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233822 results in Physics and Astronomy

Page 15 of 11692

  • Pressure fluctuations of liquids under short-time acceleration

  • Chihiro Kurihara, Akihito Kiyama, Yoshiyuki Tagawa
  • Journal:
    Journal of Fluid Mechanics / Volume 1003 / 25 January 2025
    Published online by Cambridge University Press:
    16 January 2025, A20
    • Article
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      • Open access
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  • This study investigates experimentally the pressure fluctuations of liquids in a column under short-time acceleration. It demonstrates that the Strouhal number $St=L/(c\,\Delta t)$, where $L$, $c$ and $\Delta t$ are the liquid column length, speed of sound, and acceleration duration, respectively, provides a measure of the pressure fluctuations for intermediate $St$ values. On the one hand, the incompressible fluid theory implies that the magnitude of the averaged pressure fluctuation $\bar {P}$ becomes negligible for $St\ll 1$. On the other hand, the water hammer theory predicts that the pressure tends to $\rho cu_0$ (where $u_0$ is the change in the liquid velocity) for $St\geq O(1)$. For intermediate $St$ values, there is no consensus on the value of $\bar {P}$. In our experiments, $L$, $c$ and $\Delta t$ are varied so that $0.02 \leq St \leq 2.2$. The results suggest that the incompressible fluid theory holds only up to $St\sim 0.2$, and that $St$ governs the pressure fluctuations under different experimental conditions for higher $St$ values. The data relating to a hydrogel also tend to collapse to a unified trend. The inception of cavitation in the liquid starts at $St\sim 0.2$ for various $\Delta t$, indicating that the liquid pressure goes lower than the liquid vapour pressure. To understand this mechanism, we employ a one-dimensional wave propagation model with a pressure wavefront of finite thickness that scales with $\Delta t$. The model provides a reasonable description of the experimental results as a function of $St$.

  • 8 - Hartree–Fock Approximation

  • Gianluca Stefanucci, Università degli Studi di Roma 'Tor Vergata', Robert van Leeuwen, university of jyväskylä, Finland
  • Book:
    Nonequilibrium Many-Body Theory of Quantum Systems
    Published online:
    02 January 2025
    Print publication:
    16 January 2025, pp 215-234

  • Summary

    The Green’s function method is among the most powerful and versatile formalisms in physics, and its nonequilibrium version has proved invaluable in many research fields. With entirely new chapters and updated example problems, the second edition of this popular text continues to provide an ideal introduction to nonequilibrium many-body quantum systems and ultrafast phenomena in modern science. Retaining the unique and self-contained style of the original, this new edition has been thoroughly revised to address interacting systems of fermions and bosons, simplified many-body approaches like the GKBA, the Bloch equations, and the Boltzmann equations, and the connection between Green’s functions and newly developed time-resolved spectroscopy techniques. Small gaps in the theory have been filled, and frequently overlooked subtleties have been systematically highlighted and clarified. With an abundance of illustrative examples, insightful discussions, and modern applications, this book remains the definitive guide for students and researchers alike.

  • 13 - Linear Response Theory

  • Gianluca Stefanucci, Università degli Studi di Roma 'Tor Vergata', Robert van Leeuwen, university of jyväskylä, Finland
  • Book:
    Nonequilibrium Many-Body Theory of Quantum Systems
    Published online:
    02 January 2025
    Print publication:
    16 January 2025, pp 333-356

  • Summary

    The Green’s function method is among the most powerful and versatile formalisms in physics, and its nonequilibrium version has proved invaluable in many research fields. With entirely new chapters and updated example problems, the second edition of this popular text continues to provide an ideal introduction to nonequilibrium many-body quantum systems and ultrafast phenomena in modern science. Retaining the unique and self-contained style of the original, this new edition has been thoroughly revised to address interacting systems of fermions and bosons, simplified many-body approaches like the GKBA, the Bloch equations, and the Boltzmann equations, and the connection between Green’s functions and newly developed time-resolved spectroscopy techniques. Small gaps in the theory have been filled, and frequently overlooked subtleties have been systematically highlighted and clarified. With an abundance of illustrative examples, insightful discussions, and modern applications, this book remains the definitive guide for students and researchers alike.

