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This study utilizes the sea wind-tower data of eight typhoons from September 2015 to July 2017, including Dujuan (2015), Nepartak (2016), Meranti (2016), Malakas (2016), Megi (2016), Aere (2016), Nesat (2017) and Haitang (2017), to obtain wind speed profile parameters in the Taiwan coastal area during these typhoons. The data obtained is compiled to project the wind speed profile parameters of the wind turbine and the maximum wind speed during the 50-year regression period for the potential offshore turbines in the proximity. The findings are to help accumulate data of natural wind field in the offshore region in Taiwan, and to provide reference for selecting locations for wind turbines in the future.
The conclusions of this study are as follows: the α value of the wind field in the offshore area of Hsinchu is 0.0988, and the ratio of the maximum instantaneous wind speed to the maximum average wind speed is 1.277; the 50-year regression period wind speed uses the Type I extreme value distribution to obtain the maximum instantaneous wind speed of the region is 65.95 m/ s (without threshold value), 59.72 m/s (take threshold value), maximum average wind speed is 51.57 m/s (no threshold value), 46.80 m/s (take threshold value); use Type II extreme value distribution The maximum instantaneous wind speed in the area is 79.73 m/s (no threshold value), 67.71 m/s (take threshold value), the maximum average wind speed is 61.05 m/s (no threshold value), 51.16 m/s (take the threshold) Value), compared with national norms, the alpha value is closer to the specifications of Japan and the United States.
1. Motivation and purpose of research
Due to the concern of widespread global greenhouse effect along with the rapid growth of petroleum energy consumption, the Taiwanese legislation passed the Green Energy Innovation Technology Program in October, 2016. The goal of this program is to develop green energy industry with focus of renewable energy mainly including solar power and offshore wind power. The “Two-Year Plan for Solar Photovoltaic” and the “Four-Year Plan for Wind Power Generation”, proposed by the Ministry of Economic Affairs, set goals to achieve 20 GW from sunlight and 3 GW from wind power by 2025. Not only will it help Taiwan to keep up with global energy conservation and carbon reduction, but also can stipulate domestic industries, create opportunities of investment and employment, and most importantly, achieve environmental sustainability, which leads to a clean and green Taiwan for generations to come.
Accurate prediction of typhoon-induced surge is an important issue for coastal disaster prevention. The purpose of this study is to develop a long-lead-time prediction model for storm surge by using adaptive neuro-fuzzy inference system approach (ANFIS). We carefully select effective controlling parameters, including the maximum wind speed, central pressure deficit, radius of maximum wind, location of typhoon center to the tidal station (i.e., distance and angle). In addition, the forward speed and direction of typhoon are considered to improve the long-lead-time prediction. The prediction performances are examined and discussed. Overall, predictions up to t + 9 hour ahead of lead time are satisfactory (with correlation coefficient CC > 0.7).
Coastal areas with natural resources and economic potentials are highly developed regions in a country. Over the world, more than one billion people are working and living in these areas. The continuously increasing population might approach 2 to 5 billion by 2080 (IPCC, 2007). However, coastal environment system would face destructive hazards induced by the combined influences of land/river, ocean and atmospheric forcing. One of such threats is typhoon-induced surges.
Storm surge has always been an important topic in both science and engineering due to its severe social impacts. In 2018, for example, Hurricane Michael made landfall near Mexico Beach, Florida, United States. It brought heavy rain, high winds, and an extreme storm surge up to 14 feet, resulting in at least 60 deaths and 15 billion economic losses. The catastrophic tragedies might repeat and even get worse, as typhoons and storm surges are expected to surpass historical records under a changing climate, i.e., global warming (Webster et al., 2005; Emanuel, 2005).
Accurate and efficient surge prediction with better understanding of its variation (e.g., maximum or possible range) plays a critical role in disaster mitigation. The basic idea for storm surge prediction is to capture its essential components (e.g., see Flather, 2001). Two main mechanisms are (i) pressure setup (i.e., a pressure drop of 1 mb leads to a 1-cm rise in sea level) and (ii) wind setup (i.e., strong onshore wind leads to significant sea level rise). To date, the prediction tools can be generally divided into three classes: (i) empirical formulas, (ii) hydrodynamic models, and (iii) artificial intelligence approaches.
