Regular Head Waves Research Articles

Modelling Sea-Surface Wave Motion and Ship Response Using Smoothed Particle Hydrodynamics and Finite Element Analysis

The response of a ship or other vessel to surface sea waves, including extreme waves, may compromise crew and vessel safety and long-term operational capability. Herein, a novel high-fidelity numerical time-dependent simulation approach is presented using Smoothed Particle Hydrodynamics (SPH) for modelling sea waves coupled with Finite Element Analysis for modelling vessel structural response under wave loading conditions. The results are compared with physical scale model wave tank test results. Good agreement was obtained for heave and pitch motions and vertical bending moments for various forward (head) speeds in regular head waves, heave and pitch motions, and vertical bending moments. High computational demands can be met by the increasing availability of computation power. Ongoing research is outlined. The implications for the design of vessels such as ships and for through-life assessment are discussed.

CFD-FEM simulation of hydroelastic responses and slamming loads of a bow-flare ship advancing in head regular waves

ABSTRACT In this paper, the global motions and wave loads on a large bow-flare ship with the effect of hydro-elasticity, including whipping and springing, are numerically simulated by a CFD-FEM two-way coupled method. The simulation results of ship motions and wave loads by employing the numerical method are demonstrated by verification and validation studies including comparison with tank model experiment results. Additionally, the presented method is applied to study the motions, wave loads and slamming loads of the ship in different wave conditions. The vertical bending moment and vertical shearing force of different frequency components and their distribution along ship length are studied. The characteristics of slamming pressure and green water loads and their spatial distribution are studied. The simulation results indicate that the present numerical method well reproduces ship seakeeping and hydro-elasticity and has potential application values in the design and evaluation of ship wave loads.

Numerical study on hydrodynamic interaction between two parallel surge-released ships advancing in head regular waves based on the hybrid method

This study examines the hydrodynamic interaction between two parallel surge-released ships in head regular waves. A hybrid approach combining potential flow theory and functional-decomposition URANS is used for an efficient and accurate simulation. The methodology involves a surge-released module, 4DOF motion equations, multiple coordinate systems, and a dynamic structured grid. Two ship models are used for validation, comparing numerical and experimental results in ship motions and wave loads. The analysis shows good agreement, with differences of less than 5 % in heave and pitch motions, less than 8 % in roll motions, and less than 15 % in total resistance and sway interference forces. Present study also observes consistent trajectories of the wave system between the two ships. The influence of surge motion and wave height on wave-ship-ship interaction is investigated, emphasizing the importance of considering surge motion in seakeeping performance and longitudinal separation. The occurrence of parametric roll in Ship B is accurately resolved by the hybrid method and the parametric roll amplitude is smaller under ship-to-ship interaction conditions. Wave-induced moments play a limited role in this phenomenon, while the dominating roll restoring moment exhibits a hardening effect. Accordingly, roll motion and moments remain relatively unchanged with increasing wave height, indicating the strong nonlinear nature of parametric roll.

Hydrodynamic Analysis of Different Formation Configurations of Catamaran in Regular Head Waves

When undertaking long-distance missions at sea, vessels aim to achieve an extended operational range through drag reduction and energy efficiency, while enhanced wave resilience also provides substantial benefits. In this work, the Delft-372 catamaran is utilized to investigate the feasibility of drag reduction and roll mitigation for catamaran formation sailing in waves, analyzing the effects of three different formation configurations and varying spacings. The overset grid method was employed to simulate vessel motions, while the Volume of Fluid (VOF) method captured the free surface. First, the numerical results of the catamaran’s resistance, pitch, and heave motion amplitudes under different wave conditions were compared with experimental data to verify the accuracy of the CFD numerical method, and a grid convergence analysis was performed. Next, numerical models of the Delft-372 catamaran were constructed in parallel, tandem, and lateral formations under wave conditions. The results of the single-ship simulation were employed as a benchmark to analyze the impact of different formation configurations and varying lateral and longitudinal spacings on the resistance, pitch, and heave motions of the catamarans. The study also examined the effects of wave interference between vessels and the combined influence of external waves on individual and overall hydrodynamic performance. Results indicated that the tandem formation outperformed the parallel and lateral formations, with optimal performance observed at the longitudinal distance of 1 LPP. Generally, during navigation, the follower catamaran should ideally be positioned in the trough of the stern wave of the leader catamaran.

