JST CREST 予測数学基盤 「決定論的支配方程式の非線形・散逸・偶然性の動力学」第1回セミナーを，本流体物理学グループセミナーと合同開催します。

JST CREST 予測数学基盤「決定論的支配方程式の非線形・散逸・偶然性の動力学」

第1回セミナー  

(京大理学部物理教室の流体物理学セミナーとの合同開催)

【研究趣旨】

流れは決定論的な支配方程式に従っているものの、非線形性、散逸性、偶然性という三要素を有し、これらの現象を理解し制御することは一般には困難です。特に「実問題へ適用した際の従来の数理科学理論の限界」についてはあまり議論されてきていません。本研究提案では、数学と工学の融合領域として、最新の数理科学により実問題としての物理現象の解明と制御、将来的には異分野への応用を進めたいと考えています。(代表: 焼野藍子(東北大学流体科学研究所)，主たる共同研究者: 狩野隼輔(東北大学数理科学共創社会センター)，石川勲(愛媛大学データサイエンスセンター))

【オンライン参加をお申し込みの方はこちら】

申し込みフォーム：

https://forms.gle/yoY1fexbVpKvgbk29

※Googleフォームへアクセスできない場合は，メールにて連絡いただければ幸いです．

【開催概要】

日時：2024年11月25日(月) 15時00分〜18時00分（18時30分以降：意見交換会）

方法：ハイブリッド開催（現地：京都大学理学研究科5号館401室、オンライン：Teams）

招待講演：

東大工学系研究科航空宇宙工学専攻D3 小島直泰さん (40分)

「粘性圧縮性流体と弾性構造の連成問題の縮約モデル」

東大工学系研究科航空宇宙工学専攻M2 藤野献さん（40分）

「異なる周波数現象間のNonstationary流体解析」

早稲田大学大学院 基幹理工学研究科D3 藤井愛実さん (40分)

「極超音速エンジンの実現に向けたCFDの課題と衝撃波振動現象のモデル化」

京大基礎物理学研究所D1 三浦憂さん (1時間)

「非エルミート行列の物理学」

【問い合わせ先】

東北大学流体科学研究所 准教授 焼野 藍子

E-mail: aiko.yakeno*tohoku.ac.jp  (*を@に置き換えてください)

ICIAMのため8月に来日されるSamriddhi Sanker RayさんとStatphysのため8月に来日されるLuca Biferaleさんが京大理物理にも来訪されて以下のセミナーを8/17午後に連続して行います。ご興味のある方のご来聴を歓迎いたします。

日時: ８月１７日（木） １３：１５ から １４：４５

場所: 京大 理学研究科５号館(物理学教室) ４０１号室

講師： Samriddhi Sanker Ray (ICTS Bangalore)

講演題目： In what sense are bacterial suspensions turbulent?

講演要旨:
Active turbulence — the spatio-temporally complex motion of a dense suspension of microorganisms such as bacteria — has gathered great traction recently as an intriguing class of emergent, complex flows, occurring in several living systems at the mesoscale, whose understanding lies at the interface of non-equilibrium physics and biology. However, are these low Reynolds number living flows really turbulent or just chaotic with structural, or even superficial, similarities with high Reynolds number (classical) inanimate turbulence? This is a vital question as the fingerprints of classical turbulence —– universality, intermittency and chaos —– makes it unique amongst the many different driven-dissipative systems. In this talk we address these questions with a focus on the issues of (approximate) scale-invariance, intermittency and maximally chaotic states and how they lead to anomalous diffusion in bacterial suspensions. In particular, we show the existing of a critical level of activity beyond which the physics of bacterial flows become universal, accompanied by maximally chaotic states which allow for efficient, Levy-walk mediated foraging strategies.

日時: ８月１７日（木） １５：００ から １６：３０

場所: 京大 理学研究科５号館(物理学教室) ４０１号室

講師： Luca Biferale (University of Rome ‘Tor Vergata’)

講演題目： Data-driven and equations-informed tools for modelling turbulent flows

講演要旨:
Our ability to collect data is rapidly increasing thanks to computational power and the unprecedented diversity of sensors. But how good are we at extracting, reconstructing, and understanding information from them? We present a short overview of some recent advancements for data-assimilation and modelling of turbulent multi-scale flows using both data-driven and equations-informed tools, starting from sparse and heterogeneous observations of complex fluid systems. Issues connected to validations and benchmarks in the presence of full or partial observability will be discussed. A few examples of data-augmentation based on Generative Adversarial Learning [1], Nudging, Gappy-Proper Orthogonal Decomposition [2] and Generative Diffusion Models will be quantitatively discussed [3].

