Showing papers on "Momentum published in 2002"
TL;DR: In this paper, the detection of cosmic rays with unexpectedly high energies has prompted a rethink of Einstein's theory of special relativity and a new formulation, called "doubly special relativity", might be the answer.
Abstract: The detection of cosmic rays with unexpectedly high energies has prompted a rethink of Einstein's theory of special relativity. A new formulation, called 'doubly special relativity', might be the answer.
370 citations
TL;DR: In this article, the energy dependence of the saturation momentum in the presence of saturation boundaries limiting the region of linear BFKL evolution was studied and it was shown that the scaling of the scattering amplitude is valid in a wide range of momenta for fixed coupling evolution and in a more restricted region for running coupling evolution.
316 citations
TL;DR: In this paper, a molecular dynamics simulation method which can generate configurations belonging to the canonical (T, V, N) ensemble or the constant temperature constant pressure ensemble was proposed, which is tested for an atomic fluid (Ar) and works well.
Abstract: A molecular dynamics simulation method which can generate configurations belonging to the canonical (T, V, N) ensemble or the constant temperature constant pressure (T, P, N) ensemble, is proposed The physical system of interest consists of N particles (f degrees of freedom), to which an external, macroscopic variable and its conjugate momentum are added This device allows the total energy of the physical system to fluctuate The equilibrium distribution of the energy coincides with the canonical distribution both in momentum and in coordinate space The method is tested for an atomic fluid (Ar) and works well
300 citations
TL;DR: In this article, an acoustic analogy using linearized Euler's equations (LEE) forced with aerodynamic source terms is investigated to computetheacousticfare eld. And the results obtained by solving LEE are in good agreement with the reference solution.
Abstract: An acoustic analogy using linearized Euler’ s equations (LEE) forced with aerodynamic source terms is investigated to computetheacousticfare eld. Thishybridmethod isappliedto threemodelproblemssimulatedby solving Navier‐Stokes equations. In this way, its validity is estimated by comparing the predicted acoustic e eld with the reference solution given directly by the Navier ‐Stokes equations. The noise radiated by two corotating vortices is studied: e rst, in a medium at rest and, second, in a mean sheared e ow with no convection velocity. Then the sound e eld generated by vortex pairings in a subsonic mixing layer is investigated. In this case, a simplie ed formulation of LEE is proposed to prevent the exponential growth of instability waves. The acoustic e elds obtained by solving LEE are in good agreement with the reference solution. This study shows that the source terms introduced into the LEE are appropriate for free sheared e ows and that acoustic ‐mean e ow interactions are properly taken into account in the wave operator. Nomenclature b = half-width of the monopolar source c = sound velocity E;F;H = vectors in linearized Euler’ s equations (LEE) f = frequency f0 = fundamental frequency of the mixing layer k = complex wave number, kr Ciki M = Mach number p = pressure Re = Reynolds number rc = vortex core radius r0 = initial half distance between the two vortices S = sound source vector in LEE Si = source terms in the momentum equations T = period Tij = Lighthill’ s tensor t = time U = unknown vector in LEE U1 = slow stream velocity of the mixing layer U2 = rapid stream velocity of the mixing layer u = velocity vector, .u1;u2/ Vµ = initial tangential velocity of vortices
269 citations
TL;DR: Measurements of wave breaking are presented, using aerial imaging and analysis, and a statistical description of related sea-surface processes are provided to find that the distribution of the length of breaking fronts per unit area of sea surface is proportional to the cube of the wind speed, and that the fraction of the ocean surface mixed by breaks is dominated by wave breaking at low velocities and short wavelengths.
Abstract: Surface waves play an important role in the exchange of mass, momentum and energy between the atmosphere and the ocean. The development of the wave field depends on wind, wave-wave and wave-current interactions and wave dissipation owing to breaking, which is accompanied by momentum fluxes from waves to currents. Wave breaking supports air-sea fluxes of heat and gas, which have a profound effect on weather and climate. But wave breaking is poorly quantified and understood. Here we present measurements of wave breaking, using aerial imaging and analysis, and provide a statistical description of related sea-surface processes. We find that the distribution of the length of breaking fronts per unit area of sea surface is proportional to the cube of the wind speed and that, within the measured range of the speed of the wave fronts, the length of breaking fronts per unit area is an exponential function of the speed of the front. We also find that the fraction of the ocean surface mixed by breaking waves, which is important for air-sea exchange, is dominated by wave breaking at low velocities and short wavelengths.
245 citations
CERN1
TL;DR: The facility is described, reports on the latest neutron spectral measurements, gives an overview of the most important experiments performed by the various collaborating institutions over recent years and briefly addresses the possible application of the facility to measurements related to the space programme.
Abstract: A reference facility for the calibration and inter-comparison of active and passive detectors in broad neutron fields has been available at CERN since 1992. A positively charged hadron beam (a mixture of protons and pions) with momentum of usually 120 GeV/c hits a copper target, 50 cm thick and 7 cm in diameter. The secondary particles produced in the interaction traverse a shield, at 90° with respect to the direction of the incoming beam, made of either 80 to 160 cm of concrete or 40 cm of iron. Behind the iron shield, the resulting neutron spectrum has a maximum at about 1 MeV, with an additional high-energy component. Behind the 80 cm concrete shield, the neutron spectrum has a second pronounced maximum at about 70 MeV and resembles the high-energy component of the radiation field created by cosmic rays at commercial flight altitudes. This paper describes the facility, reports on the latest neutron spectral measurements, gives an overview of the most important experiments performed by the various collaborating institutions over the past years, addresses the possible application of the facility to measurements related to the space programme and discusses the latest calculations performed in view of its development for such use.
