3,663 research outputs found
Extended RPA within a solvable 3 level model
Working within an exactly solvable 3 level model, we discuss am extension of
the
Random Phase Approximation (RPA) based on a boson formalism. A boson
Hamiltonian is defined via a mapping procedure and its expansion truncated at
four-boson terms. RPA-type equations are then constructed and solved
iteratively. The new solutions gain in stability with respect to the RPA ones.
We perform diagonalizations of the boson Hamiltonian in spaces containing up to
four-phonon components. Approximate spectra exhibit an improved quality with
increasing the size of these multiphonon spaces. Special attention is addressed
to the problem of the anharmonicity of the spectrum.Comment: 5 figure
Heat transfer and wall temperature effects in shock wave turbulent boundary layer interactions
Direct numerical simulations are carried out to investigate the effect of the
wall temperature on the behavior of oblique shock-wave/turbulent boundary layer
interactions at freestream Mach number and shock angle of the wedge
generator . Five values of the
wall-to-recovery-temperature ratio () are considered, corresponding to
cold, adiabatic and hot wall thermal conditions. We show that the main effect
of cooling is to decrease the characteristic scales of the interaction in terms
of upstream influence and extent of the separation bubble. The opposite
behavior is observed in the case of heating, that produces a marked dilatation
of the interaction region. The distribution of the Stanton number shows that a
strong amplification of the heat transfer occurs across the interaction, and
the maximum values of thermal and dynamic loads are found in the case of cold
wall. The analysis reveals that the fluctuating heat flux exhibits a strong
intermittent behavior, characterized by scattered spots with extremely high
values compared to the mean. Furthermore, the analogy between momentum and heat
transfer, typical of compressible, wall-bounded, equilibrium turbulent flows
does not apply for most part of the interaction domain. The pre-multiplied
spectra of the wall heat flux do not show any evidence of the influence of the
low-frequency shock motion, and the primary mechanism for the generation of
peak heating is found to be linked with the turbulence amplification in the
interaction region.Comment: submitted to PRFluid
Second random-phase approximation with the Gogny force. First applications
We present the first applications of the second random-phase-approximation
model with the finite-range Gogny interaction. We discuss the advantages of
using such an interaction in this type of calculations where 2 particle-2 hole
configurations are included. The results found in the present work confirm the
well known general features of the second random-phase approximation spectra:
we find a large shift, several MeV, of the response centroids to lower energies
with respect to the corresponding random-phase-approximation values. As known,
these results indicate that the effects of the 1 particle-1 hole/2 particle-2
hole and 2 particle-2 hole/2 particle-2 hole couplings are important. It has
been found that the changes of the strength distributions with respect to the
standard random-phase-approximation results are particularly large in the
present case. This important effect is due to some large neutron-proton matrix
elements of the interaction and indicates that these matrix elements (which do
not contribute in the mean-field calculations employed in the conventional fit
procedures of the force parameters) should be carefully constrained to perform
calculation
Housing system and welfare of buffalo (Bubalus bubalis) cows
Twenty-eight buffalo cows were used to evaluate the effect of housing system on a range of behavioural and physiological variables. Fourteen cows were group-housed in a loose open-sided barn with a concrete floor and 10 m2 per head as space allowance (group IS). Fourteen others were group-housed in a similar barn but they could also benefit from an outdoor yard with 500 m2 per head as space allowance, free access to potholes for wallowing and spontaneous vegetation (group TS). Animals were subjected to six sessions of instantaneous scan sampling at 10-day intervals. Behavioural variables were expressed as proportions of subjects observed in each category of posture and activity. Phytohaemagglutinin (PHA) was used to perform a skin test based on non-specific delayed type hypersensitivity, whereas 20 mg of ovalbumin were injected subcutaneously to evaluate humoral immune response. Blood samples for evaluation of cortisol concentration were collected immediately prior to exogenous porcine ACTH injection and 1, 2 and 4 h after. The metabolic status of the animals and milk production were also monitored. The proportion of idling animals was higher in group IS than in group TS (P< 0·001). More IS buffalo cows were observed eating at the manger than TS animals (P< 0·001). A higher proportion of TS animals were observed in the sun (P< 0·001). Grazing and bathing activities were recorded only for TS animals. Our findings suggest that buffalo cows kept in intensive conditions and having no access to ample yards and potholes may extend their periods of idling with negative effects on the state of welfare. Immune responses, metabolite concentrations and milk production were not affected by treatment, whereas cortisol levels were higher in IS animals (P< 0·05). The provision of a housing system similar to natural conditions was able to improve the welfare of buffalo cows as indicated by the expression of some species-specific natural behaviours. Such conditions were also associated with lower adrenal cortex response to ACTH injection, possibly as a consequence of the higher degree of initiative allowed to TS cows
Coherent transport structures in magnetized plasmas II: Numerical results
In a pair of linked articles (called Article I and II respectively) we apply
the concept of Lagrangian Coherent Structures borrowed from the study of
