Hidetsugu Iwashita

A calculation method for finite depth free-surface green function

ABSTRACT An improved boundary element method is presented for numerical analysis of hydrodynamic behavior of marine structures. A new algorithm for numerical solution of the finite depth free-surface Green function in three dimensions is developed based on multiple series representations. The whole range of the key parameter R/h is divided into four regions, within which different representation is used to achieve fast convergence. The well-known epsilon algorithm is also adopted to accelerate the convergence. The critical convergence criteria for each representation are investigated and provided. The proposed method is validated by several well-documented benchmark problems.

Improvement of Rankine Panel Method for Seakeeping Prediction of a Ship in Low Frequency Region

Rankine panel methods have been studied for solving 3D seakeeping problems of a ship with forward speed and oscillatory motions. Nevertheless, there is a drawback in the numerical method for satisfying the radiation condition of outgoing waves at low frequencies, because the waves generated ahead of a ship reflect from the outward computational boundary and smear the flow around the ship. The so-called panel shift technique has been adopted in the frequency-domain Rankine panel method, which is effective when the generated waves propagate downstream of a ship. In this paper, in addition to this conventional panel shift method, Rayleigh’s artificial friction is introduced in the free-surface boundary condition to suppress longer wave components in a computational region apart from the ship. With this practical new method, it is shown that there is no prominent wave reflection from the side and/or upstream computational boundaries even in the range of low frequencies. As a consequence, the unsteady pressure, hydrodynamic forces, wave-induced ship motions, added resistance are computed with reasonable accuracy even in following waves and in good agreement with measured results in the experiment using a bulk carrier model which is also conducted for the present study.Copyright

The Effect of Controlling Fins on the Motion of Resonance-Free SWATH (RFS) Platform

The speed reduction, additional resistance or slamming, which are caused by the large amplitude of ship motions, should be restricted completely for oceangoing large fast ship, because of the strict time-punctuality and high value of the cargo. In the present work, comparison of seaworthiness, such as the motion responses in head seas, is made among four types of ship hulls, i.e. Mono-hull, Ordinary SWATH, Trimaran and Resonance-Free SWATH (RFS). The last one can be regarded as a special type of catamaran. As a result of the extremely small water plane, the restoring moments are negative for RFS hull, which leads to free of resonance in the motion responses. Experiments in towing tank and theoretical calculations are carried out to examine the motion responses in waves. Calculations are generally based on potential theory modified by adding the viscous effects and including fin lifts. The results of the motion responses of RFS are compared with those of mono-hull ship, ordinary SWATH ship as well as trimaran ship. Where the motion of RFS is controlled using movable small underwater fins. The motion of ordinary SWATH is examined with and without fin control. It can be seen that the heave motion responses of RFS are very small in comparison with those of other ship hulls. Also, the pitch motion responses of RFS are much smaller than those of mono-hull or trimaran or ordinary SWATH without fin control, and are smaller than those of ordinary SWATH with fin control. Fin control actions, i.e. proportional and derivative control, are applied in the experiments and calculations. It is considered that the derivative control action is more effective on the motion responses of RFS than the proportional one as predicted. The larger the derivative gain value is, the smaller the motion response of RFS is.© 2010 ASME

