CFD Investigations of Ship Maneuvering in Waves Using naoe-FOAM-SJTU Solver

Ship maneuvering in waves includes the performance of ship resistance, seakeeping, propulsion, and maneuverability. It is a complex hydrodynamic problem with the interaction of many factors. With the purpose of directly predicting the behavior of ship maneuvering in waves, a CFD solver named naoe-FOAM-SJTU is developed by the Computational Marine Hydrodynamics Lab(CMHL) in Shanghai Jiao Tong University. The solver is based on open source platform OpenFOAM and has introduced dynamic overset grid technology to handle complex ship hull-propeller-rudder motion system. Maneuvering control module based on feedback control mechanism is also developed to accurately simulate corresponding motion behavior of free running ship maneuver. Inlet boundary wavemaker and relaxation zone technique is used to generate desired waves. Based on the developed modules, unsteady Reynolds-averaged Navier-Stokes(RANS) computations are carried out for several validation cases of free running ship maneuver in waves including zigzag, turning circle, and course keeping maneuvers. The simulation results are compared with available benchmark data. Ship motions, trajectories, and other maneuvering parameters are consistent with available experimental data, which indicate that the present solver can be suitable and reliable in predicting the performance of ship maneuvering in waves. Flow visualizations, such as free surface elevation, wake flow, vortical structures, are presented to explain the hydrodynamic performance of ship maneuvering in waves. Large flow separation can be observed around propellers and rudders. It is concluded that RANS approach is not accurate enough for predicting ship maneuvering in waves with large flow separations and detached eddy simulation(DES) or large eddy simulation(LES) computations are required to improve the prediction accuracy.

Numerical Analysis of a Floating Offshore Wind Turbine by Coupled Aero-Hydrodynamic Simulation

The exploration for renewable and clean energies has become crucial due to environmental issues such as global warming and the energy crisis. In recent years,floating offshore wind turbines(FOWTs) have attracted a considerable amount of attention as a means to exploit steady and strong wind sources available in deep-sea areas. In this study, the coupled aero-hydrodynamic characteristics of a spar-type 5-MW wind turbine are analyzed. An unsteady actuator line model(UALM) coupled with a twophase computational fluid dynamics solver naoe-FOAM-SJTU is applied to solve three-dimensional Reynolds-averaged NavierStokes equations. Simulations with different complexities are performed. First, the wind turbine is parked. Second, the impact of the wind turbine is simplified into equivalent forces and moments. Third, fully coupled dynamic analysis with wind and wave excitation is conducted by utilizing the UALM. From the simulation, aerodynamic forces, including the unsteady aerodynamic power and thrust, can be obtained, and hydrodynamic responses such as the six-degrees-of-freedom motions of the floating platform and the mooring tensions are also available. The coupled responses of the FOWT for cases of different complexities are analyzed based on the simulation results. Findings indicate that the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform are obvious. The aerodynamic loads have a significant effect on the dynamic responses of the floating platform, and the aerodynamic performance of the wind turbine has highly unsteady characteristics due to the motions of the floating platform. A spar-type FOWT consisting of NREL-5-MW baseline wind turbine and OC3-Hywind platform system is investigated. The aerodynamic forces can be obtained by the UALM. The 6 DoF motions and mooring tensions are predicted by the naoe-FOAM-SJTU. To research the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform, simulations with different complexities are performed. Fully coupled aero-hydrodynamic characteristics of FOWTs, including aerodynamic loads, wake vortex, motion responses, and mooring tensions, are compared and analyzed.

Numerical Prediction of Added Resistance and Vertical Ship Motions in Regular Head Waves

The numerical prediction of added resistance and vertical ship motions of one ITTC （Intemational Towing Tank Conference） S-175 containership in regular head waves by our own in-house unsteady RANS solver naoe-FOAM-SJTU is presented in this paper. The development of the solver naoe-FOAM-SJTU is based on the open source CFD tool, OpenFOAM. Numerical analysis is focused on the added resistance and vertical ship motions （heave and pitch motions） with four very different wavelengths （ 0.8Lpp 〈 2 〈 1.5L ） in regular head waves. Once the wavelength is near the length of the ship model, the responses of the resistance and ship motions become strongly influenced by nonlinear factors, as a result difficulties within simulations occur. In the paper, a comparison of the experimental results and the nonlinear strip theory was reviewed and based on the findings, the RANS simulations by the solver naoe-FOAM-SJTU were considered competent with the prediction of added resistance and vertical ship motions in a wide range of wave lengths.

