85

ＯＴＥＣ
Vol. 26（2021），85 ～ 97

Analysis of Floating Oscillating Water Column (FOWC)-breakwater
Hybrid System: A Numerical Study
Giri Ram*1, Mohd Rashdan Saad*1, Noh Zainal Abidin*2 and Mohd Rosdzimin Abdul Rahman*1*1 Department of Mechanical Engineering, Faculty of Engineering,
Universiti Pertahanan Nasional Malaysia,
Kem Sg. Besi, 57000, Kuala Lumpur, Malaysia*2 Department of Science and Maritime Technology,
Faculty of Defence Science and Technology,
Universiti Pertahanan Nasional Malaysia,
Kem Sg. Besi, 57000, Kuala Lumpur, Malaysia

Abstract
This study numerically investigates the performance of a Floating Oscillating Water Column (FOWC)breakwater Hybrid System using the Computational Fluid Dynamics (CFD) solver Ansys 2021 R1: Fluent. The
domain is a two-dimensional numerical wave tank (NWT) with a partially immersed FOWC. Volume of Fluid,
finite volume method and RANS equations were used to model and simulate the FOWC-breakwater hybrid
system. The meshing process was done by separating the NWT to three segments, with the water surface and the
middle segment having a fine element size of 0.005m, which is two times finer than the outer segments. A
numerical beach was added 2m from the outlet to minimize the effects of wave reflection to the devices. Firstly,
the design of FOWC and numerical setup was validated. Two models, with different top opening sizes and initialconditions were used for validation. Measurements of wave elevation and horizontal velocity were recorded andcompared with experimental data from a previous study. Secondly, the FOWC-breakwater hybrid system wasmodelled and calculated. Four gap ratios and five wave periods were used for comparison. Measurements ofwave elevation at various locations of the NWT and inside the FOWC chamber were recorded. Performancecoefficients, such as the reflection coefficient, wave elevation ratio and velocity ratio, were derived from themeasurements.
Key words: Floating Oscillating Water Column (FOWC), Wave Energy Converter (WEC), Breakwater,
Computational Fluid Dynamics (CFD), Numerical Wave Tank (NWT), Validation, Reflection
Coefficient, Wave Elevation Ratio, Velocity Ratio___________________________________________________________________________________*1 Received date: 2021.11.30
E-mail of corresponding authors: ramamethyst@gmail.com (G Ram)rosdzimin@gmail.com (MRA Rahman)

86

Giri Ram, Mohd Rashdan Saad, Noh Zainal Abidin, Mohd Rosdzimin Abdul Rahman

1. Introduction
1.1 BackgroundIn response to the threat of climate change, countries are looking into adopting renewable forms energy generation
with a low carbon footprint. Recently on 31st October 2021, the United Nations Climate Change Conference (COP26)
was held in Glasgow. It concluded with all countries agreeing the Glasgow Climate Pact to keep 1.5oC alive and finalize
the outstanding elements of the Paris Agreement. One way to reduce the effects of global warming is to transition fromthe use of fossil fuel to alternative sources such as wind and ocean wave energy. Ocean wave energy is pollution free anddoes not contribute to contamination in the environment, besides reducing carbon dioxide emissions (Kharati-Koopaee
and Fathi Kelestani, 2020).

