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CFD based Investigation of Wave-Structure Interaction and Hydrodynamics of an Oscillating Water Column Device

Kamath, Arun
Doctoral thesis
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URI
http://hdl.handle.net/11250/2375897
Date
2015
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  • Institutt for bygg- og miljøteknikk [2807]
Abstract
The application of computational fluid dynamics (CFD) methods to various problems

in the field of coastal and ocean engineering is gaining importance due to the level

of detail and accuracy offered by these methods. With the advances made in the

computing power over the last decade and anticipated future increase in computational

power, large and complex problems can be handled using CFD modeling.

The PhD study aims at the development of a CFD-based numerical wave tank,

validation and testing of the wave tank and application of the model to study the

hydrodynamics of an Oscillating Water Column (OWC) device and build a platform

for further research on OWC design and deployment in arrays. The development of

the numerical model covers incorporation of the best available numerical recipes to

produce accurate results in the numerical wave tank, using higher-order discretization

schemes to obtain a sharp representation of the free surface and avoiding numerical

damping of the waves propagating in the wave tank.

The numerical model is validated by investigating various phenomena in coastal

engineering such as interaction of non-breaking waves with cylinders, wave shoaling,

breaking, and interaction of breaking waves with vertical cylinders and the numerical

results are compared to experimental data with very good agreement. Wave shoaling

and decomposition over a submerged bar is simulated with very good representation

of the phase and amplitude of the decomposed waves observed in experiments. The

model is further validated for wave interaction with an OWC device by investigating

a 1:12.5 model scale device; the hydrodynamics of the device is studied and the

numerical results compared to experimental data.

Wave interaction with cylinders at low Keulegan-Carpenter (KC) numbers is

further investigated to obtain insight into the phenomena associated with large

coastal structures such as the OWC device. The investigation into high steepness

wave diffraction revealed that the wave forces on a single cylinder are over-predicted

by about 32% by the McCamy-Fuchs theory for an incident wave of steepness 0.1,

due to the difference in the wave diffraction pattern. The phenomenon of wave

near-trapping is investigated for a four cylinder group and it is found that the leading

cylinder in the group experiences two times the force on a single cylinder due to

wave near-trapping at low incident wave steepness of 0.004, but only 1.2 times the

force on a single cylinder at a higher wave steepness of 0.06 due to a break-down of

the conditions leading to wave near-trapping at a high incident wave steepness. Breaking wave forces on a vertical cylinder are known to be sensitive to the

location of the cylinder with respect to the breaking point. The maximum breaking

wave force is calculated in the scenario where the overturning wave crest impacts

the cylinder just below the wave crest level and is 1.5 times the magnitude of the

wave force calculated when the wave breaks just behind the cylinder. An impinging

jet is seen to form behind the vertical cylinder due to breaking wave impact, which

can have consequences on the wave forces on a cylinder placed behind it. It is found

that when the wave breaks at or just behind the first cylinder, the wave force on

the second cylinder is about 10% higher than the breaking wave force on a single

cylinder.

The hydrodynamics of an OWC device is studied, including the interaction

with steep incident waves, the effect of power take-off (PTO) damping due to the

turbine and the air volume in the chamber is investigated. A linear PTO system

corresponding to a bi-directional Wells turbine is assumed in the study. It is found

that for incident waves with a high steepness of 0.1, the maximum hydrodynamic

efficiency is about 40% compared to the maximum hydrodynamic efficiency of 80%

obtained at a lower wave steepness of 0.03 for the same incident wavelength.

The effect of damping from the linear PTO system is investigated and it is found

that the hydrodynamic efficiency of the OWC is found to increase from about 10%

to about 80% for the resonant wavelength on increasing the PTO damping from 0 to

4 ×108 m−2. On further increase in the PTO damping, the hydrodynamic efficiency

of the OWC is reduced to 60%. Similar trend is observed for wavelengths away from

resonance and it is concluded that there exists an optimal value for PTO damping for

every incident wavelength, which results in the maximum hydrodynamic efficiency of

the OWC for that wavelength.

