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.
Chuang, Po-Yu (Sinotech Engineering Consultants, Inc.) | Huang, Yin-Chung (Sinotech Engineering Consultants, Inc.) | Ke, Chien-Chung (Sinotech Engineering Consultants, Inc.) | Teng, Mao-Hua (National Taiwan University) | Chia, Yeeping (National Taiwan University) | Chiu, Yung-Chia (National Taiwan Ocean University)
ABSTRACT: A novel approach using nanoscale zero-valent iron (nZVI) as a tracer was developed for detecting fracture flow paths directly. This approach was examined at a hydrogeological research station in central Taiwan. Heat-pulse flowmeter tests were performed to delineate the vertical distribution of permeable fractures in two boreholes, providing the design basis of tracer test. A magnet array was placed in the observation well to attract arriving nZVI particles for identifying the location of incoming tracer. Then, the nZVI slurry was released in the injection well. The arrival of the slurry in the observation well was detected by an increase in electrical conductivity. The position where the maximum weight of attracted nZVI particles coincides with the depth of a permeable fracture zone delineated by the heat-pulse flowmeter. In addition, a saline tracer test produced comparable results with the nZVI tracer test. Numerical simulation was performed using multi-porosity approaches to estimate the hydraulic properties of the connected fracture zones between the two wells. The study results indicate that the nZVI particle could be a promising tracer for the characterization of flow paths in fractured rock.
In this study, a pioneer work on characterizing rainfall-induced shallow landslides in layered soils using unsaturated flow equation is proposed. Several models using different mathematical approximations for describing the nonlinear physical relationships of the soil-water characteristic curve were introduced. The Gardner exponential model was adopted to derive analytical solutions of unsaturated flow problems. To model the rainfall-induced shallow landslides, the slope stability analysis coupled with the hydrological model with the consideration of the fluctuation of pore water pressure for soil water characteristic curve was developed. The results obtained from this study demonstrate that the slope stability of landslides is strongly dependent on the hydraulic conductivity. It is found that the variation of pore water pressure in unsaturated layered influences the stability of a slope. Besides, the pressure head at the interface increases quickly and the lowest safety factor may occur at the interface between two consecutive soil layers during a rainfall event.
Due to the effects of climate change and global warming, severe weather conditions such as the large amount of precipitation are becoming much more frequent around the world. The extreme rainfall events affect the stability of slopes and trigger extensive landslides. It is therefore necessary to characterize how the variability of severe climate variables, such as the rainfall, affects the vulnerability to landslide hazards. Shallow landslides often occurred in the unsaturated zone. The assessment of rainfall-induced shallow landslides has drawn much attention in branches of engineering and science such as geotechnical engineering, civil engineering and engineering geology (Liu et al, 2017). In the past, numerical models have been developed to analyze the slope stability, assuming soil was nearly saturated to further solve the numerical solution of the Richards equation in the simplified form of a one-dimensional linear diffusion equation. Since the appearance of layered heterogeneous porous media is much more common than homogeneous soils in engineering problems, the hydrological process in layered unsaturated soils has been studied (Ku et al, 2017). However, the modeling of rainfall-induced shallow landslides in layered unsaturated soils using the variably saturated flow equation has hardly been reported.
Accurate and efficient prediction of typhoon-induced surge is an important task for coastal disaster prevention. The purpose of this study is to develop a long-lead-time prediction model for storm surge using effective controlling factors and artificial neural network. Effective controlling parameters will be carefully investigated to establish the artificial neural network model, including the maximum wind speed, the center air pressure, the radius of the storm, the relative location between the station and the center typhoon (i.e., distance and angle). The combined effective information can not only reduce the input dimension but also improve the model's learning and forecasting capability.
Coastal regions of a country are highly developed owing to rich natural resources and economic potentials. To date, more than one billion people are living and working in the coastal areas over the world. The continuously increasing population might approach 2 to 5 billion by 2080 (IPCC, 2007). However, coastal environment system, affected by land/river, ocean and atmosphere, would face the threats by various disasters. One of them is the typhoon-induced surges.
