セミナーのお知らせ
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年10月09日（水）13:00-15:00
- 使用言語:
- 日本語
- 開催場所:
- 横浜研究所情報技術棟5階508 MATシアター
- 講演者:
- 有木健人（筑波技術大学）
- タイトル:
- Hessian-based theory of passive scalar turbulence
- 概要：
- Turbulence, a class of ubiquitous phenomena widely observed in nature, dominates transport of various physical properties (e.g., mass, momentum, temperature, chemical species, particulate matters). Thus, the proper understanding of turbulence may be demanded in a wide range of fields of both natural and industrial sciences, which is not yet achieved because of the strong nonlinearity in its dynamics. Eventually, theoretical treatments based on the exact governing law often become challenging. The present talk provides the overview of a recent development of analytical theory of turbulence, where a dynamical model of turbulence transport of the scalar field is developed purely from the exact governing law. On the basis of the Hessian, the Lagrangian time scale of the local scalar distribution can be extracted, which leads us to a successful description of the memory fading effect of the scalar field for the first time. Then the theory enables us to predict the Obukhov-Corrsin spectrum, an expected universality of the turbulence transport, which shows reasonable agreement with experimental and numerical results. Application to the clustering of the cloud droplets will also be discussed, where the number density field is treated as a passive scalar combined with the preferential concentration mechanism. The theory explicated the pair correlation function from the balance between the preferential concentration and the inter-scale transfer of the number density field.
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年10月02日（水）13:00-15:00
- 使用言語:
- 日本語
- 講演者:
- 亀山真典 (MAT招聘上席研究員)
- タイトル:
- 3次元マントル対流の流れ場を粗い空間解像度で求める直接解法の考案と実装
- 概要：
- 本研究では、多重格子法の最粗格子レベルの計算時間の短縮を図ることを目的として、3次元マントル対流の流れ場 (速度と圧力) を粗い空間解像度で求める直接解法を開発する。この手法では変数分離法の考えに基づき、鉛直方向には有限体積法によって方程式を離散化して解くが、水平方向には我々が「なんちゃってスペクトル法」と呼ぶ方法を用いる。この手法の核心は、与えられた粗い空間解像度でのスペクトル展開に適した「なんちゃって」基底関数の組を数値的に構築することにある。このような基底関数は、水平面内の微分演算子 (ラプラシアン) を離散化したものにあたる行列の固有値問題を解くことによって得られる。我々の経験から、この手法は問題が小規模であるが故に計算負荷が非常に小さく、「なんちゃって」基底関数を数値的に構築するコストや鉛直方向の微分方程式を数値的に解く手順を含めても、ごくわずかな計算コストしか必要としないことが分かった。また、3次元直交座標系でのマントル対流シミュレーションプログラムに本手法を取り入れたところ、新しい直接解法によって (最粗格子レベルでの精度に見合った) 非常に「行儀のよい」解を得ることができ、ひいては多重格子法計算の全体的なコストの削減にも役立つ可能性のあることが分かった。発表では、(3次元球殻形状など) 直交座標系でない場合にこの手法を応用する可能性についても議論する。
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年8月21日（水）13:00-15:00
- 使用言語:
- 日本語
- 開催場所:
- 横浜研究所情報技術棟5階508 MATシアター
- 講演者:
- 井元佑介（京都大学）
- タイトル:
- トポロジカルデータ解析を用いた細胞分化の経路解析
- 概要：
-
細胞分化とは、細胞が他の細胞へと特殊化していくことであり、その過程で細胞は自身の遺伝子発現レベルを複雑に変化させる。つまり、細胞分化は高次元の遺伝子発現空間に複雑
なトポロジーとその上の流れを形成する。そのトポロジーを推定する方法として、高次元データからトポロジー情報をグラフ表現するMapperを用いたシングルセル遺伝子発現データ解析が
行われてきたが、Mapperはトポロジー上での流れを推定することはできない。
そこで本研究では、RNA velocity法を用いることでシングルセル遺伝子発現データから速度データを生成できることに着目し、点群と速度の組で構成される高次元データに対するトポロジ カルデータ解析手法V-Mapper（velocity-Mapper）を開発した。V-Mapperは、速度データをMapperグラフの辺に埋め込むことによって、重み付き有向グラフ（V-Mapperグラフ）として トポロジーと流れを同時に記述する。本講演では、V-Mapperの基本原理とマウス内分泌データを用いた解析例を紹介する。さらに、組み合わせホッジ分解を組み込むことで、V-Mapperグラ フ内の流れの解釈性が高まることも紹介する。また、トポロジカルデータ解析における次元の呪いについても紹介し、次元の呪いに対する解決策を提示する。
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年8月1日（木）13:00-15:00
- 使用言語:
- 英語
- 開催場所:
- 横浜研究所情報技術棟5階508 MATシアター
- 講演者:
- 平岩 徹也 (Institute of Physics, Academia Sinica, Taiwan)
