45 research outputs found
Phase-space Lagrangian dynamics of incompressible thermofluids
Phase-space Lagrangian dynamics in ideal fluids (i.e, continua) is usually
related to the so-called {\it ideal tracer particles}. The latter, which can in
principle be permitted to have arbitrary initial velocities, are understood as
particles of infinitesimal size which do not produce significant perturbations
of the fluid and do not interact among themselves. An unsolved theoretical
problem is the correct definition of their dynamics in ideal fluids. The issue
is relevant in order to exhibit the connection between fluid dynamics and the
classical dynamical system, underlying a prescribed fluid system, which
uniquely generates its time-evolution. \
The goal of this paper is to show that the tracer-particle dynamics can be
{\it exactly} established for an arbitrary incompressible fluid uniquely based
on the construction of an inverse kinetic theory (IKT) (Tessarotto \textit{et
al.}, 2000-2008). As an example, the case of an incompressible Newtonian
thermofluid is here considered.Comment: submitted to Physica
Modelling of anthropogenic pollutant diffusion in the atmosphere and applications to civil protection monitoring
A basic feature of fluid mechanics concerns the frictionless phase-space
dynamics of particles in an incompressible fluid. The issue, besides its
theoretical interest in turbulence theory, is important in many applications,
such as the pollutant dynamics in the atmosphere, a problem relevant for civil
protection monitoring of air quality. Actually, both the numerical simulation
of the ABL (atmospheric boundary layer) portion of the atmosphere and that of
pollutant dynamics may generally require the correct definition of the
Lagrangian dynamics which characterizes arbitrary fluid elements of
incompressible thermofluids. We claim that particularly important for
applications would be to consider these trajectories as phase-space
trajectories. This involves, however, the unfolding of a fundamental
theoretical problem up to now substantially unsolved: {\it namely the
determination of the exact frictionless dynamics of tracer particles in an
incompressible fluid, treated either as a deterministic or a turbulent (i.e.,
stochastic) continuum.} In this paper we intend to formulate the necessary
theoretical framework to construct such a type of description. This is based on
a phase-space inverse kinetic theory (IKT) approach recently developed for
incompressible fluids (Ellero \textit{et al.}, 2004-2008). {\it Our claim is
that the conditional frictionless dynamics of a tracer particles - which
corresponds to a prescribed velocity probability density and an arbitrary
choice of the relevant fluid fields - can be exactly specified}.Comment: Contributed paper at RGD26 (Kyoto, Japan, July 2008
The computational complexity of traditional Lattice-Boltzmann methods for incompressible fluids
It is well-known that in fluid dynamics an alternative to customary direct
solution methods (based on the discretization of the fluid fields) is provided
by so-called \emph{particle simulation methods}. Particle simulation methods
rely typically on appropriate \emph{kinetic models} for the fluid equations
which permit the evaluation of the fluid fields in terms of suitable
expectation values (or \emph{momenta}) of the kinetic distribution function
being respectively and\textbf{\}
the position an velocity of a test particle with probability density
. These kinetic models can be continuous or discrete in
phase space, yielding respectively \emph{continuous} or \emph{discrete kinetic
models} for the fluids. However, also particle simulation methods may be biased
by an undesirable computational complexity. In particular, a fundamental issue
is to estimate the algorithmic complexity of numerical simulations based on
traditional LBM's (Lattice-Boltzmann methods; for review see Succi, 2001
\cite{Succi}). These methods, based on a discrete kinetic approach, represent
currently an interesting alternative to direct solution methods. Here we intend
to prove that for incompressible fluids fluids LBM's may present a high
complexity. The goal of the investigation is to present a detailed account of
the origin of the various complexity sources appearing in customary LBM's. The
result is relevant to establish possible strategies for improving the numerical
efficiency of existing numerical methods.Comment: Contributed paper at RGD26 (Kyoto, Japan, July 2008
Lagrangian dynamics of incompressible thermofluids
A key aspect of fluid dynamics is the correct definition of the \textit{%
phase-space} Lagrangian dynamics which characterizes arbitrary fluid elements
of an incompressible fluid. Apart being an unsolved theoretical problem of
fundamental importance, the issue is relevant to exhibit the connection between
fluid dynamics and the classical dynamical systems underlying incompressible
and non-isothermal fluid, typically founded either on: a) a
\textit{configuration-space} Lagrangian description of the dynamics of fluid
elements; b) a kinetic description of the molecular dynamics, based on a
discrete representation of the fluid. The goal of this paper is to show that
the exact Lagrangian dynamics can be established based on the inverse kinetic
theory (IKT) for incompressible fluids recently pointed out (Ellero \textit{et
al.}, 2004-2006, \cite{Ellero2004}). The result is reached by adopting an IKT
approach based on a \textit{restricted phase-space representation} of the
fluid, in which the configuration space coincides with the physical fluid
domain. The result appears of potential importance in applied fluid dynamics
and CFD.Comment: Contributed paper at RGD26 (Kyoto, Japan, July 2008
Characterization of LAPPD timing at CERN PS testbeam
Large Area Picosecond PhotoDetectors (LAPPDs) are photosensors based on
microchannel plate technology with about 400 cm sensitive area. The
external readout plane of a capacitively coupled LAPPD can be segmented into
pads providing a spatial resolution down to 1 mm scale. The LAPPD signals have
about 0.5 ns risetime followed by a slightly longer falltime and their
amplitude reaches a few dozens of mV per single photoelectron. In this article,
we report on the measurement of the time resolution of an LAPPD prototype in a
test beam exercise at CERN PS. Most of the previous measurements of LAPPD time
resolution had been performed with laser sources. In this article we report
time resolution measurements obtained through the detection of Cherenkov
radiation emitted by high energy hadrons. Our approach has been demonstrated
capable of measuring time resolutions as fine as 25-30 ps. The available
prototype had performance limitations, which prevented us from applying the
optimal high voltage setting. The measured time resolution for single
photoelectrons is about 80 ps r.m.s.Comment: 35 pages, 23 figure
The Polarised Valence Quark Distribution from semi-inclusive DIS
The semi-inclusive difference asymmetry A^{h^{+}-h^{-}} for hadrons of
opposite charge has been measured by the COMPASS experiment at CERN. The data
were collected in the years 2002-2004 using a 160 GeV polarised muon beam
scattered off a large polarised ^6LiD target and cover the range 0.006 < x <
0.7 and 1 < Q^2 < 100 (GeV/c)^2. In leading order QCD (LO) the asymmetry
A_d^{h^{+}-h^{-}} measures the valence quark polarisation and provides an
evaluation of the first moment of Delta u_v + Delta d_v which is found to be
equal to 0.40 +- 0.07 (stat.) +- 0.05 (syst.) over the measured range of x at
Q^2 = 10 (GeV/c)^2. When combined with the first moment of g_1^d previously
measured on the same data, this result favours a non-symmetric polarisation of
light quarks Delta u-bar = - Delta d-bar at a confidence level of two standard
deviations, in contrast to the often assumed symmetric scenario Delta u-bar =
Delta d-bar = Delta s-bar = Delta s.Comment: 7 pages, 3 figures, COMPASS, revised: details added, author list
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