Makiura, Nobukazu (West-Nippon Expressway Engineering Kansai Co., Ltd.) | Sakuradani, Keiji (West-Nippon Expressway Co., Ltd.) | Kamide, Sadayuki (The Calamity Science Institute) | Oda, Kazuhiro (Osaka Univesity) | Matsui, Tamotsu (Osaka Univesity)
For the safety management of soil slopes along expressways, it is quite important to develop an effective surveying method to detect the vulnerable slopes against instability, land slide, and mud flows. The authors have used an airborne electrical resistivity survey method, so called HEM (Helicopter Electromagnetic Method), to perform a rapid exploration along a long stretch of highway to examine the geotechnical and geological features of surveyed ground. Currently the interpretation of resistivity survey is made using either the true or the contrast resistivity value presentations, but because of their accuracy and unsatisfactory prediction capabilities, an enhanced resistivity value presentation is also used in this paper. To compare the reliability of the enhanced resistivity method with others, a study is carried out herein to evaluate their performances by conducting field reconnaissance and examining existing exploration data. Based on such studies the authors propose a chart to evaluate the geotechnical and geological features of surveyed ground by using both the true and the enhanced resistivity value presentations.
From a viewpoint of safety management of soil slopes along expressway, it is indispensable to identify unstable slopes by knowing their possible failure mechanism and extent of soil movements. The unstable slopes are usually associated with weak ground conditions such as previous slides, colluvial deposits, fault zones and also with adverse hydrological condition within the ground. Thus, it is very important to study whether such weak ground condition exists along a long stretch of expressway. However, economical and time restrictions of project often prohibit of carrying out conventional detailed geotechnical and geological explorations over such long distance, and therefore alternative survey method based on airborne electromagnetic exploration method has been sought by the authors.(Kamide et al 2011), The results of airborne electromagnetic survey can be used to identify where such vulnerable unstable slopes and therefore preventive or remedial measures for keeping the expressway slopes safe can be applied to selected sites through electromagnetic survey.
During the past few years, design and construction of large deep-water wharf has become an important subject. A new type of wharf structure with arched longitudinal beams has been proposed to adapt to open sea deep-water area. Previous studies of this new structure mainly focused on static calculation. In this study, a three-dimensional model of wharf with arched longitudinal beams is established by using ANSYS/LS-DYNA software. The p-y curve is employed to simulate the pile-soil interaction. Considering different pile torsional angles and positions, the wharfs dynamic characteristics is analyzed when the impact force is imposed upon the structure.
In recent years, a trend of large tonnage vessels is increasing in port engineering. The international routes are now sailing the fifth and sixth generation container ships and over 300,000 tons for bulk vessels and oil tankers (Leifer and Wilson, 2007). In China, at present, the number of berths which can handle vessels over 50,000 tons is about 260, but in fact, most of them cannot meet the requirements of large-tonnage vessels, and construction of deep water wharves is in urgent needed (Zhang, 2006). The deep-water wharf works under adverse conditions and is hard to be constructed, so design of deep-water wharf is an important research topic in port engineering (Zhai and Lu, 2006). A new type of wharf with arched longitudinal beams was proposed (Zhai and Lu, 2008), which can make full use of arch's overhead crossing and reinforced concrete compression resistance, improve the interval between transversal bents greatly, and decrease underwater construction quantity.
Previous studies on the new structure mainly focused on the static analysis with all-vertical-piled foundation, while ignored the structural response under the complex dynamic loads. The horizontal force mainly acts on the brace piles, so analysis on the brace piles arrangement influenced to the whole forced condition of wharf structure is necessary. In this study, the p-y curve is employed to simulate the pile-soil interaction. Considering different pile torsional angles and positions, the wharf's dynamic characteristics is analyzed when the impact force is imposed upon the structure, the most suitable brace piles arrangement and torsional angle are determined. The results will provide guidelines in design and construction of the deepwater wharf.
In this paper, the power capture performance of a Pelamis-like wave energy converter (WEC) is studied. The Pelamis-like WEC is simplified as three floating cylindrical pontoons which are hinged together. And the relative motion is converted into electricity by the power take off (PTO) system which is simplified as a linear damper. Frequency domain method which is based on linear potential theory is used to establish the equations of motion of the floating system. Considering the displacement constraints between pontoons, the dynamic response of the Pelamis-like WEC can be analyzed in regular waves. Based on the calculated results, the effects of wave frequencies, wave directions, the damping coefficient of the PTO system, as well as the geometric quantities on the power capture performance of the Pelamis-like WEC are studied in detail. It is found that for a given Pelamis-like WEC, there exists an optimum combination of damping coefficient, wave direction, wave frequency, hinge axis inclination to make the maximum power capture.