Dynamical Systems to magnetic field configurations in order to separate regions
where field lines have different kind of behavior. In the present article,
article II, by means of a numerical procedure we investigate the Lagrangian
Coherent Structures in the case of a two-dimensional magnetic configuration
with two island chains that are generated by magnetic reconnection and evolve
nonlinearly in time. The comparison with previous results, obtained by assuming
a fixed magnetic field configuration, allows us to explore the dependence of
transport barriers on the particle velocity
Coherent transport structures in magnetized plasmas, I : Theory
In a pair of linked articles (called Article I and II respectively) we apply
the concept of Lagrangian Coherent Structures (LCSs) borrowed from the study of
Dynamical Systems to magnetic field configurations in order to separate regions
where field lines have different kind of behaviour. In the present article,
article I, after recalling the definition and the properties of the LCSs, we
show how this conceptual framework can be applied to the study of particle
transport in a magnetized plasma. Futhermore we introduce a simplified model
that allows us to consider explicitly the case where the magnetic configuration
evolves in time on timescales comparable to the particle transit time through
the configuration. In contrast with previous works on this topic, this analysis
requires that a system that is aperiodic in time be investigated. In this case
the Poincar\'e map technique cannot be applied and LCSs remain the only viable
tool
Development of a Particle-In-Cell code with Structured Adaptive Mesh Refinement for Plasma Focus devices breakdown simulation
The aim at simulating the breakdown phase of a Plasma Focus
(PF) discharge follows the need to fully understand the dynamics of such device, in order to retrieve useful information for the design and optimization of the machine
itself. PFs are compact devices able to generate, accelerate, compress and confine a plasma by means of strongly varying electric and magnetic fields. In the final phase of the discharge, the generated plasma collapses in a high density region (the focus) where nuclear reactions occur. The choice of the gases composing the plasma tunes the nuclear reactions in order to characterize the device as a possible neutron-free Short-Life Radioisotopes (SLRs) generator for PET (f.i. 18F and 15O), as well as a neutrons or collimated-electrons-beams source for radio-therapy applications. An electrostatic-collisional Particle-In-Cell (PIC) code for Plasma Focus devices (es-cPIF) has already been developed to investigate the breakdown phenomenon and the formation of the plasma seed, the preliminary plasma spot, within the device: the exact knowledge of the phase space distribution function (strongly deviating from the Maxwellian equilibrium one) is a fundamental basis indeed for the whole discharge simulation. In order to extend the present simulations towards the complete evolution of the plasma seed into a running plasma sheath, the code is
being re-structured for strong parallelization and inclusion of Structured Adaptive Mesh Refinement (SAMR) capabilities. In this paper the development frame as well
as the software design architecture are presented together with the features that will be provided by the new SAMRes-cPIF code
Low-lying dipole response in the stable Ca nuclei with the second random-phase approximation
Low-energy dipole excitations are analyzed for the stable isotopes Ca
and Ca in the framework of the Skyrme-second random-phase approximation.
The corresponding random-phase approximation calculations provide a negligible
strength distribution for both nuclei in the energy region from 5 to 10 MeV.
The inclusion and the coupling of 2 particle-2 hole configurations in the
second random-phase approximation lead to an appreciable dipole response at low
energies for the neutron-rich nucleus Ca. The presence of a neutron skin
in the nucleus Ca would suggest the interpretation of the low-lying
response in terms of a pygmy excitation. The composition of the excitation
modes (content of 1 particle-1 hole and 2 particle-2 hole configurations),
their transition densities and their collectivity (number and coherence of the
different contributions) are analyzed. This analysis indicates that, in
general, these excitations cannot be clearly interpreted in terms of
oscillations of the neutron skin against the core with the exception of the
peak with the largest value, which is located at 9.09 MeV. For this
peak the neutron transition density dominates and the neutron and proton
transition densities oscillate out of phase in the internal part of the nucleus
leading to a strong mixing of isoscalar and isovector components. Therefore,
this state shows some features usually associated to pygmy resonances
Lagrangian Coherent Structures as a new frame to investigate the particle transport in highly chaotic magnetic systems
Robust in-line qualification of lattice structures manufactured via laser powder bed fusion
The shape complexity enabled by AM would impose new part inspection systems (e.g., x-ray computed tomography), which translate into qualification time and costs that may be not affordable. However, the layerwise nature of the process potentially allows anticipating qualification tasks in-line and in-process, leading to a quick detection of defects since their onset stage. This opportunity is particularly attractive in the presence of lattice structures, whose industrial adoption has considerably increased thanks to AM. This paper presents a novel methodology to model the quality of lattice structures at unit cell level while the part is being built, using high resolutions images of the powder bed for in-line geometry reconstruction and identification of deviations from the nominal shape. The methodology is designed to translate complex 3D shapes into 1D deviation profiles that capture the âgeometrical signatureâ of the cell together with the reconstruction uncertainty
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