Lee, Chi-Fang (CR Classification Society) | Sun, Chien-Ting (CR Classification Society) | Hsin, Ching-Yeh (National Taiwan Ocean University) | Lee, Ya-Jung (CR Classification Society) | Chiu, Forng-Chen (National Taiwan University) | Lin, Tsung-Yueh (CR Classification Society)
This study focuses on the ultimate strength assessment of the Fiber-Reinforced Plastic (FRP) sandwich-structured composite blades of a current turbine under normal and abnormal conditions. The structural response was evaluated through finite element analyses that reproduced the interaction between the core and the FRP layers. This study discusses the design extreme loads assessment, the finite element modeling of the blades, and the related strength criteria to be considered.
The Kuroshio Current flows along the east coast of Taiwan from the Philippines to Japan following a northeast direction. The Kuroshio Current being relatively steady in amplitude and in direction, not much affected by tides, and, with an average velocity of 1.5 m/s, could offer an economically viable source of energy to Taiwan. However, numerous marine creatures live and travel in the Kuroshio Current for its warm temperature and high velocity, which increases the probability of collisions with immersed current turbines. Therefore, a research project funded by Taiwan’s Ministry of Science and Technology was conducted by National Taiwan University (NTU) and National Taiwan Ocean University (NTOU) to evaluate the feasibility of a 0.5 MW Floating Kuroshio Current Turbine (FKT).
This study focuses on the ultimate strength assessment of the Fiber-Reinforced Plastic (FRP) sandwich-structured composite blades, including the root connection under normal and abnormal conditions. The extreme loads arising during nominal power production were produced by RANS-formulation CFD simulations (Chiu, Lai, Lee, Tzeng, and Hsin, 2018), and the structural strength of the blades and root connections was verified against yielding as per the rules (DNVGL, 2015, BV, 2015). Besides, the accidental loads arising from collision were considered in a ‘fail safe’ manner, where quasi-static concentrated forces were applied at the blade tip with increasing amplitudes until the failure of the blade, while the strength of the root connection made of steel parts was verified against yielding. This design approach enabled us to ensure that the failure of the blades will occur before compromising the structural integrity of the blade root connection, thereby limiting the cost of blade replacement operations.
Shih, Chao-Feng (National Taiwan Ocean University / Taiwan International Ports Corporation, Ltd.) | Chen, Yung-Wei (National Taiwan Ocean University) | Soon, Shih-Ping (National Taiwan Ocean University) | Ho, Sheng-Yu (National Taiwan Ocean University)
In this paper, we use the multiple scale Trefftz method (MSTM) combined with the Lie group scheme to simulate the two-dimensional nonlinear sloshing problem. When considering the effects of baffles for the nonlinear sloshing phenomena, the conventional Trefftz method (CTM) may encounter numerical instabilities, degenerate scale, and numerical dissipation problem. In order to eliminate the high-order numerical oscillations and noise disturbances on the boundary, the vector regularization method (VRM) and the multiple scale characteristic lengths (CLs) are applied. At the same time, they can also overcome the degenerate scale problem. Additionally, in order to increase the numerical accuracy at the initial-boundary value problem, we introduce the weighting factors based on the Lie group scheme into the linear system to avoid high numerical dissipation and reduce the numbers of iterations. Comparing with the solutions in the previous literature, the present scheme is efficient and accurate in simulating nonlinear sloshing problem of the two-dimensional tank with baffles. Hence, the method developed here is a simple and stable way to cope with the nonlinear sloshing problem with baffles.
Liquid sloshing in a moving container has been studied in various engineering problems, such as liquid propellant launch vehicles (Mohammad et al., 2011), oil surges in large tanks due to long-term intense ground motion (Tetsuya and Takashi, 2013), and shock surges in nuclear fuel pools (Eswaran and Reddy, 2016). Besides, the sloshing effect in the ship’s ballast tanks may cause it to experience large rolling moments, even losing dynamic stability and overturning (Przemysáaw et al., 2012). Also, if the forcing frequency coincides with the natural sloshing frequency, the high dynamic pressures, by reason of resonance, may damage the tank walls and may even create moments that affect the stability of the vehicle. Therefore, the hydrodynamics of sloshing for the vehicle is important and requires understanding sloshing dynamics phenomena.
