1,018 research outputs found
Canard Cycles and Poincar\'e Index of Non-Smooth Vector Fields on the Plane
This paper is concerned with closed orbits of non-smooth vector fields on the
plane. For a subclass of non-smooth vector fields we provide necessary and
sufficient conditions for the existence of canard kind solutions. By means of a
regularization we prove that the canard cycles are singular orbits of singular
perturbation problems which are limit periodic sets of a sequence of limit
cycles. Moreover, we generalize the Poincar\'e Index for non-smooth vector
fields.Comment: 20 pages, 25 figure
Numerical study of a banded precipitation event over Italy
Satellite images of 30 October 2008 show the development
over north-central Italy of rainbands and multiple
waves during a strong south-westerly wind episode
associated with a deepening synoptic trough and
cold front passage.
The event was studied by means of the ISAC model chain constituted
of the hydrostatic model BOLAM and the nested non-hydrostatic model
MOLOCH at 1.1 km resolution.
Diagnostics of model output was performed to reveal the
physical origin of the dynamical features and precipitation
field as simulated.
Based on our results we propose a theoretical framework in
which symmetric instability underlies some of the observed
precipitation patterns
Numerical study of a banded precipitation event over Italy
Satellite images of 30 October 2008 show the development
over north-central Italy of rainbands and multiple
waves during a strong south-westerly wind episode
associated with a deepening synoptic trough and
cold front passage.
The event was studied by means of the ISAC model chain constituted
of the hydrostatic model BOLAM and the nested non-hydrostatic model
MOLOCH at 1.1 km resolution.
Diagnostics of model output was performed to reveal the
physical origin of the dynamical features and precipitation
field as simulated.
Based on our results we propose a theoretical framework in
which symmetric instability underlies some of the observed
precipitation patterns
OROGRAPHIC INFLUENCE ON DEEP CONVECTION: CASE STUDY AND SENSITIVITY EXPERIMENTS
The non hydrostatic convection resolving model MOLOCH is employed in order to evaluate its capability to realistically simulate the evolution of a mesoscale convective system responsible for an episode of heavy rainfall and flood over southeastern France (Gard event). Numerical experiments indicate large sensitivity of precipitation amounts and distribution, due to different cell organization and propagation, to the specification of the initial
conditions.
Further experiments, aimed at studying the role played by the orography in triggering the convection and controlling its evolution, have been performed in order to characterize sensitivity to ambient wind and orography. Although the convective system remained almost stationary for many hours with maximum precipitation located at some distance upstream of the mountain main slope, simulations demonstrate that the presence of the orographic barrier is essential
for both triggering and maintaining the mesoscale convective system. The intensity of precipitation turns out to be sensitive to small variations of the mean meridional wind component
Equilibrium states for non-uniformly expanding maps: decay of correlations and strong stability
We study the rate of decay of correlations for equilibrium states associated
to a robust class of non-uniformly expanding maps where no Markov assumption is
required. We show that the Ruelle-Perron-Frobenius operator acting on the space
of Holder continuous observables has a spectral gap and deduce the exponential
decay of correlations and the central limit theorem. In particular, we obtain
an alternative proof for the existence and uniqueness of the equilibrium states
and we prove that the topological pressure varies continuously. Finally, we use
the spectral properties of the transfer operators in space of differentiable
observables to obtain strong stability results under deterministic and random
perturbations.Comment: 29 pages, Annales de l'Institut Henri Poincare - Analyse non lineaire
(to appear
Orographic triggering of long lived convection in three dimensions
A significant fraction of the occurrences of intense flash floods is due to quasi-stationary or long-lived convection that may insist on the same place for many hours, producing high values of accumulated precipitation. One of the elements that favour the initiation and anchoring of the convective system (MCS) is the orography. In one of the most severe floods (Gard basin in southern France, 8-9 September 2002), the orography of the Massif Central played a rather unusual role, favouring the onset and maintenance of the MCS at some distance upstream of the main orographic slope. In the present work the initial atmospheric conditions of this event have been largely idealized, taking horizontally uniform values for wind, temperature and humidity profiles, and a simplified isolated orography representing the sole Massif Central. A convective system is initiated in the non-hydrostatic simulations, embedded in a quasi-stationary solution of flow over the orography. It is shown that the triggering of convection occurs in the convergence zone immediately upstream of the orographic obstacle, at an altitude comparable with the mountain height. The subsequent growth of the mesoscale convective system is associated with a slow eastward drift, with the intense precipitation located upstream of the mountain and with the formation of a gust front that propagates against the incoming basic flow. Sensitivity experiments show that the development of convection critically depends on mountain height and moisture content. Although the results obtained in such idealized conditions do not reflect all the observed characteristics of the real event, they contribute to clarify the role of the orography in triggering and maintaining strong convection