Numerical Simulation of Wind Turbine Blade-Tower Interaction

Numerical simulations of wind turbine blade-tower interaction by using the open source OpenFOAM tools coupled with arbitrary mesh interface （AMI） method were presented. The governing equations were the unsteady Reynolds-averaged Navier-Stokes （RANS） which were solved by the pimpleDyMFoam solver, and the AMI method was employed to handle mesh movements. The National Renewable Energy Laboratory （NREL） phase VI wind turbine in upwind configuration was selected for numerical tests with different incoming wind speeds （5, 10, 15, and 25 m/s） at a fixed blade pitch and constant rotational speed. Detailed numerical results of vortex structure, time histories of thrust, and pressure distribution on the blade and tower were presented. The findings show that the wind turbine tower has little effect on the whole aerodynamic performance of an upwind wind turbine, while the rotating rotor will induce an obvious cyclic drop in the front pressure of the tower. Also, strong interaction of blade tip vortices with separation from the tower was observed.

Numerical Study on the Effect of Current Profiles on Vortex-Induced Vibrations in a Top-Tension Riser

In this paper, numerical simulations of vortex-induced vibrations in a vertical top-tension riser with a length-to-diameter ratio of 500 using our in-house code viv-FOAM-SJTU are presented. The time-dependent hydrodynamic forces on two-dimensional strips are obtained by solving the Navier-Stokes equations, which are, in turn, integrated into a finite-element structural model to obtain the riser deflections. The riser is discretized into 80 elements with its two ends set as pinned and 20 strips are located equidistant along the risers. Flow and structure are coupled by hydrodynamic forces and structural displacements. In order to study the effects of the shear rate, of the current profiles on the vortex-induced vibrations in the riser, vibrations, with varying shear rates, in both the in-line and cross-flow directions, are simulated. In addition to the time domain analysis, spectral analysis was conducted in both the temporal and spatial domains. Multi-mode vibration characteristics were observed in the riser. The relationship between dominant vibration mode number and the shear rate of current profiles is discussed. In general, the overall vibrations in the riser pipe include contributions from several modes and each mode persists over a range of shear rates. Moreover, the results suggest that with a larger shear rate the position of the maximum in-line time-averaged displacement will move closer to the end where the largest velocity is located.

Application Progress of Computational Fluid Dynamic Techniques for Complex Viscous Flows in Ship and Ocean Engineering

Complex flow around floating structures is a highly nonlinear problem,and it is a typical feature in ship and ocean engineering.Traditional experimental methods and potential flow theory have limitations in predicting complex viscous flows.With the improvement of high-performance computing and the development of numerical techniques,computational fluid dynamics(CFD)has become increasingly powerful in predicting the complex viscous flow around floating structures.This paper reviews the recent progress in CFD techniques for numerical solutions of typical complex viscous flows in ship and ocean engineering.Applications to free-surface flows,breaking bow waves of high-speed ship,ship hull-propeller-rudder interaction,vortexinduced vibration of risers,vortex-induced motions of deep-draft platforms,and floating offshore wind turbines are discussed.Typical techniques,including volume of fluid for sharp interface,dynamic overset grid,detached eddy simulation,and fluid-structure coupling,are reviewed along with their applications.Some novel techniques,such as high-efficiency Cartesian grid method and GPU acceleration technique,are discussed in the last part as the future perspective for further enhancement of accuracy and efficiency for CFD simulations of complex flow in ship and ocean engineering.

