10:30
Hydrodynamics in Ocean III
Chair: Huiping FU
10:30
30 mins
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HYDRODYNAMIC PERFORMANCE OF AN ARRAY OF OSCILLATING WATER COLUMN DEVICE EXPOSED TO OBLIQUE WAVES
John Ashlin Samuel, Sannasiraj Sannasi Annamalaisamy, Sundar Vallam
Abstract: A detailed three-dimensional experimental investigation on the performance evaluation of an array of oscillating water column (OWC) integrated with offshore detached breakwater (ODBW) subjected to oblique wave incidence is studied. A Froude scale of 1:20 was chosen for integrating five similar devices of OWC to the ODBW (OWCBW) and a series of experiments was carried out in a shallow wave basin for angle of wave approaching between 900 and 700 with reference to the front face of the structure with an angular decrement of 100. The hydrodynamic performance was studied in terms of lip wall pressure ratio, wave amplification factor, air pressure ratio, capture width and relative capture width. The constructive interference due to three dimensional effects concentrates higher amount of energy in front of the array of OWCs for normal wave incidence. It makes system to absorb larger amount of wave power of about 2.3 times the given input power at natural frequency of the system. This confirms that the array of OWC devices exhibits better performance than in isolation. The performance of the system reduces with angular decrement due to peak of the wave front reaches the line front of OWCBW at different time along with wave dissipation over the breakwater. The natural frequency of the system was around d/L of 0.128 and unaltered for different angle of wave incidence.
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11:00
30 mins
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THE IMPACT OF TYPHOON ON THE HYDRODYNAMIC AND POLLUTANT TRANSPORT IN COAST AREAS BASED ON LAGRANGIAN METHOD
Zhaochen Sun, Jiafa Shen, Yifei Zhang, Songlin Han
Abstract: A better understanding of pollutant transport processes is highly desirable for the exploitation of the ocean resources and the protection of the ocean ecological system. Based on the finite-volume coastal ocean model FVCOM, a three-dimensional storm surge model was developed in Xiangshan Bay and adjacent areas in Zhejiang Province, China. Taking typhoon “Haikui” as an example, the storm surge was simulated before and after the typhoon landing based on the wind fields from the outputs of typhoon model. Then a concept based on the dynamical systems theory, Lagrangian coherent structures (LCS) was introduced in the paper. Combined with LCS, Lagrangian particles tracking method, Lagrangian residual current and synoptic Lagrangian maps (SLMs), several Lagrangian methods were applied to study the pollutant transport in the study area. The impact of typhoon on the pollutant transport was analysed. The results reveal that the daily-average residual current is significant affected by typhoon. However, the influence time is limited to the period of typhoon action. The strong hydrodynamic processes caused by typhoon significantly changes the transport and distribution characteristics of pollutant. The characteristic of water transport regimes is changed and water mixing is increased due to the typhoon. The influence on water exchange time varies significantly with position.
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11:30
30 mins
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DESIGN AND ANALYSIS OF A VERTICAL AXIS CURRENT TURBINE
Abhay Srivastava, Joydip Bhattacharjee
Abstract: The world oceans are enormous source of renewable energy that has remained largely untapped till date. Among different forms of ocean energy, tidal power is one of the most powerful resources that bears the advantages of high predictability, periodic availability and high concentration of power. India has a long coastline, which is thickly populated following the global trend. Hence, to meet the ever increasing demand of electricity, it has become necessary to exploit the power from tidal current. There are several locations in India including the Gujarat coast in the west and the Sundarbans, West Bengal in the east, where the tidal range is suitable for energy extraction.
It is well understood that the tidal current stream generators are advantageous over the tidal barrage systems in terms of both economic viability and environmental concerns. The working principle of tidal current turbines is same as the wind turbines. However, since water is much denser than air, the tidal turbine will give more energy than a wind turbine of same dimension assuming same wind speed as water. Although no particular device has emerged as the preferred one for the industry, out of different variants, vertical axis marine current turbine (MCT) is one of the variant that is being studied to improve the efficiency. Vertical axis MCT rotates about a vertical axis which is perpendicular to the current flow direction. The turbine comprises of a number of blades having hydrofoil sections that are mounted vertically between the top and bottom support. Vertical axis MCT is capable of extracting energy from any incoming direction of the tidal steam, which is not the case for horizontal axis turbine. However, vertical axis MCT shows high torque fluctuations and has no self starting capability.
In the present study, an attempt has been made to optimally design a vertical axis MCT. The maximum efficiency is obtained by modifying distinct design parameters that affects significantly the power capture efficiency. The turbine is consisting of five blades, which are being modeled using the selected airfoil section of NACA0021. Airfoil section NACA0021 is selected by considering both the hydrodynamics and robustness of designed turbine. The device is designed for a water depth of 6 m. Hence, considering the effect of cavitation and availing space for machinery, blade length is chosen to be 3 m. To get enough swept area, the diameter of the turbine was chosen to be 5.15 m, giving a swept area of 16 m2. The aspect ratio is defined as the ratio between the water depth and the diameter of the turbine, which is equal to 1.167.
Using modified designing algorithm, power coefficient is found to be 0.41 and total extracted power through turbine is found to be approximately 18 KW in the initial numerical computations. The effect of the tip speed ratio, rpm of the rotor, Reynolds number of the flow on the efficiency of the device are studied in detail. The flow passed the device and the surrounding pressure distribution is being studied using ANSYS-CFX.
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