[1] M. Buzzicotti, F. Bonaccorso, P. Clark Di Leoni, L. Biferale. Reconstruction of turbulent data with deep generative models for semantic inpainting from TURB-Rot database. Physical Review Fluids 6 (5), 050503, (2021).
[2] T. Li, M. Buzzicotti, L. Biferale, F. Bonaccorso, S. Chen, M. Wan. Data reconstruction of turbulent flows with Gappy POD, Extended POD and Generative Adversarial Networks. arXiv:2210.11921v1(2022) Journal Fluid Mechanics (in press)
[3] T Li, L Biferale, F Bonaccorso. M Scarpolini and M Buzzicotti, Sunthetic Lagrangian Turbulence by Generayive Diffusion Models, arxive 2307.08529 (2023)

Statphysのため8月に来日されるSiddhartha Mukherjeeさんが京大理物理にも来訪されて以下のセミナーを行います。ご興味のある方のご来聴を歓迎いたします。

日時: ８月３日（木） １３：３０ から １５：００

１６：３０から１８：００

(講師の都合により時間変更になりました)

場所: 京大 理学研究科５号館(物理学教室) ４０１号室

講師： Siddhartha Mukherjee (ICTS Bangalore)

講演題目： Aspects of Turbulence Organization: Emergent Structures, Vortex-Lines and Local Multifractality

The description of turbulence field organization from a “structures” perspective has been a long-standing challenge. While individual realizations of turbulence are rich in various kinds of structure, possibly “at all scales”, much of our knowledge is either limited to the statistical structure of ensembles of realizations, or to some eduction techniques suited for specialized applications. In this talk, I will take you through three aspects of turbulence structure and organization that we are trying to address. (1.) We shall see how a generalized correlations framework can be used to identify and seek insight about a range of velocity and vorticity field structures in turbulence. Coupling this with a Helmholtz-decomposition paradigm then allows disentangling individual structures and also entire fields from superposition. Such an exercise reveals some very intriguing features of the high kinetic energy structures, suggestive of an emergent nature in turbulence structures. (2.) We shall then move onto a vortex-lines based approach aimed at identifying coherent vortex-tubes. These long and slender objects can be in complex tangles, playing a key role in turbulence dynamics, via vortex-reconnections, stretching and interactions. Much of their geometrical features remain uncharted, like the distribution of their lengths, persistence profiles and links with dynamical aspects of the flow. We show how this can be solved by extracting individual vortex-lines. (3.) Finally, we shall move onto the subject of singular structures in turbulence fields, linked to intermittency and anomalous scaling and dissipation. While the multifractal formalism allows estimating a range of scaling exponents of the flow, the method remains largely reductive, and by its very construction precludes any conception of spatial variation in multifractality. We now show that this technique can be simply adapted as a local analysis, thereby revealing entire fields of generalized dimensions, and hence multifractal statistics of the small neighbourhood around any point in space. This throws up surprising findings that revise our understanding of the multifractal structure of the dissipation field. These topics, I hope, will make the case for studying individual fields in their own right to uncover their constituent structures, and inspire us to go beyond our usual set of lenses for studying turbulence.

Statphysのため8月に来日されるFlorian Kogelbauerさんが京大に来訪されて以下のセミナーを行います。ご興味のある方のご来聴を歓迎いたします。

日時: ８月１４日（月） １５：００ から １７：００

場所: 京大 理学研究科５号館(物理学教室) ４０１号室

講師： Florian Kogelbauer (ETH Zurich)

講演題目： Exact Hydrodynamics from Kinetic Theory

講演要旨：
The closure problem for kinetic equations constitutes one of the fundamental questions in thermodynamics. Based on the refined spectral theory of several kinetic models, involving a finite-dimensional Grad system, the Boltzmann-BGK equation and the Shakhov model, we give exact closure relations and transport operators for the hydrodynamic moments (density, velocity and pressure). This allows us to derive a new, non-local fluid model for gaseous fluid in the linear regime, which extends classical models such as the Navier—Stokes equation.

