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

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.