JAMSTEC > Research Institute for Value-Added-Information Generation (VAiG) > Center for Mathematical Science and Advanced Technology (MAT) > Seminar Schedule > details

[MAT Seminar]

- Date:
- 2017/12/19 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Takane Hori (CEAT)
- Title:
- Evolution of sediment consolidation state during accretionary prism formation
- Abstract:
- Recent reflection surveys reveal the sediment consolidation states near the deformation front in a subduction zone. Over-consolidated sediments near the deformation front with little sediment subducted are found in the segment where the plate boundary of seismogenic zone is inferred to be strongly locked. In contrast, a thick under-consolidated sediment sequence is subducting in a region where reduced locking is inferred. They argue that the results suggest that the consolidation state of the sediments near the deformation front is a key factor contributing to megathrust slip behavior and its along-strike variation, and it may also have a significant role in the deformation style of the accretionary wedge. Although the argument seems to be reasonable, there is a problem that the sediment consolidation states in the two regions are almost identical before the deformation front is formed and no clear explanation how such difference in consolidation states appear in the deformation front is given. We provide a hypothesis to solve this problem based on numerical simulation results of accretionary wedge formation using discrete element methods.

[MAT Seminar]

- Date:
- 2017/12/05 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Osamu Kuwano (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Direct observation of flash heating during high-velocity slip
- Abstract:
- Recent experiments conducted at sub‐seismic to seismic sliding velocities (mm/s to m/s) show the dramatic weakening in the friction coefficient for a wide variety of rock types due to mechanochemical effects by frictional heating. Consequently, several weakening mechanisms have been proposed depending on the type of rock specimens.

Some of them are based on the sample observation and analysis after experiments. Direct observation of frictional contacts during an experiment is indisputable to constrain an elementary process at frictional contacts during slip. To measure the temperature distribution of frictional contact directly, we conducted the in-situ observation of frictional interface of fused silica using a high-speed camera. I'll talk about our recent experiments to obtain thermal images of a frictional interface.

[MAT Seminar]

- Date:
- 2017/11/28 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Takashi Minoshima (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Development of the Vlasov simulation code and its application to magnetic reconnection
- Abstract:
- Dynamics of collisionless systems such as self-gravitating bodies and space plasmas is described by the collisionless Boltzmann (Vlasov) equation, which is a multi-dimensional advection equation of the distribution function in phase space. Since the system is highly non-linear, the numerical simulation is a powerful mean to examine their behavior. The so-called Vlasov simulation has been proposed as an alternative to conventional particle simulations, in which the phase space distribution is discretized on grid points in six-dimensional phase space. The Vlasov simulation has an advantage that its solution is free from a statistical noise inherent to the particle simulation. However, the Vlasov simulation requires highly accurate algorithms to obtain reliable solutions, because available grid points in phase space are severely limited by the physical memory on the computation node.

To overcome the difficulty, we propose a new algorithm to solve the advection equation with high order of accuracy (Tanaka, Yoshikawa, Minoshima, and Yoshida, 2017, ApJ, 849, 76). The scheme preserves two important requirements of the advection equation, monotonicity and positivity, by setting proper limiters. Numerical tests confirm the expected performance of the scheme.

We aim to apply the scheme to collisionless magnetic reconnection, which is a cause of explosive events in our space such as solar flares and aurora activities. We measure the dissipation and transport of the momentum as well as the electric current in the course of reconnection, and discuss the mechanism facilitating reconnection through comparison with our dissipative MagnetoHydroDynamics model.

[MAT Seminar]

- Date:
- 2017/11/21 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Shigenobu Hirose (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Local nonlinear outcomes of self-gravity in protoplanetary disks: dependence on thermodynamics
- Abstract:
- Angular momentum transport in protoplanetary disks controls their time evolution and thus strongly affects the planet formation process within them. In some cold and massive protoplanetary disks, angular momentum can be transported by shear stresses associated with the gravitational instability (GI). A natural consequence of the long-range nature of gravity is formation of spiral arms as a result of GI, which globally transport angular momentum. On the other hand, there can be another nonlinear outcome of GI, called gravito-turbulence, when the disk to star mass ratio is small, in which angular momentum transport can be described locally. In this talk, I present 3D radiation hydrodynamics simulations to explore the local outcome of self-gravity in protoplanetary disks with realistic thermodynamics. Especially I focus on fragmentation of the disk where long-lived gravitationally bound clumps, which are expected to be companion stars or massive planets, are formed, and discuss how thermodynamics affects the condition for the disk fragmentation.