Refs:
https://arxiv.org/pdf/2306.07103.pdf
https://arxiv.org/pdf/2305.06612.pdf
https://arxiv.org/pdf/2301.03069.pdf

2023年7月下旬から8月上旬まで東大生産研の横井喜充先生のところに滞在されるDhrubaditya Mitra さんが京大に来訪されて以下のセミナーを行います。ご興味のある方のご来聴を歓迎いたします。

日時: ７月２７日（木） １５：００ から １６：３０

場所: 京大 理学研究科５号館(物理学教室) ４０１号室

講師： Dhrubaditya Mitra (NORDITA Stockholm)

講演題目： Entropy and active elasticity

講演要旨：
The traditional field of continuum mechanics, which contains both elasticity and fluid mechanics is going through a resurgance due to possible applications to biology. Biological systems bring in two novel aspects : (a) biological samples are complex — often they cannot be clearly demarcated into solid and fluid. They may be viscoelastic. (b) biological samples are living. This second aspect is the most important one. The fundamental aspect of a living systems is that they are out of equilibrium — they generate entropy. This poses the question : how do the equations of elasticity and fluid mechanics must change to take into account the fundamental non-equilibrium nature of these systems. Within this broad framework, I will present two problems. One, how to estimate the entropy generation rate, which is a measure of activity, of living systems. Two, how the buckling of shells, a traditional non-linear problem in elasticity, change if the shells are made active.

Refs :

• Active buckling of pressurized spherical shells: Monte Carlo Simulation, V Agrawal, V Pandey, and D Mitra, arXiv:2206.14172
• Estimate of entropy generation rate can spatiotemporally resolve the active nature of cell flickering, SK Manikandan, T Ghosh, T Mandal, A Biswas, B Sinha, and D Mitra, arXiv:2205.12849

            流　体　力　学　セ　ミ　ナ　ー

日時:  ３月１３日（月） １０：３０ から １２：００

場所:  京大 理学研究科５号館(物理学教室) ４１３号室

講師： Professor Mikhael Gorokhovski
       Laboratoire de Mécanique des Fluides et d’Acoustique
       École Centrale de Lyon      

講演題目： Stochastic subgrid acceleration models in LES of two-phase flows. 

講演要旨：
It was recently recognized that the turbulence at the high Reynolds number is characterized by quick peaks in the velocity signal, and the corresponding velocity gradients may attain intermittently very large values. This is a manifestation of intense and long-lived flow structures at small spatial scales.  However, in practical computations of highly turbulent flows, the smallest scales are filtered, and consequently, the effects of intermittency are mainly neglected. In this talk we propose to discuss an approach to account the sub-filtered effects of intermittency in the stochastic way. The idea is to force the filtered Navier-Stokes equations by the stochastic acceleration term with prescribed properties known from the experimental and DNS statistics of the Lagrangian acceleration. The stochastic model for this acceleration contains the lognormal process for its norm (with long-range correlations), and the model for the acceleration direction (with short range correlations). The latter represents the Ornstein-Uhlenbeck process on the unit sphere with relaxation to the locally resolved vorticity vector. Both models – one for the norm and another for the direction of acceleration - depend on the local Reynolds number. The model is referred to as LES-SSAM (Stochastic Subgrid Acceleration Model). In this talk, the fluid mechanics from LES-SSAM is assessed by DNS and is considered as a background turbulent flow in the following situations: 
- The box turbulence laden by heavy particles; DNS/LES-SSAM. The assessment of 
single and two time statistics of the particle acceleration and velocity illustrated clearly the advantage of the proposed flow model in predictions of the non-Gaussian statistics and autocorrelations of heavy particles. 
- The homogeneous shear flow laden by heavy particles; DNS/LES-SSAM. Here we will show that the volume averaged acceleration norm of particle exhibits a pseudo-cyclic behavior, which is a signature of the periodic action of largest confined vortexes against the mean shear. Another interesting discussion will concern the preferential orientation of the particle acceleration given by mean shear-induced vortical structures. These both principal findings may have relation to the turbulence-induced intermittent structures in clouds.
- Turbulent incompressible flows with interface. Here the possibility of combination of LES-SSAM with the Volume of Fluid (VoF) approach will be discussed and illustrated for primary atomization process.  

