Regular Head Seas Research Articles

Experimental investigation on the added resistance of modified KVLCC2 hull forms with different bow shapes

The effect of different bow shapes on the added resistance in waves was observed through a series of model tests. To this end, three different hull forms of KRISO Very Large Crude Carrier 2 were considered: an original hull form and two modified hulls with different bow shapes, called ax-bow and leadge-bow. The model tests were conducted for a wide range of wavelengths with two wave amplitudes in a regular head-sea condition at the design speed. Each test condition was imposed at least twice in order to check the repeatability of measurement, considering the uncertainties in model test and the nonlinear nature of the added resistance. This article introduces a preliminary study on the effects of surge motion, amplitude of incident wave, and green-water allowance around bow region. This article briefly includes the uncertainty analysis of recent study regarding the performance of the original hull. Based on the results of the experimental study for three different bow shapes, the parameters which influence the added resistance and motion responses are discussed.

EFD and CFD for KCS heaving and pitching in regular head waves

The KCS container ship was investigated in calm water and regular head seas by means of EFD and CFD. The experimental study was conducted in FORCE Technology’s towing tank in Denmark, and the CFD study was conducted using the URANS codes CFDSHIP-IOWA and Star-CCM+ plus the potential theory code AEGIR. Three speeds were covered and the wave conditions were chosen in order to study the ship’s response in waves under resonance and maximum exciting conditions. In the experiment, the heave and pitch motions and the resistance were measured together with wave elevation of the incoming wave. The model test was designed and conducted in order to enable UA assessment of the measured data. The results show that the ship responds strongly when the resonance and maximum exciting conditions are met. With respect to experimental uncertainty, the level for calm water is comparable to PMM uncertainties for maneuvering testing while the level is higher in waves. Concerning the CFD results, the computation shows a very complex and time-varying flow pattern. For the integral quantities, a comparison between EFD and CFD shows that the computed motions and resistance in calm water is in fair agreement with the measurement. In waves, the motions are still in fair agreement with measured data, but larger differences are observed for the resistance. The mean resistance is reasonable, but the first order amplitude of the resistance time history is underpredicted by CFD. Finally, it seems that the URANS codes are in closer agreement with the measurements compared to the potential theory.

Speed loss due to seakeeping and maneuvering in zigzag motion

Ships are prone to experience a slight zigzag motion due to the interaction between steering, waves and other external disturbances. The zigzag motion will induce an added resistance and hence speed loss. This paper aims at addressing an effective tool to predict a ship's zigzag motion and azimuth propulsion system behavior in calm water and different sea conditions, and the resulting speed loss. Both model tests and numerical simulations have been carried out. Model tests with a freely running model were done in MARINTEK, Trondheim, Norway. The propulsion system was composed of two azipull thrusters, with autopilot control system to keep the power constant during the test process. Model speeds and the propulsion system behavior were recorded as time series when the models were freely running in calm water and regular head sea waves. Numerical simulations with simplified modular maneuvering model were conducted to verify the model test data. Good agreement between simulations and experimental results was found. The results also showed that when wave added resistance reach its peak value, the vessel has the most severe speed loss.

Theoretical study on the effect of parametric rolling on added resistance in regular head seas

A formula based on Maruo’s theory is presented for the added resistance in regular head seas when parametric rolling occurs. Firstly, the velocity potential of the radiation waves due to parametric rolling, sway and yaw motions should be superposed on the conventional velocity potential. Secondly, the added resistance is averaged within the time duration that is double the encounter period. Thirdly, the stationary-phase method which is used in Maruo’s method is also used to obtain the formula of added resistance in waves with parametric rolling taken into account as well as viscous roll damping. Fourthly, source distribution based on Maruo and Ishii's and Maruo and Iwase’s works is used to calculate the added resistance. Finally, the effect of parametric rolling on added resistance in regular head seas mainly results from viscous roll damping and it becomes larger as the amplitude of parametric rolling becomes larger.

Dynamic motions of planing vessels in head seas

The dynamic response of planing vessels in regular head seas is investigated numerically. Nonlinear time domain simulations were performed using a 2D + t theory (two-dimensional plus time dependent theory). A prismatic hull form was assumed. We employed a two-dimensional (2D) boundary element method to solve the initial boundary value problems in 2D cross planes, in which nonlinear free-surface conditions and exact body boundary conditions were satisfied. At each time step, the total force and moment on the hull could be obtained by using the sectional forces calculated in those 2D planes. Heave and pitch motions were then acquired by solving the equations for those motions. The calculated heave and pitch responses were compared with the experiments by Fridsma (A systematic study of the rough-water performance of planing boats. Davidson Laboratory Report R-1275, 1969) for two different Froude numbers. Three-dimensional (3D) corrections at the transom stern were applied to show the influence of the 3D effect at the stern on the numerical results. Ship motions were affected by the 3D corrections, especially near the resonance frequency, while the phase angles were slightly affected and the acceleration peaks at the bow near the resonance frequency were sensitive to the 3D corrections. Other error sources in the theoretical results are also mentioned.