[MAT Seminar]

- Date:
- 2017/11/14 (Tuesday) 10:00-12:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Manabu Morishige (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Spatial and temporal changes in the behavior of slab-derived fluid
- Abstract:
- It is generally believed that slab-derived fluid plays essential roles in magma genesis and the occurrence of various types of earthquakes. Many previous studies have worked on where and how much fluid is released from the slab, although we don’t know much about how fluid migrates after its release. In this talk I will show the fluid behavior in viscoelastic slab by using 2D and 3D numerical models based on a theory of two-phase flow. I assume that fluid migrates as porous flow in the slab which subducts at a constant velocity. Fluid mobility strongly depends on the assumed ratio of permeability to fluid viscosity (K/μ). When K/μ is sufficiently high large amount of fluid is trapped in the fluid source and migrates inside it. In contrast, fluid generally moves with the slab with the increase in porosity in both shallow and deep directions for a low value of K/μ. I also find that in 3D fluid focusing occurs where the slab bends away from the trench and it leads to the increase in porosity and pore-fluid pressure there. Based on these findings I will discuss possible implications for several types of geophysical observations including intraslab earthquakes and slow earthquakes.

[MAT Seminar]

- Date:
- 2017/11/7 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Yasuko Yamagishi(Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Towards development of software to predict downtime of future research cruise
- Abstract:
- Our project aims to provide useful information for planning future research cruises. For this purpose, we have constructed a database system for past cruise information of research vessels belonging to JAMSTEC, and started developing application to predict downtime of future research cruise. Downtime of research cruise is the period when observation or cruise stops due to rough weather, machine trouble, etc. Therefore the downtime is an indicator to judge quantitatively as to whether the observation of the research cruise was successful or not. By constructing the database system for past cruise information, the downtime was estimated for the first time. Before developing software to predict the downtime of future cruise, we need to find important factors that affect the downtime and statistically analyze the relationship between factors and the downtime. In this seminar, we will explain the definition of the downtime and show the preliminary results of statistical analysis.

[MAT Seminar]

- Date:
- 2017/10/31 (Tuesday) 10:00-12:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Yoji Kawamura (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Phase synchronization between collective rhythms of globally coupled oscillator groups
- Abstract:
- A system of coupled oscillators can exhibit a rich variety of dynamical behaviors. When we investigate the dynamical properties of the system, we first analyze individual oscillators and the microscopic interactions between them. However, the structure of a coupled oscillator system is often hierarchical, so that the collective behaviors of the system cannot be fully clarified by simply analyzing each element of the system. For example, we found that two weakly interacting groups of coupled oscillators can exhibit anti-phase collective synchronization between the groups even though all microscopic interactions are in-phase coupling. This counter-intuitive phenomenon can occur even when the number of oscillators belonging to each group is only two, that is, when the total number of oscillators is only four. In this talk, we clarify the mechanism underlying this counter-intuitive phenomenon for two weakly interacting groups of two oscillators with global sinusoidal coupling.

[MAT Seminar]

- Date:
- 2017/10/24 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Ettore Barbieri (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Cracking me softly – the mechanics of hyperelastic Kirigami structures
- Abstract:
- Kirigami (from the Japanese 切り kiru = to cut, 紙 kami = paper) is the ancient Japanese art of paper sculptures, and it consists of cutting and stretching a single sheet of paper.

The stretch causes the cracks to open and display the decorative pattern. From the mechanical point of view, because of the cracks, a kirigami metamaterial will have lower strength than a pristine sheet. Nonetheless, the cracks will render the sheet more stretchable, hence tougher.

The question is then: is there then a crack pattern that can achieve both high strength and high toughness?

In this talk, I will show simulation tools that can help answer this question, based on my previous works on meshfree methods.

In particular, I will present an arc-length solver able to handle very sudden snap-backs occurring in cracks propagating in soft materials under large strains.

I will also show some attempts to manufacture kirigami structures with the corresponding experimental uniaxial stress-strain curves.

[MAT Seminar]

- Date:
- 2017/10/17 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Natsuki Hosono (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Terrestrial magma ocean origin of the Moon: A numerical study of a giant impact incorporating the difference of the equations of state for liquids and solids
- Abstract:
- Any viable model for the origin of the Moon must explain both the chemical and mechanical characteristics of the Earth-Moon system.

The classic model of oblique giant impact explains the large angular momentum and the lack of a large Fe-rich core in the Moon, but it is difficult to explain the similarity in the isotopic compositions.

Here we propose a new model where the giant impact by a solid impactor occurred to the proto-Earth that was covered with a magma ocean.

Due to the large differences in the shock heating between liquids and solids, a substantial fraction of the ejected material is from the magma ocean even with a highly oblique collision.

This model provides a natural explanation of the compositional similarities and the differences between the Moon and Earth.

