467 research outputs found
Brane world generation by matter and gravity
We present a non-compact (4 + 1) dimensional model with a local strong
four-fermion interaction supplementing it with gravity. In the strong coupling
regime it reveals the spontaneous translational symmetry breaking which
eventually leads to the formation of domain walls, or thick 3-branes, embedded
in the AdS-5 manifold. To describe this phenomenon we construct the appropriate
low-energy effective Action and find kink-like vacuum solutions in the
quasi-flat Riemannian metric. We discuss the generation of ultra-low-energy (3
+ 1) dimensional physics and we establish the relation among the bulk five
dimensional gravitational constant, the brane Newton's constants and the
curvature of AdS-5 space-time. The plausible relation between the compositeness
scale of the scalar matter and the symmetry breaking scale is shown to support
the essential decoupling of branons, the scalar fluctuations of the brane, from
the Standard Model matter, supporting their possible role in the dark matter
saturation. The induced cosmological constant on the brane does vanish due to
exact cancellation of matter and gravity contributions.Comment: 35 pages, JHEP3 style, refs.adde
Some issues concerning Large-Eddy Simulation of inertial particle dispersion in turbulent bounded flows
The problem of an accurate Eulerian-Lagrangian modeling of inertial particle
dispersion in Large Eddy Simulation (LES) of turbulent wall-bounded flows is
addressed. We run Direct Numerical Simulation (DNS) for turbulent channel flow
at shear Reynolds numbers equal to 150 and 300 and corresponding a-priori and
a-posteriori LES on differently coarse grids. We then tracked swarms of
different inertia particles and we examined the influence of filtering and of
Sub-Grid Scale (SGS) modeling for the fluid phase on particle velocity and
concentration statistics. We also focused on how particle preferential
segregation is predicted by LES. Results show that even ``well-resolved'' LES
is unable to reproduce the physics as demonstrated by DNS, both for particle
accumulation at the wall and for particle preferential segregation. Inaccurate
prediction is observed for the entire range of particles considered in this
study, even when the particle response time is much larger than the flow
timescales not resolved in LES. Both a-priori and a-posteriori tests indicate
that recovering the level of fluid and particle velocity fluctuations is not
enough to have accurate prediction of near-wall accumulation and local
segregation. This may suggest that reintroducing the correct amount of
higher-order moments of the velocity fluctuations is also a key point for SGS
closure models for the particle equation. Another important issue is the
presence of possible flow Reynolds number effects on particle dispersion. Our
results show that, in small Reynolds number turbulence and in the case of heavy
particles, the shear fluid velocity is a suitable scaling parameter to quantify
these effects
Geomorphological features of the Rio della Rocca Valley (northern Apennines, Italy)
This paper shows the contents of the geomorphological map of the Rio della Rocca valley, a small catchment located in the lower Apennines of the Province of Reggio Emilia (northern Italy). The geomorphological map has been produced at a scale 1:7500 as a base document within multidisciplinary investigations aiming at the rehabilitation of the area, which is characterised by peculiar geological and geomorphological features and a high scenic value, despite clear evidence of human impact. Since the mid-1950s the valley has, in fact, been affected by quarrying activities which in some places have deeply changed its environmental and, in particular, geomorphological features. As a part of the research, geomorphological survey and mapping were carried out in order to detect the main slope instability and erosional processes and landforms and, at the same time, to recognise sites of geological and geomorphological interest. As a result, proposals of territorial upgrading have been developed which take into account geotourism and recreational issues
Statistical properties of an ideal subgrid-scale correction for Lagrangian particle tracking in turbulent channel flow
One issue associated with the use of Large-Eddy Simulation (LES) to
investigate the dispersion of small inertial particles in turbulent flows is
the accuracy with which particle statistics and concentration can be
reproduced. The motion of particles in LES fields may differ significantly from
that observed in experiments or direct numerical simulation (DNS) because the
force acting on the particles is not accurately estimated, due to the
availability of the only filtered fluid velocity, and because errors accumulate
in time leading to a progressive divergence of the trajectories. This may lead
to different degrees of inaccuracy in the prediction of statistics and
concentration. We identify herein an ideal subgrid correction of the a-priori
LES fluid velocity seen by the particles in turbulent channel flow. This
correction is computed by imposing that the trajectories of individual
particles moving in filtered DNS fields exactly coincide with the particle
trajectories in a DNS. In this way the errors introduced by filtering into the
particle motion equations can be singled out and analyzed separately from those
due to the progressive divergence of the trajectories. The subgrid correction
term, and therefore the filtering error, is characterized in the present paper
in terms of statistical moments. The effects of the particle inertia and of the
filter type and width on the properties of the correction term are
investigated.Comment: 15 pages,24 figures. Submitted to Journal of Physics: Conference
Serie
Recognition and Assessment of Geomorphosites in Malta at the Il-Majjistral Nature and History Park