Hsieh, Chih-Min (National Kaohsiung University of Science and Technology) | Cheng, Ming-Hung (National Taiwan Ocean University) | Sau, Amalendu (Gyeongsang National University) | Hwang, Robert R. (National Taiwan Ocean University / Institute of Physics) | Peng, Yih-Ferng (National Chi Nan University) | Yang, Wen-Chang (Taiwan Ocean Research Institute)
In order to study the interaction between the internal waves (IWs) and the surface waves (SWs) with different periods over a slope-shelf, laboratory experiments and numerical simulations by using plunger wave maker are conducted in the paper. Laboratory results reveal that the wave period of IWs shows significant decrease during this process at long incident; at short incident, the wave period of IWs and SWs keep the same value. The results of the numerical simulation show that the short interaction time causes the weak internal hydraulic jump on front slope when IWs propagates over a slope-shelf.
In a stratified fluid, the barotropic waves (i.e., Surface waves; SWs) are generated by external forces (ex., wind or flow) in surface layer and then baroclinic waves (i.e., Internal waves; IWs) are induced in interface d due to the different pressure variations. The field observations reveal, in the ocean, the IWs are generated by the interaction between flow and submerged topography (Hsu et. al., 2000); in the lake, the wind plays an important role on producing the surface pressure difference and internal waves are generated indirectly (Pannard et. al., 2011); at the estuary, the internal waves are generated by the interaction between waves and flow. Internal waves have significant effect on ecology, environment and engineering when they propagate over varied topography (Bourgault et al., 2014; Lamb, 2014). Internal solitary wave (ISW), which is a special type of internal waves, has been researched by many literatures about its generation, transport and dissipation (Grimshaw and Helfrich, 2018; Hsieh et al. 2016, Lamb, 2014). On the other hand, as short flow is indiced by larger amplitude of ISW, the resulting surface waves become small and can be ignored during this interaction.
In lake and estuary, IWs generated by surface pressure variations may affect stratified fluid mixing, SWs’ transmission, bottom nutrient pumping and lakeside scouring. Up to now, the existing literature about the interaction between surface and internal waves or together with a submerged obstacle is less and unclear. The mechanism for IWs’ or SWs’ generation, propagation and dissipation are the important topics for research. Although sequential data can be recorded by ADCP or CTD in the field observations, it is difficult to clearly analyze the interaction between IWs and SWs. Accordingly researchers often adopted both theoretical analysis, laboratory experiments and numerical simulations. However, for theoretical researchers studying the interaction between surface and internal waves, they almost adopt two wave functions directly in the surface and interface and ignore the density mixing in calculating the fluid field (Das et. al., 2018; Lee et. al., 2007; Shermeneva et al, 2015). For laboratory experiments, several wave-making mechanisms are followed to generate surface and internal gravity waves or internal waves: Vlasenko and Hutter (2001) used two pistons in upper and lower layers to study internal solitary waves transformation over a sill; Dalziel et al., (2007) adopted gate-type wavemaker to observe the internal solitary waves transformation on flat bottom; Ko and Cho (2017) used plunger type wavemaker (cylindrical shape) to find the interactions between surface and internal waves on flat bottom. Cheng et al. (2018a) discussed the variations of wave amplitude and wave period for the SWs and IWs that are generated by plunger type wave maker and propagate over a submerged ridge.
Shih, Ruey-Syan (Tungnan University) | Weng, Wen-Kai (National Taiwan Ocean University) | Li, Chi-Yu (National Taiwan Ocean University) | Wu, Chia-Ying (Tungnan University) | Wang, Ying-Chi (Tungnan University)
This paper investigates the wave attenuation and reflection induced by a submerged barrier in presence of small forward and reversed currents using hydraulic model tests, to understand the analysis and prediction of wave reflection, transmission and loss coefficient under the influence of the existing currents with different directions. The wave attenuation and reflection induced by a submerged barrier in presence of small forward and reversed currents is experimentally discussed by using both the regular and irregular wave. Wave attenuation of the embankments affected by external current are also discussed and compared with those of cases without current. The results indicated that the influence of present current on wave reflection is slightly larger for irregular waves than that of regular waves.
