ABSTRACT Nowadays, the pile-supported structures have been applied in many offshore industries, especially in the wind energy field. As a new type of stationary pile-supported structure, high-rise pile-cap structures are attracting more attention. In the present study, a fully nonlinear solver based on Navier-Stokes equations is established for the investigation into a ten-pile cap structure. The wave loads are obtained by integrating pressure over the surface of each part of the structure. The effects of the interaction between the piles and the cap are investigated. In some cases that the cap is on the top, the wave loads on the piles are 30 percent larger than that without the cap. This is caused by the effect of impact. It means that the wave loads are underestimated significantly by simply using Morison equation. The relationships between the wave loads on piles and the gap between the cap and still, the wave height, and the wavelength are also discussed in detail. INTRODUCTION In the recent decades, the pile-supported structures are commonly found in the costal and offshore environment, especially in the wind energy field for Offshore Wind Turbine (OWT). They are generally built by means of a group of piles in different arrangements.The long-term safety of the pile group-supported structures is still a major concern in coastal engineering. As a consequence, the prediction of wave loads on offshore pile group- supported structures is of great importance. Nowadays, we have already had some typical types of pile-supported structures, such as monopile, suction pile, pile cap and so on (Ryu, et. al., 2012). As a new type of stationary pile group-supported structure, high-rise pile-cap structure is attracting more attention. It has lots of advantages, such as high stiffness, reasonable cost, easy mounting, etc. (Chen & Zhou.et al.,2016). Generally, for pile group-supported structures, the total wave force could be obtained by calculating each single cylinder pile with Morison equation (Morison, J.et al., 1950). However, the effect of different interference parameters is difficult to estimate, especially the pile group effect. Zdravkovich (2003) discussed the effects of the interference parameters on the overall drag forces for pile groups. Through some small-scale and large-scale experiments, Bonakdar et al (2012) and Bonakdar (2014) reviewed that the interference parameters such as the relative spacing between piles and the number of neighbouring piles could noticeably affect the wave loads on every single pile. Anew characteristic geometric scale called effective diameter is introduced by Qu et al (2017), and the total inline wave force on different complex pile-supported structures could be estimated by the unified empirical formulas.Moreover, the interaction between the cap and the piles should also be taken into consideration when the wave hits the cap. Therefore, the Morison Equation is no longer appropriate for estimating the inline wave force of the high-rise pile-cap structure. Meanwhile, the cap is much larger than the other parts of the structure and it will also lead to a larger lift force on the structure.

ABSTRACT In this paper, a circular cylinder attached by a pair of fin-shaped strips is proposed with intent to harvest hydrokinetic energy. It is found via numerical simulations of fluid-structure interaction that vortex-induced vibration and galloping occur back-to-back by placing the strips on the front surface. With the increasing of flow speed, the hydrodynamic instability is enhanced, especially the cases with strips at 20° and 45°. More than 60W/m power is harnessed at a typical ocean current speed (un=1.5m/s), and the conversion efficient keeps above 35% at the galloping region. However, when the strips move to 90°, the galloping is disappeared, and the vibration is suppressed as the fin-shaped strips further move to 120°. INTRODUCTION Ocean currents and tidal-stream are an abundant and consistent source of renewable energy. Currently, the kinetic energy of currents is mostly converted into pollution-free electrical energy by means of horizontal- axis or vertical-axis turbines (Khan et al., 2009; Pacheco and Ferreira, 2016; Yuce and Muratoglu, 2015). Nevertheless, large-diameter open propellers pose a great threat on marine life and livelihoods linked to fisheries, and the installation and maintenance charges are relatively high. Moreover, turbines are financially viable in average flow speeds of 2.5-3.6 m/s or higher (Lee et al., 2005), the energy conversion efficiency drops significantly when the turbines run at currents below 2 m/s. It is a challenge to commercialize this energy harnessing technology in a large scale, as the majority of ocean currents are slower than 1.55 m/s (Bahaj and Myers, 2003). Bernitsas and his co-workers (Bernitsas et al., 2008, 2009; Lee and Bernitsas, 2011; Sun et al., 2016) at the University of Michigan developed a novel hydrokinetic power generating device called Vortex- Induced Vibration Aquatic Clean Energy (VIVACE) converter, which can harvest the mechanical energy of structural vibration induced by a cross flow as slow as 0.4 m/s. Vortex-induced vibration (VIV), as a common fluid-structure interaction (FSI) phenomenon, is frequently observed in nature and most practical applications. For a circular cylinder, when the Reynolds number, Re (﹩﹩ u in D/v, where u in is the free stream velocity, D is the diameter of the cylinder and υ is the fluid kinematic viscosity), is higher than 47 (Henderson, 1997), vortex is shed from the two lateral surfaces of the cylinder, exciting time-dependent hydrodynamic forces on the cylinder and hence the vibration. Lock-on phenomenon occurs when the vortex shedding frequency matches the natural frequency of the structure, resulting in larger amplitude response. More hydrokinetic energy is extracted and converted into the mechanical energy of the cylinder in this lock-on region of VIV, which is beneficial to further convert into electrical energy. In the experiments carried out by Lee et al. (2011), the maximum power of 15.85W was successfully generated by a single circular cylinder converter at a flow velocity of 1.11 m/s.