MPS-FEM Coupled Method for Study of Wave-Structure Interaction

Nowadays,an increasing number of ships and marine structures are manufactured and inevitably operated in rough sea.As a result,some phenomena related to the violent fluid-elastic structure interactions(e.g.,hydrodynamic slamming on marine vessels,tsunami impact on onshore structures,and sloshing in liquid containers)have aroused huge challenges to ocean engineering fields.In this paper,the moving particle semi-implicit(MPS)method and finite element method(FEM)coupled method is proposed for use in numerical investigations of the interaction between a regular wave and a horizontal suspended structure.The fluid domain calculated by the MPS method is dispersed into fluid particles,and the structure domain solved by the FEM method is dispersed into beam elements.The generation of the 2D regular wave is firstly conducted,and convergence verification is performed to determine appropriate particle spacing for the simulation.Next,the regular wave interacting with a rigid structure is initially performed and verified through the comparison with the laboratory experiments.By verification,the MPS-FEM coupled method can be applied to fluid-structure interaction(FSI)problems with waves.On this basis,taking the flexibility of structure into consideration,the elastic dynamic response of the structure subjected to the wave slamming is investigated,including the evolutions of the free surface,the variation of the wave impact pressures,the velocity distribution,and the structural deformation response.By comparison with the rigid case,the effects of the structural flexibility on wave-elastic structure interaction can be obtained.

Numerical Simulation of the Solitary Wave Interacting with an Elastic Structure Using MPS-FEM Coupled Method

Fluid-Structure Interaction(FSI) caused by fluid impacting onto a flexible structure commonly occurs in naval architecture and ocean engineering. Research on the problem of wave-structure interaction is important to ensure the safety of offshore structures. This paper presents the Moving Particle Semi-implicit and Finite Element Coupled Method(MPS-FEM) to simulate FSI problems. The Moving Particle Semi-implicit(MPS) method is used to calculate the fluid domain, while the Finite Element Method(FEM) is used to address the structure domain. The scheme for the coupling of MPS and FEM is introduced first. Then, numerical validation and convergent study are performed to verify the accuracy of the solver for solitary wave generation and FSI problems. The interaction between the solitary wave and an elastic structure is investigated by using the MPS-FEM coupled method.

Overview of Moving Particle Semi-implicit Techniques for Hydrodynamic Problems in Ocean Engineering

With the significant development of computer hardware,many advanced numerical techniques have been proposed to investigate complex hydrodynamic problems.This article aims to provide a detailed review of moving particle semi-implicit(MPS)techniques and their application in ocean and coastal engineering.The achievements of the MPS method in stability and accuracy,boundary conditions,and acceleration techniques are discussed.The applications of the MPS method,which are classified into two main categories,namely,multiphase flows and fluid-structure interactions,are introduced.Finally,the prospects and conclusions are highlighted.The MPS method has the potential to solve practical problems.

Numerical Simulations of Cavitation Flows around Clark-Y Hydrofoil

Cavitation is a complex flow phenomenon including unsteady characteristics, turbulence, gas-liquid two-phase flow. This paper provides a numerical investigation on comparing the simulation performance of three different models in OpenFOAM-Merkle model, Kunz model and Schnerr-Sauer model, which is helpful for understanding the cavitation flow. Considering the influence of vapor-liquid mixing density on turbulent viscous coefficient, the modified SST k-ω model is adopted in this paper to increase the computing reliability. The InterPhaseChangeFoam solver is utilized to simulate the two-dimensional cavitation flow of the Clark-Y hydrofoil with three cavitation models. The hydrodynamic performance including lift coefficient, drag coefficient and cavitation flow shape of the hydrofoil is analyzed. Through the comparison of the numerical results and experimental data, it is found that the Schnerr-Sauer model can get the most accurate results among the three models. And from the simulation point of water and water vapor mixing, the Merkle model has the best water and water vapor mixing simulation.