1.2 Floating Oscillating Water Column (FOWC)
Ocean wave energy is extracted and converted to electrical energy using a wave energy converter (WEC). There arevarious types of WECs being researched in current times, the popular ones being Oscillating Water Column (OWC),
point absorber and attenuator. The OWC typically consists of a partially submerged chamber with a power take off (PTO)
unit at the roof of the chamber. Incident wave causes motion of water surface inside the chamber which then causes
expansion and compression of the air column and this generates wave energy.
There are some notable OWCs installed around the world for commercial use, namely a 300kW OWC plant in
Mutriku, Spain and 500kW OWC plant in Limpet, Scotland (Samak et al., 2021). While a fixed OWC has been provento be a commercial success, there are some drawbacks. Because it is installed on coastal structures onshore, the marine
ecosystem may be potentially harmed. Besides that, the wave energy load is lower compared to offshore because of thebeaching effect that happens onshore. This encouraged research on Floating Oscillating Water Column (FOWC), which
as per its name, is a water column that is not mounted on the coastal structure.
1.3 Floating Breakwater
A breakwater functions as a protective barrier that shields the shoreline from the incoming waves of the ocean,
thereby sheltering the coastal infrastructure. Mikami et al. (2015) showed that detached floating breakwaters are capableof mitigating the effects of tsunami. Breakwaters also help in mitigating wave storms. An added benefit of the breakwater
is that it could also be used to assist in energy conversion, as well as to improve the longevity of WEC. The earlier designsof WEC-breakwater hybrid systems had a bottom-mounted breakwater (Zhao et al., 2019). It was later found that bottommounted breakwater is uneconomical in offshore regions, where floating breakwaters become more favorable. (Zhao etal., 2019).
Floating breakwaters are devices that are not mounted on coastal structures and typically float on the surface of thesea. Some of its advantages compared to fixed breakwaters are the low cost of construction, flexibility of design andinstallation and eco-friendliness (Zhao et al., 2019). Zhang et al. (2021) studied the effects of narrow gap wave resonanceand found that gap wave resonance considerably improved the performance of wave extraction. Koutrouvelli (2021) used
a composite modelling approach to prove the concept of novel hybrid-WEC system. It was found that the hybrid system
is an adequate approach, there is an improvement in the hydraulic performance of breakwater and the negative impact inthe structural stability of the overall system in relation integration is negligible.
1.4 FOWC-Breakwater Hybrid System
A well-documented problem in the generation of wave energy is the construction cost of WECs, rendering the priceof generated electricity to become uneconomical (Zhao et al., 2019). Therefore, the integration of FOWC-breakwater
tackles this problem by being cost-sharing, that reduces the cost of construction, space-sharing by allowing the
installation of various devices in small spaces and lastly the multifunctionality of devices, especially that of the
breakwater, which functions both to improve wave energy extraction and to shelter nearby devices from the effects ofincoming waves (Zhao et al., 2019).
The floating nature of the system also renders additional benefits, such as the ability to be installed offshore. This
makes the devices become less threatening to the marine ecosystem living near the shore as well as having a wider areafor installation, which avoids competition with other marine activities such as fishing and shipping. The portability ofAnalysis of Floating Oscillating Water Column (FOWC)-breakwater Hybrid System:
A Numerical Study87

the floating system also helps in relocating devices to a different site if and when necessary. While there are many benefits
to offshore wave energy extraction, a major disadvantage, as highlighted by Chen et al. (2013), is the strong wave
impulsive load offshore which may incur a higher maintenance cost compared to onshore. The ability of breakwater toprovide protection to the FOWC at regions of high wave impulsive load needs further study.
1.5 Numerical Study
Due to the costly nature of open ocean trials, a high number of studies rely on numerical studies and experimentationin controlled environments (Windt et al., 2021). Of the two, numerical studies are more cost-effective and time-saving.
However, depending on the method of calculation used, the accuracy of numerical data to on-site data may vary greatly.
A validation to experimental data helps in improving the accuracy of numerical data.
Computational Fluid Dynamic (CFD) method that uses Navier-Stokes equations are widely used for the analysis anddevelopment of a WEC and seen to be the best approach by many authors (De Backer, 2009) (Iturrioz et al., 2015). The
in-depth analysis of the dynamics of the structure, the discretization of domain into high-resolution meshes and the studyof complex behaviors like turbulence are some of the benefits of the CFD method (Iturrioz et al, 2015).
1.6 Performance of FOWC-Breakwater
The performance of WEC was measured in some studies either using capture width ratio or the cost of producing 1
kilowatt-hour (Aderinto and Li, 2019). Some challenges in measuring performance of WECs are firstly, the difficulty tocompare different capture technologies, secondly, difference in scales and sized that influence results and thirdly, theeffect of wave condition at experimental and installation site (Aderinto and Li, 2019). For He and Huang (2016), energy
dissipation of the system was used to measure the performance of the wave absorbers on how much of wave reflection
is reduced. Besides that, energy loss due to vortex shedding was calculated. A dimensional analysis was conducted tofind reflection coefficient, amplification coefficient, pressure coefficient and energy-extraction efficiency. In Zhang et al.
(2021), the performance of the dual-floater WEC-breakwater hybrid system was measured using the values of optimal
damping coefficient. Other performance coefficients such as the reflection coefficient, transmission coefficient anddissipation coefficient were also analyzed.
The method used to measure reflection coefficient is the two-point method, which is a method to measure the surface
displacement at two points in a time-series model. It was used for Goda and Suzuki (1977) and is still applied in recent
studies, such as He and Huang (2016) and Zhang et al. (2020). For Isaacson (1991), it was referred to as Method I: two
fixed probes, where two heights and one phase angle is measured. When compared to other methods that use three fixedprobes, it was found to have a reasonable accuracy.
1.7 Purpose of Study
The potential for wave energy generation in the offshore region is an interesting prospect for many coastal regions of
the world. In addition to that, the breakwater aids in wave energy generation. These advantages prompted the authors of
this study to look into the performance of the FOWC-breakwater hybrid system. The first objective was to understandthe characteristics and second to study the performance of the FOWC-breakwater hybrid system.
2. Numerical Set-up
2.1 General Set-up
The CFD solver used is ANSYS 2021 R1: Fluent. The models used for study are categorized as in Table 1. Models 1
and 2 are studied for validation purposes, models 3 to 6 are for the study of four different ratios of gap of breakwater
relative to FOWC.