Storm surge has always been regarded as a very important topic in science or practice due to its severe impacts. For example, storm surge struck the North Sea in Europe and caused a serious flooding in the Netherlands, Belgium and the United Kingdom at midnight on the of Feb 1, 1953. A total of 2,167 people died in the disaster. The worst storm surge over 12 meters was recorded in Bangladesh in 1970. Within 20 minutes, the number of deaths was as high as 300,000. At the beginning of this century, Hurricane Katrina swept through the Gulf of Mexico in the south of the United States in 2005. Thousands were killed and New Orleans almost destroyed. In 2012, Typhoon Sandy hit New York in the east coast of the United States. In Asia, Cyclone Nargis struck Myanmar in 2008, causing 130,000 deaths. In 2013, typhoon Haiyan hit the Philippines, leading to 6,000 deaths and $ 14 billion in economic losses. In addition, under the influence of the climate change and global warming, the frequency and magnitude of super typhoons would surpass historical records (Webster et al., 2005; Emanuel, 2005), resulting in more destructive storm surge. A deeper understanding of storm surge (i.e., characteristics and related physical processes), and accurate/efficient predictions are still the subject of concern.
Wang, Hsing-Yu (National Taiwan Ocean University) | Fang, Hui-Ming (National Taiwan Ocean University) | Chiang, Yun-Chih (Tzu Chi University) | Su, Jung-Chang (SINOTECH Engineering Consultants, Ltd.) | Lu, Chun-Sen (SINOTECH Engineering Consultants, Ltd.) | Hsiao, Sung-Shan (National Taiwan Ocean University) | Lin, Ting-Chieh (National Kaohsiung University of Science and Technology) | Hsu, Hao-Teng (Ship and Ocean Industries R&D Center)
The objective of this study was to adequately examine potential wave fields, flow fields, and coastal geomorphological changes in an ocean near an offshore wind farm after installation of a wind powergenerating set. Accordingly, this study applied hydrodynamic and geomorphological modules to simulate the waves, currents, and geomorphological changes in the study area. The simulation results revealed that geomorphological changes (i.e., scouring and silting variations) engendered by ocean waves and flows before and after the installation of offshore wind turbines were not considerably affected by the jacket-type foundation piles (diameter: 3 m) of the turbines. From a macroscopic perspective, the installation of the wind turbines did not sufficiently affect the geomorphology of the study area. From a microscopic perspective, changes in the seabed geomorphology were only limited to areas surrounding the submerged piles after the installation of the wind turbines.
As an island country, Taiwan imports more than 90% of its energy resources. Previously, Taiwan generated electricity mainly through fossil fuels and nuclear power, which provided substantial economic benefits. However, because of increasing environmental awareness in recent years, people have begun to consider the problems of air pollution and nuclear waste treatment caused by fossil fuel and nuclear power generation, respectively. Therefore, determining economically beneficial and environmentally friendly power generation methods has become an imperative task for Taiwan. Over the past 10 year, Taiwan has been actively developing terrestrial wind farms. Currently, a total of 24 wind farms have been established on the island. Nevertheless, favorable locations for developing wind farms has been exhausted, and wind turbine generators cause considerable noise engendered by wind shear; therefore, establishing offshore wind farms will become a future trend for Taiwan.
For offshore wind power research, J.H. den Boon et. al (2004), R.J.S. Whitehouse et. al (2006), E.A. Hansen et. al (2007) and R.Y. Yang et. al (2011) study has been performed on the occurrence and prevention of erosion holes (scour) around mono pile foundations of offshore wind turbines on sandy soils. J.F. Lu & D.S. Jeng (2007) developed a coupled model to investigate the dynamic interaction between an offshore pile, poroelastic seabed and sea water. R.J.S. Whitehouse et. al (2011) analysis and interpretation of monitoring data for the seabed bathymetry local to offshore windfarm foundations has shown how the scour develops in time and highlighted variations between sites with different seabed sediment characteristics, i.e. sands and clays. Y.C. Chiang et. al (2014) presented a numerical model for the simulations of the morphological changes in large coastal area and local seabed evolutions near turbine foundation, and an assessment of the possible long-term morphological evolution.
In this article, the numerical solution for solving subsurface flow problems in heterogeneous soil using a novel meshless method is presented. The numerical solutions are approximated by a set of non-singular basis function of the Laplace equation from the collocation Trefftz method. To deal with the subsurface flow problems in heterogeneous soil, the domain decomposition method is applied. The novel meshless method is validated for several test problems. Application examples are also performed. The results reveal that the proposed method has resolved one of the major issues which are finding the satisfactory location for the source points in the method of fundamental solutions. In addition, the proposed method has great numerical accuracy for solving subsurface flow problems in layered heterogeneous soil even with extreme contrasts in the hydraulic conductivity.