[MAT Seminar]

- Date:
- 2017/10/10 (Tuesday) 9:30-11:30
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Neal Turner (Jet Propulsion Lab., California Institute of Technology)
- Title:
- Oceans' Origins: Delivering Water From Protostellar Disks to Planets
- Abstract:
- The Earth's water was either accreted with its rocks, in the form of water-bearing planetesimals, or delivered later in impacting asteroids or comets. I will review evidence for each of these three scenarios, and discuss how they connect with what we know about protostellar disks. These disks, made of gas and dust, are observed orbiting much younger analogs of our Sun, where they supply the raw materials for planet formation. In a few disks, water vapor is detected spectroscopically, and in many more cases its photodissociation products are seen. Modeling suggests disks' water is mostly segregated deep inside, incorporated into bigger bodies in the earliest stages of growth towards planet size. Finally, enough exoplanets now have measured masses and radii that we can begin to test whether there exist so-called ocean worlds, planets with much bigger water fractions than the Earth. I will discuss what the delivery of water to alien worlds may tell us about the origins of our own Earth's oceans.

[MAT Seminar]

- Date:
- 2017/10/3 (Tuesday) 10:00-12:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Daisuke Nishiura (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- A coupled QDEM-FEM simulation of ballasted railway track dynamics
- Abstract:
- A quadruple discrete element method (QDEM) was developed for viscoelastic multi-body dynamics. The sleeper motion modeled using QDEM was coupled with the rail motion modeled using a finite element method (FEM). The traffic impact response of a ballast particle and a sleeper was analyzed. The three-dimensional spatial distribution of ballast particle displacement was clearly revealed. Moreover, the ballast layer absorbed the low frequency vibration of the sleeper more effectively than its high frequency vibration. This study suggests that the proposed QDEM-FEM can provide greater insight into the impact response of ballasted railway tracks.

[MAT Seminar]

- Date:
- 2017/09/26 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Takehiro Miyagoshi (D-EARTH)
- Title:
- Thermal convection in super-Earths; effects of planetary size on the vigor of convection and thickness of the plate
- Abstract:
- Super-Earths are rocky exoplanets which have larger mass than the Earth’s one (up to about ten times the Earth’s). A large number of super-Earths have been detected in this century. Thermal convection in the mantle of super-Earths is expected to be one of key factors to understand their thermal history, or their surface environment. We have studied thermal convection in super-Earths by numerical simulations.

We have found that the activity of hot plumes from core-mantle boundary becomes low while that of cold plumes from the surface of the planet almost remains high as the planetary size increases. The efficiency of heat transport is totally reduced compared with results of Boussinesq approximation models in massive super-Earths. The thickness of the lithosphere increases while flow velocity of convection does not significantly change as the planet size increases. In addition to these results, as the other topic, I will introduce the results with temperature-, pressure-, and stress-history-dependent viscosity to model the plate motion.

[MAT Seminar]

- Date:
- 2017/09/19 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Hide Sakaguchi (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Strategic Planning for Establishing CULT, Center for Underwater Laser Technology
- Abstract:
- As U.S. Defense Advanced Research Projects Agency (DARPA) pushed to develop a blue-spectrum submarine laser communication system to link submerged submarines with nearby aircraft in 2010, underwater laser technology is greatly expected to be a next generation high-speed communication tool. However, laser has much more potential for JAMSTEC science and technology in addition to a communication tool. For example, a laser distant meter and a laser scanning system have been already developed by JAMSTEC. Recently, feasibility studies to apply laser for biological purpose and geotechnical purpose have been started. In this talk, several examples of underwater laser technology are introduced and strategic planning for establishing a new JAMSTEC research and development center focusing this technology will be discussed.

[MAT Seminar]

- Date:
- 2017/09/12 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Shunsuke Shimobayashi (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Size-dependent molecular ordering transition in lipid droplets
- Abstract:
- Lipid droplets are energy resources of living things, but their excessive accumulation in our bodies, namely “obesity”, can lead to lethal diseases, such as arteriosclerosis. Despite the recent significant progress of understanding on lipid metabolic pathways, the internal structure and dynamics of lipid droplets have been still poorly understood. Here, we combine in vitro and in vivo studies, together with soft matter theories, to show the size-dependent molecular ordering transition in lipid droplets. This transition may be induced by the lipid droplet interface. The mechanism is discussed in this seminar.

