Prediction of aerodynamic noise reduction by using open-cell metal foam

As the speed of high-speed train (HST) increases continuously, aerodynamic noise has become more remarkable compared with the wheel/rail noise, which affects the inhabited environment along the railway and the riding comfort. This paper preliminarily investigates the feasibility of using open-cell metal foam covering layer to reduce the low Mach number aerodynamic noise generated by the flow around a circular cylinder which is the typical section of pantographs. The aerodynamic noises radiated from the circular cylinder with and without metal foam are calculated. The hybrid method combining two-dimensional large eddy simulation (LES) with Ffowcs Williams–Hawkings (FW–H) equation is employed. The calculated Strouhal number, time-averaged drag coefficient, base pressure and overall sound pressure level agree well with some available experimental data. Then, the influences of metal foam porosity, pore density, thickness of covering layer and the speed of train on the aerodynamic noise and the aerodynamic forces are investigated, and some detailed comparisons of flow field are made. The numerical results indicate that as a passive scheme, the open-cell metal foam with high porosity can modify the flow, adjust the vortex shedding frequency and regularize the wake, leading to a significant reduction of aerodynamic noise. The results are expected to provide useful information for the control of aerodynamic noise using this new material.     

Calculation of ground vibration spectra from heavy military vehicles

The demand for reliable autonomous systems capable to detect and identify heavy military vehicles becomes an important issue for UN peacekeeping forces in the current delicate political climate. A promising method of detection and identification is the one using the information extracted from ground vibration spectra generated by heavy military vehicles, often termed as their seismic signatures. This paper presents the results of the theoretical investigation of ground vibration spectra generated by heavy military vehicles, such as tanks and armed personnel carriers. A simple quarter car model is considered to identify the resulting dynamic forces applied from a vehicle to the ground. Then the obtained analytical expressions for vehicle dynamic forces are used for calculations of generated ground vibrations, predominantly Rayleigh surface waves, using Green's function method. A comparison of the obtained theoretical results with the published experimental data shows that analytical techniques based on the simplified quarter car vehicle model are capable of producing ground vibration spectra of heavy military vehicles that reproduce basic properties of experimental spectra.     

Atomic displacements due to interstitial hydrogen in Cu and Pd

The density functional theory (DFT) is used to study the atomic interactions in transition metal-based interstitial alloys. The strain field is calculated in the discrete lattice model using Kanzaki method. The total energy and hence atomic forces between interstitial hydrogen and transition metal hosts are calculated using DFT. The norm-conserving pseudopotentials for H, Cu and Pd are generated self-consistently. The dynamical matrices are evaluated considering interaction up to first nearest neighbors whereas impurity-induced forces are calculated with M₃₂H shell (where M = Cu and Pd). The atomic displacements produced by interstitial hydrogen at the octahedral site in Cu and Pd show displacements of 7.36% and 4.3% of the first nearest neighbors respectively. Both Cu and Pd lattices show lattice expansion due to the presence of hydrogen and the obtained average lattice expansion ΔV/V = 0.177 for Cu and 0.145 for Pd.      

Enhanced optical forces between coupled resonant metal nanoparticles

We investigate numerically the optical forces between noble metal nanoparticles sustaining localized surface plasmon resonances. Our results first point out enhanced binding optical forces compared with dielectric nanoparticles and nonresonant metallic nanoparticles. We also show that under suitable illumination conditions, short-range forces tend to make the nanoparticles cluster, leading to intense and localized hot spots in the interstices. This effect corroborates recent experimental observations of an enhanced Raman signal in trapped metal sphere ensembles.     

