resonance might come at the cost of
performance.
Keywords— Oscillating Water Column, Wave Energy
Converter, Breakwater, Hydrodynamic Experimentation,
Performance
I. INTRODUCTION
The Oscillating Water Column Wave Energy Converter is
one of the most researched and developed technologies utilised
to harness energy from ocean waves. The first technology
developed to utilise the operational principles of an OWC was
created by Yoshio Masuda in 1947 in the form of a navigational
buoy, which harnessed ocean wave power to provide electricity
to the power system of the buoy creating a self-sufficient
product [1, 2]. Evans published the first paper on the OWC
WEC describing its operation and efficiency through analytical
and theoretical analysis in 1978 [3], providing the platform for
the future development of the technology.
The progression of the technology’s hydrodynamic
efficiency and performance characteristics has increased
rapidly resulting from comprehensive research into the concept,
with some focus now shifting from hydrodynamics and
performance of an isolated structure, to viable installation
methods of the technology. This shift in research aims to aid in
the reduction of the Levelised Cost of Energy currently
associated with ocean wave energy. In comparison to wind and
solar, which have expected values by 2022 of USD$64.5/MWh
and USD$84.7/MWh respectively [4], ocean wave energy has
a considerably higher value range. The first expected
commercial scale project has an LCOE range of between
USD$120-$480/MWh [5]. Although this value range is in
relation to an offshore array configuration of devices, the lack
of commercially available and proven concepts, along with the
generalised immaturity of the technologies across the ORE
sector relative to other renewable energy sectors attributes to
the significant variation in values, hence providing a relatively
unfair comparison.
There are two promising installation concepts for large-scale
multi-OWC WEC device configurations that have the potential
to reduce the high associated LCOE, the first of which is in an
offshore array formation or farm, as previously mentioned.
Implementation as a full-scale demonstration is yet to be
produced for this concept; however, research conducted into
the effect of array configurations by means of numerical
investigation has seen progression in the feasibility of such a
concept develop rapidly in recent years [6-9]. The second of the
installation concepts is integration into maritime structures
such as breakwaters or harbours, of which a number of full-
scale and commercial concepts have been constructed and/or
are currently operational [10, 11]. Typically, this type of
structure integration implements a Caisson style OWC design
where the structure itself acts as the device. To do this, the
creation of an opening into the hollow structure allows the
incident waves to propagate within, hence operating as an
OWC chamber.
The following research investigation looked to vary this
installation concept by taking an OWC device variation and
integrating it into a solid breakwater. The Bent-Duct OWC
device used in this investigation, similar to that found in [12-
18], creates the OWC chamber by sweeping a geometrical
cross-section (rectangular for this investigation) along a
predetermined path consisting of a horizontal, diagonal and
vertical component, resulting in a 90-degree angle from the
inlet to the outlet of the device, as illustrated in Fig. 1. The
rigidity of the Bent-Duct OWC structure allows it to be more
readily incorporated into solid edifices; as such, a number of
parameters regarding constructability are likely to have an
effect on device performance.
Previous works by Howe and Nader [18] used numerical and
experimental investigations to compare the performance of an
OWC device in isolation to a device integrated into a
breakwater structure. The results of the study found that
integrating an OWC device having a 38 mm lip extrusion into
a fully reflective breakwater could produce non-dimensional
capture widths approximately twice that of the isolated device
around the resonant frequency. The resultant nodes and anti-
nodes developed by the reflected waves from the breakwater
presented the hypothesis that lip extrusion may have a non-
negligible effect on device performance, providing the
foundation for this study.
Subsequently, this experimental investigation focused on the
construction parameter of lip extrusion. By integrating the
OWC device into a solid flat-faced breakwater, the aim is to
determine whether the extrusion of the devices underwater inlet
from the breakwater would have an effect on the extraction
efficiency of the device. At the conclusion of this paper,
through subjecting the device to a monochromatic regular wave
regime across a designated frequency bandwidth for each
extrusion distance, the construction configuration in which the
device was able to achieve the greatest capture width is
determined and the trends in the extrusion length and extraction
efficiency relationship are observable.