In the past, numerical approaches to the simulation of various subsurface flow phenomena using the conventional mesh-based methods such as the finite difference method (FDM) (Clement et al., 1994; Fukuchi, 2016; Todsen, 1971; Pollock, 1988, Ku, 2013), the boundary element method (BEM) (Fan, 1992; France, 1974) and the finite element method (FEM) (Chen and Tompkins, 1994; Chen et al., 2000) were well documented. Since the meshless method has the advantages that it does not need the mesh generation, the meshless method has attracted considerable attention in recent years in solving practical problems involving complex geometry in subsurface flow problems. Several meshless methods have been reported in literature, such as the method of fundamental solutions (MFS) (Kupradze and Aleksidze, 1964; Katsurada, 1996), the collocation Trefftz method (CTM) (Trefftz, 1926; Yeih et al., 2010), element free Galerkin methods (EFGM) (Belytschko et al., 1994), radial basis function collocation method (RBFCM) (Amaziane et al., 2004; Chan and Fan, 2013), generalized finite difference method (GFDM) (Benitoet et al., 2001; Fan et al., 2014). Among these meshless methods, the method of fundamental solutions was proposed by Kupradze and Aleksidze in 1964 and the Trefftz method was proposed by Trefftz in 1926 are two important representative boundary-type meshless methods. Subsurface flow problems are governed by second-order partial differential equation. Problems involving regions of irregular geometry are generally intractable analytically. For such problems, the use of the boundary-type meshless method, to obtain approximate solutions is advantageous. In this study, we adopt the domain decomposition method (DDM) to deal with the subsurface flow problems in layered heterogeneous soil because the DDM is natural from the physics of the problem where different hydraulic conductivity in different subdomains.
The numerical solution of two-dimensional unsaturated flow problems based on the novel advanced computational meshless method was investigated. The numerical solution was approximated by superpositioning the Trefftz basis functions formulated from independent functions satisfying the governing equation in the cartesian coordinate system. To solve the two-dimensional unsaturated flow problems, this study adopted a linear approximation for the nonlinear Richards equation to model flow in unsaturated soils using the Gardner exponential model. The validity of the proposed method is established in several problems. Results indicate that the proposed method may obtain highly accurate numerical solution for two-dimensional unsaturated flow problems.
In engineering practice, soils encountered are partially saturated. Naturally, the unsaturated zone is the portion of the subsurface above the groundwater table which forms a necessary transition between the atmosphere and groundwater aquifers. Although the flow in the unsaturated zone is not a major natural resource of available groundwater resources, it is the main factor controlling water movement from the land surface to the groundwater table and strongly affecting the recharge of saturated zone. Therefore, the movement of flow in the unsaturated zone is one of the most important elements of the hydrological cycle. Several applications of unsaturated flow problems include seepage in unsaturated embankment dams, nearsurface contaminant transport and groundwater resource. Accordingly, it is necessary to understand the movement of flow in the unsaturated zone of hydrological cycle.
The governing equation describing the movement of flow in unsaturated soil, known as the Richards equation, is highly nonlinear (Richards, 1931). Studies showed that the Richards equation is highly nonlinear due to the high nonlinearity of physical behavior of unsaturated soils. Complicated physical relationships of the unsaturated soil properties can be described using soil-water characteristic curves (SWCC). Several empirical equations have been proposed to curve fit the SWCC (Gardner, 1958; Brooks and Corey, 1964; van Genuchten,1980; Fredlund and Xing, 1994). Because of the high nonlinearity of the Richards equation, analytical solutions may not be directly provided. As a result, modeling flow process in unsaturated soils is usually based on the numerical solutions of the Richards equation. Several numerical methods for modeling unsaturated flow problems in the past decades have been developed (Celia et al., 1990; Miller et al., 2006; Casulli and Zanolli, 2010; Juncu et al., 2012). Mesh-based numerical techniques such as the finite difference method and the finite element method are well documented and typically used to solve the unsaturated flow equation in the past (Liu et al., 2015).
In this paper, the speed loss is computed by the simulations of self-propulsion tests in calm water and in waves, and different approaches of the self-propulsion test are used. In addition to using the viscous flow RANS method, the potential flow methods are also applied in the computations of wave-making resistance and added resistance. The body force method is used to represent the propeller effects. The KCS and KVLCC2 ships are used to demonstrate the speed loss computations, and results from different approaches are investigated by the comparison of self-propulsion results and power curves.
The fuel efficiency in seaway becomes more important due to environmental regulations, and the speed loss in seaway is quantitatively evaluated by weather factor in EEDI (Energy Efficiency Design Index). To compute the speed loss, the ship hull resistances in both calm water and waves have to be computed, and the ship speeds in different sea states can be evaluated based on the propeller performance in waves. In this paper, instead of using the viscous flow RANS method to directly simulate the whole physical phenomena, we will use both the potential flow method and the viscous flow method. Since the viscous effects are less important in ship motion problems, to use the potential flow method is appropriate. On the other hand, the interactions between the ship wake and the propeller are dominated by the viscous effect, thus the viscous flow RANS method is used.