88

Giri Ram, Mohd Rashdan Saad, Noh Zainal Abidin, Mohd Rosdzimin Abdul Rahman

Table 1: Models used for study and differences in setting_______________________________________
Model No.
Period (s)Gap Ratio
1
1.7
None
2
1.3
None
3
1.3-2.2
2
4
1.3-2.2
2.5
5
1.3-2.2
3
6
1.3-2.2
3.5
Volume of Fluid (VOF) method and implicit setting was used. Two phases, namely air and water-liquid phaseare selected as in Table 2.
Table 2: Properties of the phases___________________________________________________________
Material Name
Density (kg/m3)
Viscosity(kg/ms)
Air1
1.48e- 3
Water-Liquid
1000
0.1
Wave theory is selected in accordance with the graph for limits of validity for wave theories (LeMéhauté, 1969).
Table 3 shows the boundary conditions of numerical model.
Table 3: Setting of boundary condition___________________________________________________________________________________
Description
Value
Free surface level (m)0
Bottom level (m)-0.6Wave boundary condition option
Shallow/Intermediate Waves
Wave theory
Second Order Stokes
Wave height (m)0.08
Wave length (m)2.418-4.893
Wave period (s)1.3-2.2
Standard initialization with flat-wave open-channel initialization method was used. Other settings to run
simulation are shown in Table 4.
Table 4: Settings used to run simulation___________________________________________________________________________________
Description
Value
Time stepping method
FixedTime step size (s)0.001
Number of time steps
60000
Maximum iterations/Time step25
2.2 Numerical Set-up for Validation Models
The design for models 1 and 2 are displayed as in Fig. 1. Two-dimensional design of FOWC is displayed in Fig. 2.
The FOWC was set to be a homogeneous rigid body that is partially immersed in water-liquid. The model operates in a
fixed floating mechanism without translation or rotation; the purpose is to narrow down on the list of variables that affectthe results and also to save on computational time. Experimental results from Iturrioz et al. (2015) was used as validation;

Analysis of Floating Oscillating Water Column (FOWC)-breakwater Hybrid System:
A Numerical Study89

the only difference being the removal of the left and right legs, which are seen to have minimal effect on the result. A
FOWC draft of 0.2m and a water depth of 0.6m was set with the orifice diameter designed to be 0.05m.

Fig. 1: Schematic of a section of NWT design for models 1 and 2 with material characteristic, dimensions of FOWC
and boundary condition

Fig. 2: Two-dimensional design of FOWC used for model 1 with dimensions
The location of 2 wave gauges is shown in Fig. 3, with the dimension specified in Table 5. One wave gauge before,
and one inside the chamber of FOWC was deemed to be sufficient for validation purposes. In order to control wave
reflection and damping, a numerical beach was added between 7.36m and 9.36m from the FOWC center.