[MAT Seminar]

- Date:
- 2017/08/22 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Ryoichiro Agata (CEAT)
- Title:
- Viscoelastic finite element modelling of crustal deformation: optimization of viscoelastic structure and earthquake cycle simulation
- Abstract:
- Estimation of the earthquake/post-earthquake fault slip using post-earthquake deformation observation data is an important topic in the field of geodetic inversion. Estimation methods for this purpose are expected to be improved by introducing numerical simulation tools (e.g. finite element (FE) method) of viscoelastic deformation, in which the computation model is of high-fidelity to the available high-resolution crustal data. We have proposed a large-scale simulation method using such FE high-fidelity models (HFM), assuming use of a large-scale computation environment such as the K computer in Japan (Ichimura et al. GJI, 2016). On the other hand, the values of viscosity in the heterogeneous viscoelastic structure in the high-fidelity model are not trivial. In this study, we developed an adjoint-based optimization method incorporating HFM, in which fault slip and asthenosphere viscosity are simultaneously estimated. In idealized numerical experiments in the Tohoku region using synthetic crustal deformation data, the method could successfully recover the true fault slip and viscoelastic structure when having access to data from observation points that will be developed in the near future in the real world.

In the talk, we will also introduce some updates on the forward modeling of earthquakes used in the above study: we incorporated a physical model of earthquake cycle, i.e. spontaneous evolution of fault slip, in the FEM code.

[MAT Seminar]

- Date:
- 2017/08/08 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Mikito Furuichi (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Spontaneous generation of arch structure in the numerical sand box experiment
- Abstract:
- A deformation mechanism of an accretionary prism was modeled using the numerical simulation of the sandbox experiment. We solved the motion of 2.4 billion solid particles with Discrete Element Method (DEM) on massively parallel supercomputer system. This huge number of particles enabled us to use the realistic sizes of the sand. Thus, we could successfully reproduce the prism evolution which was consistent with the lab sandbox experiment. One of the advantages of numerical simulation over the analog experiment is the ability to analyze the detailed deformation processes of the granular layer in 3D. Thus, we performed stress chain analysis which is the powerful tool to reveal non-continuum feature of granular material. We found the spontaneous generation of the stress arches in the sand box experiment, which might explain the wavy shapes of accretionary prism in trench axis. The nonlinearity of granular behavior could generate arch structure without any initial heterogeneity of the sand layer. The detail analysis of rearrangement stage of the stress chains showed that the discontinues granular behaviors in the force chain angle played an important role for the horizontal deformations of the mountain belt. We also consider the theoretical models to understand the control factor of arch length. It should be useful to find the potential candidate of our finding processes in a nature.

[MAT Seminar]

- Date:
- 2017/07/25 (Tuesday) 14:00-16:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Takashi Nakagawa (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Impact to the habitability from early to present Earth inferred from climate energy balance coupled with deep mantle volatile cycle
- Abstract:
- The Earth is only a habitable planet of solar system that may have life on its surface and has experienced various extreme climate events such as snowball Earth and global warming but not still clear for how evolution and dynamics of deep Earth (i.e. plate tectonics as a signature of mantle convection of the Earth) affect the habitability on Earth's surface, which controls heat transfer and volatile cycle in deep mantle described as regassing-degassing-dehydration processes. Here, two major volatiles (water and carbon) are assumed. The mantle (volcanic) degassing event is significantly influenced for physical and chemical properties of atmosphere and carbon cycle on the Earth's surface. As a computational power being powerful, the material circulation in deep Earth such as volatile circulation can be computed from the numerical mantle convection simulations. Hence, constructing a coupled model of surface climate-deep Earth evolution in numerical mantle convection simulations and energy balance theory of climate change, it would be available to discuss how various complexities of dynamics in the deep Earth affect the habitability of Earth's surface from early to present-day. A modeling result indicates that the Earth's surface can be warmed-up gradually but experienced for a number of snowball Earth events, which is expected for a new scenario on evolutionary track of climate change.

[MAT Seminar]

- Date:
- 2017/07/18 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Arthur Bauville (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Numerical simulations as a tool to understand the tectonics of the Japanese active margins
- Abstract:
- In it simplest form, tectonics can be expressed as a relation between deformation, material properties and stresses. JAMSTEC's expertise lies in assessing and monitoring the deformation and material properties in the accretionary prisms of Japan through geophysical survey and deep ocean drilling. However, stresses cannot directly be measured in nature or in experiment. For practical purposes, the state of stress in the accretionary prism is often assumed to be a combination of lithostatic pressure and a simple function describing deviatoric (e.g. constant or linear increase with depth). This kind of approximation, however, cannot take into account local variations of the stress state in and near tectonic structures such as fold and faults. This local stress is however, the one really driving the deformation.

Numerical simulations offer a way to quantitatively assess such complex dynamic stress state. In this presentation I introduce three great challenges for which operating a paradigm shift from a static to a dynamic view of the stress state may bring answers. These three challenges are:

1) Why are plate interfaces weak?

2) What controls slip stability?

3) How do plate interfaces form?