II. THEORY
A. General
The experimental investigation considers a thin-walled Bent
Duct OWC device facing in the direction of incoming incident
wave propagation. The device operates in constant water depth
h, with the vertical component of the chamber piercing the
surface. The device, as previously utilised by Howe & Nader
[18], has a rectangular cross-section of width D, height W,
thickness t that was swept along a pre-determined path to form
the chamber as displayed in Fig. 1, with the dimensions for the
design variables of the model corresponding with those
presented in the aforementioned paper. Any variation in the
design criteria, including the volume of air and water in the
chamber can result in a change of hydrodynamic device
response (cf. [19]), as such it was important to keep these
parameters consistent throughout all variations in extrusion
during the experimental investigation.
Fig. 1 Cross-section through OWC and breakwater. The OWC lip extrusion
on the seaward side of the breakwater is shown
The experimental test regime detailed five separate
variations in the extrusion parameter (e), pictured in Fig. 1. The
minimum designated extrusion of 0 mm equated to flush with
the seaward wall of the breakwater, and the maximum was
designated to 200 mm as this represented a reasonably practical
limit, which also equated to one-third of the total device length.
Fig. 2 and Fig. 3 present rendered CAD illustrations of the full
and cross-sectional views of the experimental configuration
used during testing. The OWC device was then adjusted
accordingly to correspond with the desired extrusion distance
(e).
Fig. 2 Rendered schematic of OWC configuration with 200mm extrusion
Fig. 3 Rendered schematic cross-section of OWC configuration with 200mm
extrusion
B. Theoretical Hydrodynamic Consideration
The experimental investigation aims at quantifying the
performance, presented as the non-dimensional capture width,
of the device, for the different extrusion distances considered.
Monochromatic incident plane waves propagating towards the
device in a constant direction, with an amplitude of ηo and a
frequency ω were considered. Small incident wave amplitudes
were used in order to remain in the linear water-wave theory
regime.
A linear power take-off (PTO) system, between the volume
flux Q and dynamic pressure Pc, was assumed and simulated,
for this paper symbolising the characteristics of a Wells type
turbine;
= (1)
where δ represent the pneumatic damping coefficient, which is
considered real. The presented relationship was previously used
in other works including [9, 18, 20]. The effect of air
compressibility on the power absorption of the device was
disregarded during this experimental investigation due to the
small chamber volume of air as in [18].
1) Volume Flux: The volume flux of air emanating from the
oscillation of the free surface, , in the chamber is,
=
=
=
(2)
where Sc represents the area of the free surface, vs represents
the velocity at which the free surface oscillates, and
can be
defined as the mean free surface velocity.
2) Mean Power Hydrodynamic Absorption: The
instantaneous power of the OWC device can be determined as,
()= (3)
where P is the power at a given time t. The mean power
hydrodynamic absorption, Ph, is then determined by integrating
the instantaneous power over the wave period, defined as,
=
(4)
where T is the wave period. By introducing Equation (1) into
Equation (4), and also assuming linear wave theory, the
relationship can transform to,
=
=
(5)
where Ap is the amplitude of the internal chamber pressure, Pc.
can then be non-dimensionalised with respect to the power
carried by the incident wave for a crest width corresponding to
the width of the device inlet. The incident wave power is
defined as,
=
(6)
where PI is incident wave power, ρ is water density, g is
gravitational acceleration, Cg is the group velocity and L is the
wave crest width corresponding to the OWC inlet width.
By presenting the mean hydrodynamic power absorption
with respect to the incident wave power, the non-dimensional
capture width Pw can be defined as,
=
(7)
III. METHODOLOGY
A. Experimental Considerations
To compare the effect of lip extrusion on the device
extraction efficiency, the non-dimensional capture width was
evaluated for each experimental test run.
The principles and processes utilised to evaluate volume flux,
damping and mean power hydrodynamic absorption of the
device follow that presented by Howe & Nader [18]. A concise
synopsis of each parameter is presented here.
1) Volume Flux: The volume flux was obtained by
numerically deriving the free surface velocity, measured by a
wave probe placed in the OWC chamber as an approximation
of
, and multiplying by the cross-sectional area of the
chamber . Due to the numerical error induced by numerical
derivation, the resulting volume flux was only used to obtain
the pneumatic damping.