Review:Recent Advancement of Experimental and Numerical Investigations for Breaking Waves

Breaking wave is a complex physical phenomenon that takes place at the gas fluid interface which is the chief reason for the generation of two phase turbulence wave energy dissipation and mass transfer between air and water. For marine hydrodynamics the breaking bow wave of high speed vessels induces the bubble mixed flow travelling around the ship eventually developing to be the turbulent wake which is easy to be detected by photoelectric equipment. Besides the flow induced noise stemming from wave plunging may weaken the acoustic stealth of water surface craft. In the oceanographic physics context wave breaking accounts for the energy and mass exchange of the ocean atmosphere system which has a great effect on the weather forecasts and global climate predictions. Due to multi scale properties of multiphase turbulent flows a wide range of time and length scales should be resolved making it rather complicated for experimental and numerical investigations. In early reviews[1-4] general mechanisms related to wave breaking problems are well described. However previous emphasis lies on the phenomenological characteristics of breaking wave. Thus this review summarizes the recent experimental and numerical advances of the studies of air entrainment bubble distribution energy dissipation capillary effect and so on.

Review:Recent Development of High⁃Order⁃Spectral Method Combined with Computational Fluid Dynamics Method for Wave⁃Structure Interactions

The present paper reviews the recent developments of a high⁃order⁃spectral method(HOS)and the combination with computational fluid dynamics(CFD)method for wave⁃structure interactions.As the numerical simulations of wave⁃structure interaction require efficiency and accuracy,as well as the ability in calculating in open sea states,the HOS method has its strength in both generating extreme waves in open seas and fast convergence in simulations,while computational fluid dynamics(CFD)method has its advantages in simulating violent wave⁃structure interactions.This paper provides the new thoughts for fast and accurate simulations,as well as the future work on innovations in fine fluid field of numerical simulations.

Numerical Prediction of Wall Effect on Propeller in Restricted Channel

In restricted channel, the hydrodynamic performance of propeller is affected by the wall. In the present work, two cylindrical channels with different diameters being 1.8D and 5.0D are adopted to study the influence of wall on the hydrodynamic performance and wake field of the propeller model DTMB4119. The numerical simulations are carried out by the single-phase solver pimpleDyMFoam in open source platform OpenFOAM. The Reynolds Averaged Navier-Stokes equations (RANS) are adopted to solve the flow field. The arbitrary mesh interface (AMI) method is used to simulate the rotation of propeller. The designed advance ratio, J = 0.833, is applied in all the computations. For the 5.0 D case, the predicted results of open water performance are in good agreement with experiment data. In restricted channel, the predicted results of thrust and torque coefficients are larger than the open water case. The pressure on the wall of restricted channel downstream increases and approaches the results in open water gradually. Due to the flux conservation, higher negative induced velocity is investigated in the flow field of the propeller in restricted channel.

Hydroelastic responses of an elastic cylinder impacting on the free surface by MPS-FEM coupled method

In naval engineering and offshore industry,the fluid-structure interaction(FSI)problem is a very common problem,and water entry is a very representative one.The hydroelasticity effects due to slamming are of great interest.In this paper,the water entry problem is simulated by the moving particle semi-implicit&finite element method(MPS-FEM)coupled method.The MPS method is used for the fluid because it is very suitable for the violent free-surface flow.The structure domain is solved by the FEM method because of the maturity in solving structural motion and deformation.The water entry of a rigid cylinder is numerically studied first and the results show good agreements with previous published data.After that,variable analysis is conducted in the water entry simulation of an elastic cylinder,including the structural elasticity and impact velocity.

Numerical study of vibrations of a vertical tension riser excited at the top end

This paper presents numerical simulations of vortex-induced vibrations of a vertical riser which is sinusoidally excited at its top end in both one and two directions in still water.A computational fluid dynamics method based on the strip theory is used.The riser’s responses to both top-end and two-end excitations are carefully examined.In low reduced velocity cases,the in-line vibrations consist of three components,the low-frequency oscillation,the first-natural-frequency vibration during the riser reversal,and the second-natural-frequency vibration due to vortex shedding.The sheared oscillatory flow along the span causes low-frequency oscillations in higher modes in the in-line direction,thus forming‘X’shaped,‘II’shaped,and‘O’shaped trajectories at various positions along the span when the riser is excited at its top end in one direction.In the presence of excitations in the other direction,more complex trajectories appear.