I will present preliminary results that demonstrate the feasibility of this approach, using numerical simulations that describe the hydro-thermo-mechanic evolution of the accretionary wedge and the lithosphere.

[MAT Seminar]

- Date:
- 2017/07/11 (Tuesday) 10:00-12:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Yuzuru Yamamoto (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Subduction-magathrust earthquakes in the Japanese metropolitan area: a review and project for seismic-fault drilling on Boso Peninsula
- Abstract:
- Tokyo Metropolitan Area is a densely populated economic center that has been subjected to repeated great earthquake(eq)s. Two-major different types of subduction zone eqs have been proposed based on the distribution of eq-induced uplifted topography and ancient Tsunami records: the 1923 Taisho-Kanto type eq, seismic rupture occurred around the Sagami-Bay, and 1703 Genroku type eq, rupture propagated to the off Boso Peninsula. However, except for the 1923 eq, we have no seismic records and seismic-fault modeling has been controversial. We recently found the ancient seismic fault on the Boso Peninsula accompanied with pseudotachylite, direct signature of rapid heating/melting associated with eq. We are planning to drill this fault at least in three different sites in this autumn and are going to model the seismic slip during these eqs. In this presentation, I am going to introduce a review of ancient subduction-magathrust eqs occurred around Tokyo and would like to discuss about this drilling project.

[MAT Seminar]

- Date:
- 2017/07/04 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Jiming Shi (Princeton University)
- Title:
- circumbinary disk -- structure and accretion rate
- Abstract:
- Circumbinary disks (disk around a pair of gravitating objects) can be found around many astronomical objects. They are natural outcome of hierarchical galaxy formation and are often observed around protobinary stars. In this talk, I will discuss how the gravitational torque of the binary reshapes the circumbinary disk while still maintains its accretion based on MHD/hydro simulations.

The changes of disk morphology found in simulations consist of a zoo of interesting dynamical features such as a low-density gap, narrow and high-velocity spiral gas streams, and a high-density lump adjacent to the disk edge, which could be used observationally to either identify the binary or measure its properties. At last, if time permits, I will talk about some ongoing project related to this topic.

[MAT Seminar]

- Date:
- 2017/06/27 (Tuesday) 13:00-15-00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Satori Tsuzuki (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Large-scale Viscoelastic Multi-body Simulations Using Quadruple Discrete Element Method on Supercomputers ー V&V, Implications for Future Research ー
- Abstract:
- The importance of large-scale viscoelastic multi-body simulations has been widely recognized in many fields. However, it is quite challenging because of the difficulties on several numerical problems. We had developed a simulation method of combining QDEM for deformation analysis of structures with the DEM for collisions among structures for large-scale viscoelastic multi-body problems, which brought a certain level of successful results. However, it is still difficult to reproduce surface topography of structures because particles only set on four nodes of tetrahedrons. Introduction of collision calculations among triangles (=polygons) of tetrahedrons is a promising approach, however, methodology of it has not been established. In this study, an effective method of QDEMSM (QDEM with Surface Modeling) was developed. Point-polygon collisions and line-line collisions are effectively coupled with QDEM. Neighbor-tetrahedron lists are used to reduce the computational cost from
*O*(N2) to*O*(N) to find the tetrahedrons in the neighboring cells. A linked-list technique is coupled with neighbor-tetrahedron lists. By skipping the calculations among inner tetrahedrons of each structure, it becomes possible to reduce the amount of calculations drastically. The QDEMSM was demonstrated by several domino simulations and experiments. We take up a shogi domino problem, which is quite difficult to be handled in numerical simulations because of their anisotropic geometric shapes. A shogi domino simulation using 40 pieces of shogi was successfully carried out and the result shows a qualitative agreement with the experiment. Bricks domino simulations using 5 bricks were compared with experiments. A human domino simulation using 10 peoples was also carried out. The result implicates us the feasibility of the application of QDEM simulations to a variety of viscoelastic deformable multi-body problems such as human domino accidents for disaster prevention.

[MAT Seminar]

- Date:
- 2017/06/20 (Tuesday) 13:00-15-00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Yuusuke Iida(Kwansei Gakuin University)
- Title:
- Understanding solar magnetic carpet using image recognition technique
- Abstract:
- Magnetic field on the solar surface is an origin of activities in the solar atmosphere. Formation and maintenance of the surface magnetic field is one of the most fundamental questions in the solar physics for long time. Tiny magnetic elements, which is formed by the surface convection, fill the surface. It is mathematically modeled by Magneto-Chemistry equation (Schirijver et al., 1997), which includes appearance, disappearance, coalescence, and splitting of the elements. On the other hand, satellite observation in 2000s enables us to investigate magnetic carpet more directly. Several scaling nature is found there. Especially Parnell et al.(2009) found a power-law distribution of magnetic flux contained in each element, which is a key to understand formation mechanism of the magnetic carpet. However, we cannot have found the corresponding solution in M-C equation just by considering in our head. I will talk about our results for this problem, analysis of observational data based on image recognition technique.