Time-Domain Simulations of Radiation and Diffraction Forces

Large-amplitude, time-domain, wave-body interactions are studied in this paper for problems with forward speed. Both two-dimensional strip theory and three-dimensional computation methods are shown and compared by a number of numerical simulations. In the present approach, an exact body boundary condition and linearized free surface boundary conditions are used. By distributing desingularized sources above the calm water surface and using constant-strength flat panels on the exact body surface, the boundary integral equations are solved numerically at each time step. The strip theory method implements Radial Basis Functions to approximate the longitudinal derivatives of the velocity potential on the body. Once the fluid velocities on the free surface are computed, the free surface elevation and potential are updated by integrating the free surface boundary conditions. After each time step, the body surface and free surface are regrided due to the instantaneous changing wetted body geometry. Extensive results are presented to validate the efficiency of the present methods. These results include the added mass and damping computations for a Wigley III hull and an S-175 hull with forward speed using both two-dimensional and three-dimensional approaches. Exciting forces acting on a Wigley III hull due to regular head seas are obtained and compared using both the fully three-dimensional method and the two-dimensional strip theory. All the computational results are compared with experiments or other numerical solutions.

Sea-keeping characteristics of a model rib in head seas

This paper describes the sea-keeping characteristics of a 1/4 scale model Atlantic 75 RIB, with an inflatable tube, encountering regular and irregular head seas, of various wave heights and wave periods with various tube pressures. The characteristic motion responses are identified and presented in the time and frequency domains, with respect to wave height, wave period, speed and tube pressure. The characteristic average motion responses (r.m.s. and r.m.q) are also presented. In addition an alternative average motion representation measure is proposed to describe the severity of the (slamming) motion.

규칙파중 전진하는 선박의 유체역학적 응답에 대한 비정상 수치해석

The present paper provides a CFD analysis of diffraction problem for a ship with forward speed using an unsteady RANS simulation method, a WAVIS code. The WAVIS viscous solver adopting a finite volume method has second order accuracy in time and field discretizaions for the RANS equations. A two phase level-set method and a realizable <TEX>${\kappa}-{\varepsilon}$</TEX> turbulence model are adopted to compute the free surface and to meet the turbulence closure, respectively. To validate the capability of the present numerical methods for the simulation of an unsteady progressive regular wave, computations are performed for three grid sets with refinement ratio of <TEX>${\sqrt{2}}$</TEX>. The main simulation is performed for a DTMB5512 model with a forward speed in a regular head sea condition. Validation of the present numerical method is carried out by comparing the present CFD results with available unsteady experimental data published in the 2005 Tokyo CFD Workshop: resistance, heave force, pitch moment, unsteady free surface elevations and velocity fields.

パラメトリック横揺れ防止装置としてのアンチローリングタンクの性能推定

Recently, many incidents of parametric roll of container carriers and pure car carriers have been reported. For the prevention of parametric roll, installing an Anti-Rolling Tank (ART) is one of the most effective risk control options. To estimate the performance of ART, however, a damping coefficient of ART is required in a numerical simulation of parametric roll. For this purpose, a scaled model test of ART is normally executed while other coefficients related to the tank or coupling coefficients between the ART and the ship roll motion can be obtained from its dimensions. To overcome this drawback, we attempted to estimate the damping coefficient of ART by applying a MPS method (Moving-Particle-Semi-implicit method) and examined its applicability by comparing with the model experiment. As a result, the MPS method can estimate not only damping coefficient but also natural period of ART. By utilizing the obtained damping coefficient by the MPS method, numerical simulations of parametric rolling for a post-Panamax containership in regular head seas were conducted. Calculated results with help of the MPS method well reproduce parametric rolling observed in the model experiment. These comparisons show us the possibility of the design of an ART as a parametric rolling prevention device without any model experiments

RANS Computational Fluid Dynamics Predictions of Pitch and Heave Ship Motions in Head Seas

This work extends the previous effort in unsteady Reynolds averaged Navier-Stokes (RANS) simulations developed by the ship hydrodynamics group of the University of Iowa Iowa Institute of Hydraulic Research to the capability to predict pitch and heave motions of ships with forward speed in regular head seas. The simulations are performed with CFDSHIP-IOWA, which is a general-purpose, multiblock, high-performance parallel computing RANS code. Numerical verification studies in space and time demonstrate convergence for nearly all variables. The modified Wigley hull form experimental data presented in Journee (1992) are compared with simulation results over a range of Froude numbers, wavelengths, and wave amplitudes and found to give accurate results, with uncertainties less than 2%. Viscous ship motions characteristics are investigated by decomposing the full nonlinear problem into the forward speed diffraction and pitch and heave radiation problems, in the manner of strip theory. Comparisons between the current viscous RANS solutions and those from experiments, strip theory, and nonlinear potential flow simulations show the RANS method to predict damping and added mass coefficients with a high degree of accuracy.