A comprehensive unsteady RANS numerical investigation to unveil the influence of wave steepness on the motion dynamics and added resistance of a ship in waves

In this study, we investigate the influence of wave steepness on both ship motions and added resistance in regular head waves by conducting simulations at three distinct wave steepness levels using unsteady RANS method. Numerical results show that, when wavelength comparable to the ship length, the interaction of body nonlinearity and wave nonlinearity results in highly nonlinear behavior in the time history of added resistance. The study illustrates, step-by-step, the hydrodynamic pressure distribution on the hull during an encounter period, highlighting the time-varying nature of the added resistance. Examining the visualised results, it is observed that in certain instances, high pressure may be exerted on the flat bottom of the trimmed ship and contribute significantly to the added resistance. It is observed that, when wavelength is comparable to ship length, the added resistance coefficient decreases with increasing wave steepness. Specifically, at λ/LPP = 1.15, |CAW,H/λ=1/30−CAW,H/λ=1/60|CAW,H/λ=1/60 = 27.1%. In relatively short waves the diffraction phenomena dominate and the resulting added resistance exhibits limited nonlinearity. However, obtaining stable results in such cases proves challenging. Finally, the influence of wave steepness on added resistance is quantified. The results are useful for improving the transparent methods for predicting the added resistance in wave.

Experimental and numerical study of wet-deck slamming characteristic for a trimaran in regular head waves

Wet deck may suffer from strong slamming effects when sailing under severe sea conditions, which could damage it locally and compromise the structural integrity of trimarans. The wet-deck slamming characteristic for trimaran sailing in head waves is studied in this work using both experimental and numerical simulation methods. We found that the negative pressure occurred at the front of the wet deck, which is caused by the wet deck exit from the wave crest. Regarding the spatial distribution in longitudinal, the slamming behaviour is more severe at the front part of the wet deck, as the relative vertical impact velocity decreased from the front to the back part of it. Simultaneously, the wet-deck slamming is more severe under the short-wave condition. Regarding the spatial distribution in lateral, we found that the water-entry of the main hull could cause the wet-deck slamming pressure near the outrigger larger than that of the main hull. Moreover, the wet deck has been divided into three regions according to their different lateral distribution characteristics of the wet-deck slamming. These results can provide a better understanding of the wet-deck slamming characteristics when trimaran sailing in waves, which can be considered in future designs for the trimaran.

Impacts of regular head waves on thrust deduction at model self-propulsion point

The results obtained from the self-propulsion simulations using Computational Fluid Dynamics (CFD) in the current study, for a ship free to heave, pitch and surge with the means of a weak spring system, are combined with the formerly executed CFD results of the bare hull and propeller open water simulations to investigate the impacts of regular head waves on the propeller-hull interactions in comparison to calm water, at the self-propulsion point of the model. Despite a rather significant dependency of the nominal wake on the wave conditions, the Taylor wake fraction remains almost unchanged in different studied waves which is around 12% lower than the calm water value. The thrust deduction factor in waves is reduced (12.8%–26.1%) in comparison to the calm water value. The change of thrust deduction factor is found to be associated with the boundary layer contraction/expansion and vortical structure dynamics, originating from the wave orbital velocities as well as the significant shaft vertical motions and accelerations that resulted in a modified propeller action, and consequently diminished suction effect on the aft ship. The altered thrust deduction factor and wake fraction in waves in comparison to calm water underlines the significance of waves on the propulsive factors and propeller design.

