Hiroshi Kagemoto

Interactions among multiple three-dimensional bodies in water waves: an exact algebraic method

This paper deals with three-dimensional water-wave diffraction and radiation by a structure consisting of a number of separate (vertically) non-overlapping members in the context of linearised potential flow. An interaction theory is developed which solves the complete problem, predicting wave exciting forces, hydrodynamic coefficients and second-order drift forces, but is based algebraically on the diffraction characteristics of single members only. This method, which includes also the diffraction interaction of evanescent waves, is in principle exact (within the context of linearised theory) for otherwise arbitrary configurations and spacings. This is confirmed by a number of numerical examples and comparisons involving two or four axisymmetric legs, where full three-dimensional diffraction calculations for the entire structures are also performed using a hybrid element method. To demonstrate the efficacy of the interaction theory, it is applied to an array of 33 (3 by 11) composite cylindrical legs, where experimental data are available. The comparison with measurements shows reasonable agreement. The present method is valid for a large class of arrays of arbitrary individual geometries, number and configuration of bodies with non-intersecting vertical projections. Its application should make it unnecessary to perform full diffraction computations for many multiple-member structures and arrays.

Experimental and theoretical analysis of the wave decay along a long array of vertical cylinders

A row of fifty identical, truncated vertical cylinders is submitted to regular head waves, with wave periods in a narrow range around the period of the so-called Neumann trapped mode. The free-surface elevation is measured at 14 locations along the array. Response amplitude operators of the free-surface motion are compared with numerical predictions from a potential flow model. Resonance effects, at wave periods equal to or larger than the critical one, are found to be much less than given by the numerical model. It is advocated that these discrepancies are due to dissipative effects taking place in the boundary layers at the cylinder walls. An artificial means is devised to incorporate dissipation in the potential flow model, whereby the cylinder walls are made slightly porous; the inward normal velocity of the flow is related to the dynamic pressure. The coefficient of proportionality is based on existing knowledge for circular cylinders in oscillatory flows. With this modification in the numerical code, excellent agreement is obtained with the experiments. The numerical model is further used for the case of a very long array composed of 1000 cylinders; it is found that with dissipation at the cylinder walls, the wave action steadily decreases along the array, even for wave periods substantially larger than the critical one. On the other hand, at wave periods less than the critical one, dissipation plays a negligible role; the observed decay is solely due to diffraction effects. Implications of these results for very large structures such as column-supported floating airports are discussed. In particular, it is concluded that scale effects may be an important issue in the experimental analysis of such multi-column structures.

Force and Power Estimation in Fish-Like Locomotion Using a Vortex-Lattice Method

The forces and power needed for propelling at constant speed an actively swimming flexible fish-like body are calculated. A vortex-lattice method based on a linearized theory is employed and the results are compared against slender body theory predictions, as well as experimental data from an eight-link robotic instrument, the RoboTuna. Qualitative agreement is found between our method and slender body theory; with quantitative agreement over certain parametric ranges and disagreement for other ranges of practical interest

Theoretical and experimental predictions of the hydroelastic response of a very large floating structure in waves

Abstract A prediction method for the hydroelastic behavior of a very large box-shaped flexible structure in regular waves is proposed. The structure considered is representative of such structures as a floating international airport and thus the horizontal dimensions are expected to be as large as several kilometers in both length and width. In the analysis, the structure is divided into a number of substructures and the continuous deformation is approximated by the succession of a discrete displacement of each substructure. The displacement of each component is determined from the equation of motion of a uniform free-free plate representing the substructure, while taking structural constraints into account as an additional restoring force. The hydrodynamic forces on the substructure are determined by enforcing the normal velocity of the flow to be equal to that of the corresponding body surface. Thus, the fluid motion and the body motion interact with each other, which is termed ‘hydroelastic interaction’, requiring the simultaneous solution of the structure and fluid problems.

