4,532 research outputs found
Hamiltonian closures for fluid models with four moments by dimensional analysis
Fluid reductions of the Vlasov-Amp{\`e}re equations that preserve the
Hamiltonian structure of the parent kinetic model are investigated. Hamiltonian
closures using the first four moments of the Vlasov distribution are obtained,
and all closures provided by a dimensional analysis procedure for satisfying
the Jacobi identity are identified. Two Hamiltonian models emerge, for which
the explicit closures are given, along with their Poisson brackets and Casimir
invariants
A City-Scale ITS-G5 Network for Next-Generation Intelligent Transportation Systems: Design Insights and Challenges
As we move towards autonomous vehicles, a reliable Vehicle-to-Everything
(V2X) communication framework becomes of paramount importance. In this paper we
present the development and the performance evaluation of a real-world
vehicular networking testbed. Our testbed, deployed in the heart of the City of
Bristol, UK, is able to exchange sensor data in a V2X manner. We will describe
the testbed architecture and its operational modes. Then, we will provide some
insight pertaining the firmware operating on the network devices. The system
performance has been evaluated under a series of large-scale field trials,
which have proven how our solution represents a low-cost high-quality framework
for V2X communications. Our system managed to achieve high packet delivery
ratios under different scenarios (urban, rural, highway) and for different
locations around the city. We have also identified the instability of the
packet transmission rate while using single-core devices, and we present some
future directions that will address that.Comment: Accepted for publication to AdHoc-Now 201
Derivation of reduced two-dimensional fluid models via Dirac's theory of constrained Hamiltonian systems
We present a Hamiltonian derivation of a class of reduced plasma
two-dimensional fluid models, an example being the Charney-Hasegawa-Mima
equation. These models are obtained from the same parent Hamiltonian model,
which consists of the ion momentum equation coupled to the continuity equation,
by imposing dynamical constraints. It is shown that the Poisson bracket
associated with these reduced models is the Dirac bracket obtained from the
Poisson bracket of the parent model
Electronic structure and carrier transfer in B-DNA monomer polymers and dimer polymers: Stationary and time-dependent aspects of wire model vs. extended ladder model
We employ two Tight-Binding (TB) approaches to study the electronic structure
and hole or electron transfer in B-DNA monomer polymers and dimer polymers made
up of monomers (base pairs): (I) at the base-pair level, using the on-site
energies of base pairs and the hopping integrals between successive base pairs,
i.e., a wire model and (II) at the single-base level, using the on-site
energies of the bases and the hopping integrals between neighboring bases,
i.e., an \textit{extended} ladder model since we also include diagonal
hoppings. We solve a system of ("matrix dimension") coupled equations [(I)
= , (II) = ] for the time-independent problem, and a system of
coupled order differential equations for the time-dependent
problem. We study the HOMO and the LUMO eigenspectra, the occupation
probabilities, the Density of States (DOS) and the HOMO-LUMO gap as well as the
mean over time probabilities to find the carrier at each site [(I) base pair or
(II) base)], the Fourier spectra, which reflect the frequency content of charge
transfer (CT) and the pure mean transfer rates from a certain site to another.
The two TB approaches give coherent, complementary aspects of electronic
properties and charge transfer in B-DNA monomer polymers and dimer polymers.Comment: 20 pages, 23 figure
Isoenzyme A and Urinary N-Acetyl-β-D-Glucosaminidase Activity in Normal Pregnancy
Urinary N-acetyl-β-d-glucosaminidase (NAG) activity has been found to increase during normal uncomplicated pregnancy and such behavior could limit the diagnostic value of this enzyme for detection of subclinical tubular injury. The aim of this study was to evaluate urinary NAG activity and isoenzyme A in normal pregnant women at 30th week of pregnancy and in healthy women, to discriminate between physiological and lesional enzymuria.Enzyme activities in first morning fasting urine samples from 20 nonpregnant control and 20 normal pregnant women at 30th gestational week were evaluated by fluorometric methods.Both total and isoenzyme A activity was significantly higher ( p0.01) in urines of normal pregnant women compared with control urines, whereas ratio between these two parameters was significantly lower ( p0.001).The increase of urinary NAG activity during normal uncomplicated pregnancy appears to be characterized by a prevalent increase in isoenzyme A form, a finding associated with functional (not lesional) enzymuria. The fluorometric assays may represent a simple and rapid method to evaluate whether increase in urinary NAG activity represents a renal physiological adaptation during pregnancy
Mode signature and stability for a Hamiltonian model of electron temperature gradient turbulence
Stability properties and mode signature for equilibria of a model of electron
temperature gradient (ETG) driven turbulence are investigated by Hamiltonian
techniques. After deriving the infinite families of Casimir invariants,
associated with the noncanonical Poisson bracket of the model, a sufficient
condition for stability is obtained by means of the Energy-Casimir method. Mode
signature is then investigated for linear motions about homogeneous equilibria.
Depending on the sign of the equilibrium "translated" pressure gradient, stable
equilibria can either be energy stable, i.e.\ possess definite linearized
perturbation energy (Hamiltonian), or spectrally stable with the existence of
negative energy modes (NEMs). The ETG instability is then shown to arise
through a Kre\u{\i}n-type bifurcation, due to the merging of a positive and a
negative energy mode, corresponding to two modified drift waves admitted by the
system. The Hamiltonian of the linearized system is then explicitly transformed
into normal form, which unambiguously defines mode signature. In particular,
the fast mode turns out to always be a positive energy mode (PEM), whereas the
energy of the slow mode can have either positive or negative sign
Gyrofluid analysis of electron β e effects on collisionless reconnection
The linear and nonlinear evolutions of the tearing instability in a collisionless plasma with a strong guide field are analysed on the basis of a two-field Hamiltonian gyrofluid model. The model is valid for a low ion temperature and a finite. The finite effect implies a magnetic perturbation along the guide field direction, and electron finite Larmor radius effects. A Hamiltonian derivation of the model is presented. A new dispersion relation of the tearing instability is derived for the case and tested against numerical simulations. For the equilibrium electron temperature is seen to enhance the linear growth rate, whereas we observe a stabilizing role when electron finite Larmor radius effects become more relevant. In the nonlinear phase, stall phases and faster than exponential phases are observed, similarly to what occurs in the presence of ion finite Larmor radius effects. Energy transfers are analysed and the conservation laws associated with the Casimir invariants of the model are also discussed. Numerical simulations seem to indicate that finite effects do not produce qualitative modifications in the structures of the Lagrangian invariants associated with Casimirs of the model
On the use of projectors for Hamiltonian systems and their relationship with Dirac brackets
The role of projectors associated with Poisson brackets of constrained
Hamiltonian systems is analyzed. Projectors act in two instances in a bracket:
in the explicit dependence on the variables and in the computation of the
functional derivatives. The role of these projectors is investigated by using
Dirac's theory of constrained Hamiltonian systems. Results are illustrated by
three examples taken from plasma physics: magnetohydrodynamics, the
Vlasov-Maxwell system, and the linear two-species Vlasov system with
quasineutrality
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