Comparative study of nonlinear wave-induced global loads on a container ship in regular head waves predicted with numerical methods based on weakly nonlinear potential theory and Reynolds-averaged Navier-Stokes equations

ABSTRACT Wave-induced loads on the ship hull are of significant interest for many reasons. These loads, however, are highly nonlinear in realistic wave conditions and consequently difficult to predict. In this paper, we analysed and compared wave-induced loads acting on a container vessel in regular head waves computed with a newly developed weakly nonlinear potential theory-based code and a flow solver based on the Reynolds-averaged Navier-Stokes Equations. We investigated both long and short waves with moderate steepness. Initially, the vessel was completely fixed and the ship motions were suppressed, then the vessel was free to heave and pitch motion. We compared time-histories of the computed wave-induced longitudinal and vertical global forces as well as the pitch moment and we deeply analysed nonlinear effects by applying a Fourier transformation and comparing the corresponding harmonic amplitude up to the fourth order. Our results demonstrated satisfactory agreement between the two numerical methods, especially when the ship was free to heave and pitch. Additionally, the results provided detailed information about the capabilities and limitations of the weakly nonlinear potential theory-based approach.

CFD analysis of wave loading on a 10 MW TLP-type offshore floating wind turbine in regular waves

This paper aims to study the higher-harmonics wave-induced surge force on the CENTEC-TLP platform in regular head waves with non-breaking assumptions, using a CFD-based numerical wave tank model, which is a useful tool for the analysis of the nonlinear interaction of waves and structures. Moreover, the influence of change in the wave diffraction and the incident wave angle on the surge force subject to operational and extreme wave conditions is investigated. The numerical analysis is conducted using the CFD toolbox OpenFOAM. The link between Secondary Load Cycle and the wave scattering Type2 is apparent and it is concluded that they are all inertia-driven and that 2nd harmonics play an important role.

Numerical Study of a Model and Full-Scale Container Ship Sailing in Regular Head Waves

In the present study, the added resistance, heave, and pitch of the KRISO Container Ship (KCS) in waves, at both model scale and full scale, are predicted numerically in regular head waves, for four wavelengths and three wave heights. The ISIS-CFD viscous flow solver, implemented in the Fidelity Fine Marine software provided by CADENCE, was employed for the numerical simulations. The spatial discretization was based on the finite volume method using an unstructured grid. The unsteady Reynolds-averaged Navier–Stokes (RANS) equations were solved numerically, with the turbulence modeled by shear stress transport (k-ω) (SST). The free-surface capturing was based on the volume-of-fluid method. The computed solutions were validated through comparisons with towing test data available in the public domain. To predict the uncertainties in the numerical solution, a systematic grid convergence study based on the Richardson extrapolation method was performed for a single wave case on three different grid resolutions. Specific attention was given to the free-surface and wake flow in the propeller plane. The purpose was to compare the numerical results from the model- and full-scale tests to examine the scale’s effect on the ship’s performance in regular head waves. The comparison between the model scale and full scale showed obvious differences, less accentuated for the free-surface topology and clearly observed in terms of boundary layer formation in the propeller’s vicinity.

The effect of a ship's acceleration and deceleration on parametric roll motion in regular head waves at resonance conditions

The effect of a ship's acceleration and deceleration in regular head waves on parametric roll motion is examined experimentally in the study and compared with a mathematical model. Operators queries such as "Should I accelerate or decelerate the ship in resonance condition?" and "Is the magnitude of acceleration or deceleration effective in reducing large roll motion?" are tried to answer. It is well known that waves with lengths similar to that of the ship may suffer parametric roll motion. Additionally, it is possible to say that the encounter frequency itself provides the ship's tuning factor. Since one particular wave is selected with a length equal to the ship's length, the tuning factor in this study at ship operation is the velocity. Results indicate that the amplitudes of roll motion at acceleration and deceleration periods are still large. Results also indicate that the magnitude of acceleration or deceleration affects the roll motion. The mathematical model is also solved numerically and compared with experimental results to better understand the effect of acceleration magnitude.

