5,706 research outputs found
Quasi-periodic solutions of completely resonant forced wave equations
We prove existence of quasi-periodic solutions with two frequencies of
completely resonant, periodically forced nonlinear wave equations with periodic
spatial boundary conditions. We consider both the cases the forcing frequency
is: (Case A) a rational number and (Case B) an irrational number.Comment: 25 pages, 1 figur
Phase separation in quasi incompressible fluids: Cahn-Hilliard model in the Cattaneo-Maxwell framework
In this paper we propose a mathematical model of phase separation for a
quasi-incompressible binary mixture where the spinodal decomposition is induced
by an heat flux governed by the Cattaneo-Maxwell equation. As usual, the phase
separation is considered in the framework of phase field modeling so that the
transition is described by an additional field, the concentration c. The
evolution of concentration is described by the Cahn-Hilliard equation and in
our model is coupled with the Navier-Stokes equation. Since thermal effect are
included, the whole set of evolution equations is set up for the velocity, the
concentration, the temperature and the heat flux. The model is compatible with
thermodynamics and a maximum theorem holds.Comment: Submitted to ZAM
Turbulence and coarsening in active and passive binary mixtures
Phase separation between two fluids in two-dimensions is investigated by
means of Direct Numerical Simulations of coupled Navier-Stokes and
Cahn-Hilliard equations. We study the phase ordering process in the presence of
an external stirring acting on the velocity field. For both active and passive
mixtures we find that, for a sufficiently strong stirring, coarsening is
arrested in a stationary dynamical state characterized by a continuous rupture
and formation of finite domains. Coarsening arrest is shown to be independent
of the chaotic or regular nature of the flow.Comment: 4 pages, 5 figures; discussion on the dependence of the arrest scale
on the shear rate has been added; figures have been modified accordingl
How to enhance crop production and nitrogen fluxes? A result-oriented scheme to evaluate best agri-environmental measures in Veneto Region, Italy
The cost-effectiveness of adopting agri-environmental measures (AEMs) in Europe, which combine agricultural productions with reduced N losses, is debated due to poorly targeted site-specific funding that is allocated regardless of local variability. An integrated DAYCENT model-GIS platform was developed combining pedo-climatic and agricultural systems information. The aim was to evaluate best strategies to improve N fluxes of agro-ecosystems within a perspective of sustainable intensification. Indicators of agronomic efficiency and environmental quality were considered. The results showed that agronomic benefits were observed with a continuous soil cover (conservation agriculture and cover crops), which enhanced nitrogen use efficiency (+17%) and crop yields (+34%), although in some cases these might be overestimated due to modelling limitations. An overall environmental improvement was found with continuous soil cover and long-term change from mineral to organic inputs (NLeach 45 Mg ha 121), which were effective in the sandy soils of western and eastern Veneto with low SOM, improving the soil-water balance and nutrients availability over time. Results suggest that AEM subsidies should be allocated at a site-specific level that includes pedo-climatic variability, following a result-oriented approach
Non-radial oscillation modes as a probe of density discontinuities in neutron stars
A phase transition occurring in the inner core of a neutron star could be
associated to a density discontinuity that would affect the frequency spectrum
of the non-radial oscillation modes in two ways. Firstly, it would produce a
softening of the equation of state, leading to more compact equilibrium
configurations and changing the frequency of the fundamental and pressure modes
of the neutron star. Secondly, a new non-zero frequency g-- mode would appear,
associated to each discontinuity. These discontinuity g--modes have typical
frequencies larger than those of g--modes previously studied in the literature
(thermal, core g-- modes, or g--modes due to chemical inhomogeneities in the
outer layers), and smaller than that of the fundamental mode; therefore they
should be distinguishable from the other modes of non radial oscillation. In
this paper we investigate how high density discontinuities change the frequency
spectrum of the non-radial oscillations, in the framework of the general
relativistic theory of stellar perturbations. Our purpose is to understand
whether a gravitational signal, emitted at the frequencies of the quasi normal
modes, may give some clear information on the equation of state of the neutron
star and, in particular, on the parameters that characterize the density
discontinuity. We discuss some astrophysical processes that may be associated
to the excitation of these modes, and estimate how much gravitational energy
should the modes convey to produce a signal detectable by high frequency
gravitational detectors.Comment: submitted to MNRA
Eccentric binary black-hole mergers: The transition from inspiral to plunge in general relativity
We study the transition from inspiral to plunge in general relativity by
computing gravitational waveforms of non-spinning, equal-mass black-hole
binaries. We consider three sequences of simulations, starting with a
quasi-circular inspiral completing 1.5, 2.3 and 9.6 orbits, respectively, prior
to coalescence of the holes. For each sequence, the binding energy of the
system is kept constant and the orbital angular momentum is progressively
reduced, producing orbits of increasing eccentricity and eventually a head-on
collision. We analyze in detail the radiation of energy and angular momentum in
gravitational waves, the contribution of different multipolar components and
the final spin of the remnant. We find that the motion transitions from
inspiral to plunge when the orbital angular momentum L=L_crit is about 0.8M^2.
For L<L_crit the radiated energy drops very rapidly. Orbits with L of about
L_crit produce our largest dimensionless Kerr parameter for the remnant,
j=J/M^2=0.724. Generalizing a model recently proposed by Buonanno, Kidder and
Lehner to eccentric binaries, we conjecture that (1) j=0.724 is the maximal
Kerr parameter that can be obtained by any merger of non-spinning holes, and
(2) no binary merger (even if the binary members are extremal Kerr black holes
with spins aligned to the orbital angular momentum, and the inspiral is highly
eccentric) can violate the cosmic censorship conjecture.Comment: Added sequence of long inspirals to the study. To match published
versio
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