1,461 research outputs found
High speed imaging of traveling waves in a granular material during silo discharge
We report experimental observations of sound waves in a granular material
during resonant silo discharge called silo music. The grain motion was tracked
by high speed imaging while the resonance of the silo was detected by
accelerometers and acoustic methods. The grains do not oscillate in phase at
neighboring vertical locations, but information propagates upward in this
system in the form of sound waves. We show that the wave velocity is not
constant throughout the silo, but considerably increases towards the lower end
of the system, suggesting increased pressure in this region, where the flow
changes from cylindrical to converging flow. In the upper part of the silo the
wave velocity matches the sound velocity measured in the same material when
standing (in the absence of flow). Grain oscillations show a stick-slip
character only in the upper part of the silo.Comment: 5 pages, 5 figures, accepted to Phys. Rev.
Whispering Gallery States of Antihydrogen
We study theoretically interference of the long-living quasistationary
quantum states of antihydrogen atoms, localized near a concave material
surface. Such states are an antimatter analog of the whispering gallery states
of neutrons and matter atoms, and similar to the whispering gallery modes of
sound and electro-magnetic waves. Quantum states of antihydrogen are formed by
the combined effect of quantum reflection from van der Waals/Casimir-Polder
(vdW/CP) potential of the surface and the centrifugal potential. We point out a
method for precision studies of quantum reflection of antiatoms from vdW/CP
potential; this method uses interference of the whispering gallery states of
antihydrogen.Comment: 13 pages 7 figure
Quantum Reciprocity Conjecture for the Non-Equilibrium Steady State
By considering the lack of history dependence in the non-equilibrium steady
state of a quantum system we are led to conjecture that in such a system, there
is a set of quantum mechanical observables whose retarded response functions
are insensitive to the arrow of time, and which consequently satisfy a quantum
analog of the Onsager reciprocity relations. Systems which satisfy this
conjecture can be described by an effective Free energy functional. We
demonstrate that the conjecture holds in a resonant level model of a multi-lead
quantum dot.Comment: References revised to take account of related work on Onsager
reciprocity in mesoscopics by Christen, and in hydrodynamics by Mclennan,
Dufty and Rub
Nature of acoustic nonlinear radiation stress
When a fluid is insonified with ultrasound, a flow consequence of a net stress becomes observable, which has been described as acoustic streaming, quartz wind, acoustic radiation force or acoustic fountain. Following Sir James Lighthill's formulation of the Reynold's streaming, these phenomena have been attributed to a cumulative viscous effect. Instead, a new multiscale effect, whereby the constitutive elastic nonlinearity scales from the ultrasonic to the macroscopic time, is here proposed and formulated to explain its origin. This raises a new term in the Navier-Stokes equation, which ultimately stems from the anharmonicity of the atomic potential. In our experimental validation, this theory is consistent in water and for a range of ultrasonic configurations, whereas the formerly established viscous theory fails by an order of magnitude. This ultrasonic-fluid interaction, called nonlinear mechanical radiation since it is able to remotely exert a stress field, correctly explains a wide range of industrial and biomedical active ultrasonic uses including jet engines, acoustic tweezers, cyanobacteria propulsion mechanisms, nanofluidics or acoustic radiation force elastography.Ministerio de EconomĂa y Competitividad (Spain) for Project DPI2010-17065, and Junta de AndalucĂa for Projects P11-CTS-8089 and GGI3000IDIB
The effect of pressure on statics, dynamics and stability of multielectron bubbles
The effect of pressure and negative pressure on the modes of oscillation of a
multi-electron bubble in liquid helium is calculated. Already at low pressures
of the order of 10-100 mbar, these effects are found to significantly modify
the frequencies of oscillation of the bubble. Stabilization of the bubble is
shown to occur in the presence of a small negative pressure, which expands the
bubble radius. Above a threshold negative pressure, the bubble is unstable.Comment: 4 pages, 2 figures, accepted for publication in Physical Review
Letter
On the attractors of two-dimensional Rayleigh oscillators including noise
We study sustained oscillations in two-dimensional oscillator systems driven
by Rayleigh-type negative friction. In particular we investigate the influence
of mismatch of the two frequencies. Further we study the influence of external
noise and nonlinearity of the conservative forces. Our consideration is
restricted to the case that the driving is rather weak and that the forces show
only weak deviations from radial symmetry. For this case we provide results for
the attractors and the bifurcations of the system. We show that for rational
relations of the frequencies the system develops several rotational excitations
with right/left symmetry, corresponding to limit cycles in the four-dimensional
phase space. The corresponding noisy distributions have the form of hoops or
tires in the four-dimensional space. For irrational frequency relations, as
well as for increasing strength of driving or noise the periodic excitations
are replaced by chaotic oscillations.Comment: 9 pages, 5 figure
Evolution of a barotropic shear layer into elliptical vortices
When a barotropic shear layer becomes unstable, it produces the well known
Kelvin-Helmholtz instability (KH). The non-linear manifestation of KH is
usually in the form of spiral billows. However, a piecewise linear shear layer
produces a different type of KH characterized by elliptical vortices of
constant vorticity connected via thin braids. Using direct numerical simulation
and contour dynamics, we show that the interaction between two
counter-propagating vorticity waves is solely responsible for this KH
formation. We investigate the oscillation of the vorticity wave amplitude, the
rotation and nutation of the elliptical vortex, and straining of the braids.
Our analysis also provides possible explanation behind the formation and
evolution of elliptical vortices appearing in geophysical and astrophysical
flows, e.g. meddies, Stratospheric polar vortices, Jovian vortices, Neptune's
Great Dark Spot and coherent vortices in the wind belts of Uranus.Comment: 7 pages, 4 figures, Accepted in Physical Review
SEA analysis in the cabin of a regional turboprop with metamaterial lining panels
The main goal of this paper is to evaluate the comfort, and hence the interior sound pressure
levels, in the cabin of a regional turboprop with metamaterial lining panels under Turbulent
Boundary Layer flow over the fuselage during cruise flight conditions. In the preliminary
work phase, the design of metamaterial and a numerical analysis at component level were
performed. Then, the CAD model of the fuselage was created representing the typical features
and dimensions of an airplane for regional flights and a Statistical Energy Analysis (SEA) model
was built by using Va One software. An investigation on the influence of designed metamaterial
on the soundproofing of the cabin was presented. Results reveal a reduction in Sound Pressure
Level (SPL) of almost 5 dB with respect to classical materials, in overall the frequency range
and for all the cavities analyzed, in the configuration with metamaterial applied as core of the
sandwich lining panels
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
Mechanical Instabilities of Biological Tubes
We study theoretically the shapes of biological tubes affected by various
pathologies. When epithelial cells grow at an uncontrolled rate, the negative
tension produced by their division provokes a buckling instability. Several
shapes are investigated : varicose, enlarged, sinusoidal or sausage-like, all
of which are found in pathologies of tracheal, renal tubes or arteries. The
final shape depends crucially on the mechanical parameters of the tissues :
Young modulus, wall-to-lumen ratio, homeostatic pressure. We argue that since
tissues must be in quasistatic mechanical equilibrium, abnormal shapes convey
information as to what causes the pathology. We calculate a phase diagram of
tubular instabilities which could be a helpful guide for investigating the
underlying genetic regulation
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