Comprehensive computational analysis of the impact of regular head waves on ship bare hull performance

This paper focuses on investigating the impact of waves on ship hydrodynamic performance, enhancing our understanding of seakeeping characteristics and contributing to advanced ship and propeller design. It examines the resistance, motions, and nominal wake of the KVLCC2 bare hull, which is free to surge, heave, and pitch, in both calm water and regular head waves using a RANS approach. The research reveals a substantial dependency of the wake on grid resolution, particularly in calm water and shorter waves, while motions and resistance display a weaker dependency. The computed nominal wake is compared against towing tank SPIV measurements. Utilizing Fourier analyses and reconstructed time series, the study examines correlations among various factors influencing the bare hull’s performance in waves. The axial velocity component of the wake in waves demonstrates significant time variations, mainly driven by higher harmonic amplitudes. This dynamic wake is influenced by instantaneous propeller disk velocities due to hull motions, orbital wave velocities, boundary layer contraction/expansion, bilge vortex and shaft vortex dynamics. The wake distribution at the propeller plane not only differs significantly from the calm water wake in longer waves but also exhibits notably larger time-averaged values (up to 21%).

A Study on Safe Navigation Towards Intelligent Shipping Considering Sea Conditions

A mathematical model is created to obtain safe navigation for ships in regular head waves in this study. To validate the suggested model, firstly, the added resistances are calculated for two different ships using empirical formulas in the mathematical model. Secondly, the turning test simulations are performed for calm water and in waves with various wave amplitudes. After these validation studies, the path following simulation of the ship to the target destinations is performed in both waves and calm water for the determined course. It is assumed that regular head waves affect the ship as an external disturbance. The wavelengths and wave amplitudes are changed systematically to understand their effect during the path following simulations. When the ratio of wavelength to ship length, λ/Lpp, is nearly 1.0, the path following simulation times increase. Moreover, when the value of wave amplitude increases, so does the simulation time.

Prediction of Added Resistance of Container Ships in Regular Head Waves Using an Artificial Neural Network

In this paper, an artificial neural network was used to predict the added resistance coefficient for container ships in regular head waves for various speeds. The data used for training the neural network were gathered based on performed numerical calculations using the Boundary Integral Element Method for various hull forms of container ships. The numerically obtained results were validated against the available experimental data for three benchmark container ships. The data were divided into three classes based on the ship length, and the expressions for the prediction of the added resistance coefficient for each container ship class were provided. The performance and generalization properties of the neural network were evaluated based on the normalized value of the root mean square error. The model enables reliable prediction of the added resistance coefficient within the preliminary design stage of a ship based on the ship characteristics and speed.

Experimental study on the influence of a T-foil and bulbous bow on the resistance and motion in regular head waves of trimarans

This study proposes a novel approach for reducing the total resistance and mitigating motion in regular head waves of trimarans. Instead of the traditional method of optimising the side hull layout, we investigated the effects of a T-foil and bulbous bow configuration on the trimaran's performance. Through model tests, we studied the impact of T-foils with various aspect ratios and their combination with a bulbous bow on the motion in regular head waves of a trimaran. We observed that the T-foils can reduce the resistance and mitigate the motion of trimarans in regular waves. This effect is more pronounced at high speeds than at low speeds. T-foils with the same projected areas but different aspect ratios have a similar influence on the motion of the trimaran; however, those with large aspect ratios are more effective in reducing resistance. Furthermore, the combination of a T-foil and bulbous bow can simultaneously reduce the total resistance and mitigate the motion in regular head waves of the trimaran. The configuration with a T-foil with a large aspect ratio is particularly effective in mitigating the heave and pitch motions and reducing the total resistance to waves. This study provides a new approach to enhancing the hydrodynamic performance of trimarans.

Numerical Investigations of Water Column Responses at Different Moonpool Locations Within a Fixed Vessel

Abstract The oscillating water column (OWC) responses within forward, central, and aft located moonpools for a fixed rectangular vessel are studied numerically under regular head wave conditions using Reynolds-averaged Navier–Stokes-based computational fluid dynamics calculations. The free surface elevation of the confined water inside the moonpool for the piston mode frequencies is studied independently for the forward, central, and aft moonpools, showing a strong dependence on location, draft, and incident wave frequency. The relative response is largely magnified for wave frequencies around the resonant range. It is observed that the free surface response amplitude is significantly higher in the forward moonpool location, and the results are in well comparison with experiments performed in the towing tank. The variation of dynamic pressure and its influence on the moonpool responses is investigated to understand the position-specific fluctuations, which showed the presence of harmonics. The phase difference between the water column oscillations within the moonpool and the propagating wave in the domain depends on its position. Finally, the effect of the moonpool in modifying the pressure field is studied by comparison with a vessel without moonpool for a common incident wave frequency.

