Flow and air-entrainment around partially submerged vertical cylinders

In this study, a partially submerged vertical cylinder is moved at constant velocity through water, which is initially at rest. During the motion, the wake behind the cylinder induces free-surface deformation. Eleven cylinders, with diameters from $D=1.4$ to 16 cm, were tested at two different conditions: (i) constant immersed height $h$ and (ii) constant $h/D$. The range of translation velocities and diameters are in the regime of turbulent wake with experiments carried out for $4500<Re<240 \,000$ and $0.2<Fr<2.4$, where $Re$ and $Fr$ are the Reynolds and Froude numbers based on $D$. The focus here is on drag force measurements and relatively strong free-surface deformation up to air-entrainment. Specifically, two modes of air-entraiment have been uncovered: (i) in the cavity along the cylinder wall and (ii) in the wake of the cylinder. A scaling for the critical velocity for air-entrainment in the cavity has been observed in agreement with a simple model. Furthermore, for $Fr>1.2$, the drag force varies linearly with $Fr$

The Field of Flow Structures Generated by a Wave of Viscous Fluid Around Vertical Circular Cylinder Piercing the Free Surface

AbstractThe diffraction of water waves induced by a large-diameter, surface-piercing, vertical circular cylinder is studied numerically. The Navier-Stokes equations in primitive variables are considered for the simulation of a given wave case, and the technique is followed of the Direct Numerical Simulation (DNS). The criterion of the imaginary part of the complex-eigenvalue pair of the velocity-gradient tensor for the extraction of the flow vortical structures is applied to the computed fields, so unveiling a complex configuration of structures at the free surface, and at the cylinder external walls. The most energetic modes of the flow are further extracted from the DNS-simulated fields by using the Karhunen-Loève decomposition (KL). A “reduced” velocity field is reconstructed using the first three most energetic eigenfunctions of the decomposition, and its evolution is followed through a sequence of time steps

Numerical study of wave run-up on a fixed surfacepiercing cylinder in non-breaking waves

In wave-structure interaction, wave run-up is an important phenomenon that needs to be considered in the design of offshore structures. A thorough understanding of the physics of the nonlinear flow phenomena is necessary for the better insight into the runup phenomenon. The present work, primarily, is focused on the hydrodynamic simulation of wave run-up and mainly seeks to evaluate the importance of highfrequency wave scattering types identified by Swan-et al. [2005] and lateral progressive edge waves on nonlinear wave amplification around a single fixed cylinder. The physics of the problem involves the interaction of single surface piercing cylinder with surface gravity incident waves which are propagating over a flat bed in an unbounded domain in deep water. The analysis is performed numerically using CFD based Navier-Stokes equations and Potential-flow theory. The numerical results are compared with experimental data provided by ITTC (OEC),[2013]. Taking into account the numerical simulation of the physical mechanism of wave scattering around the cylinder, the first part of the thesis deals with the investigation of the importance of the aforementioned high-frequency wave scattering and also lateral edge waves on nonlinear wave field and also inline wave force over a range of wave steepnesses and wavelengths. Then the Influence of potential flow, viscous and turbulence effects on wave run-up is explored. Afterward, in the second part of the thesis, the effects of the change in cylinder submerged geometry and finally, change in cross-section on the wave field around the cylinder is studied.Na interação onda-estrutura, o wave run-up é um fenômeno importante a ser considerado no projeto de estruturas offshore. Uma maior compreensão da física desse fenômeno não-linear é necessária. O presente estudo está, principalmente, focado na simulação hidrodinâmica da onda e procura avaliar a importância dos tipos de espalhamento das ondas identificados por Swan-et al. [2005], nas ondas de borda progressivas laterais e na amplificação das elevações das ondas em torno de um único cilindro circular fixo. A física do problema inclui a interação do cilindro que atravessa a superfície livre com ondas gravitacionais de superfície incidentes que estão se propagando sobre um fundo plano em um domínio ilimitado. A análise é realizada numericamente usando CFD para resolver a equação de Navier-Stokes e na teoria de escoamento potencial. Os resultados numéricos são comparados com os dados experimentais apresentados em ITTC (OEC), [2013]. Inicialmente, investiga-se a importância do espalhamento da onda incidente na forma de ondas de alta frequência e ondas de borda laterais não lineares. É explorada a influência da amplitude e do comprimento da onda incidente no espalhamento da onda e nas forças geradas sobre o cilindro. Em seguida, investiga-se a Influência dos efeitos das ondas no wave run-up assumindo que o escoamento é potencial, a influência dos efeitos viscosos e influência dos efeitos da turbulência. Posteriormente, na segunda parte da tese, estudam-se os efeitos das mudanças na geometria submersa e na seção transversal do cilindro no campo de onda em seu entorno