Global Hydroelastic Response of Catamarans Due to Wetdeck Slamming

Global hydroelastic effects due to wetdeck slamming on a high-speed catamaran with forward speed in regular head sea waves are studied experimentally and theoretically. The elastic response is dominated by a two-node bending mode. Predictions of vertical shear forces, bending moments, and slamming force are in fair agreement. Experimental and theoretical error sources are discussed. An important uncertainty is associated with improper measurements of trim angles. The hydrodynamic model of the side hulls needs to be improved by considering hydrodynamic hull interaction.

Green Water Investigation for a Containership

The occurrence of deck wetness for ships with forward speed is predicted using a non-linear strip method in both regular and irregular head seas. The approach is validated with experimental data for the container ship model S-175. Once green water has taken place, a simplified model of green water flow on deck is simulated using CFD. Loads on deck and a vertical wall downstream are validated as well.

A New-Generation Motion-Control System for Twin-Hull Vessels Using a Neural Optimal Controller

A new ride control system using a neural optimal controller (NOC) is developed and applied to improve the heave and pitch motion responses of two twin-hull vessels operating in regular head seas. A time domain model for the vessel dynamics in the presence of active fin control is used to simulate the vessel and fin motion responses. An online switching procedure is introduced to select among a number of linear quadratic regulator optimal controllers, designed for different operating conditions of the vessel, to improve the system robustness. Although the online switching offered better robustness and performance characteristics, in between switching operating points, it still remained suboptimal. Therefore, an artificial neural network (ANN) controller was developed as an alternative and initially trained to emulate the same level of control at a number of design operating points, as a NOC. The advantage of this novel application is that practical difficulties in applying an online switching procedure are no longer present and, more importantly, the ANN has been capable of nonlinear generalization to give a near optimal solution away from the trained operating conditions.

Experimental Study on Shipping Water Volume and its Load on Deck

In order to develop a practical prediction method for shipping water volume, load and pressure due to deck wetness, a model test was carried out for a domestic tanker in regular head seas. Shipping water height, load and pressure were measured at various ship speed and wave steepness. As for the occurrence of deck wetness, N. S. M. calculation of relative water height at bow gives a good estimation if the bow top height is reduced by considering the static swell up.A practical prediction method for shipping water height, load and pressure was proposed. The inputs of this method are relative water height at bow, pitching angle and bow vertical velocity, those can be calculated by strip theory and so on. The theory of “flood waves” was applied for evaluating the shipping water height from the relative water height at the bow and so on. This theory gives a better estimation of the water height distribution on deck than the conventional model of “dam collapsing” because it can include the effect of ship forward speed. On the shipping load and pressure, not only the static component but also the time derivative of the momentum of the shipping water were considered. It was clarified that the momentum component is important to estimate the peak of shipping water load and pressure. Having compared with measured data, it was confirmed that the combination of these two methods is practical enough for predicting shipping water load and pressure.

Nonlinear Prediction of Vertical Motions of a Fishing Vessel in Head Sea

Several years ago the authors developed a practical method for calculating vertical motions and wave loads of a high-speed craft which travels in regular head sea, and verified its validity by comparing the computed motions and wave loads with the results of model tests. In order to clarify further its validity, the method is applied herein to compute the vertical motions of a fishing vessel, and the computed motions are compared with the results of experiments conducted by Bales and others at the David W. Taylor Naval Ship Research and Development Center (DTNSRDC). Also, the vertical motions and shipto-wave relative motions predicted by the method are compared with the numerical results of conventional linear strip theory computations performed at DTNSRDC. As a result, it is found that the present method, which in principle is based on the conventional Ordinary Strip Method synthesis but modified to be able to evaluate nonlinear hydrodynamic impact forces as well as dynamic lift in waves, can be applied to estimate vertical motions and ship-to-wave relative motions of fishing vessels traveling in head sea with enough accuracy for practical use.

