Journal of Advanced Simulation in Science and Engineering Vol. 3(2016) No. 2

River plumes flowing into channels with strong tidal currents cause pulsed decreases in the salinity around the river mouth. This study constructs a numerical model for reproducing the river plumes of Asahi and Yoshii Rivers flowing into Bisan Seto, a tidal channel in the Seto Inland Sea. Recently inverse estimation of the model parameters using Green's functions has been applied to numerical modelling of sea shelves as a simple and effective method of parameter optimization. This paper examines the method that aims to improve the precision of the model results, but uses a model domain (ca. 50 km) that is much smaller than in previous studies. The reproducibility of the tidal elevations and currents was improved significantly by the parameter optimization procedures. The reproducibility of the temporal changes in salinity was also improved, although they showed strong nonlinearity. These results indicate that the inverse estimation of model parameters using Green's functions can greatly improve the reproducibility of the modelling of river plumes flowing into tidal channels.

It is necessary to evaluate the endurance of tungsten (W) under plasma irradiation to realize the nuclear fusion reactor. The tungsten that has flat surface and amorphous structure is often assumed in the experiment and the simulation in plasma-wall-interaction study. However, from experiments, it has been reported that the sputtering yield for the flat surface W under argon (Ar) irradiation is different from that for the rough (fuzz) surface W. In our previous work, using binary-collision-approximation (BCA) simulation, we calculated the sputtering yield under Ar irradiation onto W target which has “simplified rough surface.” We could obtain the roughness dependence of the sputtering yield, which is coincident qualitatively with the experimental result. In this paper, we also calculated the physical quantities, i.e., sputtering yield, retention rate and mean penetration depth, under the noble (helium, neon, or argon) gas irradiation onto four types of the flat W targets: Amorphous W and BCC W lattices with the crystal orientation (110), (100) and (111). This simulation shows that the W surface structure affects noticeably the physical quantities. From these simulations, we propose that it is necessary to consider the surface form and the crystal structure to evaluate the sputtering phenomena though it has not been considered sufficiently.

A high-speed analysis of a shielding current density in a high-temperature superconducting film containing cracks has been proposed. To this end, a numerical code has been developed for analyzing the shielding current density in an HTS film. In the code, the GMRES(k) with the H-matrix method has been implemented to solve a linear-system obtained by discretizing an initial-boundary-value problem of a nonlinear integro-differential equation with respect to time and space. The results of the computations show that, if the Jacobi scaling is not applied to the linear-system, the Newton method does not converge at a certain time step. Therefore, the Jacobi scaling is an necessary tool for analyzing the shielding current density. Furthermore, by using the GMRES(k) method with the H-matrix method as a linear-system solver, the time evolution of the shielding current can be analyzed with high speed. In conclusion, the speed of the GMRES(k) with H-matrix method is about 221.3 times as fast as that of the LU decomposition.

In an extended element-free Galerkin method (X-EFG), the essential and natural boundary conditions can be imposed by the collocation based method. However, a coefficient matrix of linear systems obtained by X-EFG become asymmetric, although a symmetric structure exists in a part of the coefficient matrix. In fact, the structure of the coefficient matrices almost becomes symmetric when the size of linear systems is large. Hence, efficient effects may be obtained by using preconditioning for symmetric matrices. The purpose of the present study is to investigate effects of preconditioning for symmetric matrices to linear systems obtained by X-EFG. To this end, the incomplete Cholesky factorization (IC) is applied to the linear systems by regarding the coefficient matrix as symmetric one. In numerical experiments, it is found that the linear systems obtained by X-EFG can be efficiently solved by using IC as preconditioning for GMRES(m) and Bi-CGSTAB. In some cases, the efficiency of IC is superior than that of the incomplete LDU factorization as preconditioning for these iterative methods.

This study investigated the influences of a giant tsunami on salinity in coastal waters based on the scenario of the largest potential tsunamigenic earthquake that could occur along the Nankai Trough in the near future, by conducting an ocean-tsunami coupled three-dimensional simulation in Osaka Bay. This realistic simulation using an unstructured-grid model was capable of evaluating the oceanic salinization induced by the tsunami. Simulated results indicated that tsunami intrusion into the bay head increased salinity and it returned to its previous value within 1 week after the earthquake. The salinization, which continues for a few days, may adversely affect the local brackish biota.