Transmission Coefficient Analysis Floating Breakwater using

Computational Fluid Dynamics (CFD)

Asfarur Ridlwan

, Haryo Dwito Armono

and Shade Rahmawati

Department of Ocean Engineering, Institut Teknologi Sumatera, Jati Agung, South Lampung, Indonesia

Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember, Sukolilo, Surabaya, Indonesia

Keywords: Computational Fluid Dynamics, Floating Breakwater, Porous Breakwater, Transmission Coefficient.

Abstract: Breakwater is one of the coastal structures that was built with the aim to protect the coastal area against

waves. This structure is generally designed to protect ships and facilities at the Port. Breakwaters that are

often applied are conventional breakwaters such as the rubble mound type. Climate change that causes tidal

variations and sea level height which tends to change at any time, as well as unsuitable soil conditions to

receive large structural loads will be costly and more economical if using floating breakwater. Over time, the

demand for floating breakwater development increased so that scientists and engineers did a lot of research

development both in physical experiments and numerical models. In this study numerical simulations will be

conducted by exploring the porous shape of floating breakwater from the Christensen experiment. The aim is

to get the most optimal transmission coefficient. Numerical simulations using the Computational Fluid

Dynamics (CFD) method with the help of Flow 3D Software. Validation is done first between numerical tests

and experiments to get a valid approach before exploring the development of shape.

1 INTRODUCTION

Breakwater is one of the coastal structures that was

designed to protect ships, marine ecosystems, port

facilities, and for coastal protection against waves.

Conventional breakwater is generally used in shallow

water and the geometry is bottom founded.

Unconventional breakwater is known as floating

breakwater for the deeper the water. However, there

are several advantages using a Floating Breakwater

(FB). For example, more environmentally friendly to

pollution and sedimentation problems because it does

not inhibit water circulation (Dai et. al, 2018), a little

effort to move another location (Christensen et. al,

2018). When sea water level (SWL) rises due to tides

or climate change, FB can adjust so that is more

economical than bottom founded. Further, floating

breakwater may be the only solution for high load

structures to poor soil conditions.

Floating breakwater (FB) was the first applied in

1811 to protect marinas and ships against waves at the

port of Plymouth, England (Hales, 1981). Starting in

20th century, the demand of floating breakwater

increased to anticipate infrastructures development of

ocean space. So, Scientist and Engineers did a great

developing FB research either experimentally,

numerically, or combination of both. Most research is

usually done on various geometric shapes,

configurations, or bilge keels of FB on the wave

characteristics.

Experimentally and numerically of floating

breakwaters have been studied. For instance,

(Christensen et al., 2018) conducted physical

experiments and numerical modeling to evaluate

transmission, reflection, and performances of Regular

Pontoon (RG), WP, and WP-P100. The study showed

that wing plates (WP) reduced the motions of floating

breakwater, while wing plates and porous media

(WP-P100) reduced the refection and transmission

most effectively. (Ji Chunyan et al., 2018) conducted

experimental study for a dual rectangular pontoon

floating breakwater with two treatments (single-row

and double-row). The result indicated that double-

row floating breakwater significantly reduced

transmission especially for short-period wave than

single-row floating breakwater. (Cho, 2011)

investigated transmission of incident waves

interacting with vertical porous side plates using

matched eigenfunction expansion method (MEEM).

(Wang and Sun, 2010) conducted experimental study

of a porous floating breakwater with fabricated large

numbers of diamond-shaped blocks to reduce

Ridlwan, A., Armono, H. and Rahmawati, S.

Transmission Coefﬁcient Analysis Floating Breakwater using Computational Fluid Dynamics (CFD).

DOI: 10.5220/0010059301430149

In Proceedings of the 7th International Seminar on Ocean and Coastal Engineer ing, Environmental and Natural Disaster Management (ISOCEEN 2019), pages 143-149

ISBN: 978-989-758-516-6

143

transmitted wave height and mooring force. (Dong et

al., 2008) conducted two-demensional physics model

test on three types of structures: the single box, the

double box, and the board net floating breakwaters.

The result showed that transmission coefficient

depend on the width of board, and the length of the

mooring chain lying on the sea bottom. (Rahman et

al., 2006) conducted numerical study of single

pontoon floating breakwater to estimate the nonlinear

dynamics using the volume of fluid (VOF) method.