Fig. 3: Schematic of full NWT design for model 1 with the location of wave gauges and numerical beach
Table 5: Label of wave gauge and its distance from the center of FOWC_____________________________
Wave
Distance from center
Gauge
of FOWC (m)
WG0
0
WG1
-1.79
For model 2, the design is similar to model with the exception to the differences shown in Fig. 4. Another differenceis the flow viscosity, where standard k-epsilon turbulence method is used for all models except model 2, which uses SST

90Giri Ram, Mohd Rashdan Saad, Noh Zainal Abidin, Mohd Rosdzimin Abdul Rahman

k-omega method. Wave gauge located at the center of FOWC, similar to WG5 in Fig. 6 was used to gather data on wave
elevation and velocities in x and y-direction.

Fig. 4: Two-dimensional design of FOWC used for model 2 with the dimension of the top slot and location of wave
gauge
For all models, a fixed mesh setting of 3 different segments was created. As shown in Fig. 5, segment 1 is from theinlet to center, segment 2 is at the center and segment 3 is from center to outlet. For segment 1 and 3, an edge element
size of 0.01m with a bias of 10 was set for the height, whereas an edge element size of 0.01m with no bias was set for
the length. Face meshing was added for both segments with the cell shape chosen to be quadrilateral. As for segment 2,
a face element size of 0.005m with no bias was set.

Fig. 5: Schematic of mesh for models 1 and 2. (Segment 1 to the left, segment 2 at center and segment 3 to the right)
2.3 Numerical Set-up for FOWC-breakwater Models
For models 3 to 6, a breakwater is added into the NWT. The design is shown in Fig. 6 and the dimensional
specification is shown in Table 6. The gap ratio is calculated based on the gap of FOWC to breakwater, relative to theinterior diameter of FOWC. For WG3, WG5, WG6 and WG7, the distance to nearest FOWC or breakwater is maintained
to be at 0.01m, this is to ensure that the effect of rigid body on the incident and reflected waves is constant.

Analysis of Floating Oscillating Water Column (FOWC)-breakwater Hybrid System:
A Numerical Study91

Fig. 6: Schematic of a segment of NWT design for models 3 to 6 with dimensional expressions and location of wave
gauges.

Table 6: Gap length and ratio for models 3 to 6_________________________________________________________ModellGR(m)
Gap Ratio
3
0.6
2
4
0.725
2.5
5
0.9
3
6
1.05
3.5

For models 3-6, segment 2 was expanded according to the distance between FOWC and breakwater or increase in
the size of breakwater. It was ensured that a gap of 0.13m is available between the edges of segment 2 and FOWC andbreakwater, as illustrated in Fig. 7.

Fig. 7: Schematic of mesh for model 3 and 8.
2.4 Governing Equations
ANSYS Fluent solver uses finite volume method (Yamac and Koca, 2019). RANS equations were used. Eq. (1)92Giri Ram, Mohd Rashdan Saad, Noh Zainal Abidin, Mohd Rosdzimin Abdul Rahman– (3) show the mass continuity and Navier–Stokes equations (Fluent, 2013).

�����
��

𝜌𝜌 �

𝜌𝜌 �

��
��

��
��

�

�����
��

� 𝑢𝑢

� 𝑢𝑢

��
��

��

��

�0

� 𝑣𝑣

� 𝑣𝑣

(1)
��

��

�� �

�

��

��

(2)

��

��

�� �

�

��

��

(3)

� � � �� � 2𝜇𝜇 ��� � �� �𝜇𝜇 ��� � ���� � 𝐹𝐹���� � � �� � 2𝜇𝜇 ��� � �� �𝜇𝜇 ��� � ���� � 𝜌𝜌� � 𝐹𝐹���

For mass equations, 𝜌𝜌 is the fluid density, 𝑢𝑢 is velocity at horizontal direction, 𝑣𝑣 is velocity at vertical direction,

𝑥𝑥 and 𝑦𝑦 are coordinate system directions, 𝜇𝜇 is the flow viscosity, and 𝐹𝐹� and 𝐹𝐹� are forces that affect fluid in thevertical and horizontal directions.

