Hiroshi Itakura

Theoretical and Experimental Study on a Porous Cylinder Floating in Waves

In the present work, theoretical and experimental studies on the interaction of water waves with a truncated circular cylinder were performed. The cylinder, which is partly made of porous materials, possesses a porous sidewall and an impermeable bottom. A nondimensional parameter b is adopted in the theoretic formulation to describe the porosity, which is not directly related to the opening ratio τ of the porous materials. To validate the theoretical work and computed results, a series of model tests are carried out in a wave basin. Effort is made to establish an empirical relation between b and τ based on the comparison of the calculation and experimental data. The phenomenon of the sloshing mode that occurred at a certain wave number is observed, which might have an application in breakwaters. The validation of the Haskind relations is examined for the porous body. It is found that the damping coefficient consists of two parts. In addition to the component of conventional wave-radiating damping, there exists a second component caused by the porous effects.

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

Wave Fields Diffracted by an Array of Porous Circular Cylinders

In the present work, the diffraction of waves by an array of porous circular cylinders with or without an inner porous plate is considered in detail based on the linear wave theory. For the sake of designing more effective floating breakwaters, the transmission rate of waves propagating through the array is also calculated. Each cylinder is partly made of porous materials. Namely, it possesses a porous sidewall and an impermeable bottom. In addition, an inner porous plate is fixed horizontally inside the cylinders, which works as obstruction to make wave dissipate more effectively and eliminates the phenomenon of sloshing mode. To deal with hydrodynamic interactions among a great number of bodies, a hierarchical interaction theory, which saves computation time efficiently, is adopted to compare with the approach of Kagemoto & Yue (1986). Consequently, a series of model test of an array of porous cylinder are carried out in the basin to validate the theoretical work and the calculation results. The draft of the cylinders, the location of the inner porous plate and the spacing between adjacent cylinders are adjusted to investigate its effect on the wave dissipation.Copyright

A PMM Experimental Research on Ship Maneuverability in Waves

Recently, much attention has been paid on the maneuvering characters of ships in waves. Apparently, ships steering in waves are subjected to linear wave loads at the encountering frequency. Nevertheless, the maneuvering characters of ships are more affected by the nonlinear wave drift forces at low frequencies. In the present work, to examine the wave effects on the ship maneuverability, a serial of towing tests with a Planer Motion Mechanism system (PMM) is carried out in waves. Forces and horizontal displacements of the model are measured throughout the tests. The measured data are analyzed by the Fourier analysis to obtain components at different frequencies. Discussion on the importance of the force components is made.Copyright

Experimental Investigation and Numerical Modeling of Hydrodynamic Force Characteristics and Deformation of an Elastic Net

With the increasing worldwide demand of seafood and environmental problems in coastal aquaculture, offshore area has been increasingly expected to be utilized for aquaculture. An offshore aquaculture system has fewer effects on the surrounding marine environment through the rapid diffusion of organic wastes from the cultured fish than a coastal one. The offshore area then provides clean waters for cultured fish. On the other hand, the offshore aquaculture system is subject to the severe natural condition such as typhoon attack. Actually, in the current aquaculture system, the offshore sea cages are always submerged around 10m below the sea surface to avoid the effects of high waves and strong currents. However, the safety of the sea cage against the incident wave has seldom been examined, while that against the water current has been analyzed by model tests in tank and numerical simulation. We investigated the hydrodynamic force properties of a heaving sea cage in the previous work and its results proved that the deformation of net much affects on sea cage’s motion characteristics. To study the most fundamental problem, we investigated the hydrodynamic force characteristics of a rigid net as a first step and explained that these relate to its porosity and wave conditions. As a second step, in the present study, we investigate hydrodynamic force properties of an elastic net. Six square-shaped textile net models which are made of elastic strings and metallic frame are arranged. Three different porosities (0.53, 0.64 and 0.74) and two different net’s tension (10 and 50 N/m) on each porosity are employed; that is, we arranged 6 elastic net models. Forced heaving tests and diffraction tests are carried out. In addition to measuring heave force, the deformation of net surface is measured by a video image analysis system. To compare with the experimental results, we calculate hydrodynamic forces and the deformation of net using a boundary element method. The boundary condition on elastic net is that the normal velocity to the surface is not zero but proportional to the pressure difference. The deformation is also taken into consideration.Copyright

Experimental Investigation and Numerical Modeling of Hydrodynamic Force Characteristics of a Heaving Net

With the increasing world demand for seafood and environmental problems in coastal aquaculture, offshore area has been increasingly expected to be utilized for aquaculture. The offshore aquaculture system has fewer effects on the surrounding marine environment through the rapid diffusion of organic wastes from the cultured fish than the coastal one. The offshore area then provides clean waters for cultured fish. On the other hand, the offshore aquaculture system is subject to the severe natural condition such as typhoon attack. Actually, in the current aquaculture system, the offshore sea cages are always submerged around 10m below the sea surface to avoid the effects of high waves and strong currents. However, the safety of the sea cage against the incident wave has seldom been examined, while that against the water current has been analyzed by model tests in tank and numerical simulation. We investigated the hydrodynamic force properties of a heaving sea cage in the previous work and its results proved that the deformation of net much affects on sea cage’s motion characteristics. To study the most fundamental problem, we investigated the hydrodynamic force properties of an undeformable net in the present study. Nine net models are arranged. These models have square-shaped frame and its inner dimensions are 216 × 216 mm. In these net models, three different twine diameters and three different twine intervals are arranged. The porosities of these models are in the range of 0.39 to 0.91. In addition, an impermeable plate model is also arranged. Forced heaving tests and wave exciting force tests are carried out on each net and plate model. Added mass and damping coefficients, as obtained by forced heaving tests, differ according to porosity of net and oscillation frequency and amplitude. Wave exciting forces, as obtained by wave exciting force tests, also differ according to porosity of net and wave slope and the forces increase with wave amplitude in higher order than one. To compare with experimental results, we theoretically calculate the hydrodynamic forces from a linearized potential flow approach supplementing the effect of porosity. In the boundary condition on the net surface, the vertical velocity to the surface is not equal to zero, but determined by the permeate coefficient of the surface.© 2010 ASME