Simulating the Hydrodynamics of Offshore Floating Wind Turbine Platforms in a Finite Volume Framework

There is great potential for the growth of wind energy in offshore locations where the structures are exposed to a variety of loading from waves, current and wind. A variety of computer-aided engineering (CAE) tools, based largely on engineering models employing potential-flow theory and/or Morison\u27s equation, are currently being used to evaluate hydrodynamic loading on floating offshore wind turbine platforms. While these models are computationally inexpensive, they include many assumptions and approximations. Alternatively, high-fidelity computational fluid dynamics models contain almost no assumptions, but at the cost of high computational expense. In this work, CFD simulations provide detailed insight into the complex fluid flow that has not been captured experimentally, nor can be attained with reduced-order models. This work includes a thorough validation of the various CFD toolboxes necessary for simulating offshore floating wind turbine platforms in the ocean environment, from numerical wave propagation to fluid-structure interactions. The fundamental physics of flow around complex structures is examined through various studies to better understand the effects of a fluid interface, truncated ends, structure size, multi-member arrangements and environmental conditions. These factors are explored in terms of drag, lift and frequency of the loads. Additionally, motion of structures in free decay tests and waves are investigated. The work provides insight into the complex fluid flow around floating offshore structures of small draft in a variety of environmental conditions. CFD simulations are used to assess assumptions and approximations of reduced-order engineering models, and explain why, and in which conditions, these models perform inaccurately. Finally, the work provides suggestions for improvements to engineering tools often used for hydrodynamics modeling of floating offshore wind turbines

Flow and drag force around a free surface piercing cylinder for environmental applications

This paper investigates flows around a free surface piercing cylinder with Froude number F<0.5 and Reynolds number around Re = 50,000. The aim of this work is to gain a better understanding of the flow behaviour in environmental systems such as fishways. The advances are based upon experimental and numerical results. Several flow discharges and slopes are tested to obtain both subcritical and supercritical flows. The drag force exerted on the cylinder is measured with the help of a torque gauge while the velocity field is obtained using particle velocimetry. For the numerical part, two URANS turbulence models are tested, the k-w SST and the RNG k-e models using the OpenFOAM software suite for subcritical cases, and then compared with the corresponding experimental results. With fishways applications in mind, the changes in drag coefficient Cd versus Froude number and water depth are studied and experimental correlations proposed. We conclude that the most suitable URANS turbulence model for reproducing this kind of flow is the k-w SST model

The loading on a vertical cylinder in steep and breaking waves on sheared currents using smoothed particle hydrodynamics

Waves and currents coexist in a wide range of natural locations for the deployment of offshore structures and devices. This combined wave–current environment largely determines the loading of vertical surface piercing cylinders, which are the foundations typically used for offshore wind turbines along with many other offshore structures. The smoothed particle hydrodynamics (SPH) code DualSPHysics is used to simulate focused waves on sheared currents and assess subsequent loading on a vertical cylinder. Outputs from another numerical model are used to define the SPH inlet–outlet boundary conditions to generate the wave–current combinations. A modified damping zone is used to damp the waves, but allow the currents to exit the domain. Numerical results are validated against experimental measurements for surface elevation and associated loading on the cylinder. Four phase repeats are used in the SPH model to understand the harmonic structure of the surface elevation at the front face of the cylinder and associated loading. It is shown that the SPH model provides agreement with experimental measurements of harmonic components for both force and elevations. Taking advantage of the SPH method, wave amplitudes were increased up to, and beyond, the breaking threshold highlighting a complex relationship between peak force and wave phase, requiring detailed investigation. The numerical modeling of interactions of steep and breaking waves on sheared currents with the cylinder demonstrates the SPH model's capability for modeling highly nonlinear fluid–structure interaction problems