Hydrodynamic interaction analyses of very large floating structures

Abstract A series of studies conducted at MIT over the past seven years for the analysis of hydrodynamic interactions among legs for the support of a very large floating structure is reviewed. For general three-dimensional leg geometries and arbitrary configurations, an algebraic method for exact hydrodynamic interaction, which includes the effects of evanescent waves, has been developed. This method produces identical results compared with direct computations for hydrodynamic coefficients, first-order wave exciting forces and second-order steady-drift forces. Motions with and without the presence of linear elastic constraints, such as moorings or deck structures, are also obtained. When the number of legs involved is extremely large, a matching idea is introduced which divides the total structure into an interior ‘core’ plus a relatively small number of legs near the outer boundary. The interior core is treated as part of an infinite array and the ‘end’ conditions are accounted for by considering the exact interactions between the interior core and the boundary legs. Finally, a method for the solution of the ‘inverse’ hydrodynamic interaction problem is presented. In this, optimal arrangements of legs for minimum wave forces, displacements, etc., are obtained, subject to constraints specified, say, for the total length of the array. Through comparison with results of other numerical methods and available experimental data, the validity and effectiveness of these methods are demonstrated.

On the estimation method of hydrodynamic forces acting on a very large floating structure

The floating structures that may be used for such purposes as an international airport or an offshore city are expected to be as large as several kilometers long and wide. For the estimation of hydrodynamic forces due to waves or motions that will act on such huge structures, a direct application of conventional numerical methods is practically prohibitive, because the required computational burden is enormous. In order to avoid this difficulty, an approximate method is developed in which computational time is drastically reduced without appreciable loss of accuracy. Although a direct application of conventional numerical methods is difficult for the reason that the corresponding structure is so large, the method proposed in this paper exploits the very fact that a structure is very large to simplify the calculation. The effectiveness of the new method is demonstrated in comparisons with results obtained by the direct application of a conventional numerical method.

Minimization of wave forces on an array of floating bodies — The inverse hydrodynamic interaction theory

Abstract Given the location and the diffraction/radiation characteristics of each member of a floating-body assembly, hydrodynamic interactions due to the multiple scattering of waves among the bodies can be accounted for exactly within the linear potential theory by the method developed by Kagemoto and Yue. The interest of this paper is the solution of the inverse problem of the above one, i.e. given the diffraction/radiation characteristics of each member of a floating-body assembly, what should the location of each body be if, say, you want the wave forces acting on the assembly to be minimal? It is shown that combining the hydrodynamic interaction theory of Kagemoto and Yue and the existing nonlinear programming techniques, the inverse problem can be solved. Through several example problems the effectiveness of the present method is demonstrated.

Consensus building in safety and security: a case study of Fukushima evacuees returning home

This study examines consensus building in environmental and energy policies by analyzing the minutes of the safety and security committee of the Nuclear Regulation Authority (NRA) of the Japanese government, in the context of the discussion of the safe return of Fukushima evacuees after the 2011 Great East Japan Earthquake and Tsunami and the subsequent Fukushima nuclear disaster. One important issue associated with evacuation policies was the high number of evacuees, if not all, who were not willing to return to their old homes. Although the contents of governmental committee meetings are freely accessible through the internet, they have been rarely analyzed for these purposes. In this study, we used text-mining techniques to analyze NRA committee minutes quantitatively and qualitatively. We have three primary findings. First, the committee attempted to take evacuees’ feelings into account and pragmatically discuss what was needed to restore their lives and livelihoods, as well as to make its meetings transparent and open to the public by, for example, streaming them live on the internet. Second, in earlier committee meetings, government representatives insisted on specific policies made by themselves to control the return of evacuees. However, outside experts at the meetings convinced representatives that decisions regarding issues surrounding the safe return of evacuees should consider the opinions of the evacuees themselves. Third, the NRA reported the outcome of the meetings at a Cabinet meeting to accelerate policies requiring urgent implementation as well as those related to the alleviation of people’s anxieties regarding exposure to radioactivity or those related to equal treatment among those who wanted to voluntarily move away from the affected areas versus those who want to return to their homes. Finally, this analysis further identified a number of issues concerning citizen participation and governance associated with environment and energy policies, all of which need to be overcome in order to establish consensus among concerned stakeholders.

Visualization of the ambient wave field and the elastic responses of a very large floating structure in waves by use of a very small water tank

Very large floating structures of several kilometers long and . wide are now considered as possible alternatives to such land-based big facilities as an airport. Since the principal horizontal dimension is so large that the conventional experimental tanks can not comply with the appropriate similarity law, an attempt is made to carry out experiments in a very small water tank. With the use of a very small tank, the entire wave field around a model or the entire response pattern of the model in waves can be visualized by making use of certain optical techniques, which can not be achieved if the conventional water tanks are used.