In recent years, wave energy utilization has been a hot topic of ocean energy development. In order to capture wave energy, various types of wave energy device have been proposed and studied, such as oscillating water column (OWC), floating structure wave energy converter (WEC) and overtopping WEC, etc. The Pelamis-like WEC, as a kind of floating WEC, consists of several floating-body modules which are hinged together. And the relative angular motion between adjacent floating elements is converted into electricity by the power take off (PTO) system. In spite of a lot of researches focusing on Pelamis-like WEC, only a few of them are concerned with hydrodynamics. Dalton (2010) has investigated on the performance and economic viability of the Pelamis-like WEC over a 20-year project time period using 2007 wave energy data from various global locations: Ireland, Portugal, USA and Canada. Palha (2010) described the study of the impact of energy absorption by wave farms on the near shore wave climate and, in special, the influence of the incident wave conditions and the number and position of the wave farms, on the near shore wave characteristics is studied and discussed; Henderson (2006) described the hydraulic power take-off system employed in the Pelamis wave energy converter. He presented the process of the system’s development, including simulation and laboratory tests at 1/7th and full-scale, as well as results of efficiency measurements. O’Connor (2013) presented the results of a case study comparing the performance of two wave energy devices at various scaled power ratings deployed at several European wave energy locations. Yemm (2012) introduced the Pelamis-like WEC in detail, and pointed out that pitch axis and yaw axis were not designed horizontal and vertical but with a certain offset angle. Thiam (2010) studied the power generation efficiency of the Pelamis systematically. He focuses on a simplified model of the Pelamis wave energy converter, with the model consisting of a modified Euler-Bernoulli beam oriented head-on to incoming ocean waves and with energy conversion accounted for by a damping term, where the additional bending moment is proportional to the time derivative of the beam curvature. But no literature exists to study the efficiency of power generation from the aspect of the hydrodynamic model. Multi-body floating model is supposed to be a more accurate method to analyze this problem. He (2013) utilized the AQWA hydrodynamic software to calculated swing angles, hydrodynamic coefficients, and wave exciting forces, but he didn’t mention the efficiency of power generation. Gou (2004, 2014) studied the hydrodynamic interaction effects between wave and two connected floating structures by the boundary integral method. This method is used to study the performance of a Pelamis-like WEC.
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.
Aiming at the problem that the amplitude frequency characteristic and the group delay characteristic of the traditional fractional delay filter deteriorate at the higher frequency, a fractional delay filter based on Hermite interpolation polynomial is proposed to approximate the ideal non-integer delay filter system. This filter improves the energy spectrum of sonar beamforming, and allows the system to avoid using high sampling rates to improve delay accuracy, which helps to reduce computer storage performance requirements. By comparing the frequency response characteristics of the Least-squares linear-phase FIR filter, Lagrange and Hermite fractional delay filter with same order, the Hermite fractional delay filter is proved to be less attenuated and less error at the high frequency.
The signal beamforming methods of traditional array sonar can be divided into two types: frequency domain beamforming and time domain beamforming. Although the frequency-domain method is easy to use for multi-beamforming, the beam offset problem will be caused due to the influence of frequency on the array manifold vector. Time-domain beamforming is a method of real-time compensation of signals, eliminating the problem of beam offsets. And for the broadband signal, frequency domain beamforming needs to convert the signal to the frequency domain through the Fourier transform, and the narrowband phase shifting is performed after dividing the broadband into a subband. The whole process is to ensure the high resolution of sonar direction finding which will cause the shortcomings of large amount of computation and more storage space. Therefore, using Time Domain Direct Delay compensation (DTD) beamforming to process wideband signals has become a hot research topic in signal processing (Wang, Wu and Yang, 2006).
The core of the DTD is to make accurate delay compensation for the array signals received in all directions so as to ensure fast and accurate search and location. The current A/D sampling rate is limited and can not be infinitely high, so the delay precision of the array signal is limited. The traditional methods of improving delay precision can be divided into two types: analog and digital. Cao and Shang (2006) proposed a method of analog Time Delay Units (TDUs), but the cost is high, the volume is large and the stability is not high. In contrast, the digital delay method, represented by the fractional delay technique, is simpler and easier to implement. The performance of traditional fractional time delay filters such as Lagrange interpolation filter and Farrow structure filter, deteriorate at the condition of close to half Nyquist sampling rate (Du, Song and Cao, 2013), which results in the attenuation of higher frequency signal in beamforming. Soontornwong, Chivapreecha, and Pradabpet (2012) described the amplitude frequency response and group delay response of the third order Hermite fractional delay filter with the backward difference, the second order central difference and the fourth order central difference. Tseng and Lee (2012) proposed the design method of Hermite filter to reduce the frequency response error. Subsequently, Du, Song, et al. (2015) verified the performance optimization of the Hermite fractional delay filter for beamforming in Ultra-wideband (USB) radar, but it only considered the analysis of a single delay amount.