In coastal protection technology, artificial structures such as seawalls, jetty, armor units, and detached breakwaters are traditionally adopted as absorbing facilities to eliminate wave energies. Scholars have studied various offshore embankments, including changing the shape of the submerged embankment (Shih and Weng, 2016), the permeability characteristics of the submerged embankment, the number of submerged embankments and the distance between the submerged structures (Shih et al., 2013, 2014), to explore the interaction between waves and structures and their dissipation effectiveness. Traditionally, coastal protective was achieved by applying the constructions of armor units and rubble mound breakwater, etc., with the change of leisure patterns and the demand for activity spaces, many dissipation technologies and new energy-dissipation structures have been extensively developed and investigated to reduce wave energy, that provided the maintenance of natural ecology and landscape. The applications of the stepped rough surface on coastal structures has been studied by Krecic et al. (2004), and Lokesha et al. (2015), they explored the effect of smooth and stepped submarine embankment on the wave transmission characteristics with hydraulic model tests. The investigations on stepped revetments were limited in the past. Kerpen et al. (2014) conducted a series of experiments using 2D hydraulic model tests. Three different breakwater heights and breakwater widths consisting 18 sets of stepped and smooth breakwaters. Detailed descriptions of the roughness slopes for different types of revetments can be found in EurOtop (2018).
Lan, Yuan-Jyh (National Taiwan Ocean University)
In this study, the problem of linear wave propagation over a fixed and floating poroelastic medium is studied theoretically. The floating poroelastic medium is assumed to be homogeneous, isotropic and elastic, and allows slightly heaving motion. Lan-Lee’s poro-elastomer theory is extended to derive a new analytical solution for describing the problem. Using general solutions for each region and the matching boundary conditions, a set of simultaneous equations is thus developed and solved numerically. Wave reflection, transmission and energy dissipation induced by different key parameters of a poroelastic medium are studied.
Humans have imitated the floating natural vegetation to build floating islands centuries ago. The ancient Peruvian population built the Islands of Uros in Lake Titicaca to escape violent attacks from more aggressive tribes of the Collas and the Inca. In order to protect the land demand for economic development of densely populated coastal cities and areas, the relevant research on wave defense and interaction between floating breakwaters, offshore floating platforms (Lee and Lee, 1993; Oliver et al., 1994; Khabakhpasheva and Korobkin, 2002; Nakamura et al., 2003; Diamantoulaki et al., 2008; Zhao et al., 2012; Ruol et al., 2013; Yeh et al., 2013; Burcharth et al., 2015; Tsai et al., 2016; Dolatshah et al., 2018; Emami and Gharabaghi, 2018; Yu et al., 2018) and VLFS (Very Large Floating Structures) (Takagi, 1996; Utsunomiya and Watanabe, 2006; Wang and Tay, 2011; Lamas-Pardo et al., 2015; Shirkol et al., 2016; Sun et al., 2018) in coastal and offshore areas have been carried out in recent decades. In addition, ecological floating islands have been widely recognized as an effective tool for habitat restoration in many European countries and the United States (Winston et al., 2013), and have been commercially used in water environments in many countries. The composition of the ecological floating island basically inherits the natural vegetation floating island with permeability and softness (deformability). Most of the research on artificial ecological floating islands focuses on issues such as ecological environment, water quality improvement, and landscape maintenance (Nakamura et al., 1996; Francis, 2009; Winston et al., 2013). Research on the reduction of wave energy to protect waterfront functions is less intensive and generally appears in case studies (Gaffney and Munoz, 2010; Zhu and Zou, 2016). Due to the flexibility of soft material, the deformation induced by water waves disturbs flow field in the vicinity of the structure when compared with impermeable reflective concrete counterparts (Lan, 2018). For the wave attenuation effect caused by the permeable structures, different structural material properties have different wave damping characteristics, such as the flow configurations of Darcy, Darcy-Forchheimer and vegetation drag resistance, etc. Therefore, in academic and practical studies, ecological floating islands need to consider both the flexibility and permeability of the floating medium composition, such as the poroelastic type.