- タイトル:
- Computer simulations on influence of mechanical perturbations due to actions of subnuclear molecules on chromatin organization and dynamics
- 概要：
-
Genetic information in a eukaryotic cell is stored in its
chromatin, a polymer-like composite of DNA and proteins, densely
packed within the nucleus. Physical spacing of chromatin is critical
in regulating bio-chemical and transcriptional abilities of genes, and
proper functionality of the genomic content depends on the nonrandom
organization of chromatin. Meanwhile, in a living cell, other
subnuclear molecules, such as enzymes like polymerase and
topoisomerase, act to facilitate cellular functions. Mechanical
perturbation due to such actions of molecules may affect the chromatin
organization and dynamics. In this talk, I would like to explain our
computer-simulation studies about such effect, based on
polymer-physics concepts and the GPU-aided computations, where we
focused on a type of actions of molecules that we call
catch-and-release action and implemented in the way inspired by a
class of molecules like topoisomerase-II. I will share with the
participants the results of our simulations on how it affects
chromatin organization and dynamics. The results clarified (i) that
the mechanical perturbation of such actions can modulate the phase
separation organizations of chromatin called heterochromatic and
euchromatic regions [1], and (ii) that the mechanical perturbation
enhances fluctuating dynamics of inclusions in chromatin through the
newly-proposed dynamic mode of chromatin remodeling [2].
References ---
[1] R Das, T Sakaue, GV Shivashankar, J Prost, T Hiraiwa (2022) "How enzymatic activity is involved in chromatin organization", eLife 11, e79901
[2] R Das, T Sakaue, GV Shivashankar, J Prost, T Hiraiwa (2024) “Chromatin Remodeling Due to Transient-Link-and-Pass Activity Enhances Subnuclear Dynamics", Physical Review Letters 132, 058401.
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年7月24日（水）13:00-15:00
- 使用言語:
- 英語
- 開催場所:
- 横浜研究所情報技術棟5階508 MATシアター
- オンライン接続先：
- https://us02web.zoom.us/j/84174291596?pwd=OkifY8FybgEAaDTXHlaoftPfIzWsOK.1
- 講演者:
- Vikash Churasia (OIST)
- タイトル:
- Shape Preserving Everting Motion of Orientable or Nonorientable, Knotless, and Knotted Bands
- 概要：
- Eversion, the process of turning bodies inside out, is a fascinating phenomenon observed widely in nature that has intrigued elasticians and mathematicians for decades. We introduce a novel framework for achieving continuous, periodic, everting motions of both orientable and nonorientable, as well as knotless and knotted bands, extending beyond the eversion of simple objects like halved tennis balls and cylindrical tubes. Our analytical approach yields a traveling wave solution that describes this unique class of motions. Each motion is isometric, preserving inter-point distances on the band, and isoenergetic, maintaining constant elastic bending energy throughout the process. This shape-preserving motion enriches the known categories of band transformations, previously limited to rigid rotations and translations. Potential applications of this work include the development of adaptive materials and soft robotic systems with dynamically tunable properties, which could drive innovation across diverse fields such as medical, aerospace, and architectural engineering.