Zhang, Jianli (National Marine Data & Information Service) | Wang, Hui (National Marine Data & Information Service) | Fan, Wenjing (National Marine Data & Information Service) | Li, Wenshan (National Marine Data & Information Service) | Gao, Tong (National Marine Data & Information Service) | Liu, Qiulin (National Marine Data & Information Service)
Based on the observing data from 1980 to 2016 of 24 tide gauges, the long term tide variation/change along the China coast are investigated. Results show that the mean sea levels, M2 constituent amplitudes, mean tidal ranges, mean high tide levels, mean low tide levels along the China coast show rising trends, at the average rate of 0.32 cm/year, 0.09 cm/year, 0.31 cm/year, 0.51 cm/year, 0.19 cm/year, respectively. From the northern Yellow Sea to Hangzhou Bay, the mean tidal ranges increase significantly, with the values of 0.50-1.30 cm/year. The mean tidal ranges along China coast have obvious 18.61 year cycle variation, which the amplitude is more than 20 cm on the Beibu Gulf coast.
In the last hundred years, the global climate is experiencing the change with a main characteristic of warming. It is pointed out in the fifth IPCC (2013) that the global mean surface temperature rising rate is about 0.012 °C /year during 1951-2012, and during 1971-2010, the global mean sea level has been rising at a rate of about 0.02 cm/year caused by the thermal expansion of sea water, the melting of land glaciers and polar ice sheets. Sea level rise can increase the frequency and effects of occurrence of storm surge, flooding, erosion, salinization and inundation of low-lying land in coastal areas (Chen, 1997).
The variation and reduction of the effect the seabed and lateral friction in the shelf shallow water regions caused by global sea level rise lead to the change of the propagation of the incident wave, the reflected wave and the refraction wave, and the shift of the amphidromic point (Zhang, 2000). The possible effects of sea level rise on the tidal wave system in the China Sea are studied by means of the numerical methods (Yu, 2008; Yan 2010; and Zhang, 2013). Assuming the sea level rises 100cm, it is found that the highest astronomical tidal level is up to 10-16cm, and the depth datum of the sea chart drops to 10-12cm in the China Sea (Yu, 2007). The M2 and K1 constituent amplitude in the eastern Pacific has a long-term increasing trend, and the variations of the M2 and K1 component amplitude at all tidal stations in the north of 18°N are consistent (Jay, 2009). The linear changes of the major tidal constituents are commonplace around the world, although not necessarily with large spatial scales (Woodworth, 2010). The S2 was found to be the component that usually shows the largest linear changes among the main tidal constituents in the east Pacific (Müller, 2011). During 1954-2012 years, the semidiurnal tidal parameters show significant secular trends in the Bohai, Yellow Sea and Taiwan Strait, and the largest increase for M2 amplitude is found by 0.4-0.7 cm/year in the Yellow Sea (Feng, 2014). In the northern part of the South China Sea, the amplitude and phase of O1, K1 and M2 has significant periodic variation, and the amplitude of S2 is more stable (Fu, 2015).
Sakata, Keigo (Graduate school of University of Tokyo) | Cicolin, Murilo M. (University of São Paulo) | Gonçalves, Rodolfo T. (Graduate school of University of Tokyo) | Hirabayashi, Shinichiro (Graduate school of University of Tokyo) | Assi, Gustavo R. S. (University of São Paulo) | Suzuki, Hideyuki (Graduate school of University of Tokyo)
PIV measurements were carried out in a recirculating water channel at the Reynolds number of 43,000 to understand the three-dimensional flow structures around the free end of the cylinders with low aspect ratio piercing the free surface. Flow fields at different vertical and horizontal planes were presented for two aspect ratios, namely 2.0 and 0.5. The results showed the core of the longitudinal recirculation region (bubbles) varied at different planes for aspect ratio 2.0 and did not vary for aspect ratio 0.5. The difference of vortices showing up between aspect ratio 2.0 and aspect ratio 0.5 was also presented.