  • 12 - Conserving Approximations

  • Gianluca Stefanucci, Università degli Studi di Roma 'Tor Vergata', Robert van Leeuwen, university of jyväskylä, Finland
  • Book:
    Nonequilibrium Many-Body Theory of Quantum Systems
    Published online:
    02 January 2025
    Print publication:
    16 January 2025, pp 313-332

  • Summary

    The Green’s function method is among the most powerful and versatile formalisms in physics, and its nonequilibrium version has proved invaluable in many research fields. With entirely new chapters and updated example problems, the second edition of this popular text continues to provide an ideal introduction to nonequilibrium many-body quantum systems and ultrafast phenomena in modern science. Retaining the unique and self-contained style of the original, this new edition has been thoroughly revised to address interacting systems of fermions and bosons, simplified many-body approaches like the GKBA, the Bloch equations, and the Boltzmann equations, and the connection between Green’s functions and newly developed time-resolved spectroscopy techniques. Small gaps in the theory have been filled, and frequently overlooked subtleties have been systematically highlighted and clarified. With an abundance of illustrative examples, insightful discussions, and modern applications, this book remains the definitive guide for students and researchers alike.

  • 4 - The Contour Idea

  • Gianluca Stefanucci, Università degli Studi di Roma 'Tor Vergata', Robert van Leeuwen, university of jyväskylä, Finland
  • Book:
    Nonequilibrium Many-Body Theory of Quantum Systems
    Published online:
    02 January 2025
    Print publication:
    16 January 2025, pp 93-122

  • Summary

    The Green’s function method is among the most powerful and versatile formalisms in physics, and its nonequilibrium version has proved invaluable in many research fields. With entirely new chapters and updated example problems, the second edition of this popular text continues to provide an ideal introduction to nonequilibrium many-body quantum systems and ultrafast phenomena in modern science. Retaining the unique and self-contained style of the original, this new edition has been thoroughly revised to address interacting systems of fermions and bosons, simplified many-body approaches like the GKBA, the Bloch equations, and the Boltzmann equations, and the connection between Green’s functions and newly developed time-resolved spectroscopy techniques. Small gaps in the theory have been filled, and frequently overlooked subtleties have been systematically highlighted and clarified. With an abundance of illustrative examples, insightful discussions, and modern applications, this book remains the definitive guide for students and researchers alike.

  • 5 - Nonequilibrium Green’s Functions

  • Gianluca Stefanucci, Università degli Studi di Roma 'Tor Vergata', Robert van Leeuwen, university of jyväskylä, Finland
  • Book:
    Nonequilibrium Many-Body Theory of Quantum Systems
    Published online:
    02 January 2025
    Print publication:
    16 January 2025, pp 123-146

  • Summary

    The Green’s function method is among the most powerful and versatile formalisms in physics, and its nonequilibrium version has proved invaluable in many research fields. With entirely new chapters and updated example problems, the second edition of this popular text continues to provide an ideal introduction to nonequilibrium many-body quantum systems and ultrafast phenomena in modern science. Retaining the unique and self-contained style of the original, this new edition has been thoroughly revised to address interacting systems of fermions and bosons, simplified many-body approaches like the GKBA, the Bloch equations, and the Boltzmann equations, and the connection between Green’s functions and newly developed time-resolved spectroscopy techniques. Small gaps in the theory have been filled, and frequently overlooked subtleties have been systematically highlighted and clarified. With an abundance of illustrative examples, insightful discussions, and modern applications, this book remains the definitive guide for students and researchers alike.

  • 9 - Kadanoff–Baym Equations

  • Gianluca Stefanucci, Università degli Studi di Roma 'Tor Vergata', Robert van Leeuwen, university of jyväskylä, Finland
  • Book:
    Nonequilibrium Many-Body Theory of Quantum Systems
    Published online:
    02 January 2025
    Print publication:
    16 January 2025, pp 235-256

  • Summary

    The Green’s function method is among the most powerful and versatile formalisms in physics, and its nonequilibrium version has proved invaluable in many research fields. With entirely new chapters and updated example problems, the second edition of this popular text continues to provide an ideal introduction to nonequilibrium many-body quantum systems and ultrafast phenomena in modern science. Retaining the unique and self-contained style of the original, this new edition has been thoroughly revised to address interacting systems of fermions and bosons, simplified many-body approaches like the GKBA, the Bloch equations, and the Boltzmann equations, and the connection between Green’s functions and newly developed time-resolved spectroscopy techniques. Small gaps in the theory have been filled, and frequently overlooked subtleties have been systematically highlighted and clarified. With an abundance of illustrative examples, insightful discussions, and modern applications, this book remains the definitive guide for students and researchers alike.