[MAT Seminar]

- Date:
- 2017/06/13 (Tuesday) 13:00-15-00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Takane Hori (CEAT)
- Title:
- Mechanical understanding of decollement formation during accretion of sediments from subducting plate through a particle-based simulation model
- Abstract:
- Subduction zones are often characterized by wedge-shaped sedimentary complexes, called accretionary prisms, that form when sediments are scraped off from the subducting plate. In the well-accreted subduction zones, faults parallel to the subduction direction are commonly observed as strong reflectors in seismic profiles, called decollements. The mechanism of decollement formation was investigated through numerical experiments using a particle-based simulation model. In the numerical experiments, a decollement-like structure appeared as a spontaneously localized shear deformation near the bottom of the sediment when the thickness of the sediment was sufficient to balance the gravitational force and tectonic loading. Furthermore, spontaneous formation of another fault parallel to the decollement-like structure was demonstrated. In contrast, no such decollement-like structure was formed when the sediment was too thin. These results are consistent with various observations in real subduction zones. I will discuss the mechanism based on precise analyses of the deformation and stress state evolution in the numerical experiments.

[MAT Seminar]

- Date:
- 2017/06/06 (Tuesday) 13:00-15-00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Osamu Kuwano (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Direct observation of frictional contacts during high-velocity slip
- Abstract:
- Recent experiments conducted at sub‐seismic to seismic sliding velocities (mm/s to m/s) show the dramatic weakening in the friction coefficient for a wide variety of rock types due to mechanochemical effects by frictional heating. Consequently, several weakening mechanisms have been proposed depending on the type of rock specimens.

Some of them are based on the sample observation and analysis after experiments. Direct observation of frictional contacts during an experiment is indisputable to constrain an elementary process at frictional contacts during slip. In this seminar, I'll talk about our recent experiments for direct microscopic observation of frictional interface.

[MAT Seminar]

- Date:
- 2017/05/30 (Tuesday) 10:00-12:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Murino Kobayakawa (Osaka University)
- Title:
- Analysis of plate drag in granular materials using DEM simulation
- Abstract:
- The response of granular materials to plate drag is numerically studied using DEM simulation. The effect of initial packing volume fraction of the materials on the horizontal force acting on the plate is examined. The results show that a volume fraction-dependent bifurcation occurs in the force: at lower initial fraction, the force reaches approximately constant as the plate advances, while at higher initial fraction, the force profile has repeated fluctuations. In addition, the force averaged over a steady state increases approximately linearly with the volume fraction. The observed force behaviour is consistent with that obtained experimentally in previous studies (Physical Review Letters 105, 128301 (2010)&Physical Review E 89, 042202 (2014)). The analysis of local volume fraction in the materials during the drag shows that at the higher initial fraction, a clear shear band, reaching from the plate tip to the free surface, is observed but not at the lower initial fraction. The force fluctuations are attributed to the consecutive appearing and disappearing of the shear bands.

[MAT Seminar]

- Date:
- 2017/05/16 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Takashi Minoshima (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Toward a physics-based modeling of solar energetic particles
- Abstract:
- The Solar Energetic Particles (SEPs) are high-energy charged particles associated with explosive events on the Sun, such as flares and Coronal Mass Ejections (CMEs). Understanding the nature of SEPs is especially important from the viewpoint of space weather forecasting, because they may pose serious hazards to space satellites and astronauts. In particular, widely-spread shock waves driven by CMEs can cause powerful and long-lasting SEP events, which affect the radiation environment in space. In this talk, we discuss our new approach to model (and forecast in future) the dynamics of CME-driven SEPs based on the focused-transport and drift-acceleration theories.

Preliminary numerical results will be shown.

[MAT Seminar]

- Date:
- 2017/05/09 (Tuesday) 09:30-11:15
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Shigenobu Hirose (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Local nonlinear outcomes of self-gravity in protoplanetary disks
- Abstract:
- Angular momentum transport in protoplanetary disks controls their time evolution and thus strongly affects the planet formation process within them. In some cold and massive protoplanetary disks, angular momentum can be transported by shear stresses associated with the gravitational instability (GI). A natural consequence of the long-range nature of gravity is formation of spiral arms as a result of GI, which globally transport angular momentum. On the other hand, there can be another nonlinear outcome of GI, called gravito-turbulence, when the disk to star mass ratio is small, in which angular momentum transport can be described locally. In this talk, we present 3D radiation hydrodynamics simulations to explore the local outcome of self-gravity in protoplanetary disks with realistic thermodynamics. We will discuss the nature of the gravito-turbulence as well as the fragmentation conditions of the self-gravitating protoplanetary disks.