Floating offshore wind turbines (FOWTs) are subjected to severe environmental loads due to winds, waves, and currents. Vortex-induced motion (VIM) is the result of the exciting forces by vortex shedding on the hull of a bluff body exposed to currents. VIM results in additional oscillating mooring line tensions and affects mooring lines’ fatigue damage severely. Concerning economy, changing mooring lines is not desirable during the service lifetime of FOWT. The mechanism of VIM needs to be clarified to predict the service lifetime of mooring lines and better design the floating unit. The VIM behavior is susceptible in offshore platforms such as a spar, monocolumn, and multi-columns platforms, see examples by Gonçalves et al. (2010, 2012c, 2013b, 2018). The main characteristic of these platforms is to present a low aspect ratio (ratio between the draft and the characteristic diameter).
Structures with low aspect ratio are affected by the free-surface and freeend effects. The free-end effect on vortex shedding in the case of circular cylinders has been studied by many authors for the cylinder positioned on the ground plane (fixed on the bottom), see the example by Kawamura et al. (1984). Some investigations have considered the flow structure around the low-aspect-ratio and wall-mounted cylinders. Pattenden et al. (2005) sketched the flow around cylinders on the ground plane with aspect ratio equal one, and showed that there are some kinds of vortex namely, tip vortex, arch vortex, trailing vortex, and horseshoe vortex. The paper by Sumner (2013) is an excellent review of this theme. On the other hand, the case of the cylinders with low aspect ratio piercing the free surface needs more attention and to be better studied for the offshore scenario. Hence, this experimental study focused on the flow around stationary circular cylinders with low aspect ratio piercing the free surface to understand the free-end effects. There are few works for L/D<1 and piercing the free surface. Gonçalves et al. (2015) measure the drag and lift forces and visualized the flow around two horizontal planes and one vertical plane for low aspect ratio cylinders including the freesurface with L/D=2.0, 1.0, 0.5, 0.3 by PIV. The present study visualized eleven vertical planes for cylinders including the free-surface with L/D=2.0 and 0.5 by PIV and tried to complement this previous work.
Methods for generating electricity from renewable energy sources are of growing interest for achieving a stable energy supply and as countermeasures against climate change. The tidal current power generation system we have investigated employs a variable-speed DFIG. This paper has been investigated for identifying the gear ratio and the rated capacity of the generator that will allow maximizing the generated energy without overloading the generator while the tidal current power generation system is operated at the maximum power point tracking control scheme. Then, we investigated effect of the gear ratio and the rated capacity of generator by stator d-axis current.
Renewable energy power generation systems are garnering attention as means to combat climate change and provide stable energy sources. One such renewable energy resource is tidal current—the flow of seawater caused by tidal phenomena. The tides reverse direction every half cycle, with one cycle taking between about a half day or a full day depending on the location (Kondo, Uehara, Kihoh, Miyazaki and Yano, 1996). This makes the power generated by tidal currents predictable, which offers advantages when used in conjunction with a power grid; it is also unaffected by weather. In the present study, tidal current power generation was investigated for a Darrieus type water turbine, which uses a generator to convert rotational energy from the turbine into electrical energy. We have previously studied the power generation characteristics of this type of water turbine in water-channel and marine experiments, to assess its usefulness for tidal current power generation (Kihoh, Shiono and Suzuki, 1993; Shiono, Suzuki and Kiho, 1999).
Similar to the case for wind power, the power generated by tidal currents is not constant, but fluctuates. For this reason, tidal current power generation systems with variable speed operation are now gaining attention for their high efficiency in converting tidal energy to electrical energy. Such generators fall into two categories: doubly-fed induction generators (DFIGs) and synchronous generators. With DFIGs, the inverter is located between the rotor circuit and the grid, allowing for reduced capacity compared with synchronous generators (Ackermann, 2005). The present study considers a tidal current power generation system using a DFIG with variable speed operation.
One of the most interesting characteristics of the Wave Energy Converters (WEC) based on the overtopping principle is their capability of being integrated into conventional harbor structures. However, in these cases particular attention must be drawn to the structural stability and the nature of loadings exerted on the device. This paper presents CFD experiments on a composite breakwater including an upright section and a sloping crown simulating a WEC. The hydrodynamic forces caused by the overtopping process onto the inner face of the structure are analyzed, with a particular focus on the impulsive events
Wave Energy Converters (WECs) based on the overtopping principle, also referred to as OverTopping Devices (OTDs), have been paid a remarkable attention in the last years. They function by leading waves to flow in one or more reservoirs via a sloping plate often called “focuser”; then wave energy is extracted using a turbine that works with low head differences (below 3m) and large flow rates. Several OTDs have been developed so far, such as the pioneering “TAPCHAN” (TAPered CHANnnel Wave Power Device; Falcao 2004) or the floating device “Wave Dragon” (Kofoed et al., 2006); more recently, Buccino et al.(2015a) studied a very low-head hydropower plant, in which waves overtop in a shoreline reservoir, so forming a “composite seawall system”.