Waves and Ocean Structures

Ocean Structures subjected to actions of ocean waves require safety inspection as they protect human environment and everyday lives. Increasing uses of ocean environment have brought active research activities continuously. The newly developed technology of ocean energy even pushed the related needs forward one more step. This Special Issue focuses on Analysis of Interactions between wave structures and ocean waves. Although ocean structures may cover various practical and/or conceptual types, we hope in the years to come, the state-of-the-art applications in wave and structure interactions and/or progress review and future developments could be included. There are fifteen papers published in the Special issue. A brief description includes: Lee et al. [1] presented a concept of a water column type wave power converter. Li et al. [2] considered submerged breakwaters. Lin et al. [3] studied an ocean current turbine system. Thiagarajan and Moreno [4] investigated oscillating heave plates in wind turbines. Chiang et al. [5] proposed an actuator disk model. Tseng et al. [6] investigated Bragg reflections of periodic surface-piercing submerged breakwaters. Lee et al. [7] analyzed caisson structures with a wave power conversion system installed. Yeh et al. [8] reported motion reduction in offshore wind turbines. Wu and Hsiao [9] considered submerged slotted barriers. Tang et al. [10] studied floating platforms with fishnets. Chen et al. [11] calculated mooring drags of underwater floating structures with moorings. Jeong et al. [12] estimated the motion performance of light buoys using ecofriendly and lightweight materials. Zhang et al. [13] considered vibrations of deep-sea risers. On the other hand, Shugan et al. [14] studied the effects of plastic coating on sea surfaces

Numerical modelling of interactions of waves and sheared currents with a surface piercing vertical cylinder

Vertical surface piercing cylinders, such as typical coastal wind turbine foundations and basic elements of many coastal structures, are often exposed to combined loading from waves and currents. Accurate prediction of hydrodynamic loads on a vertical cylinder in a combined wave-current flow is a challenging task. This work describes and compares two different approaches for numerical modelling of the interaction between focussed wave groups and a sheared current, and then their interactions with a vertical piercing cylinder. Both approaches employ an empirical methodology to generate a wave focussed at the location of the structure in the presence of sheared currents and use OpenFOAM, an open source Computational Fluid Dynamics (CFD) package. In the first approach, the empirical wave-on-current focussing methodology is applied directly in the OpenFOAM domain, replicating the physical wave-current flume. This approach is referred to as the Direct Method. In the second approach, a novel Lagrangian model is used to calculate the free surface elevation and flow kinematics, which are then used as boundary conditions for a smaller 3-D OpenFOAM domain with shorter simulation time. This approach is referred to as the Coupling Method. The capabilities of the two numerical methods have been validated by comparing with the experimental measurements collected in a wave-current flume at UCL. The performance of both approaches is evaluated in terms of accuracy and computational effort required. It is shown that both approaches provide satisfactory predictions in terms of local free surface elevation and nonlinear wave loading on the vertical cylinders with an acceptable level of computational cost. The Coupling Method is more efficient because of the use of a smaller computational domain and the application of the iterative wave-current generation in the faster Lagrangian model. Additionally, it is shown that a Stokes-type perturbation expansion can be generalized to approximate cylinder loads arising from wave groups on following and adverse sheared currents, allowing estimation of the higher-order harmonic shapes and time histories from knowledge of the linear components alone

Experiments and Simulations of Free-Surface Flow behind a Finite Height Rigid Vertical Cylinder

We present the results of a combined experimental and numerical study of the free-surface flow behind a finite height rigid vertical cylinder. The experiments measure the drag and the wake angle on cylinders of different diameters for a range of velocities corresponding to 30,000 &lt;Re&lt; 200,000 and 0.2&lt;Fr&lt;2 where the Reynolds and Froude numbers are based on the diameter. The three-dimensional large eddy simulations use a conservative level-set method for the air-water interface, thus predicting the pressure, the vorticity, the free-surface elevation and the onset of air entrainment. The deep flow looks like single phase turbulent flow past a cylinder, but close to the free-surface, the interaction between the wall, the free-surface and the flow is taking place, leading to a reduced cylinder drag and the appearance of V-shaped surface wave patterns. For large velocities, vortex shedding is suppressed in a layer region behind the cylinder below the free surface. The wave patterns mostly follow the capillary-gravity theory, which predicts the crest lines cusps. Interestingly, it also indicates the regions of strong elevation fluctuations and the location of air entrainment observed in the experiments. Overall, these new simulation results, drag, wake angle and onset of air entrainment, compare quantitatively with experiments