The volume fraction of water–liquid at the probe was used to measure wave elevation while the heave motionof FOWC from centroid (average-weighted area) was used to measure the heave movement. The equation used forvolume fraction parameter during computation is shown in Eq. (4) (Fluent, 2013).
��� �𝜌𝜌� 𝛼𝛼� ���. �𝜌𝜌� 𝛼𝛼� → �� � 𝑆𝑆�� ∑��������� � ���� �

�� ��

(4)

��

Whereby, ���� is the instantaneous mass transfer from q-phase to p-phase, ���� is the instantaneous mass transferfrom p-phase to q-phase, and 𝑆𝑆�� is the term of source. Relative error is calculated on the basis of percentage of errorof numerical data from present study relative to experimental data (Iturrioz et al., 2015), as shown in Eq. (5) and (6).
(Connell and Cashman, 2016)������ � ���������
Where,

������������

x 100�����������

�∈��������� � � ���,������,� �����,� �����,� �����,�
�

(5)

�

� � � ���,������,�� ����,� �����,� �����,�
�

�

(6)

Where 𝜖𝜖��������� is the numerical data for normalized wave elevation and heave movement and 𝜖𝜖���������� is theexperimental data (Iturrioz et al., 2015) for normalized wave elevation and heave movement. 𝜖𝜖���,� to 𝜖𝜖���,� refer tothe points of the highest and lowest values from the data.

The method used for calculating reflection coefficient is based on Method I from Isaacson (1991), which uses a two

fixed probe where two heights and one phase angle is measured. Based on the expression of wave elevation in terms ofincident and reflected wave parameters, Eq. (7) was developed, followed by derivations in Eq. (8)-(9):𝐴𝐴� exp�𝑖𝑖𝑖𝑖𝑖𝑖� � � 𝐴𝐴� exp��𝑖𝑖�𝑘𝑘𝑘𝑘� � ��� � 𝐴𝐴� exp �𝑖𝑖�Φ� � �� ��
𝐴𝐴� �

𝐴𝐴� �

�

�|��� �|�

�|��� �|�𝐴𝐴� � � 𝐴𝐴� � � 2𝐴𝐴� 𝐴𝐴� cos �� � ��

�𝐴𝐴� � � 𝐴𝐴� � � 2𝐴𝐴� 𝐴𝐴� cos �� � ��

(7)
(8)
(9)

Where 𝐴𝐴� is the amplitude of incident wave, k is the wave number, 𝑥𝑥� is the nth probe location, 𝐴𝐴� is the93

Analysis of Floating Oscillating Water Column (FOWC)-breakwater Hybrid System:
A Numerical Study

amplitude of reflected wave, 𝛽𝛽 is the phase angle, 𝐴𝐴� is the measured amplitude of the nth record, Φ� is the phaseangle at first probe, 𝛿𝛿� is the measured phase of the nth wave record relative to the first record. From Eq. (8) and (9),Δ is the dimensionless distance between two probes, 𝐴𝐴� is the measured amplitude of the first record, 𝐴𝐴� is themeasured amplitude of the second record. The formula for performance coefficients are shown in Eq. (10)-(16) (He andHuang, 2016) (Zhang et al., 2021).
𝐶𝐶� �

��

(10)

��

𝐶𝐶�,� �𝐶𝐶�,� �𝐶𝐶�,� �𝐶𝐶�,� �𝐶𝐶�,� �𝐶𝐶�,� ���,���

(11)

��,���

(12)

��,���

(13)

��,���

(14)

��,���

(15)

��,���

(16)

�� ,���

�� ,���
�� ,���

�� ,���
�� ,���

�� ,���

Where 𝐶𝐶� is the reflection coefficient, 𝐶𝐶�,� is the wave elevation ratio of the gap region between the FOWC andbreakwater versus incident wave, 𝐻𝐻�,��� is the maximum wave elevation in the gap region between the FOWC andbreakwater, 𝐻𝐻�,��� is the average incident wave before the FOWC, 𝐶𝐶�,� is the velocity ratio of the gap region between

the FOWC and breakwater versus incident velocity, 𝑉𝑉�,��� is the average velocity in the vertical direction in the gap

region between the FOWC and breakwater and 𝑉𝑉�,��� is the maximum velocity in the vertical direction of incident wave

before FOWC, 𝐶𝐶�,� is the wave elevation ratio of chamber region versus incident wave, 𝐻𝐻�,��� is the maximum wave

elevation inside FOWC chamber, 𝐶𝐶�,� is the velocity ratio of chamber region versus incident velocity, 𝑉𝑉�,��� is theaverage velocity in the vertical direction inside the FOWC chamber, 𝐶𝐶�,� is the wave elevation ratio of transmission

region versus incident wave, 𝐻𝐻�,��� is the maximum wave elevation in the transmission region, 𝐶𝐶�,� is the velocity

ratio of transmission region versus incident velocity, 𝑉𝑉�,��� is the average velocity in the vertical direction in thetransmission region.

