1,282 research outputs found
Non-linear relativistic perturbation theory with two parameters
An underlying fundamental assumption in relativistic perturbation theory is
the existence of a parametric family of spacetimes that can be Taylor expanded
around a background. Since the choice of the latter is crucial, sometimes it is
convenient to have a perturbative formalism based on two (or more) parameters.
A good example is the study of rotating stars, where generic perturbations are
constructed on top of an axisymmetric configuration built by using the slow
rotation approximation. Here, we discuss the gauge dependence of non-linear
perturbations depending on two parameters and how to derive explicit higher
order gauge transformation rules.Comment: 5 pages, LaTeX2e. Contribution to the Spanish Relativity Meeting (ERE
2002), Mao, Menorca, Spain, 22-24.September.200
On the validity of the adiabatic approximation in compact binary inspirals
Using a semi-analytical approach recently developed to model the tidal
deformations of neutron stars in inspiralling compact binaries, we study the
dynamical evolution of the tidal tensor, which we explicitly derive at second
post-Newtonian order, and of the quadrupole tensor. Since we do not assume a
priori that the quadrupole tensor is proportional to the tidal tensor, i.e. the
so called "adiabatic approximation", our approach enables us to establish to
which extent such approximation is reliable. We find that the ratio between the
quadrupole and tidal tensors (i.e., the Love number) increases as the inspiral
progresses, but this phenomenon only marginally affects the emitted
gravitational waveform. We estimate the frequency range in which the tidal
component of the gravitational signal is well described using the stationary
phase approximation at next-to-leading post-Newtonian order, comparing
different contributions to the tidal phase. We also derive a semi-analytical
expression for the Love number, which reproduces within a few percentage points
the results obtained so far by numerical integrations of the relativistic
equations of stellar perturbations.Comment: 13 pages, 1 table, 2 figures. Minor changes to match the version
appearing on Phys. Rev.
Exact regularized point particle method for multi-phase flows in the two-way coupling regime
Particulate flows have been largely studied under the simplifying assumptions
of one-way coupling regime where the disperse phase do not react-back on the
carrier fluid. In the context of turbulent flows, many non trivial phenomena
such as small scales particles clustering or preferential spatial accumulation
have been explained and understood. A more complete view of multiphase flows
can be gained calling into play two-way coupling effects, i.e. by accounting
for the inter-phase momentum exchange between the carrier and the suspended
phase, certainly relevant at increasing mass loading. In such regime, partially
investigated in the past by the so-called Particle In Cell (PIC) method, much
is still to be learned about the dynamics of the disperse phase and the ensuing
alteration of the carrier flow.
In this paper we present a new methodology rigorously designed to capture the
inter-phase momentum exchange for particles smaller than the smallest
hydrodynamical scale, e.g. the Kolmogorov scale in a turbulent flow. In fact,
the momentum coupling mechanism exploits the unsteady Stokes flow around a
small rigid sphere where the transient disturbance produced by each particle is
evaluated in a closed form. The particles are described as lumped, point masses
which would lead to the appearance of singularities. A rigorous regularization
procedure is conceived to extract the physically relevant interactions between
particles and fluid which avoids any "ah hoc" assumption. The approach is
suited for high efficiency implementation on massively parallel machines since
the transient disturbance produced by the particles is strongly localized in
space around the actual particle position. As will be shown, hundred thousands
particles can therefore be handled at an affordable computational cost as
demonstrated by a preliminary application to a particle laden turbulent shear
flow.Comment: Submitted to Journal of Fluid Mechanics, 56 pages, 15 figure
Application of the exact regularized point particle method (ERPP) to particle laden turbulent shear flows in the two-way coupling regime
The Exact Regularized Point Particle method (ERPP), which is a new inter-phase momentum coupling ap- proach, is extensively used for the first time to explore the response of homogeneous shear turbulence in presence of different particle populations. Particle suspensions with different Stokes number and/or mass loading are considered. Particles with Kolmogorov Stokes number of order one suppress turbulent kinetic energy when the mass loading is increased. In contrast, heavier particles leave this observable almost un- changed with respect to the reference uncoupled case. Turbulence modulation is found to be anisotropic, leaving the streamwise velocity fluctuations less affected by unitary Stokes number particles whilst it is increased by heavier particles. The analysis of the energy spectra shows that the turbulence modulation occurs throughout the entire range of resolved scales leading to non-trivial augmentation/depletion of the energy content among the different velocity components at different length-scales. In this regard, the ERPP approach is able to provide convergent statistics up to the smallest dissipative scales of the flow, giving the opportunity to trust the ensuing results. Indeed, a substantial modification of the turbu- lent fluctuations at the smallest-scales, i.e. at the level of the velocity gradients, is observed due to the particle backreaction. Small scale anisotropies are enhanced and fluctuations show a greater level of in- termittency as measured by the probability distribution function of the longitudinal velocity increments and by the corresponding flatness
In vitro acellular dissolution of mineral fibres: A comparative study