Nonlinearity of Sagging Moment of Fine Ships

Rational design of ship's longitudinal strength should, be based on accurate evaluation of bending moment and shearing force. The nonlinear strip theory and model experiments reveal that the internal force by waves in rough seas differ from the ones obtained by the linear theory. Rules of classification societies are based on the long term prediction of structural responses, which is performed under the assumption of linearity in response and uses of the linear superposition method. According to the recent studies, the effect of nonlinearity becomes to be taken into consideration in the rules of classification societies.In the present paper, calculations are performed for a fine container ship and for a fullbodied bulk carrier using the nonlinear strip theory. Numerical results are obtained for regular waves of various wave height, wave length and wave encounter angle and for irregular waves of ISSC spectra by the nonlinear strip theory, and are compared with the result of O. S. M. : the linear theory. Results obtained are as follows : 1) Sagging moment increases as wave height increases by the effect of nolinearities, and the moment is about 170% of the one obtained by O. S. M. at the wave height 8 m in regular head sea for the container ship.2) On the contrary, the effect of nonlinearity in hogging moment is not so significant as in sagging moment. Therefore, long term prediction of hogging moment is obtained appropriately by the linear theory.3) Approximate prediction of long term response of bending moment is performed taking into account of the nonlinearity, and compared with the present rules.

A Consideration on Vertical Wave Loads Acting on a High-speed Craft of Medium Size

In the previous paper, the authors presented a method for calculating vertical motions and vertical wave loads of a high-speed craft which travels in regular head sea, and verified its validity comparing the computed motions and wave loads with the results of model tests. Furthermore in this paper, the above-stated method is applied to an existing high-speed craft of medium size in order to clarify the fundamental characteristics of vertical motions and wave loads of the medium-sized craft. The illustrative calculation is performed extensively for various cases of wave length, wave height and advance speed.As a result, it is found that the magnitudes of the vertical acceleration at the bow and the vertical bending moment correlate definitely to each other. This fact approves the propriety of the current design practice : the highest possible bending moment that is required to be predetermined for structural design of the hull is prescribed well as a function of the vertical acceleration at the bow.Besides, remarkable dependence of the maximum bending moment on the advance speed of a craft is demonstrated successfully by dividing the vertical bending moment into three parts and then clarifying the effects of advance speed on each part.

Bow Flow and Added Resistance of Slender Ships at High Froude Number and Low Wave Lengths

Flow around a slender ship bow at high Froude number and regular incident head sea waves is analyzed by matched asymptotic expansions. The near-field solution implies solving a two-dimensional Laplace equation with complete linear free-surface conditions. A solution technique with fundamental sources and dipoles is used. The solution technique is tested with good results for transient forced heave oscillation of a circular cylinder and for steady flow around a wedge. Comparisons with other numerical and experimental results for steady bow flow around a Wigley hull and a Series 60, Cb = 0.6 model show partly satisfactory results. The theoretical model for unsteady flow around a ship bow is used to calculate added resistance on a slender ship in high Froude number and incident regular head sea waves of low wave lengths. Experimental results for added resistance at low wave length for a cargo ship with Cb = 0.61 are not in complete agreement with the theoretical predictions.

On Motions of a High Speed Planning Boat in Regular Head Sea

Recent advances in the research of ship motion in a seaway have enabled to predict the ship motion. These researches, however, are restricted to those of displacement type ships such as a merchant ship and a destroyer, and as to behaviours of high speed planing boat in a seaway few available researches have been published.In the present study the towing tank test of a torpedo boat model was carried out in regular head wave and experimental data were discussed along with the results calculated by Ordinary Linear Strip Method (O. S. M.).The conclusions deduced from these studies are as follows, 1) High frequency components appear in pitching and heaving oscillations at higher encounter frequency, that is, at higher speed and shorter wave. In these regions, each mean value of the pitching and heaving oscillation becomes larger compared with the trim angle and vertical displacement in still water.2) The motions calculated by the O. S. M. give good agreement with experimental data for Fn< 0.5, but they do not always for higher Froude number.3) In spite of much higher frequency components in the motions, the amplitudes of pitching and heaving motions are almost proportional to those of the wave, that is, the linearity to wave height is nearly conserved.

Behaviour and performance of compact frigates in head seas1

This report describes an experimental investigation into the motions and powering of a so-called Compact Frigate in waves. Special attention was given to the measurement of the relative motions, with respect to the wave surface along the length of the ship, bearing in mind that not the available power but mainly deck wetness and slamming are governing its sustained sea speed. For that reason, two models having the same main dimensions, weight distribution and underwater cross sections, but different forebody section shapes above water, were tested in regular and irregular head seas, in order to study the effect of flare upon motions and propulsion. In this analysis the deformation of the waves, when traveling along the hull, has been accounted for since the models were also run in restrained condition. The applicability of the linear superposition principle is examined, whereas the model motion measurements are compared with strip theory calculations.