One of the most interesting OTDs features is their capability of being incorporated into conventional maritime structures (such as harbor breakwater), so that wave energy is produced while protecting coastal areas; as an example of such kind of WEC, Vicinanza et al. (2012) discussed the case of the Seawave Slot-cone Generator (SSG), a trapezoidal multi-reservoir system, whereas Vicinanza et al. (2014) and Contestabile et al. (2016) analyzed the response of the Overtopping Breakwater for Energy Conversion (OBREC).
In these cases the structural response of the devices has to be analyzed with a particular attention, given the double purpose of the structure. To these goal, a deep knowledge of the wave induced loadings is needed (see also Castellino et al., 2018).
Altomare, Corrado (Ghent University) | Tagliafierro, Bonaventura (University of Salerno) | Suzuki, Tomohiro (Flanders Hydraulics Research) | Dominguez, Jose M. (Vigo University) | Crespo, Alejandro J. C. (Vigo University) | Briganti, Riccardo (University of Nottingham)
The Relaxation Zone method (RZ) has been implemented in the meshless SPH-based DualSPHysics code. RZ acts as an internal wave maker and allows coupling DualSPHysics with any other model or analytical solution to generate sea waves. In this work, the coupling with the SWASH model is performed to simulate multi-scale and long-duration phenomena in coastal engineering, which represent a challenge for researcher and practitioners. In fact, despite the fact that SPH-based models are getting more and more popular in coastal and civil engineering, they still present a huge computational cost. In the present work, RZ is validated for phenomena of overtopping flow impacts on vertical walls. The results proved that the RZ is efficient and reliable alternative for wave generation in SPH-based models for coastal engineering applications.
Smoothed Particle Hydrodynamics method (SPH) is a promising meshless technique for modelling fluid flows and fluid-structure interaction (FSI) as it is capable to deal with large deformations, complex geometries, violent free-surface flows inducing large abrupt hydrodynamic loads and highly nonlinear phenomena (Violeau, 2012).
In general, SPH methods can be categorized into two groups: weakly compressible and incompressible. The Weakly Compressible SPH (WCSPH) methods solve an appropriate equation of state (Tait's equation) in a fully explicit form. The DualSPHysics model used in the present work is based on WCSPH. The incompressible SPH (ISPH) methods (e.g. Shao & Lo, 2003) solve a Poisson pressure equation (PPE) by applying project-based methods. Latest advancements have been made during the last decade in the context of SPH methods in terms of model stability, accuracy, energy conservation, boundary conditions and improved simulations of multiphase flows and fluid-structure interactions. A comprehensive review of it is presented in Gotoh & Khayyer (2016, 2018).
SPH methods have been widely applied to coastal engineering problems, such as wave breaking (e.g. Khayyer et al., 2008), wave overtopping (e.g. Gómez-Gesteira et al., 2005), wave run-up (e.g. Zhang et al., 2018), wave impacts (e.g. Altomare et al., 2015), tsunamilike wave processes (e.g. St-germain et al., 2014), wave energy applications (e.g. Crespo et al., 2017). Notwithstanding, further research is still needed to enhance the reliability of SPH methods and to widen their applicability for coastal engineering problems. Lately Rota Roselli et al. (2018) presented an automatic optimization framework to find the set of SPH parameters in DualSPHysics for an accurate wave propagation modelling. Yet, there are still limitations to be solved, one of which consisting in the unphysical oscillations in the pressure field due to high-frequency acoustic noise. Meringolo et al. (2017) proposed a procedure to filter out this noise, however the work is dedicated to post-processing analysis rather than solve the problem a priori. For WCSPH, besides the most classical diffusion schemes such as artificial viscosity (Monaghan, 1992), the so-called δ-SPH scheme has been proposed (e.g. Molteni & Colagrossi, 2009) to increase accuracy of the pressure field. To improve both accuracy and stability in SPH, particle regularization schemes have been proposed in order to regularize the anisotropic distributions of particles prone to be formed due to Lagrangian characteristics of particle methods (Lind et al., 2012).