4,880 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
Phase Diagrams of Forced Magnetic Reconnection in Taylor's Model
Recent progress in the understanding of how externally driven magnetic
reconnection evolves is organized in terms of parameter space diagrams. These
diagrams are constructed using four pivotal dimensionless parameters: the
Lundquist number , the magnetic Prandtl number , the amplitude of the
boundary perturbation , and the perturbation wave number .
This new representation highlights the parameters regions of a given system in
which the magnetic reconnection process is expected to be distinguished by a
specific evolution. Contrary to previously proposed phase diagrams, the
diagrams introduced here take into account the dynamical evolution of the
reconnection process and are able to predict slow or fast reconnection regimes
for the same values of and , depending on the parameters that
characterize the external drive, never considered so far. These features are
important to understand the onset and evolution of magnetic reconnection in
diverse physical systemsComment: Comments: 13 pages, 2015 Workshop "Complex plasma phenomena in the
laboratory and in the universe
Turbulence and secondary motions in square duct flow
We study turbulent flows in pressure-driven ducts with square cross-section
through direct numerical simulation in a wide enough range of Reynolds number
to reach flow conditions which are representative of fully developed
turbulence. Numerical simulations are carried out over extremely long
integration times to get adequate convergence of the flow statistics, and
specifically high-fidelity representation of the secondary motions which arise.
The intensity of the latter is found to be in the order of 1-2% of the bulk
velocity, and unaffected by Reynolds number variations. The smallness of the
mean convection terms in the streamwise vorticity equation points to a simple
characterization of the secondary flows, which in the asymptotic high-Re regime
are found to be approximated with good accuracy by eigenfunctions of the
Laplace operator. Despite their effect of redistributing the wall shear stress
along the duct perimeter, we find that secondary motions do not have large
influence on the mean velocity field, which can be characterized with good
accuracy as that resulting from the concurrent effect of four independent flat
walls, each controlling a quarter of the flow domain. As a consequence, we find
that parametrizations based on the hydraulic diameter concept, and
modifications thereof, are successful in predicting the duct friction
coefficient
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
Influence of space allowance on the welfare of weaned buffalo (Bubalus Bubalis) calves
Twenty weaned female buffalo calves were used to evaluate the effect of space allowance in relation to their body surface area on a range of behavioural and physiological parameters. Body surface area in m2 was calculated as 0.12 body weight0.60. Ten calves received 50% of body surface as space allocation (Group 50), 10 others received 90% of body surface area (Group 90). Animals in Group 50 lay with a lower number of outstretched legs than calves in Group 90. Buffaloes from Group 50 were observed standing more frequently than animals from Group 90 ( P < 0.001). The proportions of idling ( P < 0.01) and lying idle observations ( P < 0.001) were higher for Group 90 than for Group 50. Group 90 performed a higher number of non-agonistic interactions than Group 50 ( P < 0.01), whereas the opposite was observed for the number of agonistic interactions ( P < 0.01). When exposed to open field testing, Group 50 animals displayed an increased duration of movement, number of galloping events and more vocalisation. Neither immune responses to phytohemagglutinin and ovalbumin nor the cortisol response to exogenous ACTH were affected by treatment. It was concluded that 50% of body surface area may be an inadequate space allowance for weaned calves
The qualitative assessment of responsiveness to environmental challenge in horses and ponies.
The responsiveness of 10 horses and 10 ponies to environmental challenge (represented by an open field
test) was assessed using a qualitative approach based on free choice profiling methodology (FCP), which
gives observers complete freedom to choose their own descriptive terms. Data were analysed with
generalised Procrustes analysis (GPA), a multivariate statistical technique associated with FCP. A
cross-validation of the outcomes of this approach to data recorded through quantitative behaviour analysis,
and through a questionnaire given to the animals’ owner/riding instructor, was also performed using
principal component analysis (PCA). Twelve undergraduate students generated their own descriptive
vocabularies, by watching 20 horse/pony video clips lasting 2.5 min each. GPA showed that the consensus
profile explained a high percentage of variation among the 12 observers, and differed significantly from the
mean randomised profile ( p < 0.001). Two main dimensions of the consensus profile were identified,
explaining 60% and 5.2% of the variation between animals, respectively. The 12 observer word charts
interpreting these dimensions were semantically consistent, as they all converged towards the same
meaning, albeit using different terms. The most used term to describe the positive end of axis 1 was
‘‘quiet’’, whereas ‘‘attentive’’ was the best positive descriptor of axis 2. The most frequently used
descriptors for the negative ends of axes 1 and 2 were ‘‘nervous’’ and ‘‘bored’’, respectively. Thus, axis
1 was labelled as ‘‘quiet/nervous’’ and axis 2 was named as ‘‘attentive/bored’’. A marked effect of animal
category was observed on the scores of the animals on the first dimension ( p < 0.001). Horses received
significantly higher scores, and were thus assessed as more quiet and calm, than ponies. Conversely, ponies
tended to receive lower scores on the second dimension ( p < 0.12), therefore they appeared less curious and attentive. The results of the PCA showed that the variables from different types of measurement clearly had
meaningful relationships. For instance, the variables with the highest loading on the positive end of axis 1
were all indicative of tractable and docile animals, whereas axis 2 showed high loadings on the positive end
for variables indicating attentive animals. Qualitative behaviour assessment proved to be an appropriate
methodology for the study of horse behavioural responsiveness, in that it provided a multifaceted
characterisation of horse behavioural expression that was in agreement with other quantitative and
subjective assessments of the animals’ behaviour
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
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