Hongda Shi

Practical Simulation of Oscillating Water Column Chamber for Wave Energy Conversion

Oscillating water column (OWC) device has been widely employed in the wave energy conversion. In the present paper, a numerical wave tank (NWT) using two-phase volume of fluid (VOF) model is utilized to simulate the generation and propagation of incident regular waves, and water column oscillation inside the chamber. The NWT consists of the continuity equation, the Reynolds-averaged Navier-Stokes equations, and the two-phase VOF functions. The standard k-ϵ turbulence model, the finite volume method, NITA-PISO algorithm, and dynamic mesh technique are employed to generate 2 Dimensional (2D) regular waves. The numerical results are compared and validated with corresponding experimental data. Effects of incident wave conditions and several structure parameters on the operating performance of OWC chamber are investigated numerically.

Effects of Damping Plate and Taut Line System on Mooring Stability of Small Wave Energy Converter

Ocean wave energy can be used for electricity supply to ocean data acquisition buoys. A heaving buoy wave energy converter is designed and the damping plate and taut line system are used to provide the mooring stability for better operating conditions. The potential flow assumption is employed for wave generation and fluid structure interactions, which are processed by the commercial software AQWA. Effects of damping plate diameter and taut line linking style with clump and seabed weights on reduction of displacements in 6 degrees of freedom are numerically studied under different operating wave conditions. Tensile forces on taut lines of optimized mooring system are tested to satisfy the national code for wire rope utilization.

Application of 2D numerical model to unsteady performance evaluation of vertical-axis tidal current turbine

Tidal current energy is renewable and sustainable, which is a promising alternative energy resource for the future electricity supply. The straight-bladed vertical-axis turbine is regarded as a useful tool to capture the tidal current energy especially under low-speed conditions. A 2D unsteady numerical model based on Ansys-Fluent 12.0 is established to conduct the numerical simulation, which is validated by the corresponding experimental data. For the unsteady calculations, the SST model, 2×105 and 0.01 s are selected as the proper turbulence model, mesh number, and time step, respectively. Detailed contours of the velocity distributions around the rotor blade foils have been provided for a flow field analysis. The tip speed ratio (TSR) determines the azimuth angle of the appearance of the torque peak, which occurs once for a blade in a single revolution. It is also found that simply increasing the incident flow velocity could not improve the turbine performance accordingly. The peaks of the averaged power and torque coefficients appear at TSRs of 2.1 and 1.8, respectively. Furthermore, several shapes of the duct augmentation are proposed to improve the turbine performance by contracting the flow path gradually from the open mouth of the duct to the rotor. The duct augmentation can significantly enhance the power and torque output. Furthermore, the elliptic shape enables the best performance of the turbine. The numerical results prove the capability of the present 2D model for the unsteady hydrodynamics and an operating performance analysis of the vertical tidal stream turbine.

Study on Hydraulic System Efficiency of Heaving-Buoy Wave Energy Converter

The hydraulic system is the key component in the widely used wave energy converters (WEC). In this paper, we theoretically analyze and describe our investigation of the efficiency of the hydraulic system by simulation and model testing of the combined heaving-buoy WEC. We derive a new governing equation that includes nonlinear hydraulic resistance in the power take-off (PTO). We conducted a physical model experiment based on a 100-kW prototype and applied a hydraulic system with an energy accumulator. The model test results reveal an important parameter related to efficiency with respect to nonlinear hydraulic resistance. We also studied the relationship between the efficiency and the initial conditions. Finally, based on our numerical simulation results, we discuss the effect on efficiency of the gas content of the hydraulic fluid and ways to reduce its impact.

Theoretical Analysis of Coral Reef Stability in South Sea

In this paper, we utilize the variational limiting equilibrium (LE) procedure to theoretically determine the slope stability of homogeneous coral sand in the South China Sea. By establishing a reasonable functional and calculating its extremum, we can deduce the stability factor of a coral reef identical to that obtained by upper bound analysis. To determine the accuracy of the results obtained using the variational LE procedure, we present some classical examples of homogeneous sand slopes without a hard shell and compare our results with those obtained by the upper bound theorem of plasticity. A series of extensive calculations and comparisons reveal that the results obtained by our proposed analysis method and those obtained using classical methods are slightly different, but the variational results are believable. On the other hand, our method solves some stability factors for reef slopes covered with a hard shell, which indicates that the existence of a hard reef shell may significantly enhance the stability of a reef slope, and that there is strong nonlinear relationship between reef stability and the mechanical characteristics of the reef shell, such as its thickness and strength. Finally, based on the variational results presented in this paper, we present a useful chart that offers a convenient and straightforward way to determine the maximum stability factor of reef slopes. Taking into account the influence of the hard reef shell, we found that, in some limit equilibrium cases, the limit stability factor and critical height of a coral slope could be improved significantly. Therefore, this issue deserves careful attention in engineering practice.