3. Results and Discussion
3.1 Results for Validation Models
Result for model 1 shows a relative error from crest to trough of 0.48% at location WG1. Figure 8 shows the resultof wave elevation at WG1.

94

Giri Ram, Mohd Rashdan Saad, Noh Zainal Abidin, Mohd Rosdzimin Abdul Rahman

0.06

WG1
Iturrioz 2015

Wave Elevation (m)

0.04
0.02
0
-0.02
-0.04
-0.06

40

45

50

Flow Time (s)Fig. 8: Graph of wave elevation versus flow time for wave gauge WG1When model 2 was numerically simulated, the result for horizontal velocity shows a similar range to the result inexperimental data, as shown in Fig. 9. The percentage of relative error from crest to trough was not calculated forhorizontal velocity because of the inconsistent pattern. From results of both model 1 and 2, it was deduced that thedesigned numerical models have a reasonable accuracy to experimental data.
Numerical Data

0.8

Iturrioz 2015

X-Velocity (m/s)

0.6
0.4
0.2
0
-0.2

39

44

49

-0.4
-0.6

Flow Time (s)Fig. 9: Graph of horizontal velocity versus flow time

3.2 Results for FOWC-breakwater models
Models 3-6 were used to study the performance of FOWC-breakwater at gap ratios 2, 2.5, 3 and 3.5 and wave periods
1.3s, 1.5s, 1.7s, 1.9s and 2.2s. Firstly, the performance was assessed in terms of Cr, as shown in Fig. 10. Result shows ahigher Cr value for gap ratio 3.5, from wave angular frequency of 3-3.5rad/s and 4-4.5rad/s. Ratio 2 shows the lowest Cr
value from wave angular frequency range of 3.3-4rad/s, the lowest being 0.07 at 3.3rad/s. Between wave angularfrequency range of 3.3-4rad/s, an increasing trend is observed for ratios 2, 2.5 and 3.
Because the reflection coefficient is measured from the incident region, it is favorable to obtain a lower Cr value.
This was achieved for all gap ratios at lower wave angular frequencies or higher wave periods from the tested ranges.
Gap ratio 2 also shows a lower Cr value on average compared to other gap ratios. This indicates that wave energy
transmission to FOWC chamber and beyond is higher at higher wave periods and lower gap ratios from the tested range.

Analysis of Floating Oscillating Water Column (FOWC)-breakwater Hybrid System:
A Numerical Study95

Cr

For He and Huang (2016), the x-axis used to plot results is different but comparable to present study; in the latter,wave angular frequency is used but in the former, the ratio of width of WEC versus wavelength was used. The differencein wavelength is proportional to the difference in period in present study. The similarity is that lower or absence of gap
could give a lower value of Cr, with an extra advantage being the saving of additional space.
For Zhang et al. (2021), a minimum Cr occurs at wave angular frequency of 3.14rad/s, which is close to the 3.3rad/s
for gap ratio 2 in present study. There is also an increase from 3.3-4 rad/s, similar to present study. However, unlike
present study, there is no correlation between gap ratio and Cr value. One major point that needs highlighting between
present study and Zhang et al. (2021) is the difference in the gap between FOWC and breakwater, which is narrower for
all ranges in Zhang et al. (2021) compared to present study.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

Ratio-2
Ratio-2.5
Ratio-3
Ratio-3.5

2.5

3.5

4.5

5.5

𝜔𝜔(rad/s)