The air chamber volume of a 1:12.5 scale model device is increased by increasing

the height of the chamber while maintaining the cross-sectional area to study the

effect of air compressibility. This effect is found to be negligible both in the model

scale device and in the device with an enlarged air chamber. The pressure developed

in the OWC chamber, though, is reduced by about 30% and free surface oscillations

increased by about 30% in the device with the enlarged chamber compared to the

1:12.5 scale model device. The differences observed could be attributed to the air

velocity distribution in the two configurations of the device, as it is found that the

high velocity air stream interacts with the free surface in the model scale device but

not in the device with the enlarged chamber.

The results show that the numerical model produces a good representation of

the hydrodynamics involved in the different wave transformation and interaction

processes encountered in coastal waters and in the specialized case of an OWC

device. In future work, the model can be used to further investigate the wave

forces on an OWC, interaction with irregular waves, the combination of OWC with

detached breakwaters and other device parameters to improve the hydrodynamic

characteristics of an OWC.
Has parts
Paper 1: Bihs H., Kamath A., Alagan Chella M., Aggarwal A. and Arntsen Ø.A. A New Level Set Numerical Wave Tank with Improved Density Interpolation for Complex Wave Hydrodynamics

Paper 2: Kamath, Arun; Alagan Chella, Mayilvahanan; Bihs, Hans; Arntsen, Øivind Asgeir. CFD Investigations of Wave Interaction with a Pair of Large Tandem Cylinders. Ocean Engineering 2015 ;Volum 108. s. 738-748 http://dx.doi.org/10.1016/j.oceaneng.2015.08.049

Paper 3: Kamath, Arun; Bihs, Hans; Alagan Chella, Mayilvahanan; Arntsen, Øivind Asgeir. Upstream and Downstream Cylinder Influence on the Hydrodynamics of a Four Cylinder Group. Journal of waterway, port, coastal, and ocean engineering 2016 http://dx.doi.org/10.1061/(ASCE)WW.1943-5460.0000339

Paper 4: Kamath A., Alagan Chella M., Bihs H. and Arntsen Ø.A. Shoaling and Decomposition of Breaking and Non-Breaking Waves over a Submerged Bar. - Preprint of an article submitted for consideration in Coastal Engineering Journal © 2015 copyright World Scientific Publishing Company http://www.worldscientific.com/worldscinet/cej

Paper 5: Kamath A., Alagan Chella M., Bihs H. and Arntsen Ø.A. Breaking Wave Interaction with a Vertical Cylinder and the Effect of Breaker Location

Paper 6: Bihs H., Kamath A., Alagan Chella M. and Arntsen Ø.A. Breaking Wave Interaction with Tandem Cylinders under Different Impact Scenarios. This manuscript is accepted and Inn press - Journal of waterway, port, coastal, and ocean engineering 2016 http://dx.doi.org/ 10.1061/(ASCE)WW.1943-5460.0000343

Paper 7: Kamath, Arun; Bihs, Hans; Arntsen, Øivind Asgeir. Numerical Investigations of the Hydrodynamics of an Oscillating Water Column Device. Ocean Engineering 2015 ;Volum 102. s. 40-50 http://dx.doi.org/10.1016/j.oceaneng.2015.04.043

Paper 8: Kamath, Arun; Bihs, Hans; Arntsen, Øivind Asgeir. Numerical Modeling of Power Take-off Damping in an Oscillating Water Column Device. International Journal of Marine Energy 2015 ;Volum 10. s. 1-15 http://dx.doi.org/10.1016/j.ijome.2015.01.001

Paper 9: Kamath A, Bihs H. and Arntsen Ø.A. Study of Air Chamber Volume and Compressibility Effects in an Oscillating Water Column Wave Energy Device
Publisher
NTNU
Series
Doctoral thesis at NTNU;2015:326

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