Optical manipulation of plasmonic nanoparticles

We investigate numerically the optical forces exerted by an incident light beam on metal nanoparticles (MNP) sustaining the so-called localized surface plasmons (LSP). We first describe how the particle dispersion can be used to tune the respective contribution from extinction and gradient forces. By a suitable adjustment of the illumination conditions, single MNP can be selectively guided, sorted and trapped. The second part of our work investigates the interparticle forces existing within a MNP ensemble. Our results show that MNP located in a conventional optical trap can self-arrange under optical forces according to specific architectures. In particular, at very low distances, they tend to agglomerate into metal clusters leading to very high field concentration in the interstices. PACS 71.45.Gm; 71.36.+c; 87.80.Cc     

Scaling of optical forces in dielectric waveguides: rigorous connection between radiation pressure and dispersion

We show that eigenmodes of dielectric optical waveguides exert surface dilation forces on waveguide boundaries owing to radiation pressure, and we develop an exact scaling law relating modal dispersion of an arbitrary dielectric waveguide to the magnitude of optical forces generated by radiation pressure. This result points to highly dispersive waveguides as an optimal choice for the generation of large optical forces in nano-optomechanical systems. Exact agreement with ab initio calculations is demonstrated.     

Electrical Current in Sinai Billiards Under General Small Forces

The Lorentz gas of $\mathbb{Z}^{2}$ -periodic scatterers (or the so called Sinai billiards) can be used to model motion of electrons on a metal. We investigate the linear response for the system under various external forces (during both the flight and the collision). We give some characterizations under which the forced system is time-reversible, and derive an estimate of the electrical current generated by the forced system. Moreover, applying Pesin entropy formula and Young dimension formula, we get several characterizations of the non-equilibrium steady state of the forced system.     

Crystallographic determination of metal surfaces with helium scattering

Helium-surface diffraction data has been used to construct the interaction potential between a Helium atom and a metal surface. The constructed potential fits experimental data for all beam energies and angles of incidence and more importantly is independent of beam energy. Such a potential essentially consists of two parts: a short range repulsive corrugated potential due to electronic overlap, and a long range attractive potential due to dispersion forces. We have generated potentials for He scattering for Ni, Cu and Au and in all cases found excellent agreement with experiments. We also report that the repulsive part of the He scattering potential can be obtained rather simply from first principles without adjustable parameters. This suggests that one can determine surface crystallography from first principles without adjustable parameters.     

Electrostatic guiding of cold polar molecules on a chip

We propose a novel scheme to guide cold polar molecules on the surface of an insulating substrate (i.e. a chip) using an electrostatic field generated by the combination of a pair of parallel charged wires and a grounded metal plate. The spatial distributions of the electric fields from the above charged-wire layout and their Stark potentials for cold CO molecules and dipole forces are calculated, and the relationships between the electric field and the geometric parameters of our charged-wire system are analyzed. Our study shows that our charged-wire scheme can be used to guide cold polar molecules in the weak-field-seeking state, and to construct various molecular optical elements, such as a molecular funnel, a molecular beam splitter and a molecular interferometer and so on, to form various integrated molecular optical elements and their molecular chips, and even to generate a continuous wave (CW) cold molecular beam by using a low-pass energy filter based on bent two-wire guiding.     

Electrostatic surface guiding of cold polar molecules with a single charged wire

This paper proposes a scheme to guide cold polar molecules by using a single charged wire half embanked in an insulating substrate and a homogeneous bias electric field, which is generated by a plate capacitor composed of two infinite parallel metal plates. The spatial distributions of the electrostatic field produced by the combination of the charged wire and the plate capacitor and the corresponding Stark potentials (including dipole forces) for metastable CO molecules are calculated, the relationships between the electric field and the parameters of our charged-wire layout are analysed. It also studies the influences of the insulator on the electric field distribution and the discharge effect. This study shows that the proposed scheme can be used to guide cold polar molecules in the weak-field -- seeking states, and to form various molecule-optical elements, such as molecular funnel, molecular beam-splitters and molecule interferometer, even to construct a variety of integrated molecule-optical elements and their molecule chips.     

