Artificial reefs

Hydrodynamic Design of New Type of Artificial Reefs

O. B. Yaakob1,2,*, Yasser M. Ahmed1,2, M. Rajali Jalal1,

Faizul A. A.1,2, K. K. Koh1,2, Tarmizi J. Zaid2

1Marine Technology Center, Universiti Teknologi Malaysia,2Faculty of Mechanical Engineering, Universiti Teknologi Malaysia,

omar@fkm.utm.my, yasser@mail.fkm.utm.my

Introduction

Determination of Water Particle Velocity

Computational Domains and Grids

Results and Comparisons

Conclusion

OutlinePresentation Outline

Introduction• Fisheries industry is one of the important industries in Malaysia.

Majority of the citizen who live near the sea are involved in this industry and make profit from it.

• Artificial reefs are man-made structures, placed on the seabed deliberately for reducing beach erosion, maintaining coasts, protecting habitats, boosting recreational fishing and increasing biotic diversity.

• There are different types of ARs such as cube, pipes, cross-shaped and…et, which provide good mediums for fish perching, foraging, breeding and defense from enemies

Some types of ARs in Malaysia

Due to the importance of studying the flow pattern around ARs, many studies have been conducted experimentally in wind tunnel and using CFD simulations to study the effect of geometry of these ARs on the flow pattern around them.

• In this research work, the flow field patterns around and inside a new type of artificial reef, based on the design of streamlined bicycle helmet and hollow cube artificial reef of the same volume were studied by employing FVM code Ansys CFX and Potential Flow Code Aqwa.

New type of artificial reef (helmet artificial reef)

Hollow cube artificial reef

Determination of Water Particle Velocity• The horizontal forces which acted on both ARs had been calculated using Ansys CFX for the viscous drag force (FD), while the inertia force (FI) due to wave effect on the bodies of the ARs had been calculated using the potential flow code, Ansys Aqwa.

• Based on the linear wave theory, the horizontal water particle velocity (u) which moves towards the reef body in x direction can be calculated using

• The range of ω was taken from 0.5 to 1.5 rad/sec and a was equal to 1 m based on the environmental data for Malaysia seas.

• Three values for h were considered in this study, which were 20, 30 and 40 m.

Determination of Water Particle Velocity

Computational Domains and Grids

Numbers of unstructured, panel mesh and total mesh elements used in each case in this study

Results and Comparisons

The numerical results for the hydrodynamic drag force FD , inertia force FI and wave force FW

FW = FD + FI

0

100

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0 0.25 0.5 0.75 1 1.25 1.5 1.75

FD(N)

Wave frequency ω (rad/sec)

FD H C h = 20 m FD Hel h = 20 mFD H C h = 30 m FD Hel h = 30 mFD H C h = 40 m FD Hel h = 40 m

FD for helmet (Hel) and hollow cube (H C) ARs at different water depths (Ansys CFX)

0

20

40

60

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100

120

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180

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0 0.25 0.5 0.75 1 1.25 1.5 1.75

FI(N)

Wave frequency ω (rad/sec)

FI H C h = 20 m FI Hel h = 20 mFI H C h = 30 m FI Hel h = 30 mFI H C h = 40 m FI Hel h = 40 m

FI for helmet (Hel) and hollow cube (H C) ARs at different water depths (Ansys Aqwa)

0

100

200

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700

0 0.25 0.5 0.75 1 1.25 1.5 1.75

F W(N)

Wave frequency ω (rad/sec)

FW H C h = 20 m FW Hel h = 20 mFW H C h = 30 m FW Hel h = 30 mFW H C h = 40 m FW Hel h = 40 m

FW on helmet (Hel) and hollow cube (H C) ARs at different water depths

0

100

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900

0 0.25 0.5 0.75 1 1.25 1.5 1.75

FD(N)

Wave frequency ω (rad/sec)

FD H C 0 deg FD Hel 0 degFD H C 45 deg FD Hel 45 degFD H C 90 deg FD Hel 90 deg

Drag force FD for helmet (Hel) and hollow cube (H C) ARs at different flow directions at h = 20 m (Ansys CFX)

0

50

100

150

200

250

0 0.25 0.5 0.75 1 1.25 1.5 1.75

FI(N)

Wave frequency ω (rad/sec)

FI H C 0 deg FI Hel 0 degFI H C 45 deg FI Hel 45 degFI H C 90 deg FI Hel 90 deg

Inertia force FI for helmet (Hel) and hollow cube (H C) ARs at different flow directions at h = 20 m (Ansys CFX)

0

100

200

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900

0 0.25 0.5 0.75 1 1.25 1.5 1.75

F W

(N)

Wave frequency ω (rad/sec)

FW H C 0 deg FW Hel 0 degFW H C 45 deg FW Hel 45 degFW H C 90 deg FW Hel 90 deg

Wave force FW for helmet (Hel) and hollow cube (H C) ARs at different flow directions at h = 20 m

Velocity vectors for flow direction 0o

Velocity vectors for flow direction 45o

Velocity vectors for flow direction 90o

• The numerical results showed that the hollow cube artificial reef was always subjected to higher FD and FI than the helmet artificial reef at all water depths and flow directions, except for h = 20 m and flow direction 45o, where there were little increases in the values of FI of the helmet unit than that of hollow cube artificial reef in this case.

• The streamlined design of the new artificial reef always had less FW than that of the hollow cube artificial reef at all water depths and flow directions.

• The flow pattern in front of the helmet reef was always subjected to less resistance than that of the hollow cube artificial reef, which led to the reduced area of the stagnation pressure on the unit body in front of the water flow.

Conclusion

• The streamlined body of the helmet artificial reef improved the flow pattern at its rear region and provided zones with moderate flow, which can help fishes and marine organisms from finding good shelter.

• The different openings in the body of the helmet artificial reef

improved the condition of the flow velocity distribution inside the unit than that of the hollow cube unit, which can increase the amount of the nutrient to the living fishes and organisms inside the reef.

• However, more studies are still required, such as using PIV measurements, for discovering more details about the flow pattern of the helmet artificial reef in different conditions.

Conclusion (cont.)

Thank You