Concept of an Offshore Aquaculture System With an Automated Feeding Platform

With the increasing world demand for seafood and environmental problems in coastal aquaculture, offshore area has been increasingly expected to be utilized for aquaculture. The present study proposed the concept of an offshore aquaculture system with an automated feeding platform. The offshore aquaculture system has fewer effects on the surrounding marine environment through the rapid diffusion of organic wastes than the coastal one. The offshore area then provides clean waters for cultured fish. On the other hand, the offshore aquaculture system is subject to the severe natural condition such as typhoon attack. Actually, in the current aquaculture system, the cultured fish is starved during a few days before and after typhoon attack since the feeding ship is not able to approach the offshore sea cages, which is submerged every time around 10m below the sea surface to escape from high waves and strong currents. The automated feeding system should be therefore developed to enable the cultured fish to grow faster under the severe natural condition. The proposed system consists of several submerged fish cages and a feeding platform, which includes fuel tanks, feed storage silos, and an automated feeding machine. The feeding platform was designed by numerical analysis and tank model test in order to suppress its motion as small as possible. The motion of the feeding platform was first predicted by numerical analysis. The unmoved point was formed at each period of the incident wave. It moves upward as the period of the incident wave increases. Consequently, the point at 8m below the sea surface (3m above the bottom of the feeding platform) is the best one to attach the feeding hose. As a result of tank model test, pitch and heave motions of the feeding platform were dominant in the waves with short (1.5 seconds) and long (3.1 seconds) periods, respectively. An unmoved point was observed on the surface of the feeding platform as predicted by the numerical model, while the top of the feeding platform moved more largely. The unmoved point for the feeding hose was specified at above the bottom of the feeding platform.Copyright

Wave Drift Forces Affecting on Maneuverability Derivatives

Maneuverability in waves is important for the safety of ships and other marine vehicles. Conventionally, the maneuvering experiments are performed in still water at a constant forward velocity. In order to investigate the wave effects on ships steering, a series of towing tests with a Planer Motion Mechanism System (PMM) are carried out in waves in the present work. The ship model is forced to oscillate in horizontal plane at a low frequency when being towed forward at a small constant speed. As an extension of the previous work, the low-frequency oscillation of yaw motion is also considered. Hydrodynamic forces and the displacements of the ship model are measured throughout the tests. Then, the measured data are analyzed by the Fourier analysis to obtain components at different frequencies. By comparing the hydrodynamic forces measured in still water and that in waves, the wave effects on the maneuverability of ships are estimated. The variation of these nonlinear hydrodynamic coefficients against the wave amplitude and wavelength is examined.Copyright

Experimental Investigation and Numerical Modeling of Hydrodynamic Characteristics of a Heaving Sea Cage

With the increasing world demand for seafood and environmental problems in coastal aquaculture, offshore area has been increasingly expected to be utilized for aquaculture. The offshore aquaculture system has fewer effects on the surrounding marine environment through the rapid diffusion of organic wastes from the cultured fish than the coastal one. The offshore area then provides clean waters for cultured fish. On the other hand, the offshore aquaculture system is subject to the severe natural condition such as typhoon attack. Actually, in the current aquaculture system, the offshore fish cages are always submerged around 10m below the sea surface to avoid the effects of high waves and strong currents. However, the safety of the sea cage against the incident wave has seldom been examined, while that against the water current has been analyzed by model tests in tank and numerical simulation. In the present study, therefore, we investigated hydrodynamic property of a heaving sea cage as the first step. Forced oscillation tests and wave exciting force tests have been carried out, and numerical modeling have also been made to estimate hydrodynamic characteristics theoretically and to estimate the drag and mass coefficients. Results of the forced oscillation tests show that the added mass and damping coefficient of sea cage models depend on forced oscillation amplitude. This dependence may be mainly attributed to the deformation of net, and we successfully reduce the dependence on the forced oscillation amplitude in the result of reanalysis which takes into account the deformation of net. Results of measured wave exciting force show that wave exciting forces are not linear in wave amplitude. This may be due to the viscous drag effects as well as the deformation of net. On the other hand, we calculated the flow around a sea cage. This is based on velocity potential and supplemented the effect of viscosity by equivalent linearization. In the boundary condition on the sea cage surface, the vertical velocity to the surface is not equal to zero, but determined by the permeate coefficient of the surface. This permeate coefficient is a function of wave amplitude, wave period and porosity of the net. In the future works, deformation of net should be also taken into account in this calculation.Copyright

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