2) Pneumatic Damping: As in [18, 21], a porous mesh was
used to simulate the behaviour of a Wells type PTO turbine as
in Fig. 4. In the same way as in [18], the pneumatic damping
was processed using a linear regression between the resulting
pressures and volume fluxes over different frequencies.
Fig. 4 Porous mesh damping substitute applied to the model OWC
The method for establishing the damping coefficient of the
porous mesh, and thus the system, followed that outlined in [18],
including the numerical derivation and associated sources of
errors. It was established that the damping of the system was
not frequency dependant; subsequently the pneumatic damping
could be uniformly applied across all test frequencies.
B. Model Test Basin
The experimental investigation was conducted in the
Australian Maritime College’s Model Test Basin (MTB). The
MTB’s dimensions are 35 m × 12 m × 1 m, and is fitted with
16 individual wavemaker paddles capable of producing waves
in both regular and irregular regimes. The facility is also
equipped with a damping beach at the Southern end of the basin,
which is utilised to reduce reflection during experimental
investigations, and dissipate energy from the basin between
experimental runs. A scaled schematic of the MTB with the
experimental apparatus set up found in Fig. 5, with a detailed
view of the breakwater and OWC configuration can be found
in Fig. 6.
Fig.
6 Schematic of OWC and breakwater detailing the OWC wave
and pressure probe
Fig. 5 Scaled schematic of the AMC Model Test Basin with the experimental apparatus set up
C. Physical Model
The model OWC used during experimental testing was
OWC model AMC 15-15, as used in [18, 21]. This device is a
bent duct type OWC having a rectangular cross-sectional area,
of which the height and width of the inlet are 0.23 m and 0.3 m
respectively. The device was design to represent a 1:20 scale
physical model of the numerical device presented in [18]. OWC
model AMC 15-15 can be seen integrated into the model
breakwater in Fig. 7.
Fig. 7 OWC scale model AMC 15-15 integrated into the model breakwater
D. Sensors & Calibration
TABLE I
SENSOR PROPERTIES
Sensor
Range
Sensitivity
Output
Incident Wave Probe
±40mm
0.25VDC/mm
±10VDC
Phase Wave Probe
±40mm
0.25VDC/mm
±10VDC
OWC Wave Probe
±70mm
0.143VDC/mm
±10VDC
OWC Pressure Probe
±300Pa
33.3mVDC/Pa
±10VDC
The experimental investigation conducted into the OWC
device utilised a series of wave probes and pressure sensors to
measure both the wave field in front of the breakwater and the
oscillations within the OWC chamber. In total, six wave probes
and one pressure sensor were configured in the experimental
apparatus, however three wave probes were disregarded for the
purpose of this study, as their primary application was to
determine reflection coefficient of the breakwater, which lies
outside the scope of this paper. An incident and phase wave
probe were situated close to the wavemakers and in line with
the breakwater respectively, with the third wave probe and the
pressure probe located inside the OWC chamber. The wave
probes were positioned in the selected configuration for two
reasons, the first of which looked to compare the wave height
from the incident to phase wave probes, ensuring the target
incident wave height was met, the second looked to compare
the oscillations within the OWC chamber with respect to the
incident wave to determine the amplification factor. The
properties of each of the considered sensors can be found in
Table I. All probes and sensors were calibrated daily to the
respective ranges found in Table I to aid in the reduction of
uncertainty associated with variations in the experimental test
facility. Each of the probes had a sampling rate of 200 Hz and
the filter value was designated to 10 kHz.
E. Experimental Test Regime
A total of 99 experimental runs were conducted as part of the
investigation spanning across five separate variations in lip
extrusion. Along with lip extrusion, the investigation also
varied incident wave frequency across a bandwidth of 0.5-1.2
Hz for each extrusion case in increments of 0.1 Hz.
Additionally, 0.525 Hz, 0.55 Hz, 0.575 Hz and 0.65 Hz were
investigated to improve the resolution of results around the
natural resonant frequency of the OWC device. The porous
mesh damping simulant was applied uniformly across all
relevant conditions, and target incident wave height was 20mm
for all experimental runs. The data received from the probes
and sensors could then be processed and analysed to identify
any discernible trends regarding the lip extrusion effect on
device performance. The conditional information and
investigation variables can be found in Table II.