Force generation by polymerizing microtubules

Polymerization ratchets formed by the assembly of actin filaments and microtubules are possibly the simplest realizations of biological thermal ratchets. A variety of experimental evidence exists that significant forces are generated by these processes, but quantitative studies lag far behind similar studies for molecular motors such as kinesin and myosin. Here we present a discussion of the theory of polymerization ratchets as well as experimental techniques used in our laboratory for the study of forces generated by single growing microtubules. Data obtained with these techniques provide us with valuable information that may eventually allow us to distinguish between different models for the growth of microtubules. Received: 15 January 2002 / Accepted: 11 February 2002 / Published online: 22 April 2002     

Infrared and Raman spectra of MnS2

The infrared and Raman spectra of the pyrite-type crystal MnS₂ are reported. Nine of the ten optically allowed $\text{q≌0}$ phonons are observed. A comparison of the phonon frequencies with those of FeS₂ allows us to conclude that the electrostatic forces as well as the covalent sulfur-sulfur bonds are about the same strength in the two compounds. The short-range forces between metal and sulfur ions in the two cases, however, are quite different.     

Electromigration forces on ions in carbon nanotubes

We calculate the electromigration forces on ions such as adsorbed alkali metal atoms in a carbon nanotube transistor. The forces are especially large in the turn-on regime of the transistor and much smaller in the off and on states. Electromigration in the channel is driven almost exclusively by the wind force. The sign of the "effective valence" Z* is independent of the actual charge sign but can be reversed with gate voltage, providing a dramatic illustration of the quantum character of the wind force. Our self-consistent nonequilibrium Green's function calculations treat a ballistic device within a tight-binding approximation.     

Bimetallic adhesive force at small separation

Metal films in intimate contact exert strong adhesive forces on each other. The force between metal films at zero separation is obtained first from the analytic expression of Heinrichs and Kumar (HK) and then from our earlier numerical calculations. The agreement is excellent. HK's reported large disagreement between forces computed by the two methods is incorrect.     

Stable multilayer structure based on restoring Casimir forces

The stability of the mesoscopic multilayer structure resulting from the Casimir effect is investigated. For the multilayer structure composed of periodic metal and metamaterial layers, we focus on the formation of the restoring Casimir forces that simultaneously stably equilibrate each layer of the structure with the suitably chosen frequency-dependent electromagnetic properties of the materials. The stable equilibrium distances for the layers are discussed, and the influences of the metamaterial characteristic frequencies and of the layer thickness are shown. Since the system stability is induced only by the Casimir forces, this mesoscopic multilayer can be considered as a stable non-touching “Casimir solid”.     

The scratching mechanism for a plastic metal

A scratching cone can be considered as a knife edge with a negative leading angle; a conical indentor rises towards the surface of the metal on scratching with an unchanged normal load on account of work-hardening in the metal and of frictional forces, which have a component in opposition to the vertical load.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 95–101, June, 1971.     

A pseudopotential based theory of the driving forces for electromigration in metals

We present a pseudopotential calculation of the driving forces for atomic migration in metals in the presence of electron currents. When electrons are scattered by impurities in a metal, we find that a force is generally exerted on each atom in the vicinity of the scattering center. Because the scattering is predominantly elastic, it is possible to express this force field as the classical electrostatic force arising from the total electronic charge, as has been assumed by Friedel and Bosvieux. The electron charge density is determined from a pseudopotential calculation, and the resulting force is expressed as a sum of effective interactions between the diffusing atom and all crystal defects.The forces on an atom arising from the electron scattering and from the applied electric field together comprise the driving force which causes a net current of atoms. The driving forces are calculated for intestitial and vacancy migration in several metals, and the results are found to compare favorably with most experimental data.     

A Stochastic Version of the Noether Theorem

A stochastic version of the Noether theorem is derived for systems under the action of external random forces. The concept of moment generating functional is employed to describe the symmetry of the stochastic forces. The theorem is applied to two kinds of random covariant forces. One of them generated in an electrodynamic way and the other is defined in the rest frame of the particle as a function of the proper time. For both of them, it is shown the conservation of the mean value of a random drift momentum. The validity of the theorem makes clear that random systems can produce causal stochastic correlations between two faraway separated systems, that had interacted in the past. In addition possible connections of the discussion with the Ives Couder’s experimental results are remarked.