Modelling Analysis of the Influence of Wave Farm to Nearshore Hydrodynamics Forces

The gradually growing demands of energy and global warming are series problems the globe facing. Facing this condition, renewable and green energy sources may play a key role in both of meeting the growing demand for energy and preventing global warming. Among the novel renewable and green energy sources, wave energy is one of the most promising marine energy sources. However, utility of wave energy resource might cause environment evolutions, which has to be paid much attention. So we can balance environment and resources. The present work focuses on the evolution of wave climate and wave-induced longshore current. To demonstrate effects of wave farm on evolution of the two parameters mentioned above more clearly, Zhangjiapu (ZJP) nearshore areas are regarded as the potential wave farm location. There are large wetland areas having sensitive environment problems. Therefore, this chapter studied the evolution of nearshore hydrodynamic environment in ZJP nearshore areas, including wave climate and wave-induced longshore current, which was caused by the installation of wave farm. Numerical simulation has been adopted to investigate the effects induced by wave farm on nearshore hydrodynamic environment. The wave and wave-induced longshore current have been simulated by flow model Delft3D. The significant wave height and period gotten by numerical model agree with measured data generally. There are obvious changes occurring when wave farm is installed along 20-m-depth contour in ZJP. This chapter is structured as the following five parts: First, theories of wave modelling and methods of wave energy resources assessment and hydrodynamic environment are introduced. Second, the model setting and validation are discussed. Third, wave energy resources were investigated based on wave parameters outputted by wave numerical modelling. Fourth, evolution of wave climate and longshore current induced by the potential wave farm is analysed by hydrodynamic modelling. Finally, some summaries about the evolution of hydrodynamic environment when wave farm is set up are given.

Preface for special issue for the eighth Chinese-German joint symposium on hydraulic and ocean engineering

Social development has tightened the interaction between human beings and the water bodies on planet earth. Human beings benefit much from inland rivers, estuaries, coastal waters and the vast open oceans through hydraulic, ocean engineering and marine renewable energy. However, it is difficult for scientists and engineers to get reasonable engineering scheme balancing between safety and economy of hydraulic, ocean engineering and marine renewable energy under extreme hydrodynamic environment usually. The situation becomes more serious when accounting for global climate changes. Particularly, in the field of coastal and ocean engineering, it is the global trend of making the engineering be harmonic with natural environment and ecological systems. Respective complex and difficult issues in engineering practice, which arose in many countries and areas worldwide, are quite similar and borderless. They are strove for mutual understanding and international research collaboration. Under such circumstances, an international symposium on coastal and ocean engineering is recognized as an important step aiming at the contribution of more international research collaborations. Chinese-German research cooperation and academic exchange in the field of hydraulic, coastal and ocean engineering has a long history. In 1980, a cooperation agreement was signed by representatives of the East China Technical University of Water Resources (now Hohai University) in Nanjing, China and Institute for Hydrology Water Resources in the state of Lower Saxony, Germany. The first Chinese-German joint symposium on hydrology and coastal engineering was held in Nanjing in 1983 and the second one in Hannover in 1987. Twelve years later, the joint symposia on coastal engineering were re-held in Hasenwinkel in 1997 and Tainan in 1999 respectively, jointly

Study on deformation law of circular foundation under combined loading

The foundations of some ocean engineering structures are built to withstand not only the vertical gravity load V, but also the horizontal load H induced by sea waves and current. The horizontal load includes the concentrated force load, the moment load M, and the torque load T termed also as combined loading. It is of academic and engineering significance to study the deformation law of submarine seabed due to combined loading. On the basis of the three-dimensional elastic mechanics solution of circular foundation, numerical methods are used to analyze the deformation law of submarine soil under circular foundation with six degrees of freedom. The finite element analysis results give the elastic deformation law of soil in three dimensional spaces, modify the theoretical elasticity solution, and presents nonlinear soil deformation mechanism under the circular foundation with six degrees of freedom.

A wave focusing device for OWC wave energy convertor

Oscillating Water Column (OWC) device has been widely employed in the wave energy conversion. Wave Focusing Device (WFD) is proposed to improve the operating performance of OWC chamber. In the present paper, a 3D Numerical Wave Tank (NWT) using two-phase VOF model is utilized to simulate the generation and propagation of incident regular waves. The NWT consists of the continuity equation, the Reynolds-averaged Navier-Stokes equations and the two-phase VOF functions. The standard k-ε turbulence model, the finite volume method, NITA-PISO algorithm and dynamic mesh technique are employed to generate the regular waves. The effects of WFD on the operating performance of OWC chamber are investigated numerically.