流　体　力　学　セ　ミ　ナ　ー

(京都大学応用数学セミナー(KUAMS)との共催)

日時:  ９月１４日（金） １５：００ から １６：３０

場所:  京大 理学研究科 物理学教室(理５号館)　４０１号室

講師： Uriel Frisch 氏
Laboratoire Lagrange, Observatoire and Universite Cote d’Azur
Nice, France

講演題目：The mathematical and numerical construction of
turbulent solutions for the 3D incompressible Euler
equation and its perspectives

講演要旨：

Starting with Kolmogorov’s 1941 (K41) work, infinite Reynolds number
flow is known to have velocity increments over a small distance r that
vary roughly as the cubic root of r. Formally,  such  flow  is  expected  to
satisfy  Euler’s  partial differential  equation,  but  the  flow  being
not  spatially differentiable, the equation is satisfied only in
a distributional sense. Since Leray’s 1934 work, such solutions are called
weak. Actually  they  were  already  present  –very  briefly–  in
Lagrange’s 1760/1761 work on non-smooth solutions of the wave equation.
A  major  breakthrough  has  happened  recently: mathematicians  succeeded
in  constructing  rigourously  weak solutions  of  the  Euler  equation
whose  spatial  regularity  –measured by their Hölder continuity exponent–
is arbitrarily close to the value predicted by K41 (Isett 2018), Buckmaster et
al. 2017). Furthermore these solutions present the anomalous energy dissipation
investigated by Onsager in 1949 (Ons49). We shall highlight some aspects of
the derivation of these results which took about ten years and was started
originally by Camillo de Lellis and Laszlo Szekelyhidi and continued with a
number  of  collaborators.  On  the  mathematical  side  the derivation makes
use of techniques developed by Nash (1954) for isometric embedding and by Gromov
(1986, 2017) for convex  integration.  Fortunately,  many  features  of  the
derivation  have  a  significant  fluid  mechanical  content.  In particular
the successive introduction of finer and finer flow structures, called Mikados
by Daneri and Szekelyhidi (2017) because  they  are  slender  and jetlike.
The Mikados generate Reynolds stresses on larger scales; they can be chosen
to cancel discrepancies between approximate and exact solutions of the Euler
equation. A particular engaging aspect of the construction of weak solutions
is its flexibility. The Mikados can be chosen not only to  reproduce  K41/Ons49
selfsimilar  turbulence,  but  also  to synthesize  a  large  class  of
turbulent  flows,  possessing,  for example,  small-scale  intermittency
and  multifractal  scaling. This huge playground must of course be explored
numerically for testing all manners of physical phenomena and theories, a
process being started in a collaboration between Leipzig, Nice, Kyoto and Rome.

(in collaboration with　Laszlo Szekelyhidi,Department of Mathematics,
University of Leipzig, Germany　and　Takeshi Matsumoto,Department of
Physics, Kyoto University, Japan)

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世話人：山田 道夫(京大数理研)， 竹広 真一（京大数理研），
藤 定義(京大理)，松本 剛(京大理)
連絡先：山田道夫 yamada_at_kurims.kyoto-u.ac.jp
=========================================================