This paper presents numerical study or simulation

to analysis transmission coefficients using

computational fluid dynamics (CFD) method. The

basic geometry is regular ponton floating breakwater

that modified in several notched shape on the side of

structure. Data parameters and numerical model

verifications based on the result of experiments that

conducted by (Christensen et al., 2018).

2 NUMERICAL SIMULATIONS

2.1 Floating Breakwater Geometry

Floating breakwater geometry is based on Regular

Pontoon (RG) experiment test conducted by

(Christensen et al., 2018). The experiment conducted

two-dimensional physical model test where cross-

sections were tested and analysed in wave flume. The

data parameters are shown in Table 1 and illustrated in

Figure 1. Furthermore, the experimental model will be

developed on porous shape on side plates with

numerical simulations using computational fluid

dynamics (CFD) to optimalization wave transmission.

The developed shapes can be seen in Figure 2.

Table 1: Regular Pontoon Dimensions.

No Geometry Dimensions [m]

1 Length (L) 0.58

2 Width (W) 0.46

3 Draft (D) 0.31

4 Height (H) 0.39

Figure 1: Basic Cross-section Regular Pontoon FB.

(a) (b) (c) (d)

Figure 2: Developed Model Floating Breakwater (Notched Shape).

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

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2.2 Data Parameters

The data were composed of wave parameters that are

scaled. The scale is assumed to use Froude scalling

by 1:65. Froude scalling was considered valid as long

as viscous effects are negligible. The 2

stokes wave

was used on this simulation. Table 2 shows conditions

for numerical simulations.

2.3 The Wave Flume

The flume is 28 m long, 0.6 m wide, 0.8 m high, and

0.615 m initial surface elevation. The flume was filled

water with density of 1000 kg/m

. The flume was

equipped with wave maker at one and wave absorber

at the other end. Where, the floating breakwaters were

placed almost cover the entire width of flume to

reduce the effect of sidewalls. See, Fig. 3 for

ilustraton.

2.4 Wave Measurements and Analysis

The objective of this study was to obtain the

transmission coefficient (C

), the ratio of transmitted

wave height (H

) to incident wave height (H

). See

equation 1 below:

= H

/ H

(1)

Measurement of transmitted and incident wave

heights were represented by surface elevation using

wave gauges. 4 wave gauges were placed on the

incident side, while 3 wave gauges were placed on the

lee side. See figure 4.

2.5 Mooring System

The mooring lines were installed only to keep floating

breakwater on position. The mooring system have an

effect on the performances of stability and the

mooring forces, but in this study no analysis of both.

There are four mooring lines on each corner of

floating breakwater, two on each side of the cross-

section. The submerged weight of the mooring line

was w =0.589 N/m. The mooring line made from

polyethylene (PE) with density of 880 kg/m

and

diameter of 0.01 m. The dimension of mooring line

was illustrated on figure 1.

Table 2: Parameters Wave Data.

No.

Wave

Length

Wave

Period

Frequency

Depth

Ratio

Wave

Height

L[m] T[s] f[Hz] h/L[] H

{

}

1 1.174 0.868 1.152 0.524 0.023

2 1.883 1.116 0.896 0.327 0.036

3 2.252 1.240 0.806 0.273 0.041

5 3.333 1.613 0.620 0.185 0.052

6 4.024 1.861 0.538 0.153 0.055

Figure 3: Sketch of the wave flume.

Figure 4: Position Wave Gauges in Unit [mm].

Transmission Coefﬁcient Analysis Floating Breakwater using Computational Fluid Dynamics (CFD)

145

2.6 Boundary Conditions

The purpose of boundary conditions was to determine

model conditions that represented experiment

conditions. See figure 5. Boundary conditions used in

the model are as follow:

(1) Free surface (Wave): A wave boundary condition

was defined at left (X Min). A surface wave

entered the computational domain and

propagated in the direction normal to the

boundary. The wave was set 2

stokes as wave

generator represent the physical wave conditions

at the boundary.

(2) Outflow: The outflow boundary condition was

defined at right (X Max). It allowed users to

numerically investigate the effects of wave

interactions with structures. The capability

permited a reduction in the extent of the

computing mesh needed for accurate

computations. A wave-absorbing layer used to

reduce reflection of periodic wave at an open

boundary.

(3) Symmetry: The symmetry condition was defined

at front (Y Min), behind (Y Max), below (Z

Min), and up (Z Max). No-slip conditions were

imposed using the wall shear-stress options

described in the Prandtl Mixing Length model. A

symmetry condition can be specified as free-slip

conditions that have a non-zero wall shear-stress.

