Abstract In this study, three-dimensional numerical simulations are performed to study the VIV of two side-by-side cylinders of different diameters in a steady flow. The two cylinders are rigidly connected together and elastically mounted as a single body. The main aim of the study is to identify the difference between the response of the two cylinder system and that of a single cylinder. It was found that the lock-in range of the reduced velocity of the two cylinder system is similar to that of a single cylinder. However, if the sum of the two cylinder diameter is used as the representative dimension of the system to calculate the reduced velocity, the lock-in regime is narrower that of a single cylinder. Introduction Vortex-induced vibration of a circular cylinder has been studied extensively due to its engineering significance. It is well known that the vortex shedding frequency and the vibration frequency of the cylinder synchronize in a range of reduced velocity, and this range of reduced velocity is commonly called the lock-in range. The reduced velocity is defined as V r= U /( f n D ), where U is the fluid velocity, f n is the natural frequency of the cylinder and D is the cylinder diameter. Comprehensive reviews of VIV of a cylinder can be found in Sumer and Fredsøe (1997) Sarpkaya (2004), Bearman (1984) and Williamson and Govardhan (2004, 2008). Recently, many numerical studies have been conducted to study VIV of cylinders. Due to the intrinsic three-dimensionality of the wake flow behind a vibrating cylinder, it is preferable to perform threedimensional simulations to study VIV. Recently, numerical studies of VIV of a circular cylinder have been extended from two-dimensional (2D) to three-dimensional (3D) simulations. Kondo (2012), Lucar et al. (2005), Navrose and Mittal (2013) and Zhao et al. (2014) studied VIV of a circular cylinder by solving the 3D Navier-Stokes equations and found some mechanisms of VIV that could not be found using 2D numerical models. For example, Lucor et al (2005) found that the correlation of the lift force along the span of the cylinder was poor at the reduced velocities in the hysteretic range between the upper and lower branches, and Zhao et al. (2014) found that the flow in the wake of a vibrating cylinder at Re=1000 is dominated by the streamwise vortices in the lock-in regime. Due to the limitation of the computer power, three-dimensional numerical studies of VIV are still performed at relatively low Reynolds numbers in the turbulent wake flow regime. For example, all the above examples of 3D numerical studies have been performed at Reynolds numbers less or equal to 1000.

Abstract This paper presents numerical results from three-dimensional Large Eddy Simulations (LES) of steady flow around a pipeline close to an uneven seabed with staggered gaps underneath it. It was found that even small gaps below a pipeline can lead to great changes in the hydrodynamics along the span, as well as the seabed shear stress. It is observed that the total forces can be calculated by independently considering the gap section and the embedded section; however, the spanwise variation in the configuration leads to much energetic vibrations in sectional forces. The instantaneous flow feature across the span, however, appears to be less affected by the gaps, where a coherent turbulent structure is found to develop in the upstream and break down above the pipeline. Introduction The research on pipeline hydrodynamics has been largely driven by the wide use of pipelines for transporting oil and gas products across ocean floors, which has been well documented in the monograph by Sumer & Fredsøe (1997) and papers by Bearman & Zdravkovich (1978), Zdravkovich (1985) and Lei, Cheng & Kavanagh (1999), among others. Most of the research was concerned with pipelines either embedded in or spanned above the seabed uniformly along the pipeline. Due to either uneven seabed or local scour below the pipeline, however pipeline/seabed contacts are unlikely uniform. It is expected that the non-uniformity of pipeline-seabed configuration will affect the hydrodynamic forces on the pipeline and the associated flow features around the pipeline. To date, little research has been reported with regard to this issue. This motivated the present study. Problem Definition As illustrated in Fig. 1, this paper investigates steady flow around a pipeline (represented by a circular cylinder) laid on an uneven seabed, mimicking a situation where the pipeline is partially embedded at certain locations, while free spanned at other locations.

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 agility of the humpback whale has been attributed to the leading edge protuberances of its pectoral flippers. Previous studies have shown that the wavy leading edge of a static wing can enhance hydrodynamic performance by increasing lift and delaying stall, but the performance of wavy leading edge in dynamic stall is still not clear. The objective of present study is to elucidate the flow control mechanism of the wavy leading edge in dynamical stall by solving the Unsteady Reynolds Averaged Navier-Stokes (URANS) equations. The results show that the wavy leading edge has positive effects on the performance at high angle attacks in deep stall while negative effects in light stall, and no distinctive stall is observed during the flapping motion.

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.