投稿者: kyoryu

  • 流体力学セミナー 8/17

    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)

  • 流体力学セミナー 8/3

    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.

  • 流体力学セミナー 8/14

    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

  • 流体力学セミナー 7/27

    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

                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.  
  • 論文紹介2022/11/15

    皆様

    M2の松村です。

    11/15の論文紹介のお知らせです。

    日時: 11/15 15:00〜 (?)
    場所: 理学研究科5号館413とZoomのハイブリッド
    論文: A. Alexakis, “Searching for the fastest dynamo: Laminar ABC flows,” Phys. Rev. E 84, 026321 (2011).
    doi: https://doi.org/10.1103/PhysRevE.84.026321

    概要: 2つの異なった強制スケールにおいて、Arnold-Beltrami-Childress flowの族のダイナモの数値シミュレーションを行った。ダイナモの作用が最大となる流れのパラメータを調べ、流れ場の構造について議論を行った。

  • 流体物理学ゼミナール2022/11/15

    皆さま
    
    流体研 M1 の金沢です。
    
    11/15 の流体物理学ゼミを以下の通り行います。
    
    日時: 11月15日 13:30~
    場所: 理学研究科5号館413とZoom のハイブリッド
    発表者: 金沢
    教科書: Introductory Fluid Mechanics-Cambridge University Press (2010)
    Chapter 5.9.4 “Network of Pipes” からスタートします。
    
    以上よろしくお願いします。
  • 流体物理学ゼミナール2022/11/8

    皆さま

    流体研 M1 の金沢です。

    11/8 の流体物理学ゼミを以下の通り行います。

    日時: 11月8日 13:30~

    発表者: 金沢

    教科書: Introductory Fluid Mechanics-Cambridge University Press (2010) Chapter 5.9.1 “Simple Pump Model” からスタートします。

    以上よろしくお願いします。

  • 流体物理学ゼミナール2022/11/1

    皆さま

    流体研 M1 の金沢です。

    11/1 の流体物理学ゼミを以下の通り行います。

    日時: 11月1日 13:30~

    発表者: 金沢

    教科書: Introductory Fluid Mechanics-Cambridge University Press (2010) Chapter 5.4 “Fully Developed Laminar Flow between Two Concentric Circular Pipes” からスタートします。

    以上よろしくお願いします。

  • 流体物理学ゼミナール2022/10/25

    皆さま
    
    流体研 M1 の金沢です。
    
    10/25 の流体物理学ゼミを以下の通り行います。
    
    日時: 10月25日 13:30~
    発表者: 金沢
    教科書: Introductory Fluid Mechanics-Cambridge University Press (2010)
    Chapter 5.4 “Laminar Flow in Circular Pipes (The Hagen-Poiseuille Flow)” の続きからスタートします。
    
    
    以上よろしくお願いします。