The Il-Majjistral Nature and History Park, located on the northwestern coast of the Island of Malta, is a protected area with natural, cultural and tourist interests. The Park, which was registered in 2008, is characterised by a landscape dominated by limestone and clay. Geomorphological features are highly controlled by the presence of faults and rock masses characterised by different physical and mechanical properties. Coastal cliffs, bays and sandy beaches are the most outstanding geomorphological features of the Park, creating a breathtaking landscape which is an attraction for a number of visitors. In the framework of an international research project, a study for the identification, selection and enhancement of the rich geomorphological heritage of the area has been carried out. In particular, a recognition and a quantitative assessment of geomorphosites on the basis of two different methodologies has been developed, and the results have been compared to establish the reliability of the methodologies. The results represent the first step and the necessary basic knowledge for possible enhancement of geomorphosites in Malta and the promotion of tourism activities at the Il-Majjistral Nature and History Park through the auspices of environmental agencies
Intrinsic filtering errors of Lagrangian particle tracking in LES flow fields
Large-Eddy Simulations (LES) of two-phase turbulent flows exhibit
quantitative differences in particle statistics if compared to Direct Numerical
Simulations (DNS) which, in the context of the present study, is considered the
exact reference case. Differences are primarily due to filtering, a fundamental
intrinsic feature of LES. Filtering the fluid velocity field yields approximate
computation of the forces acting on particles and, in turn, trajectories that
are inaccurate when compared to those of DNS. In this paper, we focus precisely
on the filtering error for which we quantify a lower bound. To this aim, we use
a DNS database of inertial particle dispersion in turbulent channel flow and we
perform a-priori tests in which the error purely due to filtering is singled
out removing error accumulation effects, which would otherwise lead to
progressive divergence between DNS and LES particle trajectories. By applying
filters of different type and width at varying particle inertia, we
characterize the statistical properties of the filtering error as a function of
the wall distance. Results show that filtering error is stochastic and has a
non-Gaussian distribution. In addition, the distribution of the filtering error
depends strongly on the wall-normal coordinate being maximum in the buffer
region. Our findings provide insight on the effect of subgrid-scale velocity
field on the force driving the particles, and establish the requirements which
a LES model must satisfy to predict correctly the velocity and the trajectory
of inertial particles.Comment: 39 pages, 1 table, 12 figures, submitted to Physics of Fluid
The quantum Hall effect in graphene samples and the relativistic Dirac effective action
We study the Euclidean effective action per unit area and the charge density
for a Dirac field in a two--dimensional spatial region, in the presence of a
uniform magnetic field perpendicular to the 2D--plane, at finite temperature
and density. In the limit of zero temperature we reproduce, after performing an
adequate Lorentz boost, the Hall conductivity measured for different kinds of
graphene samples, depending upon the phase choice in the fermionic determinant.Comment: Conclusions extended. References added. 9 pages. 1 figur
Comparison of quantum field perturbation theory for the light front with the theory in lorentz coordinates
The relationship between the perturbation theory in light-front coordinates
and Lorentz-covariant perturbation theory is investigated. A method for finding
the difference between separate terms of the corresponding series without their
explicit evaluation is proposed. A procedure of constructing additional
counter-terms to the canonical Hamiltonian that compensate this difference at
any finite order is proposed. For the Yukawa model, the light-front Hamiltonian
with all of these counter-terms is obtained in a closed form. Possible
application of this approach to gauge theories is discussed.Comment: LaTex 2.09, 20 pages, 5 figure
Geomorphology of the north-eastern coast of Gozo (Malta, Mediterranean Sea)
The paper presents a geomorphological map of the north-eastern coast of the Island of Gozo (Malta) integrating inland and offshore areas at the scale 1:15,000. The map derives from the integration of different methods, such as aerial photo interpretation, field surveys and analysis of seafloor bathymetry. The landforms identified on land were shaped by coastal, fluvial, karst and gravity-induced processes, and some of them prolong on the seafloor. Most of the submerged landforms appear to have been modelled in subaerial conditions during sea-level lowstands, having been sealed by the rising sea in post-glacial times. Two sketches accompany the Main Map showing the type and distribution of coastal geomorphotypes and the land cover of the area
Pairs Emission in a Uniform Background Field: an Algebraic Approach
A fully algebraic general approach is developed to treat the pairs emission
and absorption in the presence of some uniform external background field. In
particular, it is shown that the pairs production and annihilation operators,
together with the pairs number operator, do actually fulfill the SU(2)
functional Lie algebra. As an example of application, the celebrated Schwinger
formula is consistently and nicely recovered, within this novel approach, for a
Dirac spinor field in the presence of a constant and homogeneous electric field
in four spacetime dimensions.Comment: 26 pages, no figure
- …