  • 14 - Two-Particle Green’s Function: Diagrammatic Expansion

  • Gianluca Stefanucci, Università degli Studi di Roma 'Tor Vergata', Robert van Leeuwen, university of jyväskylä, Finland
  • Book:
    Nonequilibrium Many-Body Theory of Quantum Systems
    Published online:
    02 January 2025
    Print publication:
    16 January 2025, pp 357-410

  • Summary

    The Green’s function method is among the most powerful and versatile formalisms in physics, and its nonequilibrium version has proved invaluable in many research fields. With entirely new chapters and updated example problems, the second edition of this popular text continues to provide an ideal introduction to nonequilibrium many-body quantum systems and ultrafast phenomena in modern science. Retaining the unique and self-contained style of the original, this new edition has been thoroughly revised to address interacting systems of fermions and bosons, simplified many-body approaches like the GKBA, the Bloch equations, and the Boltzmann equations, and the connection between Green’s functions and newly developed time-resolved spectroscopy techniques. Small gaps in the theory have been filled, and frequently overlooked subtleties have been systematically highlighted and clarified. With an abundance of illustrative examples, insightful discussions, and modern applications, this book remains the definitive guide for students and researchers alike.

  • Emergent asymmetry in confined bioconvection

  • Martin A. Bees, Prasad Perlekar
  • Journal:
    Journal of Fluid Mechanics / Volume 1003 / 25 January 2025
    Published online by Cambridge University Press:
    16 January 2025, A21

  • Bioconvection is the prototypical active matter system for hydrodynamic instabilities and pattern formation in suspensions of biased swimming microorganisms, particularly at the dilute end of the concentration spectrum where direct cell–cell interactions are less relevant. Confinement is an inherent characteristic of such systems, including those that are naturally occurring or industrially exploited, so it is important to understand the impact of boundaries on the hydrodynamic instabilities. Despite recent interest in this area, we note that commonly adopted symmetry assumptions in the literature, such as for a vertical channel or pipe, are uncorroborated and potentially unjustified. Therefore, by employing a combination of analytical and numerical techniques, we investigate whether confinement itself can drive asymmetric plume formation in a suspension of bottom-heavy swimming microorganisms (gyrotactic cells). For a class of solutions in a vertical channel, we establish the existence of a first integral of motion, and reveal that asymptotic asymmetry is plausible. Furthermore, numerical simulations from both Lagrangian and Eulerian perspectives demonstrate with remarkable agreement that asymmetric solutions can indeed be more stable than symmetric; asymmetric solutions are, in fact, dominant for a large, practically important region of parameter space. In addition, we verify the presence of blip and varicose instabilities for an experimentally accessible parameter range. Finally, we extend our study to a vertical Hele-Shaw geometry to explore whether a simple linear drag approximation can be justified. We find that although two-dimensional bioconvective structures and associated bulk properties have some similarities with experimental observations, approximating near-wall physics in even the simplest confined systems remains challenging.

  • Stability of cylindrical, multicomponent vesicles

  • Anirudh Venkatesh, Aman Bhargava, Vivek Narsimhan
  • Journal:
    Journal of Fluid Mechanics / Volume 1003 / 25 January 2025
    Published online by Cambridge University Press:
    16 January 2025, A18
    • Article
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      • Open access
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  • Vesicles are important surrogate structures made up of multiple phospholipids and cholesterol distributed in the form of a lipid bilayer. Tubular vesicles can undergo pearling – i.e. formation of beads on the liquid thread akin to the Rayleigh–Plateau instability. Previous studies have inspected the effects of surface tension on the pearling instabilities of single-component vesicles. In this study, we perform a linear stability analysis on a multicomponent cylindrical vesicle. We solve the Stokes equations along with the Cahn–Hilliard equation to develop the linearized dynamic equations governing the vesicle shape and surface concentration fields. This helps us to show that multicomponent vesicles can undergo pearling, buckling and wrinkling even in the absence of surface tension, which is a significantly different result from studies on single-component vesicles. This behaviour arises due to the competition between the free energies of phase separation, line tension and bending for this multi-phospholipid system. We determine the conditions under which axisymmetric and non-axisymmetric modes are dominant, and supplement our results with an energy analysis that shows the sources for these instabilities. Lastly, we delve into a weakly nonlinear analysis where we solve the nonlinear Cahn–Hilliard equation in the weak deformation limit to understand how mode-mixing alters the late time dynamics of coarsening. We show that in many situations, the trends from our simulations qualitatively match recent experiments (Yanagisawa et al., Phys. Rev. E, vol. 82, 2010, p. 051928).

Page 15 of 11692