Together with the experimental runs regarding variation in
lip extrusion, an additional set of runs were conducted to test
the wave field within the basin. The aim of this investigation
was to confirm the parameters of the incident waves at the
breakwater location correlated with those observed during the
experimental investigation.
TABLE II
CONDITIONAL INFORMATION
Condition
Information
1
Extrusion
(e)= 0mm
2
Extrusion
(e)= 50mm
3
Extrusion
(e)= 100mm
4
Extrusion
(e)= 150mm
5
Extrusion
(e)= 200mm
6
Wave Field Test –
No Breakwater or
OWC in Basin
F. Data Processing – Phase Averaging
In order to increase the accuracy and reliability of the raw
data acquired from the experimental investigation, the data
processing technique of phase averaging was utilised for
analysis, whilst also aiding the validation of results. Phase
averaging has previously been applied as a post-experiment
processing technique for OWC investigations in [13, 14, 16, 17,
22], where it has yielded results comparable to those achieved
through the process of repeated runs. An uncertainty analysis
was conducted for the experimental investigation in [21], where
it was found that the standard error acquired via phase
averaging could be narrowed to within a threshold of ± 10% of
those obtained through 10 repeated runs, provisional upon the
number of repeated wave cycles utilised during phase
averaging. Similarly, the phase averaged results from 10 repeat
runs were presented, showing the standard deviation as a
percentage of maximum amplitude, where results were found
to be approximately less than 2%. These results quantify the
feasibility of phase averaging post processing techniques for
experimental hydrodynamic investigations using regular
waveforms when compared to repeat testing, subsequently
saving both time and resources.
By applying the phase averaging method on the incident
wave probe and pressure sensor, and Ap could be derive. A
detailed explanation of the phase averaging process is outlined
in [18].
IV. RESULTS AND DISCUSSION
A. Pneumatic Damping Coefficient - δ
The requirement to replicate the approximately linear
relationship between volume flux and pressure associated with
Wells type turbines was satisfied using a porous mesh simulant
as previously described which was applied identically across
the varying lip extrusion conditions. The gradient of the
subsequent pressure/volume flux plot would identify the
pneumatic damping coefficient, δ, for each respective variation,
which could then be applied to Equation (5) to evaluate the
mean power hydrodynamic absorption of the device.
Fig. 8 Pressure vs. Volume flux plot across test frequency range with extrusion
equal to 150mm having an average pneumatic damping value of 18632
Fig. 8 displays the approximately linear relationship
between volume flux and pressure inside the OWC chamber for
the device with an extrusion distance of 150 mm. It can be seen
that the linearly interpolated line (thick black dash) can provide
the approximate gradient of the relationship, from which the
pneumatic damping coefficient can be determined. It should be
noted that the damping coefficient is independent of the
incident wave frequency, as such can be applied uniformly
across the test frequencies.
Fig. 8 also illustrates three separate frequency cases
displayed by line variations. These represent low (0.5 Hz),
resonant (0.575 Hz) and high (1 Hz) frequency cases
investigated during experimental testing, from which it can be
found that the magnitude of both pressure and volume flux are
largest at resonance, and progressively decrease as frequency
shifts away from resonance.
The variation in the values obtained for the pneumatic
damping coefficient across the five separate cases was
relatively small, with no discernible trends in variation between
the differing cases. As such, it was determined that the effect of
lip extrusion on the PTO damping characteristics was
negligible. The pneumatic damping coefficients for each of the
lip extrusion cases can be found in Table III, where the values
varied from 16693
to 18632
as the minimum and
maximum respectively, and the units correspond to pressure per
unit flow rate.
TABLE III
PNEUMATIC DAMPING COEFFICIENTS FOR LIP EXTRUSION VARIATIONS
Condition
Number
Extrusion
Distance (mm)
Pneumatic Damping
Coefficient δ
1
0
16965
2
100
16898
3
200
18209
4
150
18632
5
50
16693
Potential sources of uncertainty are associated with the
evaluation of the damping coefficient, which can incur non-
negligible experimental errors as cited in [21]. Fig. 8 presents
the relationship between Pc and Q, which was assumed to be
approximately linear, however upon further investigation in
[21], was found to result in high Type B uncertainties due
mainly to the uncertainty in the calibration of the pressure
sensor. The uncertainty associated with the pressure sensor, in
culmination with the numerical errors induced by numerical
derivations required for volume flux provide substantial
foundations for induced uncertainty during the experimental
investigation. This presents validation for the complexity
required to replicate a PTO system at the model scale, as such,
future works are suggested to investigate a more accurate and
representative simulation for PTO systems in the WEC model
testing field.