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年7月17日（水）13:00-15:00
- 使用言語:
- 英語
- 開催場所:
- 横浜研究所情報技術棟5階508 MATシアター
- オンライン接続先：
- https://us02web.zoom.us/j/84174291596?pwd=OkifY8FybgEAaDTXHlaoftPfIzWsOK.1
- 講演者:
- Riccardo Muolo (Tokyo Tech)
- タイトル:
- Effects of higher-order interactions on synchronization dynamics: from phase reduction to chimera states
- 概要：
- Synchronization is a ubiquitous emergent phenomenon in which an ensemble of elementary units behaves in unison due to their interactions. Given the pervasiveness of synchronization, understanding how it is achieved is a fundamental question. In particular, the nature of the interactions among oscillators has strong consequences on the transition to synchronization. To tackle this issue, it is convenient to consider phase models in which each oscillator is described solely in terms of a phase variable. According to phase reduction theory, the phase model captures the dynamics completely when the coupling among the oscillators is sufficiently weak. If one considers only pairwise interactions, the synchronization transition is described by the Kuramoto-type model. Despite the versatility of such an approach, the classical theory of synchronization is solely based on pairwise interactions, while, in many natural systems, the interactions are intrinsically higher-order (many-body) rather than pairwise. In fact, many examples show that a pairwise description is not sufficient to match the theory with observations and, additionally, higher-order interactions appear naturally when phase reduction is performed up to higher orders. It was also shown that extensions of the Kuramoto model including higher-order interactions exhibit an explosive transition to synchrony or collective chaos. I will start by introducing the phase reduction theory and highlight the universality of phase models. Then, after discussing the basics of higher-order interactions, I will present a recent work where we analyzed the collective dynamics of the simplest minimal extension of the Kuramoto-type phase model for identical globally coupled oscillators subject to two- and three-body interactions and showed how the many-body interactions greatly enriches the behaviors of the system. In the last part of the seminar, I will briefly introduce an intriguing type of synchronization patterns, in which coherent and incoherent oscillators coexist, called chimera states. Such patterns, studied, among others, by Kuramoto and Battogtokh, are known to be elusive and characterized by a very short life-time when the interactions are pairwise; however some of these limitations can be overcome in systems where higher-order interactions are present.
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年7月9日（火）11:00-12:00
- 開催場所:
- 横浜研究所情報技術棟5階508 MATシアター
- 使用言語:
- 英語
- オンライン接続先：
- https://us02web.zoom.us/j/84174291596?pwd=OkifY8FybgEAaDTXHlaoftPfIzWsOK.1
- 講演者:
- Davide Bigoni (University of Trento, Italy)
- タイトル:
- Metamaterials knocking on material instabilities’ door
- 概要：
-
Homogenization of periodic elastic grids subject to axial prestress is introduced to obtain equivalent elastic materials exhibiting material instabilities, driven by the microstructure of the grid and its level of prestress. Instabilities include shear band fromation and Hopf bifurcation, thus leading to odd elasticity.