Chen, Yung-Wei (National Taiwan Ocean University) | Shih, Chao-Feng (National Taiwan Ocean University) | Liu, Yu-Chen (National Taiwan Ocean University) | Soon, Shih-Ping (National Taiwan Ocean University)
This paper presents an equal-norm multiple-scale Trefftz method (MSTM) associated with the group-preserving schemes (GPS) to tackle some difficulties in nonlinear sloshing behaviors. The MSTM combined with the vector regularization method is first adopted to eliminate the higher-order numerical oscillation phenomena and noisy dissipation in the boundary value problem. Then, the weighting factors of initial and boundary value problems are introduced into the linear system to prevent the elevation from vanishing without iterative computational controlled volume. More important, the explicit scheme, based on the GL (n, R), and the implicit scheme can be combined to reduce iteration number and increase computational efficiency. A comparison of the results shows that the proposed approach is better than previously reported methods.
Sloshing of liquid in tanks has received considerable attention from many researchers in related engineering fields. The problem arises because excessive sloshing of the confined liquid can strongly damage the structure or the loads induced by sloshing, which may seriously modify the dynamics of the vehicle that supports the tanks—for example, fuel sloshing in liquid propellant launch vehicles (Lu et al., 2015), oil oscillations in large storage tanks as a result of long-period strong ground motions (Hashimoto et al., 2017), and sloshing in nuclear fuel pools owing to earthquakes (Eswaran and Reddy, 2016). Besides, sloshing effects in the ballast tanks of a ship may cause it to experience large rolling moments and eventually capsize because of loss of dynamic stability (Krata, 2013; Sanapala et al., 2018). Also, if the forcing frequency coincides with the natural sloshing frequency, the high dynamic pressures, by reason of resonance, may damage the tank walls. Thus, accurate prediction of sloshing behaviors in tanks driven by external forces is very critical for successful structural design and reducing impacts on vehicle maneuvering.
Cheng, Ming-Hung (National Taiwan Ocean University) | Hsieh, Chih-Min (National Kaohsiung University of Science and Technology) | Hwang, Robert R. (National Taiwan Ocean University) | Hsu, Tai Wen (National Taiwan Ocean University)
Surface waves (SWs) and internal waves (IWs) are the most common natural phenomena in the ocean. Some oceanographers believe that internal solitary waves have significant influence over the surface waves. In order to observe the interaction between IWs and SWs with different periods over a submerged ridge, laboratory experiments by using plunging wavemaker are run in the study. Laboratory results reveal that the amplitude of SWs decreases as the fluid changes from homogeneous into stratified. At the slow wavemaker speed, the wave period of SWs has a significant decrease when the wave propagates over a ridge; at fast wavemaker speed, the wave period of SWs keep the same value. For the evolution of IWs in strong breaking cases, the wave transmitted amplitude increases as the wave approaches to the ridge. The wave fundamental period of IWs also shorten in slow wavemaker speed but is fixed in fast wavemaker speed. Moreover, the oscillation noises in long period is significant than that in short period.
In a stratified fluid, external forces (ex., wind or flow etc.,) produce the barotropic waves in surface layer (i.e., Surface waves; SWs) and then induce baroclinic waves in interface (i.e., Internal waves; IWs) due to the different pressure variations. In the ocean, the IWs generation is induced by the interaction between flow and submerged topography (Hsu et. al., 2000); in the lake, the wind plays an important role on producing the surface pressure difference and then internal waves are generated indirectly (Pannard et. al., 2011); at the estuary, the internal waves are generated by the interaction between waves and flow. Internal waves have significant effect on ecology, environment and engineering when they propagate over varied topography (Bourgault et al., 2014; Lamb, 2014). Internal solitary wave (ISW), which is a special type of internal waves, has been studied in previous literature about its generation, transport and dissipation (Grimshaw and Helfrich, 2018; Hsieh et al. 2016, Lamb, 2014). Due to the short flow induced by its larger amplitude of ISW, surface waves becomes small and can be ignored during the interaction.
