104 research outputs found
Critical dynamics of an isothermal compressible non-ideal fluid
A pure fluid at its critical point shows a dramatic slow-down in its
dynamics, due to a divergence of the order-parameter susceptibility and the
coefficient of heat transport. Under isothermal conditions, however, sound
waves provide the only possible relaxation mechanism for order-parameter
fluctuations. Here we study the critical dynamics of an isothermal,
compressible non-ideal fluid via scaling arguments and computer simulations of
the corresponding fluctuating hydrodynamics equations. We show that, below a
critical dimension of 4, the order-parameter dynamics of an isothermal fluid
effectively reduces to "model A," characterized by overdamped sound waves and a
divergent bulk viscosity. In contrast, the shear viscosity remains finite above
two dimensions. Possible applications of the model are discussed.Comment: 19 pages, 7 figures; v3: minor corrections and clarifications; as
published in Phys. Rev.
Transport Properties of the Quark-Gluon Plasma -- A Lattice QCD Perspective
Transport properties of a thermal medium determine how its conserved charge
densities (for instance the electric charge, energy or momentum) evolve as a
function of time and eventually relax back to their equilibrium values. Here
the transport properties of the quark-gluon plasma are reviewed from a
theoretical perspective. The latter play a key role in the description of
heavy-ion collisions, and are an important ingredient in constraining particle
production processes in the early universe. We place particular emphasis on
lattice QCD calculations of conserved current correlators. These Euclidean
correlators are related by an integral transform to spectral functions, whose
small-frequency form determines the transport properties via Kubo formulae. The
universal hydrodynamic predictions for the small-frequency pole structure of
spectral functions are summarized. The viability of a quasiparticle description
implies the presence of additional characteristic features in the spectral
functions. These features are in stark contrast with the functional form that
is found in strongly coupled plasmas via the gauge/gravity duality. A central
goal is therefore to determine which of these dynamical regimes the quark-gluon
plasma is qualitatively closer to as a function of temperature. We review the
analysis of lattice correlators in relation to transport properties, and
tentatively estimate what computational effort is required to make decisive
progress in this field.Comment: 54 pages, 37 figures, review written for EPJA and APPN; one parag.
added end of section 3.4, and one at the end of section 3.2.2; some Refs.
added, and some other minor change
Memory expansion for diffusion coefficients
We present a memory expansion for macroscopic transport coefficients such as the collective and tracer diffusion coefficients DC and DT, respectively. The successive terms in this expansion for DC describe rapidly decaying memory effects of the center-of-mass motion, leading to fast convergence when evaluated numerically. For DT, one obtains an expansion of similar form that contains terms describing memory effects in single-particle motion. As an example we evaluate DC and DT for three strongly interacting surface systems through Monte Carlo simulations, and for a simple model diffusion system via molecular dynamics calculations. We show that the numerical method provides a speedup of about two orders of magnitude in computational time as compared with the standard methods, when collective diffusion is concerned. For tracer diffusion, the speedup is not quite as significant. Our studies using the memory expansion provide information of the nature of memory effects in diffusion and suggest a nontrivial power-law behavior of memory terms at intermediate times. We also discuss the application of the present approach to studies of other transport coefficients.Peer reviewe
Multi-Particle Collision Dynamics -- a Particle-Based Mesoscale Simulation Approach to the Hydrodynamics of Complex Fluids
In this review, we describe and analyze a mesoscale simulation method for
fluid flow, which was introduced by Malevanets and Kapral in 1999, and is now
called multi-particle collision dynamics (MPC) or stochastic rotation dynamics
(SRD). The method consists of alternating streaming and collision steps in an
ensemble of point particles. The multi-particle collisions are performed by
grouping particles in collision cells, and mass, momentum, and energy are
locally conserved. This simulation technique captures both full hydrodynamic
interactions and thermal fluctuations. The first part of the review begins with
a description of several widely used MPC algorithms and then discusses
important features of the original SRD algorithm and frequently used
variations. Two complementary approaches for deriving the hydrodynamic
equations and evaluating the transport coefficients are reviewed. It is then
shown how MPC algorithms can be generalized to model non-ideal fluids, and
binary mixtures with a consolute point. The importance of angular-momentum
conservation for systems like phase-separated liquids with different
viscosities is discussed. The second part of the review describes a number of
recent applications of MPC algorithms to study colloid and polymer dynamics,
the behavior of vesicles and cells in hydrodynamic flows, and the dynamics of
viscoelastic fluids
Strongly Correlated Quantum Fluids: Ultracold Quantum Gases, Quantum Chromodynamic Plasmas, and Holographic Duality
Strongly correlated quantum fluids are phases of matter that are
intrinsically quantum mechanical, and that do not have a simple description in
terms of weakly interacting quasi-particles. Two systems that have recently
attracted a great deal of interest are the quark-gluon plasma, a plasma of
strongly interacting quarks and gluons produced in relativistic heavy ion
collisions, and ultracold atomic Fermi gases, very dilute clouds of atomic
gases confined in optical or magnetic traps. These systems differ by more than
20 orders of magnitude in temperature, but they were shown to exhibit very
similar hydrodynamic flow. In particular, both fluids exhibit a robustly low
shear viscosity to entropy density ratio which is characteristic of quantum
fluids described by holographic duality, a mapping from strongly correlated
quantum field theories to weakly curved higher dimensional classical gravity.
This review explores the connection between these fields, and it also serves as
an introduction to the Focus Issue of New Journal of Physics on Strongly
Correlated Quantum Fluids: from Ultracold Quantum Gases to QCD Plasmas. The
presentation is made accessible to the general physics reader and includes
discussions of the latest research developments in all three areas.Comment: 138 pages, 25 figures, review associated with New Journal of Physics
special issue "Focus on Strongly Correlated Quantum Fluids: from Ultracold
Quantum Gases to QCD Plasmas"
(http://iopscience.iop.org/1367-2630/focus/Focus%20on%20Strongly%20Correlated%20Quantum%20Fluids%20-%20from%20Ultracold%20Quantum%20Gases%20to%20QCD%20Plasmas
Causal Viscous Hydrodynamics for Relativistic Heavy Ion Collisions
The viscosity of the QGP is a presently hotly debated subject. Since its
computation from first principles is difficult, it is desirable to try to
extract it from experimental data. Viscous hydrodynamics provides a tool that
can attack this problem and which may work in regions where ideal hydrodynamics
begins to fail.
This thesis focuses on viscous hydrodynamics for relativistic heavy ion
collisions. We first review the 2nd order viscous equations obtained from
different approaches, and then report on the work of the Ohio State University
group on setting up the equations for causal viscous hydrodynamics in 2+1
dimensions and solving them numerically for central and noncentral Cu+Cu and
Au+Au collisions at RHIC energies and above. We discuss shear and bulk viscous
effects on the hydrodynamic evolution of entropy density, temperature,
collective flow, and flow anisotropies, and on the hadron multiplicity, single
particle spectra and elliptic flow. Viscous entropy production and its
influence on the centrality dependence of hadron multiplicities and the
multiplicity scaling of eccentricity-scaled elliptic flow are studied in
viscous hydrodynamics and compared with experimental data. The dynamical
effects of using different versions of the Israel-Stewart second order
formalism for causal viscous fluid dynamics are discussed, resolving some of
the apparent discrepancies between early results reported by different groups.
Finally, we assess the present status of constraining the shear viscosity to
entropy ratio of the hot and dense matter created at RHIC.Comment: Ph.D thesis (The Ohio State University, 2009; Advisor: U. Heinz), a
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