久保田 晃弘

The Abduction Machine Project is in progress at RACE (Research into Artifacts, Centre for Engineering) of the University of Tokyo. In this project, we are focusing on the creative process of human reasoning, referred to as abduction (hypothesis generation). The Abduction Machines are experiential apparatus to embody and describe the process of abduction. One of the main objectives of the Abduction Machine Project is to establish a new view of abduction, which prays a crucial role in creative processes, by combining the experientialist views on human knowledge and architectural space design methodologies. (C) 1997 Elsevier Science B.V.

The RACE Asynchronous Collaboration Environment Project (the ''RACE'' Project) is in progress at Research into Artifacts, Center for Engineering (RACE), the the University of Tokyo. The objective of this project is to find knowledge representation that is useful for the process of reorganizing existing information into design knowledge. As a testbed of such knowledge representation, an asynchronous and distributed design environment is being built on the basis of the World-Wide Web.

In this paper we present a new concept of FE modeling, based on the object-oriented principle, and develop the prototype progam MODIFY (MODeling tool for Integrated Finite element analysis). MODIFY has three novel features: (1) FE modeling by the part object concept, (2) a fully object-oriented data structure, and (3) a part-by-part Sully automated mesh subdivision. When using MODIFY it is necessary to define and assemble part objects, which consist of geometry objects, analytical condition objects and relation objects, to describe the continuity between adjacent part objects. MODIFY automatically generates an appropriate FE model for each geometry object, satisfying continuity conditions with adjacent parts by referring to relation objects. If some part of the model is to be modified, the user needs only to change the corresponding part objects. Because of the object-oriented data structure, MODIFY also has a powerful capability for adaptive meshing. The existing version of MODIFY is applicable to FE models for 3-D shell structures.

A new cavity model that can explain the interaction between viscous effects including vortices and cavitation bubbles is presented in this study. This model, which is named a bubble two-phase flow (BTF) model, treats the inside and outside of a cavity as one continuum by regarding the cavity as a compressible viscous fluid whose density changes greatly. Navier-Stokes equations including cavitation bubble clusters are solved in finite-difference form by a time-marching scheme, where the growth and collapse of a bubble cluster is given by a modified Rayleigh's equation. Computation was made on a two-dimensional flow field around a hydrofoil NACA0015 at angles of attack of 8-degrees and 20-degrees. The Reynolds number was 3 x 10(5). The experiments were also performed at the same Reynolds number for comparison. The computed results by the BTF cavity model can express the feature of cloud-type cavitation shed from the trailing edge of the attached cavities when the angle of attack is 8-degrees. It shows the mechanism of cavitation cloud generation and large-scale vortices. The boundary layer separates at the cavity leading edge. Then it rolls up and produces the cavitation cloud. In other words, the instability of the shear layer may produce the cavitation cloud. When the angle of attack is 20-degrees, the flow was fully separated from the leading edge of the hydrofoil and vortex cavitation occurs in the separated region. The BTF cavity model can also express the generation of such vortex cavitation and the effect of cavitation nuclei in the uniform flow.

The biggest difficulty in the process of finite element (FE) analysis is the modeling of industorial products that have complex configulation. To overcome this, prototype of integrated finite element modeler, MODIFY ( MODeling tool for Integrated Finite element analYsis) is developed by the authors [1]. MODIFY has three features : (1) FE modeling by the part object concept, (2) fully object-oriented data structure, (3) fully automated part by part mesh subdivision. In MODIFY, part objects consist of the geometry objects, the analytical condition objects and the relation objects, and the user only need to define and assemble them. The prototype system is applicable to FE modeling for 3-D plate structures. In this paper, MODIFY is extended for 3-D shell structures. Three enhncement was made. First, the shell object that is represented by B-rep model and rational B-spline surface is defined as a geometry object. Second, the mesh generator for 3-D curved surface is constructed. Finally, MODIFY is linked with external CAD syntem using IGES file, which is most widely used as a conversion in commercial CAD system. Several examples of finite element model generated by MODIFY are shown.

The present paper studies the characteristics of the foil load and the vertical motion of the high-speed ships with hydrofoils quantitatively. First, the hydrofoil load was measured directly using dynamometers. Time-domain non-linear simulation was carried out using TSLAM_<HF>^<1,2,3,4)> and was compared with the experimental data. It was shown that the simulation can predict not only the vertical ship motion but also the foil load quantitatively in case no hydrofoil emergence occurs. The experimental result, however, shows the necessity of further investigation on the characteristics of histeresis of the hydrofoil lift to simulate ship motion in case foil emergence occurs. The eigenvalue analysis for the vertical ship motion of the high-speed ships with hydrofoils was performed based on the linearized equation of motion. The effect of the ship speed, the area and locations of the foils on the ship motion in wave was clarified.