皆様、6/26日(月)の流体力学セミナーを御案内いたします。
御来聴歓迎いたします。

日時：2016年 6月 26日（月） 15:00 から 16:30

場所： 京都大学 数理解析研究所 204号室

講師： Prof. Tamer A. Zaki
（Mechanical Engineering, Johns Hopkins University）

講演題目：
LARGE-SCALE STRUCTURES IN TRANSITIONAL AND TURBULENT BOUNDARY LAYERS

講演要旨：
In transitional and turbulent boundary layers, an interface separates
the non-turbulent and turbulent regions of the flow. In the transition
zone, this interface surrounds localized patches of turbulence that
are formed sporadically and spread to consume the surrounding laminar
flow. In the turbulent regime, the interface marks the free-stream
edge of the boundary layer and separates the vortical near-wall motion
from the outer potential flow. Using direct numerical simulations, we
examine characteristics of this turbulent/non-turbulent (T/NT)
interface. In the case of transition we relate its formation, or the
inception of turbulence patches, to the streaky structures known as
Klebanoff distortions. In the fully turbulent boundary layer,
statistical evidence is presented that the interface is locally
modulated by the large-scale velocity perturbations in both the
streamwise and spanwise directions. The modulation is different when
the coherent structure is high- versus low-speed motion: high-speed
structures lead to a wedgeshaped deformation of the T/NT interface,
which causes an anti-correlation between the angles of the interface
and the internal shear layer. On the other hand, low-speed structures
are correlated with crests in the interface. Finally, the sudden
changes in turbulence statistics across the interface are in line with
the changes in the population of low-speed structures, which consist
of slower mean streamwise velocity and stronger turbulence than the
high-speed counterparts.

+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–

世話人：山田 道夫(京大数理研)， 竹広 真一（京大数理研），
　　　　藤 定義(京大)，松本 剛(京大理)
アドバイザー：水島 二郎(同志社大工)， 余田 成男（京大理）
連絡先：山田道夫 yamada_at_kurims.kyoto-u.ac.jp
=========================================================

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_/
_/ 流体力学セミナー ２００４ No. ３
_/
_/ 日 時 : ０４年 ６月 ７日 (月) １５：００〜１６：３０
_/
_/ 場 所 : 京大数理研 ００９号室
_/
_/ 講 師 : Baofeng Feng 氏 (Department of Mathematics,
_/ University of Texas-Pan American)
_/
_/ 題 目 : Application of variational methods in fiber-optic communication
_/ systems
_/
_/ 内 容 :
_/ In this talk, we apply the variational methods to study the propagation
_/ properties of dispersion management (DM) optical solitons, as well
_/ as the interactions between them, in fiber-optic communication systems.
_/ As a result, a set of ordinary differential equations (ODEs) is obtained to
_/ predict the dynamical behavior of optical solitons instead of the original
_/ patial differential equations (PDEs). The original PDEs are also solved
_/ numerically by a symmetrized split-step pseudospectral methods and the
_/ results agreed with predictions of the variational methods.
_/
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世話人：大木谷 耕司(京大数研)、藤 定義(京大理)、松本 剛(京大理)、
山田 道夫(京大数研)
アドバイザー：河原 源太(京大工)、小森 悟（京大工）、藤坂博一（京大情報学）、
船越 満明（京大情報学）、水島 二郎(同志社大工)、余田 成男（京大理）
連絡先：ohkitani@kurims.kyoto-u.ac.jp
メールリスト連絡先： semi-adm@kyoryu.scphys.kyoto-u.ac.jp

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_/
_/ 流体力学セミナー ２００４ No. ８
_/
_/ 日 時 : ０４年 １１月 ８日 (月) １５：００〜１６：３０
_/
_/ 場 所 : 京大数理研 ００９号室
_/
_/ 講 師 : 坂本 圭 氏 （京大大学院 理学研究科 地球物理学）
_/
_/ 題 目 : 潮流によって形成される海底境界層の不安定とその混合効果
_/
_/ 内 容 :
_/ 回転系・渦粘性係数一定のもとで、時間振動流である潮流によって形成される
_/ 海底境界層は、定常流による海底エクマン層と相似な流速構造を持ちます。
_/ 高レイノルズ数においては、エクマン層が不安定であるのと同様に、潮流による
_/ この境界層も不安定になると考えられます。そこで、この境界層の安定性を調べ
_/ るために数値実験を行ったところ、慣性周期と潮流周期の比に依存して、3つの
_/ 異なったタイプの不安定が現れました。また、成層を導入した実験から、慣性
_/ 周期と潮流周期がほぼ等しく成層が弱い極海域で、境界層の不安定によって効果
_/ 的な鉛直混合が引き起こされうることを示します。
_/
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世話人：大木谷 耕司(京大数研)、藤 定義(京大理)、松本 剛(京大理)、
山田 道夫(京大数研)
アドバイザー：河原 源太(京大工)、小森 悟（京大工）、藤坂博一（京大情報学）、
船越 満明（京大情報学）、水島 二郎(同志社大工)、余田 成男（京大理）
連絡先：ohkitani@kurims.kyoto-u.ac.jp
メールリスト連絡先： semi-adm@kyoryu.scphys.kyoto-u.ac.jp