A design strategy is introduced for metamaterials displaying tailored instabilities. When the latter occur at the microscale, they are analyzed with a Floquet-Bloch wave technique, while analysis of macroinstabilities leads to the definition of an equivalent elastic material. This material can be obtained via homogenization theory for periodic elastic structures, subject to a state of axial prestress and incremental deformation involving axial and shear forces and bending moment. Macroinstabilities in the form of shear bands usually occur only for compressive prestress, so that the stability domain for the equivalent material results unbounded in tension. We show that it is possible to design a material for which the stability domain is bounded, in other words, for which shear bands may form under tensile loads. The architecture of this structure leads to multiple band gaps, flat bands, and Dirac cones. The possibility of a Hopf bifurcation is introduced, as related to the presence of follower loads, or nonholonomic constraints, or discontinuity in the constraint curvature. When this instability is implemented in a material, apparently work is produced in a closed strain cycle, so that conservation of energy is apparently violated. As mentioned, this violation is only apparent, as the material is able to “suck and release” energy from the environment. When a material is subject to a Hopf bifurcation, mechanical waves propagate through it without decaying, rather with amplification, because energy is extracted from the surroundings. - 講演者:
- Diego Misseroni (University of Trento, Italy)
- タイトル:
- Origami Metamaterials and Structures: Theoretical Insights and Experimental Validation
- 概要：
-
Origami metamaterials provide a versatile framework for finely tuning mechanical properties through intricate folding arrangements. This seminar unveils two novel experimental setups tailored for probing Poisson effects in deployable metamaterials and delve into the mechanics of Kresling tubes.
The first setup focuses on assessing Poisson's ratio across diverse origami patterns: the standard Miura-ori, Eggbox, the newly devised Morph, and Trimorph pattern. Notably, our investigations showcase the unique capability of Morph pattern to reverse Poisson's ratio sign and exhibit entirely positive or negative values via topological transformations. Our comprehensive analysis, integrating theoretical predictions, simulations, and experimental data, underscores the remarkable tunability of Poisson's ratio in origami metamaterials, solidifying the alignment across these domains.
The second setup is dedicated to unraveling the mechanical intricacies of Kresling tubes, comprising even and odd numbers of Kresling units with both similar and dissimilar chirality. This setup features two fixtures enabling independent control of axial displacement (contraction/expansion) and twist, without imposing constraints on the chiral arrangement of individual cells within the Kresling origami array. The fundamental nature of this work makes it applicable to several field of engineering, including soft robotics and mechanical computing.
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年5月24日（金）13:00-15:00
- 使用言語:
- 日本語
- 講演者:
- 蓑島 敬
- タイトル:
- 2022年3月に発生した太陽高エネルギー粒子イベントのBepiColomboおよびSTELEO衛星による観測と数値モデリング
- 概要：
-
Solar Energetic Particles (SEPs) are high-energy charged particles spanning from a few keV to several GeV, which are generated in association with energetic phenomena on the Sun (flares and CMEs), and then are ejected into interplanetary space. Given that SEPs offer insights into acceleration and transport mechanisms in collisionless plasmas, understanding their origin and dynamics is of great interest to space plasma physics and relevant fields (e.g., high-energy astrophysics and laser plasma physics). Furthermore, accurate prediction of the SEP profile is required for space weather operations, as SEPs exceeding 10 MeV pose primary threats to the space environment, including radio communication failure, malfunction and degradation of equipment onboard aircraft and satellites, and radiation exposure of astronauts during extravehicular activities. This is expected to become more and more important in the future as humanity expands its activity beyond the Earth’s magnetosphere.
Unraveling the SEP physics presents a formidable challenge. On the theoretical front, the rigorous description of SEPs requires a first-principle kinetic treatment while the system scale of the Sun-Earth environment is far beyond the magnetohydrodynamic scale. On the observational front, SEPs have been conventionally observed by in-situ measurement of an individual satellite, resulting in information about physical mechanisms of acceleration and transport of SEPs being convolved during their journey from the Sun to the observation point. Presently, many satellites are operating to measure SEPs at different radii and longitudes, giving a valuable opportunity to integrate theoretical modeling with multi-satellite observations to extract their physical knowledge.