The study of the mechanisms by which mineral fibres promote adverse effects in both animals and humans is a hot topic of multidisciplinary research with many aspects that still need to be elucidated. Besides length and diameter, a key parameter that determines the toxicity/pathogenicity of a fibre is biopersistence, one component of which is biodurability. In this paper, biodurability of mineral fibres of social and economic importance (chrysotile, amphibole asbestos and fibrous erionite) has been determined for the first time in a systematic comparative way from in vitro acellular dissolution experiments. Dissolution was possible using the Gamble solution as simulated lung fluid (pH = 4 and at body temperature) so to reproduce the macrophage phagolysosome environment. The investigated mineral fibres display very different dissolution rates. For a 0.25 μm thick fibre, the calculated dissolution time of chrysotile is in the range 94-177 days, very short if compared to that of amphibole fibres (49-245 years), and fibrous erionite (181 years). Diffraction and SEM data on the dissolution products evidence that chrysotile rapidly undergoes amorphization with the formation of a nanophasic silica-rich fibrous metastable pseudomorph as first dissolution step whereas amphibole asbestos and fibrous erionite show minor signs of dissolution even after 9-12 months
Support Vector Machines for Hyperspectral Remote Sensing Classification
The Support Vector Machine provides a new way to design classification algorithms which learn from examples (supervised learning) and generalize when applied to new data. We demonstrate its success on a difficult classification problem from hyperspectral remote sensing, where we obtain performances of 96%, and 87% correct for a 4 class problem, and a 16 class problem respectively. These results are somewhat better than other recent results on the same data. A key feature of this classifier is its ability to use high-dimensional data without the usual recourse to a feature selection step to reduce the dimensionality of the data. For this application, this is important, as hyperspectral data consists of several hundred contiguous spectral channels for each exemplar. We provide an introduction to this new approach, and demonstrate its application to classification of an agriculture scene
Black hole particle emission in higher-dimensional spacetimes
In models with extra dimensions, a black hole evaporates both in the bulk and
on the visible brane, where standard model fields live. The exact emissivities
of each particle species are needed to determine how the black hole decay
proceeds. We compute and discuss the absorption cross-sections, the relative
emissivities and the total power output of all known fields in the evaporation
phase. Graviton emissivity is highly enhanced as the spacetime dimensionality
increases. Therefore, a black hole loses a significant fraction of its mass in
the bulk. This result has important consequences for the phenomenology of black
holes in models with extra dimensions and black hole detection in particle
colliders.Comment: 4 pages, RevTeX 4. v3: Misprints in Tables correcte
Transport of micro-bubbles in turbulent shear flows
The dynamics of micro-bubbles, which are typical in many industrial applications, is addressed by means the Direct Numerical Simulations (DNS) of two prototypal flows, namely a homogeneous shear flow and a fully developed pipe flows. This preliminary study has a two-fold purpose. The homogenous turbulent shear flow is useful to characterize the bubble dynamics in terms of their eventual clustering properties which is expected to be controlled by the Stokes number. The time history of the fluid pressure experienced by the bubbles during their evolution is recorded and successively employed to force the Rayleigh-Plesset equation [1]. The ensuing data are used to address a posteriori the bubble diameter statistics in view of bubble collapse induced by strong and intermittent turbulent pressure fluctuations. The turbulent pipe flow simulations serve to address the bubble dynamics in wall bounded flows. Here the bubbles are observed to accumulate in the near-wall region with different intensity depending on the bubble dimensions
The effect of grinding on tremolite asbestos and anthophyllite asbestos
The six commercial asbestos minerals (chrysotile, fibrous actinolite, crocidolite, amosite, fibrous tremolite, and fibrous anthophyllite) are classified by the IARC as carcinogenic to humans. There are currently several lines of research dealing with the inertisation of asbestos minerals among which the dry grinding process has received considerable interest. The effects of dry grinding on tremolite asbestos and anthophyllite asbestos in eccentric vibration mills have not yet been investigated. Along the research line of the mechanical treatment of asbestos, the aim of this study was to evaluate the effects of dry grinding in eccentric vibration mills on the structure, temperature stability, and fibre dimensions of tremolite asbestos from Val d\u2019Ala, (Italy) and UICC standard anthophyllite asbestos from Paakkila mine (Finland) by varying the grinding time (30 s, 5 min, and 10 min). After grinding for 30 s to 10 min, tremolite asbestos and anthophyllite asbestos showed a decrease in dehydroxylation and breakdown temperatures due to the increase in lattice strain and the decrease in crystallinity. Moreover, after grinding up to 10 min, tremolite and anthophyllite fibres were all below the limits defining a countable fibre according to WHO
Multi-mode TES bolometer optimization for the LSPE-SWIPE instrument
In this paper we explore the possibility of using transition edge sensor
(TES) detectors in multi-mode configuration in the focal plane of the Short
Wavelength Instrument for the Polarization Explorer (SWIPE) of the
balloon-borne polarimeter Large Scale Polarization Explorer (LSPE) for the
Cosmic Microwave Background (CMB) polarization. This study is motivated by the
fact that maximizing the sensitivity of TES bolometers, under the augmented
background due to the multi-mode design, requires a non trivial choice of
detector parameters. We evaluate the best parameter combination taking into
account scanning strategy, noise constraints, saturation power and operating
temperature of the cryostat during the flight.Comment: in Journal of Low Temperature Physics, 05 January 201
- …