Kao, Jui-Hsiang (National Taiwan Ocean University)
An operating propeller is the main source of vibration on the ship hull, especially on the stern. In this article, an improved numerical scheme is presented to predict the pressure fluctuations on the ship hull due to the unsteady sheet cavitation of a propeller. The calculated results are compared with the published results and experimental data carried out in the hydrodynamics and cavitation tunnel of Hamburgische Schiffbau-Versuchsanstalt to verify the improvement. The present method is based on a two-cycle iterating scheme which satisfies the boundary integral equation in time domain. The hull pressure fluctuations calculated by the first-cycle iterations are treated as the initial values for the second-cycle iterations. The solid angles of the elements will deviate from the standard value, .5, as the dramatic variation in geometry appears, and accumulate numerical errors in the calculating process. During the second-cycle iterations, a filter based on the solid angles on hull elements is proposed to minimize the iterating error. A container ship is treated as the computing sample in this study, and evidence is offered regarding the 16% improvement achieved by the present method.
A propeller operating in a nonuniform wake field in the ocean is one of the prime sources of vibration and noise affecting the ship. The prediction of hull pressure fluctuations caused by propellers is an important consideration for many vessels and worthy of being researched.
Hull pressure fluctuation caused by the marine propeller has been widely investigated since the 1980s. Huse and Guoqiang (1982) developed a semi-empirical prediction method, in which the cavitation volume on the propeller blade surface can be estimated using experimental data. Breslin et al. (1982) proposed a model based on the potential flow theory, which simulates the ship hull by means of a simple panel method and the propeller by means of a lifting surface vortex lattice method. Kinns and Bloor (2004) used an acoustic boundary element (BE) model to simplify the problem of hull vibration excitation due to propeller sources and dipoles. The convergence of results for a cruise liner model was demonstrated with different element distributions. Kehr and Kao (2004) derived the incident blade rate pressure induced by unsteady sheet cavitation (monopole) and unsteady forces (dipole) of an operating propeller, for calculating the hull pressure fluctuations. Brouwer (2005) applied a stationary set of rings of monopole and dipole sources in the frequency domain to solve the propeller-induced noise and vibrations. Lee et al. (2006) integrated computational fluid dynamics (CFD) and the finite difference method for the computation of propeller-induced hull vibration. It was recommended that the phase difference of the propeller-induced pressure should be considered for preventing overprediction. Kao and Kehr (2006) developed a time domain iteration method to calculate the hull pressure fluctuations induced by the operating propeller. The phase difference is included in the iterating scheme, and the computation is robustly convergent. Seol and Moon (2009) derived the governing equation of pressure fluctuation induced by sheet cavitation according to the Ffowcs Williams approach. In the study by van Wijngaarden (2011), a potential flow boundary element method (BEM) with measured hull pressure data as input was applied to solve the hull pressure fluctuations, and the model experiments were also carried out. It was concluded that the computed hull pressure fluctuations due to non-cavitating propellers is reasonably accurate compared with the experimental data. Kehr and Kao (2011) calculated the pressure fluctuations on ship hull due to propeller sheet cavitation by using the method presented in Kao and Kehr (2006). The numerical result is higher than the experimental data provided by the hydrodynamics and cavitation tunnel (HYKAT) of HSVA (Wiemer 1999) by approximately 27%. The pressure fluctuations on ship stern induced by cavitating propellers were solved by Kanemaru and Ando (2011) with a surface panel method. The maximum amplitude appears in the first blade frequency and overestimates the experimental data by about 30%. Kim et al. (2012) predicted the hull pressure fluctuations induced by marine propeller sheet cavitation, and the Doppler effect was considered at the same time. Wei and Wang (2013) employed CFD to simulate the propulsion of a submarine. The finite element method and BEM were then combined to solve the submarine’s structure and acoustic responses under the propeller excitations. The matched-field inversion technique applied in Lee et al. (2014) uses the fluctuating hull pressure field measured by receivers and the acoustic field calculated by BEM to define the sheet cavitation noise, source strengths, source positions, and number of sources. The equivalent source model for the propeller can result in more accurately extrapolated hull pressure distribution. In Wei et al. (2016), the unsteady forces of a submarine propeller were predicted by CFD, and the hull pressure fluctuations due to the non-cavitation propeller were calculated by the method proposed in Kao and Kehr (2006).