[MAT Seminar]

- Date:
- 2017/04/25 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Manabu Morishige (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Fluid migration in subducting slab
- Abstract:
- Fluids delivered from Earth surface are considered to play important roles in subduction zones. For example, it triggers earthquakes at the plate interface and slab interior, and it also controls magma genesis by lowering melting temperature of the mantle rock. However, little is known about the detailed fluid behavior after its release from the subducting slab. In this talk I will show preliminary results of fluid migration in the slab. I developed 1D and 2D numerical models based on a theory of two-phase flow, which allows us to treat the deformation of rock and fluids simultaneously. The rock is assumed to deform as a viscoelastic medium when pore fluids migrate through it. I focus on two important parameters, bulk viscosity and bulk modulus, which are measures of the resistance to volume change in viscous fluid and elastic body, respectively. I find that fluids show several characteristic behaviors depending on the assumed values of these two parameters. When bulk viscosity is low, viscosity plays an important role and fluids simply go up almost vertically with producing local maximum and minimum values of porosity. When bulk viscosity is relatively high, elasticity controls the fluid behavior. A large amount of fluids are trapped in and migrate along the fluid source.

[MAT Seminar]

- Date:
- 2017/04/18 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Yasuko Yamagishi (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Development of database system and input software of operation information of research cruises
- Abstract:
- JAMSTEC has seven research vessels and controls the research cruise of these vessels. All cruises in next fiscal year has to be planned one year before. In order to provide useful information on making plans of future cruises, we have developed a database system for past cruise information of research vessels belonging to JAMSTEC. This database system provides not only time-sequence data of operation state information on research cruises over the past 10 years, but also observation downtime for each cruise and what causes the downtime. One of the difficulties on construction of this database is that it takes a great deal of time to create a data table from the operation state information, because the past operation state is written in natural language. Operation status is reported at least once a day by e-mail from the vessel at sea to the operation management department on ground. To avoid this problem, we are also developing of input software of the operation state information of the vessel. By using this software, the operation state will be automatically added into the data table immediately after it is reported.

[MAT Seminar]

- Date:
- 2017/04/11 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Takehiro Miyagoshi(Department of Deep Earth Structure and Dynamics Research (D-EARTH))
- Title:
- Mantle convection in super-Earths
- Abstract:
- Understanding the thermal convection in the mantle is one of the most important keys to clarify the thermal evolution, surface environment, or the habitability of the planet. Recently, many terrestrial exoplanets have been detected, and their habitability is one of hot topics. Super-Earths are extra-solar terrestrial planets which have a large mass (up to about ten times the Earth’s), and a large number of super-Earths have been discovered (e.g., Borucki et al. 2011). We have studied mantle convection in super-Earths by numerical simulations with ACuTEMan code (Kameyama et al. 2005) to discuss its feature, possibility of plate motion, or their habitability. I will introduce the results which is published in the last month (Miyagoshi et al. 2017, Earth Planets Space) together with the past published results (Miyagoshi et al. 2015, JGR; 2014, Astrophys. J. Lett.). We found that the activity of ascending hot plumes and the efficiency of heat transport by thermal convection is considerably lowered in massive super-Earths by the strong adiabatic compression effect. They suggest that the plate motion hardly occurs in massive super-Earths. In addition, the convection remains in the transient stage which is layered convection for very long time (near ten billion years).

[MAT Seminar]

- Date:
- 2017/04/04 (Tuesday) 10:00-12:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Satori Tsuzuki(Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Large-scale Viscoelastic Multi-body Simulations Using Quadruple Discrete Element Method on Supercomputers
- Abstract:
- Contact problems among viscoelastic materials in the multi-body system is one of the challenging topics in science and many engineering applications. We have developed an effective simulation method of combining QDEM (Quadruple Discrete Element Method) for the deformation analysis of structures with the DEM for the collisions among structures. However, it is still difficult to reproduce surface topography of structures because particles only set on four nodes of tetrahedrons in our current method. In this study, QDEMSM (QDEM with Surface Modeling) is newly developed. Point-polygon collisions and line-line collisions are effectively coupled with QDEM. Our improved method was validated by several simulation results; domino simulations using the 40 pieces of shogi (= Japanese chess) were successfully carried out. It was also found the friction forces acted on the surface critically effected on the propagation speeds of contact forces. In parallel computing, by applying the space-filling curve to decomposition of the computational domain, we make it possible to contain the same number of nodes in each decomposed domain. Our parallel simulation code achieves a good weak scalability on the TSUBAME2.5 supercomputer.