Evaluation of an Internet Document Delivery Service
An Internet-based Document Delivery Service (DDS) has been developed within the framework of the CNR ( the Italian Research National Council) Project BiblioMIME, in order to take advantage of new Internet technologies and promote cooperation among CNR and Italian university libraries. Adopting such technologies changes the traditional organisation of DDS and may drastically reduce costs and delivery times. An information system managing DDS requests and monitoring the temporal evolution of the service has been implemented, running on the local-area network of a test-site library. It aims to track number and types of documents requested and received, user distribution, delivery times and types (surface mail, fax, Internet), to automate repetitive manual procedures and to deal with the various accounting methods used by other libraries. Transmission of documents is carried out by means of an e-mail/Web gateway system supporting document exchange via Internet, which assists receiving libraries in retrieving requested documents. This paper describes the architecture and main design features of the e-mail/Web gateway server (the BiblioMime server). This approach permits librarians to continue using e-mail service to send large documents, while resolving problems that users may encounter when downloading large size files with e-mail agents. The library operator sends the document as an attachment to the destination address; on fly the e-mail server extracts and saves the attachments in a web-server disk file and substitutes them with a new message part that includes an URL pointing to the saved document. The receiver can download these large objects by means of a user-friendly browser. We further discuss the data gathered during the triennium 1998-2000; this consists of about 5,000 DDS transactions per annum with 300 other Italian scientific and bio-medical libraries and commercial document suppliers. Use of the instruments described above allowed us to evaluate the performance of service "before" and "after" the use of Internet Document Delivery and to extract some critical data regarding DDS. Those include: a) libraries with which we have greater numbers of exchanges and their turnaround times; b) extraordinary reduction in costs and delivery times; c) the most frequently requested serial titles (allowing cost-effective decisions on new subscriptions); d) impact on DDS of library participation in consortia which allow user access to greater numbers of online serials
Pressure tuning of light-induced superconductivity in K3C60
Optical excitation at terahertz frequencies has emerged as an effective means
to manipulate complex solids dynamically. In the molecular solid K3C60,
coherent excitation of intramolecular vibrations was shown to transform the
high temperature metal into a non-equilibrium state with the optical
conductivity of a superconductor. Here we tune this effect with hydrostatic
pressure, and we find it to disappear around 0.3 GPa. Reduction with pressure
underscores the similarity with the equilibrium superconducting phase of K3C60,
in which a larger electronic bandwidth is detrimental for pairing. Crucially,
our observation excludes alternative interpretations based on a high-mobility
metallic phase. The pressure dependence also suggests that transient, incipient
superconductivity occurs far above the 150 K hypothesised previously, and
rather extends all the way to room temperature.Comment: 33 pages, 17 figures, 2 table
Closed-Loop Control of Local Magnetic Actuation for Robotic Surgical Instruments
We propose local magnetic actuation (LMA) as an approach to robotic actuation for surgical instruments. An LMA actuation unit consists of a pair of diametrically magnetized single-dipole cylindrical magnets, working as magnetic gears across the abdominal wall. In this study, we developed a dynamic model for an LMA actuation unit by extending the theory proposed for coaxial magnetic gears. The dynamic model was used for closed-loop control, and two alternative strategies-using either the angular velocity at the motor or at the load as feedback parameter-were compared. The amount of mechanical power that can be transferred across the abdominal wall at different intermagnetic distances was also investigated. The proposed dynamic model presented a relative error below 7.5% in estimating the load torque from the system parameters. Both the strategies proposed for closed-loop control were effective in regulating the load speed with a relative error below 2% of the desired steady-state value. However, the load-side closed-loop control approach was more precise and allowed the system to transmit larger values of torque, showing, at the same time, less dependence from the angular velocity. In particular, an average value of 1.5 mN·m can be transferred at 7 cm, increasing up to 13.5 mN·m as the separation distance is reduced down to 2 cm. Given the constraints in diameter and volume for a surgical instrument, the proposed approach allows for transferring a larger amount of mechanical power than what would be possible to achieve by embedding commercial dc motors
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