Fig. 10: Graph of reflection coefficient versus angular wave frequency at location WG3 and WG4 for gap ratios 2, 2.5,
3 and 3.5
Results for wave elevation ratio are as shown in Fig. 11. The lowest value recorded was 0.16, which was for Ch,t at awave angular frequency of 4.83rad/s. The highest recorded value was 1.89, which was for Ch,g at a wave angular frequencyof 2.86 rad/s. There is a similar trend observed in the Ch,g and Ch,t values, where the trend across the wave angularfrequency is decreasing before flattening. This is not observed for Ch,c, which has a minimum point, before showing an
increasing trend.
The noticeably lower values for Ch,t compared to Ch,g indicates that a significant amount of energy is either extracted
or loss in the FOWC chamber and in the gap between FOWC and breakwater. The higher value of Ch,c compared to Ch,gand Ch,t at a higher wave angular frequency indicates that a lower wave period from the tested range is more favorable
for higher energy extraction in the FOWC chamber. The opposite is true for Ch,g, where the most favorable reading wasrecorded for the highest wave period. As for Ch,t, a low Ch value is preferred as it indicates that less amount of wave
energy is transmitted out of the FOWC-breakwater system. Therefore, the lowest wave period from the tested range
showed the most favorable result.
For Zhang et al. (2021) only the wave elevation ratio in the gap region between FOWC and breakwater was measured.
The result for all the tested gap ratios showed a maximum value at wave angular frequency ranges of 2.5rad/s to 3.5rad/s
before a downward trend was observed. This is similar to the trend shown by Ch,g results in present study.

96

Giri Ram, Mohd Rashdan Saad, Noh Zainal Abidin, Mohd Rosdzimin Abdul Rahman

2

Ch,g
Ch,c

Ch

1.5

Ch,t

1
0.5
0

2.5

3.5

4.5

5.5

𝜔𝜔(rad/s)

Fig. 11: Graph of wave elevation ratio versus angular wave frequency
Results for velocity ratio, as shown in Fig. 12, show a similar trend as results for wave elevation ratio. The lowest
value of 0.07 was recorded for Cv,t, at a wave angular frequency of 4.83rad/s whereas the highest value of 1.03 wasrecorded for Cv,c, at a wave angular frequency of 4.83rad/s.
A major difference between results for wave elevation and velocity ratio is that a higher value was obtained inside
the chamber at a lower wave period compared to the gap region between the FOWC and breakwater at a higher wave
period, for the wave period ranges tested.
The highest value was also the only value to be above 1, which means that the average vertical velocity in the chamber
region was higher than the maximum vertical velocity in the incident region, for wave angular frequency of 4.83rad/s
and wave period of 1.3s.

Cv

1.2

Cv,g

1

Cv,c

0.8

Cv,t

0.6
0.4
0.2
0

2.5

3.5

4.5

5.5

𝜔𝜔(rad/s)

Fig. 12: Graph of velocity ratio versus angular wave frequency
4. Conclusion
A numerical study was conducted on the performance of FOWC-breakwater hybrid system. Firstly models 1 and 2
were used for the purpose of validation with experimental data. Secondly, models of gap ratios 2, 2.5, 3 and 3.5 between
FOWC and breakwater were studied for wave periods 1.3s, 1.5s, 1.7s, 1.9s and 2.2s. The performance coefficients thatwere analyzed are Cr, Ch and Cv.
Results for validation for both models 1 and 2 show a reasonable accuracy with experimental data. For model 1, thea relative error from crest to trough of 0.48% was achieved. For model 2, the result for velocity in the horizontal direction
show a similar range to experimental data.
Results for the performance of FOWC-breakwater hybrid system show differences, depending on the regions in the
FOWC-breakwater hybrid system.

Analysis of Floating Oscillating Water Column (FOWC)-breakwater Hybrid System:
A Numerical Study97

For the incident region before the FOWC, the Cr values show a more favorable result for lower gap ratio and at ahigher wave period. For the FOWC chamber region, the Ch,c and Cv,c values show a more favorable result for lower wave
period. As for the gap region between FOWC and breakwater, Ch,g and Cv,g values show a more favorable result for higher
wave period. For the transmission region after the breakwater, Ch,t and Cv,t values show a more favorable result for a lower
wave period.
All of the results are subject to the range of gap ratios and wave periods tested.
5. Acknowledgements
The authors would like to acknowledge the Ministry of Higher Education, Malaysia and Universiti Pertahanan
Nasional Malaysia for their financial support under FRGS/1/2020/TK0/UPNM/02/03.
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