Abstract The effect of yaw angle ( α ) on hydrodynamic forces of a yawed square cylinder oscillating in still water is studied experimentally. The independent principle (IP) was examined by evaluating the hydrodynamic coefficients that derived from the measured in-line and lift forces using a 3-component force link at a Keulegan-Carpenter (KC) number range of 3~30 and a Stokes parameter ( β ) range of 500~1600. Significant differences are observed in normalized drag and inertia coefficients for the KC ranges of 8~20 and 10~18, respectively, indicating the invalidity of the IP. The peak on the spectra of the lift force becomes pronounced as α becomes larger, which implies that the yaw angle has an intensified effect on the vortex shedding process around a square cylinder in oscillatory flow. Introduction Oscillatory flow around a cylindrical structure is of importance due to its intrinsic interest in theoretical hydrodynamic research and its relevance to practical engineering applications. Knowledge of the hydrodynamics is essential for both the design and operation of offshore structures, such as semisubmersibles and tension leg platforms. For some cylindrical structures, owning to their directionality, the sea waves and currents can approach the structures obliquely. In these cases, the fluid velocity in the axial direction of the structure is not negligible, which may have profound effect on the vortex instability and force characteristics of the structure.

Abstract This article represents the consideration of the ice cover stresses caused by aircraft landing and take-off. It is assumed that water is an ideal incompressible liquid and that the motion of the liquid is potential. The theory is linear, and the plate is assumed to be elastic. Aircraft is represented by pressure distribution under landing gears with lift force taken into account. The influence of ice thickness, basin depth, and unsteady moving on the possibility of ice cover destruction is analyzed.

Abstract The numerical investigation of the passive hydrodynamic performance of the manta ray beneath the water surface is presented. The main aim is to investigate the effect of attack angle on hydrodynamic forces of the manta ray near the water's surface. Three dimensional incompressible Navier-Stokes equations are resolved with RNG k-e turbulence closure model. The undulation of the free surface is captured by the volume of fluid (VOF) method. Numerical simulations are carried out with the manta ray oriented with the longitudinal axis at attack angles of -5, -2.5, 0, +2.5 and +5 degrees with various swimming speeds ranging from 0.5m/s to 4.5m/s. The drag coefficient and the lift coefficient of the manta ray are measured and compared. Results show obvious differences between negative and positive angles of attack. Additionally, the deformation of the free surface induced by the gliding manta ray is also captured and compared.

ABSTRACT: The characteristics of drag and lift forces on a circular cylinder model near a wavy boundary have been experimentally investigated in a water tunnel. In the range of the Reynolds numbers, based on the diameter of cylinder model, from 10 4 to 1.9 X 10 4 , the drag, lift force on the cylinder and lift force frequencies have been measured at the wave crest and trough and various distances from the wavy boundary. Flow visualization experiments revealed the wake structure varying as gap-to-diameter ratios (G / D) and mechanism Interacting between the cylinder and the boundary. INTRODUCTION In the offshore engineering, pipelines are used to convey oil and gas for power generation, sea-water for desalination, sewage for disposal at sea, and for the protection of communication cables. There are numerous problems associated with the design and installation of pipeline in deep-water offshore. The submerged pipelines are often fixed by concrete saddles and buried by dredging and trenching the sand at a seabed. It has been known from the practice that pipeline spans can occur due to local scour of the bed sediment The proximity of a pipeline near a seabed boundary gives rise to hydrodynamic and environmental problems m offshore engineering. Research on flow field and forces on the cylinder at various distances from the seabed boundary are of a great interest to many. There are many investigations (see Davis,1976) on forces on submerged pipelines In flow or wave. A lot of research works also have been carried out about wave forces on a cylinder near a plane boundary (see Sarpkaya,1981, Shankar et al.,1988). However, researches of hydrodynamic forces on a cylinder near a plane boundary in steady flows are relatively scarce. Second, the plane boundary changes the symmetry of flow about the cylinder nearby.