903 research outputs found
Chiral perturbation theory for partially quenched twisted mass lattice QCD
Partially quenched Quantum Chromodynamics with Wilson fermions on a lattice
is considered in the framework of chiral perturbation theory. Two degenerate
quark flavours are associated with a chirally twisted mass term. The pion
masses and decay constants are calculated in next-to-leading order including
terms linear in the lattice spacing .Comment: 7 pages, LaTeX2e, final published versio
Solving the Boltzmann Equation on GPU
We show how to accelerate the direct solution of the Boltzmann equation using
Graphics Processing Units (GPUs). In order to fully exploit the computational
power of the GPU, we choose a method of solution which combines a finite
difference discretization of the free-streaming term with a Monte Carlo
evaluation of the collision integral. The efficiency of the code is
demonstrated by solving the two-dimensional driven cavity flow. Computational
results show that it is possible to cut down the computing time of the
sequential code of two order of magnitudes. This makes the proposed method of
solution a viable alternative to particle simulations for studying unsteady low
Mach number flows.Comment: 18 pages, 3 pseudo-codes, 6 figures, 1 tabl
Light quarks with twisted mass fermions
We investigate Wilson twisted mass fermions in the quenched approximation
using different definitions of the critical bare quark mass m_c to realize
maximal twist and, correspondingly, automatic O(a) improvement for physical
observables. A particular definition of m_c is given by extrapolating the value
of m_c obtained from the PCAC relation at non-vanishing bare twisted quark mass
mu to mu=0. Employing this improved definition of the critical mass the Wilson
twisted mass formulation provides the possibility to perform reliable
simulations down to very small quark masses with correspondingly small pion
masses of m_pi \simeq 250 MeV, while keeping the cutoff effects of O(a^2) under
control.Comment: 10 pages, 8 figures, v2: version accepted for publication in PLB,
minor correction
Twisted mass chiral perturbation theory for 2+1+1 quark flavours
We present results for the masses of pseudoscalar mesons in twisted mass
lattice QCD with a degenerate doublet of u and d quarks and a non-degenerate
doublet of s and c quarks in the framework of next-to-leading order chiral
perturbation theory, including lattice effects up to O(a^2). The masses depend
on the two twist angles for the light and heavy sectors. For maximal twist in
both sectors, O(a)-improvement is explicitly exhibited. The mixing of
flavour-neutral mesons is also discussed, and results in the literature for the
case of degenerate s and c quarks are corrected.Comment: LaTeX2e, 12 pages, corrected typo
Twisted mass lattice QCD with non-degenerate quark masses
Quantum Chromodynamics on a lattice with Wilson fermions and a chirally
twisted mass term is considered in the framework of chiral perturbation theory.
For two and three numbers of quark flavours, respectively, with non-degenerate
quark masses the pseudoscalar meson masses and decay constants are calculated
in next-to-leading order including lattice effects quadratic in the lattice
spacing a.Comment: 9 pages, LaTeX2e, reference adde
Localization properties of lattice fermions with plaquette and improved gauge actions
We determine the location of the mobility edge in the spectrum of
the hermitian Wilson operator in pure-gauge ensembles with plaquette, Iwasaki,
and DBW2 gauge actions. The results allow mapping a portion of the (quenched)
Aoki phase diagram. We use Green function techniques to study the localized and
extended modes. Where we characterize the localized modes in
terms of an average support length and an average localization length, the
latter determined from the asymptotic decay rate of the mode density. We argue
that, since the overlap operator is commonly constructed from the Wilson
operator, its range is set by the value of for the Wilson
operator. It follows from our numerical results that overlap simulations
carried out with a cutoff of 1 GeV, even with improved gauge actions, could be
afflicted by unphysical degrees of freedom as light as 250 MeV.Comment: RevTeX, 37 pages, 10 figures. Some textual changes. Final for
O(a^2) cutoff effects in lattice Wilson fermion simulations
In this paper we propose to interpret the large discretization artifacts
affecting the neutral pion mass in maximally twisted lattice QCD simulations as
O(a^2) effects whose magnitude is roughly proportional to the modulus square of
the (continuum) matrix element of the pseudoscalar density operator between
vacuum and one-pion state. The numerical size of this quantity is determined by
the dynamical mechanism of spontaneous chiral symmetry breaking and turns out
to be substantially larger than its natural magnitude set by the value of
Lambda_QCD.Comment: 38 pages, 1 figure, 2 table
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Solving the Boltzmann equation on GPU’s
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.We present algorithms specifically tailored for solving kinetic equations onto graphics processing units. Unlike particle methods, the proposed methods of solution are ideally suited for solving the unsteady low speed flows which typically occur inMEMS containing oscillating components. The efficiency of the algorithms is demonstrated by solving the two-dimensional low Mach number driven cavity flow of a monatomic gas. Computational results show that it is possible to cut down the computing time of the sequential codes up to two order of magnitudes. The algorithms can easily be extended to three-dimensional flows and to non-equilibrium flows of mixtures
A kinetic theory description of liquid menisci at the microscale
A kinetic model for the study of capillary flows in devices with microscale geometry is presented. The model is based on the Enskog-Vlasov kinetic equation and provides a reasonable description of both fluid-fluid and fluid-wall interactions. Numerical solutions are obtained by an extension of the classical Direct Simulation Monte Carlo (DSMC) to dense fluids. The equilibrium properties of liquid menisci between two hydrophilic walls are investigated and the validity of the Laplace-Kelvin equation at the microscale is assessed. The dynamical process which leads to the meniscus breakage is clarified
Direct simulation Monte Carlo applications to liquid-vapor flows
The paper aims at presenting Direct Simulation Monte Carlo (DSMC) extensions and applications to dense fluids. A succinct review of past and current research topics is presented, followed by a more detailed description of DSMC simulations for the numerical solution of the Enskog-Vlasov equation, applied to the study of liquid-vapor flows. Results about simulations of evaporation of a simple liquid in contact with a dense vapor are presented as an example
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