Lift Characteristics of Controlling Fins of Resonance-Free SWATH

Speed reduction, additional resistance or slamming, which caused by the large ship motions, should be avoided for a high-speed oceangoing vessel, because of the delivery punctuality and high value of the cargo. A promising ship type for such the oceangoing vessel is the so-called “Resonance-Free SWATH (RFS)”. It has negative restoring moment due to the extraordinary small water plane area. As a consequence, the resonance peak is removed from the motion response. RFS is designed to cross 4,800 nautical miles of Pacific Ocean within 5 days punctually at a speed of 40 knots, with good seaworthiness such as no speed reduction or absolutely no slamming even when running in the rough sea of sea state 7 with significant wave height of 6–9 m.The attitude of RFS with negative restoring moment is adjusted by four pairs of controlling fins attached to the fore and aft ends of lower hulls. In the previous works, the quasi-steady values of lift-curve slope are usually adopted in the motion equations of frequency domain. However, when working in waves, the controlling fins are not in a steady state. The lift coefficient is no longer a constant. In addition, there exist a phase lag between the movement of attack angle and the fin-generated lift.The theoretical prediction and the experiment to analyze the phenomena of lift generation including the phase difference and the interaction among fins and lower hulls are carried out. The results show that the characteristics of fins depend on the encounter frequency. Then, the effect of lift characteristics of controlling fins on the RFS model is discussed. The results of theoretical estimation and experiment are discussed and it is observed that estimated results agree to some extent with experimental results.© 2014 ASME

On the Resonance-Motion-Free Swath (RMFS) as an oceangoing large fast platform

The speed reduction, additional resistance or slamming, which are caused by the large amplitude of ship motions, should be restricted completely for oceangoing large fast ship, because of the strict time-punctuality and high value of the cargo. In the present work, comparison of seaworthiness properties, such as no speed reduction and absolutely no slamming, is made among four types of ship hulls, i.e. mono-hull, ordinary SWATH, trimaran and Resonance-Motion-Free SWATH (RMFS). The last one can be regarded as a special type of catamaran. As a result of extremely small water plane, the restoring moments are negative for the RMFS hull, which leads to free of resonance in the motion responses. Experiments in towing tank and calculations are carried out to examine hydrodynamic forces and motion responses in waves. Calculations are generally based on potential theory modified by adding the viscous effects and including fin lifts. The results of RMFS are compared with those of mono-hull ship, ordinary SWATH ship as well as trimaran ship. The heave motion responses of RMFS are very small in comparison with those of other ship hulls. On the other hand, the pitch motion responses of RMFS are considerably small in comparison with those of mono-hull or trimaran, but are not as small as expected in comparison with those of ordinary SWATH. The reason is that a soft spring system is applied in experiments, to replace the proportional control action supplied by the fin lift. The spring system can not make use of the advantage of the negative restoring moment, which is a characteristic of RMFS. Accordingly, a new control system of ship motion by means of the lift force from fins should be developed.Copyright

On the Novel Fast Ocean Platform

The mission of the large trans-ocean fast ship is to transport the high-valued and time-sensitive goods punctually at reasonable price. Accordingly, the speed reduction, additional resistance or slamming, which are caused by the large amplitude of ship motions, should be restricted completely. Some seaworthiness, such as no speed reduction and absolutely no slamming, is especially required for ships running fast in ocean waves. The final intention of the study is to compare the transport efficiency, including accuracy of time schedule and transport quality like damage of goods caused by slamming, among three types of ship hulls, i.e. mono-hull, catamaran and trimaran. As a first step, a “Resonance-Motion-Free SWATH (RMFS)” hull is considered as an example of catamaran type in the present work. As a result of very small water plane, the restoring moments are near zero for this type of hull, which leads to free of resonance in the pitch responses. Experiments in towing tank and calculations based on potential theory are carried out to examine hydrodynamic forces and motion responses in waves. The results are compared with those of typical mono-hull and trimaran ships. The predominance of RMFS regarding seaworthiness will be pointed out in the conclusion.Copyright

A Study on the Hydrodynamic Interaction between Submerged Lifting and Non-Lifting Bodies Advancing in Waves

The hydrodynamic interaction between a submerged non-lifting body and lifting bodies, i.e., hydrofoils, is studied. So far it has been confirmed that the hydrofoil producing the downward lifting force reduces the wave resistance drasticly due to the hydrodynamic interaction with the non-lifting body. This paper investigates the hydrodynamic interaction effect on the unsteady hydrodynamic forces, the added wave resistance and the wave field through model test and numerical computation by Rankine panel method. Numerical results and experimental results are compared and the hydrodynamic interaction effect is discussed. We have found good agreement between numerical and experimental results and it has been confirmed that the interaction effect is not so significant in the present case.