B. Power
By determining the non-dimensional capture width for each
test frequency, the performance of the device could be assessed
across a range of incident wave periods, providing a
comprehensive representation of the devices extraction
response, whilst illustrating the resonant characteristics of the
device.
The experimental investigation saw the lip extrusion vary
from 0 mm (flush with the breakwater wall) to 200 mm, which
was equivalent to approximately one-third of the total device
length extruded. Fig. 9 displays the results for the non-
dimensional capture width across the test frequencies for each
respective extrusion variation.
Fig. 9 Non-dimensional capture width Pw over tested incident wave
frequencies for lip extrusion variation e
As displayed in Fig. 9, the shape of the relationship between
non-dimensional capture width and incident wave frequency,
together with the location of resonance within the test
frequency spectrum remained consistent across the five
variations in lip extrusion. The results regarding the lip
extrusion cases between 0-100 mm present values that are
relatively close in magnitude, particularly around the resonance
frequency of the device. However, these results are within the
uncertainty boundaries established for the experimental
investigation, as found in [21]. Irrespective of the close results
in these cases, a clear trend is identifiable in Fig. 9, which
indicates a decrease in performance corresponding to an
increase in lip extrusion. To provide a quantitative comparison,
the value for non-dimensional capture width of the device at
resonance (0.55 Hz) with no extrusion, e = 0 mm, was
approximately 2.2, whereas the corresponding value with
maximum extrusion, e = 200 mm, was approximately 1.4.
As detailed in [21], the major source of uncertainty within
the experimental investigation is derived from the linear
pneumatic damping of the system simulated by the porous
mesh. Investigating this further, the calibration of the pressure
transducer in culmination with the assumed linearity of the
pneumatic damping relationship between the numerical derived
volume flux and internal chamber pressure contributed
significantly to the Type B uncertainty, and subsequently the
expanded uncertainty of the results obtained. Recent research
from Elhanafi, Macfarlane, Fleming and Leong [23] found that
the accuracy of the airflow representation using free surface
elevation could be considerably increased by employing two or
more wave probes within the OWC chamber, from which
spatial averaging is utilised to produce results that are more
representative. This method may reduce the uncertainty
associated with the linear pneumatic damping and is therefore
recommended for future experimental investigations.
There are two hydrodynamic coefficients that can explain
the variation in results from the different lip extrusions: the
excitation volume flux, the proportionality coefficient of the
incident wave amplitude, giving the volume flux for the case
where no pressure is present in the OWC chamber and the
radiation impedance, the proportionality coefficient of the
pressure, giving the volume flux for the case where no incident
waves are present and the oscillatory pressure imposed in the
OWC chamber [24]. These hydrodynamic coefficients have
been previously used in other works investigating the
oscillating water column including [8, 19, 20, 25, 26]. The
excitation volume flux can relate to the excitation force for a
mass-spring-damper approach and represents the amount of
available energy to the system whereas the radiation impedance,
which can relate to the added mass and radiation damping for
the mass-spring-damper approach, represents the potential of
energy absorption by the device. Keeping the physical design
of the OWC device constant, it can be shown that the variation
in radiation impedance is very small for the different condition
tested. This is in fact the excitation volume flux, see Figure 11
in [18], which can explain the change in results. This excitation
volume flux is understandably dependent on the position of the
OWC inlet relative to the position of the nodes and anti-nodes
present in the wave field produced by the breakwater reflection.
The results clearly show that when the OWC inlet is at the anti-
node present at the wall of the breakwater, the performance of
the device is at the maximum. This result is also greatly
beneficial for the concept design as this is the only anti-node
position which does not move with incident wave frequency.