The speed loss problem has been studied by many researchers. Journée (1976) applied an approximate method to study the ship motion effects to the propeller performance, and he has also carried out experiments to make the comparison. Faltinsen (1980) has investigated the resistance and propulsion in seaway, and he claims that since the encounter frequency of the incoming wave is far smaller than the propeller rotational frequency, only the vertical velocities due to motions are critical to the propeller performance in waves. The variation of the propulsion thrust in wave can be computed by quasi-steady flow method, that is, to solve the thrusts at different time step, and the thrust in waves will be the mean value of these. Nakatake et al. (1986) later developed a panel method using source and sink distributions to simulate the ship hull, propeller and rudder, and studied the interactions of hull/propeller/rudder by computations. Ando et al. (1989, 1990) have verified the above computations by experiments. Recently, Kashiwagi et al. (2004) investigated the propeller performance in waves by using the Enhanced Unified Theory (EUT), which is derived from ship motion theory. Chuang and Steen (2011) have studied the power and speed loss in waves by experiments. The body force method (Kerwin et al., 1994; Hsin et al., 2000, 2008; Wei, 2012) is used to represent the propeller effects, and the reason is not only because the simplicity, but also because we can separate the flow field into “propeller inflow” and “propeller induced velocity” by using this method. The inflow due to the propeller-hull interaction can be added to the inflow due to the ship motions and wave induced velocities, and become the “total inflow” of a propeller in waves.
Green Island locates in the typhoon active southeastern Taiwan coastal water. A high resolution (250–2250 m) shallow-water flow model is used to investigate the effect of typhoon Soulik on the hydrodynamics of Kuroshio and Green Island wakes. Simulation results indicate salient characteristics of Kuroshio and meandering downstream island wakes seems less affected by typhoon Soulik. Moreover, Kuroshio currents increase when flow is in the same direction as the counterclockwise rotation of typhoon, and vice versa. This finding is in favorable agreements with the TOROS (Taiwan Ocean Radar Observing System) observed data.
According to Zdravkovich (2003) flow past a bluff obstacle with the Reynolds number Re = u∞L/vt in the range of 50 and 800, a well-organized downstream wakes occur, where u∞ is the characteristic flow velocity, L the characteristic length, and vt the eddy viscosity of fluid. Flow becomes periodic with detachment of the free shear layers, and consequently alternate shedding of vortices. The periodic phenomenon is referred to as the vortex shedding, where the anti-symmetric clockwise and counterclockwise wakes pattern is called the von Karman vortex streets (Zdravkovich, 2003).
Vortex streets occur frequently in the atmosphere and oceans (Nunalee and Basu, 2014; Chelton et al., 2011). The phenomenon of vortex shedding behind bluff bodies has long been of interest to the fluid dynamics community and has been intensively studied by many researchers (Roshko, 1955; Tritton, 1959; Williamson, 1996; Zdravkovich, 2003). This kind of phenomenon is often captured by satellite imagery (Hubert and Krueger, 1962; Thomson et al., 1977; Li et al., 2000; Young and Zawislak, 2006; Zheng et al., 2008), field measurement (Barkley, 1972), and numerical modeling (Ruscher and Deardorff, 1982; Wolanski et al., 1984; Heywood et al., 1996; Dong et al., 2007).
The Kuroshio, a western boundary current of the sub-tropic North Pacific Ocean, originates from the North Equatorial Current and flows northward to the eastern coast of Taiwan. The passage of the Kuroshio mainstream parallels to the eastern shoreline of Taiwan. Green Island is located at (121°28'E, 22°35'N) and is 40 km off the eastern coast of Taiwan. The climatology of the Kuroshio velocity revealed from in-situ measurements shows that Green Island is approximately located in the mainstream of the energetic Kuroshio passage and acts as an obstacle to the stream. Hence downstream island wakes are prone to occur. Characteristics of the vortex streets downstream Green Island can be found from the MODIS (moderate resolution imaging spectroradiometer) satellite images and in-situ measurements (Chang et al., 2013). Seasonal variation of mainstream, current patterns, and transport of the Kuroshio in the east of Taiwan has been numerically investigated (Hsin et al., 2008). Spatial and temporal scales of downstream Green Island wakes due to passing of the Kuroshio have been numerically studied (Liang et al., 2013).