Owing to recent development of techniques of finite element analysis and computer hardware, the finite element method has been used more widely for the industrial products. The biggest difficulty here is modeling of complex configuration of industrial products which is most time consuming to engineers and hence takes up large amount of the cost. To overcome this, integrated finite element modeler MODIFY (MODeling tool for Integrated Finite element analYsis) that produces finite element model automatically is developed here. This modeler is based on two new concepts, part object and object oriented data struture. By assembling part objects that have simpler configuration, geometric model of complex configuration can be built up and mesh is generated part by part. The concept of part object enables the geometric configuration to be divided into parts, which are simpler object and easy to generate mesh. The concept of object orineted data structure results in easy handling of local modification of finite element data that is required in both local modification of geometric model and modification of finite element mesh such as in zooming or adaptive meshing. With these concepts, the user do not need to handle finite element model, but only geometric model. The present proto type system is applicable to finite element modeling of two dimensional field, and three dimensional plate strucure which appear in ship structure. Several examples are presented for demonstration.

In the previous paper, the authors presented a new method for calculating vertical motions and wave loads of large high-speed ships with hydrofoils and developed the program code TSLAM-F_<HF>. The computational method of TSLAM-F_<HF> is based on the time-domain strip method taking account of the effects of the nonlinear hydrodynamic forces and dynamic lift of the hydrofoils. In this paper, the authors apply the program code TSLAM-F_<HF> to a 12 m long hydrofoil catamaran in regular head seas. The computed results show that there exists a strong nonlinearity in ship motions and wave loads due to the unsteady lift of the hydrofoils and the impact forces by the hydrofoil slamming. When we design large high-speed ships, the structural aspects are often more important than the hydrodynamical ones. The program code TSLAM-F_<HF> is a very useful tool for the structural design of large high-speed ships with hydrofoils. In the next step, the author simulate motions of the ships actively controlled by the hydrofoil flaps to stabilize the ship motions in order to prevent the hydrofoil emergence which leads to strong impact forces on the hydrofoils. The feedback gains for the flap control are determined by solving the optimal state regulator problem. The result shows that the flap control is very effective in stabilizing the ship motions. It reduces the amplitide of the ship motions to about 20 % of that for the non-controlled conditions and it can also prevent the hydrofoil emergence among rough seas.

The Boundary Element Method based on perturbation is applied to sloshing analysis. In usual sloshing analysis by the Boundary Element Method, a flow field is solved at each time step, assembling the matrix corresponding to the boundary integral equations for the deformed domain. At this time, if an analysis for the deformed field is carried out approximately with the Boundary Element Method based on perturbation, it is not necessary to re-assemble equations and therefore the calculation time would be decreased. For example, the calculation time is decreased to 1/280 in a problem which has 40 D. O. F.. The reduction ratio of the calculation time improves with an increase in the number of D. O. F..

An object-oriented data model for the finite element method is described in this paper. In the data model, which is based on the Entity-Relationship model, a finite element is treated as an object. The proposed data model is applied to the h-version adaptive mesh technique. In the h-version adaptive mesh technique, an element is divided into 2 or 4 elements recursively to reduce the error norm for the whole analysis domain. The present mesh refinement system is constructed using the C++ object-oriented programming language for the implimentation of the data model. To verify the mesh refinement system, we have chosen some 2-dimensional stress concentration and singularity problems. The results show that the newly proposed data model is very useful for the h-version adaptive mesh technique. Moreover, the data model shows great promise for the integrated FEM systems.

The structure of flow around unsteady cloud cavitation on a steady two-dimensional hydrofoil was investigated experimentally, using a conditional sampling technique. The unsteady velocity around cloud cavitation was measured by a LDV and matched with the unsteady cavitation appearance taken by a high speed camera. This matching procedure was performed, using data of pressure fluctuation measurement on the foil surface. The velocity fluctuations could be divided into a large-scale (low frequency) component and small-scare (high frequency) one, using Fourier analysis. The unsteady cloud cavitation corresponds with the large-scale structure. In this manner, both large-scale (coherent) and small-scale (incoherent) structure of the flow around the cloud cavitation were clarified and discussed respectively. Main results are given as follows ; (1) Cloud cavitation observed at the present research is a kind of large-scale vortex cavitation whose rotation direction is clockwise when main flow direction is from left to right. The velocity of cloud cavitation is much lower than the uniform flow velocity. (2) The small-scale velocity fluctuation is not distributed uniformly in the large-scale structure, but concentrated near the edge of it. One of the reasons might be that cloud bubbles collapse there. (3) Three-dimensional structure of cloud cavitation is a stretched vortex whose configuration looks like a "croissant".