Additional factors that may be considered to have attributed
to the variation and uncertainty of the results include the effects
of turbulence and sloshing, specifically around the resonance
frequency of the device. It was witnessed during the
experimental investigation that the resonant behaviour of the
device induced high amplitudes of free surface oscillation
within the OWC chamber, subsequently resulting in sloshing.
Similarly, recent work by Fleming and Macfarlane [27] found
that the sharp edge of the inlet lip can induce vortices around
the opening of the device, which may also contribute to the
losses associated with increased lip extrusion. Nonetheless, the
effects of turbulence, sloshing and vortices on the results
obtained are not conclusive, and lie within the current bounds
of uncertainty.
The implications of this experimental investigation have
direct impact on the constructability of the OWC device
integration. As has been displayed, the greatest extraction
capacity occurs when no inlet lip extrusion is present; which
indicates the entirety of the OWC chamber would be internal
within the structure. This is advantageous in the design and
construction aspects of the integrated device, as it negates the
effect of wave loading on the external lip extrusion, and hence
a reduction in cost of materials necessary to reinforce the lip
structure. Similarly, another constructability aspect that is
assisted by presenting an internal chamber is the incorporation
of a ‘shut-off’ safety mechanism designed to seal the chamber
in the case of extreme weather events, which has been found to
result in the destruction of such concepts previously [28]. Such
a mechanism would see reduced complexity for the completely
internal configuration in comparison to the design of a
mechanism that considered the lip extrusion.
When considering implications of this design parameter
external to the breakwater, the presence of an inlet lip would
present a navigational hazard for boats, ships and maintenance
vessels coming within close proximity of the breakwater. The
results of the research have nullified the navigational hazard of
the inlet lip by presenting the optimal design with no lip
extrusion.
To further justify the effect of lip extrusion on performance,
it is recommended that a number of additional investigations be
conducted. Firstly, the turbulent and sloshing effects of the
fluid within the chamber should be explored, which may
provide credible evidence into the sources of uncertainty
presented in the current investigation, and would yield results
more applicable to realistic responses of the device. Similarly,
it would also be of interest to conduct investigations that
simulate the losses associated with the PTO system, focusing
on the Type B uncertainty associated with the porous mesh
simulant.
Investigation into the vortex generation around the inlet lip
as a function of the extrusion distance would help validate the
hypothesis presented for potential reductions in the
performance of the OWC device associated with increased
extrusion. This could be done numerically or through PIV
investigation to provide conclusive evidence of this phenomena.
Variation in device geometry, similar to that presented in [27],
may be investigated to further increase the extraction efficiency
of the presented concept. This could be further developed to an
increased scale test, such as medium scale, that may yield more
representative results of a full scale device.
With regards to more realistic environmental conditions,
polychromatic and irregular wave testing is suggested to
identify the concepts capacity to extract energy from more
applicable sea states. Similarly, tidal variation may have a non-
negligible effect on performance, as such, would be another
consideration in site specific design. The development of a
numerical model may also provide the platform to investigate
these hypotheses, which can then be verified through further
experimental investigation.
V. CONCLUSION
The aim of the experimental investigation conducted was to
determine if the design parameter of lip extrusion had non-
negligible effect on the performance of a breakwater mounted
OWC device. The device was varied from no extrusion,
through to an extrusion equivalent to one-third of the total
length, from which a general trend in the relationship between
lip extrusion distance and non-dimensional capture width was
established. The performance of the integrated OWC device
reduced as the length of the lip extrusion increased, where the
variation between best and worst non-dimensional capture
widths was approximately 0.8, with the worst performing case
corresponding to the maximum extrusion length of 200 mm.
Though an increase in extrusion distance reduced the extraction
capacity of the device, it was observed that the general device
extraction response remained consistent across all extrusion
variations, and subsequently, the natural resonant frequency of
the device remained consistent throughout the experimental
investigation. The implication of the investigation may result in
design changes for maritime structure integrated OWC devices,
from which costs could potentially be reduced due to a
reduction in materials and maintenance associated with the lip
extrusion, ultimately lowering the LCOE of this technology.
Although the present investigation presented no extrusion as
the optimal design, further investigation into the sloshing,
turbulent and PTO loss effects could be investigated to provide
a more comprehensive justification of the hypothesis and
decisively establish their roles in the variation and uncertainty
of the results.
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