[MAT Seminar]

- Date:
- 2017/03/28 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Yoji Kawamura(Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Phase reduction approach to synchronization of beating flagella
- Abstract:
- We formulate a theory for the phase reduction of a beating flagellum. The theory enables us to describe the dynamics of the beating flagellum by a single variable called the phase. The theory can also be considered as a phase reduction method for limit-cycle solutions in infinite-dimensional dynamical systems, namely, limit-cycle solutions to partial differential equations representing beating flagella. We derive the phase sensitivity function, which quantifies the phase response of the beating flagellum to weak perturbations applied at each point and at each time. Using the phase sensitivity function, we analyze the phase synchronization between a pair of beating flagella via hydrodynamic interactions at low Reynolds number.

[MAT Seminar]

- Date:
- 2017/03/14 (Tuesday) 10:00-12:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Shunsuke Shimobayashi(Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Heterogeneous lipid structures in cells
- Abstract:
- Lipids are abundant in nature as we often eat oil dressing in daily lives. In cells, the lipids are generally used as physical boundaries to separate one from the others. To make their dynamic functions possible, cells transiently form a heterogeneous structure (which is the clustering of certain specific lipids and proteins). This physical process is considered to be relevant to phase separation, which is one of the phase transition phenomenon. In this seminar, I'll talk about the formation mechanism of this biologically important heterogeneous structure and recent progress.

[MAT Seminar]

- Date:
- 2017/03/07 (Tuesday) 14:00-16:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Daisuke Nishiura (Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Gravity Current in Centrifuge Modelling Test (Hide Sakaguchi,Kazuhiro Tsurugasaki,Junji Miyamoto)
- Abstract:
- SPH simulation of wave channel Abstract：The EPFL in Switzerland have studied TSUNAMI by using a wave channel in laboratory experiment. Their objectives are estimations of wave reaching area and wave force acting on structure to avoid TSUNAMI disaster. However, the real TSUNAMI is very complex because its force and reaching area depend on the ground geometry and spatial configuration of buildings. So the laboratory experiment has some limitations for these objectives. Therefore, EPFL researchers are interested in our numerical simulation technique. To collaborate with EPFL, we have to show the accuracy and reproducibility of our SPH simulation. First, I simulated in the case of without buildings and compared the wave profiles of our SPH and EPFL experiment. Next, I investigated the wave force acting on a building. Although the wave height profile was well reproduced by our simulation, the arrival time of wave front was slower in especially the case of dry bed surge. And also, the wave impact force is smaller than the experiment. By the comparison with the experimental wave, I recognized the necessity of not only improving a boundary condition and spatial resolution but also implementing turbulence flow model and air bubbles.

[MAT Seminar]

- Date:
- 2017/02/21 (Tuesday) 14:00-16:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Hide Sakaguchi (Department of Mathematical Science and Advanced Technology)
- Title:
- Gravity Current in Centrifuge Modelling Test (Hide Sakaguchi,Kazuhiro Tsurugasaki,Junji Miyamoto)
- Abstract:
- Submarine gravity current of dense mud was investigated using a drum centrifuge of Toyo Construction Co. At a centrifugal acceleration equivalent to 50G, mud with water content from 80% to 140% was submerged in a water flume separated by a shutter gate. After opening the shutter gate mud flowed as a gravity current. Flow velocities and flow patterns were detected by the high speed camera installed in the drum centrifuge. We found out the critical water content to differentiate the volume of entrained water due to the development of vortex, which is significantly related to the final flow distance.

[MAT Seminar]

- Date:
- 2017/02/14 (Tuesday) 13:00-15:00
- Place:
- MAT theater on the 5th floor, IT Building, Yokohama Institute
- Language:
- English
- Speaker:
- Mikito Furuichi(Department of Mathematical Science and Advanced Technology (MAT))
- Title:
- Recent progress of HPC architecture and application
- Abstract:
- The computing capability of supercomputers has been growing rapidly in the last few years with the development of many-core architectures.

Especially, the Sunway TaihuLight supercomputer utilizing many customized lightweight cores became the world’s first system with a peak performance greater than 100 PFlops and the first in Top500. In addition, the winner of Gordon bell prize also went to the non-hydrostatic model on the Sunway TaihuLight in 2016. Moreover, Oakforest-PACS utilizing new Xeon phi processors (KNL) got the first place in Japan. In this talk, I provide an overview of recent many-core supercomputer systems and review of recent successful applications for them. In addition, the performance of our code on the latest KNL system is presented, to discuss the efficient implementations and algorithms for the particle simulation method (PSM) for future supercomputer systems (Post-K and PEZY).