M and X-class flares, along with subsequent CMEs, occurred on 28 and 30 March, 2022. The propagations of CMEs and background solar wind were reconstructed by the global MHD simulation of the heliosphere based on the ground-based interplanetary scintillation (IPS) observation, IPS-SUSANOO. Two associated SEP events were observed simultaneously by BepiColombo at 0.6 AU and STEREO A at 1 AU. Fortunately, both satellites were positioned approximately along the same Parker spiral. In both events, the Solar Particle Monitor onboard BepiColombo/MMO detected 40-200 MeV ions approximately within one hour after the flare, with their peak exhibiting velocity dispersion indicative of ballistic motion. Time-of-flight analysis suggests the expected starting point to be 0.5-0.7 AU from the BepiColombo, close to the Sun. This is corroborated by the same analysis of 1.8-10 MeV ions detected by the Low Energy Telescope (LET) onboard STEREO A. The BepiColombo Environment Radiation Monitor (BERM) detected 1.5 MeV ions, providing a comparative study with 1.8 MeV ions detected by LET. While the difference in their peak times suggests ballistic motion along the magnetic field, the decay profile observed by LET is more prolonged than that observed by BERM. This observation may imply an increase in scattering over time, potentially indicating a reduction in mean free path to below 0.1 AU, as inferred from focused transport simulations. In this paper, we will present detailed observations and numerical modelling results, and discuss potential scenarios for the transport of SEPs between BepiColombo and STEREO locations.
[APL-MAT合同セミナー開催のお知らせ]
- 開催日時：
- 2024年5月23日（木）9:30-10:30
- 使用言語:
- 日本語
- 講演者:
- 松田拓朗（北海道大学）
- タイトル:
- 量子アニーリング方式を用いた偏微分方程式の解法
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年5月22日（水）13:00-15:00
- 使用言語:
- 英語
- 講演者:
- 小澤 歩
- タイトル:
- Development and application of the phase-reduction theory for understanding interacting oscillatory systems
- 概要：
- When oscillatory units interact, they often self-organize and exhibit nontrivial behavior. For example, they adjust their rhythms to each other and yield coherent oscillations. This phenomenon, called synchronization, has been observed in various systems, including the atmosphere-ocean system. In this talk, I will present my past and current research projects on interacting oscillatory systems and my research plan at JAMSTEC. The talk will be organized as follows. First, I will give an overview of my work. I will then focus on my research on feedback control of interacting oscillators. This study is motivated by a wide range of examples of desirable and undesirable synchronized oscillations. Previous studies indicated that a simple global feedback loop can suppress synchronized oscillations by desynchronization, i.e. making the oscillations asynchronous. Later, another study reported that a similar feedback loop may also induce oscillation quenching, where the oscillations of individual oscillators cease. However, it was unclear under what conditions which of the two, desynchronization or oscillation quenching, occurs. We therefore constructed and analyzed a simple mathematical model of oscillators under feedback by utilizing the phase-reduction theory, which allows us to describe the dynamics of one oscillator with a single variable, the phase. The model is simple enough to allow detailed analyses, but its dynamics is rich enough to exhibit both desynchronization and oscillation quenching. We obtained the phase diagrams of the collective state and proposed a methodology to tune the feedback. Next, I will briefly introduce my ongoing project on the collective dynamics of small aquatic animals that exhibit circular trajectories. Lastly, I will explain my main research project at JAMSTEC. In this project, I will develop a phase-reduction theory for delay partial differential equations and apply it to a model of synchronized sea-surface temperatures over two ocean currents to elucidate their interaction.
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年5月15日（水）13:00-15:00
- 使用言語:
- 英語
- 講演者:
- 江口剛
- タイトル:
- Fish schooling and drafting behaviors: Fish can adjust angle of attack and curvature of body to minimize the total force acting on its body
- 概要：
-
Fish schooling has various advantages, one of which is the improvement in hydrodynamic propulsive efficiency. It is said that wake flow induced by tail beat makes reverse Kármán vortices, follower fish could get the advantage of thrust because of the reduction of experienced flow velocity and local pressure. We verified the velocity field of wake flow when fish swim in parallel by using visualization method, Particle Image Velocimetry (PIV). The results demonstrated that the existence of a region of reduction flow velocity, but not enough to explain all the energy-saving in fish schooling. Therefore, we focused on the behaviors, “drafting”.