2.7 Meshing

Mesh block was used to determine the area that

modelled. The smaller mesh will be more detailed,

but the output files will be larger and simulations run

longer. Floating breakwater modelling used one mesh

block with a meshing size of 0.04 m at total length

(X-axis) is 28 m, total width (Y-axis) is 0.6 m, and

total height (Z-axis) is 0.8 m.

3 RESULTS AND DISCUSSION

3.1 Validation

Validation is done by comparing the results of

transmission coefficient of the (Christensen, 2018)

study with the numerical study of the Computational

Fluid Dynamics (CFD) aided by Flow 3D Software.

Christensen has conducted study on three shapes of

floating breakwater, but only one shape of floating

breakwater validated in this study is Regular Pontoon

(RG). The purpose of validation is to find out whether

the numerical study conducted is in accordance with

the experimental test. Validation is done in a simpler

shape (RG). If validation are represented, then the

development of the model can be done to more

optimal shapes.

In this study, validation is based on comparison of

the transmission coefficient of numerical test

simulation and experimental test. The error rate refers

to Mean Absolute Percentage Error (MAPE) theory.

MAPE theory can be seen on table 3. As for

validation results can be seen in figure 6. Based on

MAPE theory, this study can be said to be validated

and can be developed more optimal shapes.

Table 3: MAPE Theory.

No MAPE Value Prediction

MAPE ≤ 10%

High

10% < MAPE ≤ 20%

Good

20% < MAPE ≤ 50%

Reasonable

MAPE > 50%

Low

3.2 Transmission Coefficient Regular

Pontoon (RG)

Regular Pontoon is one type of floating breakwater

that was examined by (Christensen, 2018). In this

section, the results of transmission coefficient of

numerical simulation results can be seen in table 4.

The transmission coefficient is obtained from the

analysis of the output surface elevation results in

Figure 5: Boundary Conditions Model.

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

146

Flow 3D software. Surface elevation data used to

analyze the transmission coefficient are obtained

from wave gauge 1 (in front of the structure) and

wave gauge 5 (behind the structure). Surface

elevation data is converted into wave height using

Wave Analysis (WAVAN) software. Then, to get the

transmission coefficient the calculation is according

to formula (1). The results of regular pontoon

coefficient transmission are shown in table 4.

3.3 Transmission Coefficient

Development Notched Shape

It is still planned to analyze the transmission

coefficient of porous shapes floating breakwater.

There are 3 exploratory shapes of porous floating

breakwater, see figure 2. In this analysis, it used Flow

3D software such as what was done in getting the

transmission coefficient of without porous shape

floating breakwater (RG).

4 CONCLUSIONS

Four porous shapes of floating breakwater will be

examined in experimentally and numerically. But,

still planned. The porous shape is placed in lee side

and back side of the structure. The basic shape of the

structure is regular box. This experiment was

conducted by (Christensen, 2018) in a wave flume in

the hydraulic laboratory at the Technical University

of Denmark. As well as numerical simulations carried

out by the authors by taking Christensen’s

experimental data. Numerical analysis using

Figure 6: Validation Results.

Table 4: Value of Validation Results.

Data Input

T (experiment)

T (numeric)

Error (%)

H [m] T [s]

1 0,0230 0,8680 0,0353 0,0197 0,6046 0,5581 8,3370

2 0,0360 1,1160 0,0329 0,0248 0,7114 0,7538 5,6248

3 0,0410 1,2400 0,0491 0,0328 0,6125 0,6680 8,3117

4 0,0490 1,4880 0,0521 0,0235 0,5054 0,4511 12,0483

5 0,0520 1,6130 0,0589 0,0408 0,7756 0,6927 11,9677

6 0,0550 1,8610 0,0498 0,0461 0,9673 0,9257 4,4936

Mean Absolute Precentage Error (MAPE) 8,4638

Transmission Coefﬁcient Analysis Floating Breakwater using Computational Fluid Dynamics (CFD)

147

Computational Fluid Dynamics (CFD) method with

the help of Flow 3D software. The aim is to optimize

the transmission coefficient by exploring the porous

shape, due to limited experiments. Previously,

validation was done between numerical and

experimental.

ACKNOWLEDGEMENTS

This study is a requirement to pass the Master of

Oceang Enginering. The author is very grateful to the

supervisor who has helped in providing the sciences

so that this research is complete.

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