Experimental and Numerical Investigation of the Added Resistance in Regular Head Waves for the DTC Hull

Reducing the added resistance in waves has become a crucial aspect of today’s ship design. The added resistance in waves is traditionally considered proportional to the square of wave height. However, this assumption is believed to be only partly valid, and further investigations are required. In the present study, experimental tests and numerical simulations were carried out to determine the added resistance in regular head waves of the DTC hull (the scale factor is 135). The numerical analysis was performed with SHIPFLOW 7.01, and the experimental campaign was carried out in the ETSIN-UPM towing tank. The investigation revealed that the added resistance in waves was not proportional to the square of the wave height, and a better correlation was obtained by changing the power of the wave weight to 1.75. Furthermore, an unexpected double resonance phenomenon on the added resistance was found at a Froude number of 0.13. The study also revealed an acceptable agreement between the numerical simulations and the experiments, except for the double resonance phenomenon.

Application of virtual disk propulsion model for self-propelled surface ship in regular head wave

Application of virtual disk propulsion model for self-propelled surface ship in regular head wave

Regular Wave Seakeeping Analysis of a Planing Hull by Smoothed Particle Hydrodynamics: A Comprehensive Validation

In this work, the dynamics of a planing hull in regular head waves was investigated using the Smoothed Particle Hydrodynamics (SPH) meshfree method. The simulation of the interaction of such vessels with wave trains features several challenging characteristics, from the complex physical interaction, due to large dynamic responses, to the likewise heavy numerical workload. A novel numerical wave flume implemented within the SPH-based code DualSPHysics fulfills both demands, guaranteeing comparable accuracy with an established proprietary Computational Fluid Dynamics (CFD) solver without sharpening the computational load. The numerical wave flume uses ad hoc open-boundary conditions to reproduce the flow characteristics encountered by the hull during its motion, combining the current and waves while adjusting their properties with respect to the vessel’s experimental towing speed. It follows a relatively small three-dimensional domain, where the potentiality of the SPH method in modeling free-surface flows interacting with moving structures is unleashed. The results in different wave conditions show the feasibility of this novel approach, considering the overall good agreement with the experiments; hence, an interesting alternative procedure to simulate the seakeeping test in several marine conditions with bearable effort and satisfying accuracy is established.

Numerical investigations on seakeeping and added resistance in head waves based on nonlinear potential flow methods

A fully nonlinear potential flow (FNPF) method and a body-exact (BE) method based on the time-domain desingularized-Rankine panel (DRP) method have been developed to compute the motion responses and added resistance for ships traveling in regular head waves. In the present methods, the disturbed wave potential is separated from the incident potential, and the boundary value problem (BVP) for the disturbed potential is established in each method. To validate the present FNPF and BE methods, different hull forms are adopted, including two Wigley hulls and the S-175 containership. The results of radiation, diffraction, and free motion simulations are presented and systematically compared with the experimental data and linear solutions using the double-body (DB) and Neumann–Kelvin (NK) methods. Through comparison, we demonstrate that the present FNPF method can accurately predict the motion responses and added resistance of ships. For the ships with large flare angles, the FNPF and BE methods give better predictions of peak motion responses than those using DB and NK. In addition, the FNPF method can well predict the added resistance, and has significant advantages over the other numerical methods at high forward speed (Fn=0.3). The analyses also confirm that, compared with the heave and pitch motions, the added resistance and the total wave elevations are more sensitive to the incident wave amplitude. The dimensionless added resistance decreases with the increase of incident wave amplitude, which is mainly driven by the total wave elevations along the waterline in the bow region. As the incident wave amplitude increases, the 1st harmonic amplitudes of the heave and pitch motions and the total wave elevations in the bow region decrease, while the 2nd harmonic amplitudes increase.