Drafting is the act of swimming with force transmission among individuals without any physical contact. For example, it has been reported that some fish can follow other individual while swimming with minimal tail beat. These drafting occur in front or side of other individual. We investigated this mechanism by stably reproducing drafting using actual fish and a flat plate or hydrofoil model. The velocity and pressure field around the fish and forces acting on fish body were verified three experimental approach, PIV, Computational Fluid Dynamics (CFD) and model experiments using two-axis load cell. These results showed that fish can balance the anterior-posterior and lateral direction forces acting on its body by having an angle of attack or adjusting curvature of body in response to changes in pressure and flow fields. It is suggested that this mechanism may applied to energy saving in fish schooling.
In this seminar, I introduce the above in detail. In the latter, I explain “Observation of fish schooling dynamics in fish preserve nets” which I have started working on as a postdoctoral researcher, and explain the research plan, and outline of the business trip scheduled for May 16-23.
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年5月10日（金）13:00-15:00
- 使用言語:
- 日本語
- 講演者:
- 安田勇輝（東京工業大学）
- タイトル:
- 黒潮とメキシコ湾流の同期現象のマクスウェル悪魔としての解釈
- 概要：
- 北半球の西岸境界流 (黒潮とメキシコ湾流) の変動メカニズムの解明は、気候力学の理解に不可欠である。Kohyama et al. (2021, Science) は黒潮とメキシコ湾流の海面水温の同期現象を発見し (境界流同期)、大気を介した中緯度海盆間の相互作用 (情報伝達) の重要性を指摘した。一方、ここ10年で、情報理論が非平衡物理学と融合し、情報熱力学が生まれた。情報熱力学は、確率的ノイズにゆらぐ系を部分へ分割し、その部分間の相互作用を情報伝達の視点から解析できる。本研究は、情報熱力学を用いて、境界流同期が「マクスウェル悪魔」と呼ばれる情報処理系として理解できる可能性を示し、気候系における各海流の非対称な役割を議論する。
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年5月8日（水）13:00-15:00
- 使用言語:
- 日本語
- 講演者:
- 井戸悠生（名古屋大学大学院）
- タイトル:
- 億年スケールの鉱物飛跡検出器による未知宇宙線事象探索のための研究開発
- 概要：
-
暗黒物質問題は素粒子物理学のみならず、現代科学における最重要課題の一つである。暗黒物質にはいくつかの条件があるものの、質量などは不明であることから様々なモデルが提案されている。 我々は非常に重い暗黒物質を想定した場合、探索に形成年代が億年スケールとなる白雲母を用いることで素粒子物理学実験領域にブレークスルーを起こせると考えている。
よって本研究では白雲母を素粒子物理学実験における検出器として運用することを目指し、研究開発を行っている。実験により入射粒子エネルギーと飛跡光学像の関係、飛跡形成閾値が段階的に明らかになってきており、今後さらなる検証を進め、実際に現在の探索上限を更新する計画である。
[MATセミナー開催のお知らせ]
- 開催日時：
- 2024年3月13日（水）13:00-14:00
- 使用言語:
- 日本語
- 講演者:
- 西浦泰介
- タイトル:
- 『DEPTH』
- 概要：
- 大規模粒子法シミュレーションプログラム『DEPTH』は現在Ver.3までアップグレードされ、JAMSTECおよびアドバンスソフト（株）から販売されており、DEMによる粒状体および剛体シミュレーション、SPHおよびCFDによる流体シミュレーション、DEM-SPHおよびDEM-CFDによる混相流シミュレーションを行うことができる。本セミナーでは『DEPTH』の開発状況と販売実績を紹介する。