243 citations
TL;DR: In this article, the authors analyze observational constraints from TeV astrophysics on Lorentz violating nonlinear dispersion for photons and electrons without assuming any a priori equality between the photon and electron parameters.
Abstract: We analyze observational constraints from TeV astrophysics on Lorentz violating nonlinear dispersion for photons and electrons without assuming any a priori equality between the photon and electron parameters. The constraints arise from thresholds for vacuum \ifmmode \check{C}\else \v{C}\fi{}erenkov radiation, photon decay and photo-production of electron-positron pairs. We show that the parameter plane for cubic momentum terms in the dispersion relations is constrained to an order unity region in Planck units. We find that the threshold configuration can occur with an asymmetric distribution of momentum for pair creation, and with a hard photon for vacuum \ifmmode \check{C}\else \v{C}\fi{}erenkov radiation.
221 citations
TL;DR: In this paper, a second-order integral model for a round turbulent buoyant jet is proposed based on new experimental data on turbulent mass and momentum transport, and the model employs the widely used entrainment assumption with the entrainments coefficient taken to be a function of the local Richardson number.
Abstract: The development of a second-order integral model for a round turbulent buoyant jet is reported based on new experimental data on turbulent mass and momentum transport. The mean and turbulent characteristics of a round vertical buoyant jet covering the full range from jets to plumes were investigated using a recently developed combined digital particle image velocimetry (DPIV) and planar laser-induced fluorescence (PLIF) system. The system couples the two well-known techniques to enable synchronized planar measurements of flow velocities and concentrations in a study area. The experimental results conserved the mass and momentum fluxes introduced at the source accurately with closure errors of less than 5%. The momentum flux contributed by turbulence and streamwise pressure gradient was determined to be about 10% of the local mean momentum flux in both jets and plumes. The turbulent mass flux, on the other hand, was measured to be about 7.6% and 15% of the mean mass flux for jets and plumes respectively. While the velocity spread rate was shown to be independent of the flow regime, the concentration-to-velocity width ratio λ varied from 1.23 to 1.04 during the transition from jet to plume. Based on the experimental results, a refined second-order integral model for buoyant jets that achieves the conservation of total mass and momentum fluxes is proposed. The model employs the widely used entrainment assumption with the entrainment coefficient taken to be a function of the local Richardson number. Improved prediction is achieved by taking into account the variation of turbulent mass and momentum fluxes. The variation of turbulent mass flux is modelled as a function of the local Richardson number. The turbulent momentum flux, on the other hand, is treated as a fixed percentage of the local mean momentum flux. In addition, unlike most existing integral models that assume a constant concentration-to-velocity width ratio, the present model adopts a more accurate approach with the ratio expressed as a function of the local Richardson number. As a result, smooth transition of all relevant mean and turbulent characteristics from jet to plume is predicted, which is in line with the underlying physical processes.
204 citations
TL;DR: In this paper, the spin relaxation time of conduction electrons through the Elliot-Yafet, D'yakonov-Perel, and Bir-AronovPikus mechanisms is calculated theoretically for bulk GaAs, GaSb, InAs, and InSb of both n and p type.
Abstract: The spin relaxation time of conduction electrons through the Elliot-Yafet, D'yakonov-Perel, and Bir-Aronov-Pikus mechanisms is calculated theoretically for bulk GaAs, GaSb, InAs, and InSb of both n and p type. The relative importance of each spin relaxation mechanism is compared, and diagrams showing the dominant mechanism are constructed as a function of the temperature and impurity concentration. Our approach is based upon theoretical calculations of the momentum relaxation rate, and allows one to understand the interplay between various factors affecting the spin relaxation over a broad range of temperature and impurity concentration.
159 citations
TL;DR: In this article, the behavior of the oscillating shear layer produced by gravity waves below the surface convection zone of the Sun was studied and it was shown that under asymmetric filtering produced by this layer, gravity waves of low spherical order that are stochastically excited at the base of the convective zone of late-type stars can extract angular momentum from their radiative interior.
Abstract: We review the behavior of the oscillating shear layer produced by gravity waves below the surface convection zone of the Sun. We show that, under asymmetric filtering produced by this layer, gravity waves of low spherical order that are stochastically excited at the base of the convection zone of late-type stars can extract angular momentum from their radiative interior. The timescale for this momentum extraction in a Sun-like star is on the order of 107 yr. The process is particularly efficient in the central region, and it could produce there a slowly rotating core.
154 citations
TL;DR: Numerical methods for dissipative particle dynamics, a system of stochastic differential equations for simulating particles interacting pairwise according to a soft potential at constant temperature, are studied.
Abstract: We study numerical methods for dissipative particle dynamics, a system of stochastic differential equations for simulating particles interacting pairwise according to a soft potential at constant temperature where the total momentum is conserved. We introduce splitting methods and examine the behavior of these methods experimentally. The performance of the methods, particularly temperature control, is compared to the modified velocity Verlet method used in many previous papers.
TL;DR: An exact uncertainty principle, formulated as the assumption that a classical ensemble is subject to random momentum fluctuations of a strength which is determined by and scales inversely with uncertainty in position, leads from the classical equations of motion to the Schrodinger equation as discussed by the authors.
Abstract: An exact uncertainty principle, formulated as the assumption that a classical ensemble is subject to random momentum fluctuations of a strength which is determined by and scales inversely with uncertainty in position, leads from the classical equations of motion to the Schrodinger equation.
TL;DR: In this article, the effects of electric conductivity (which corresponds to the percentage by weight of NaCl in liquid layer) and microwave power level on the heating process were examined.
TL;DR: In this article, it was shown that the time accuracy of the basic projection method is first-order as a result of the momentum/pressure coupling, but that by modifying the coupling directly, or by modifying intermediate velocity boundary conditions, it is possible to recover second-order behaviour.
Abstract: Fractional-step methods solve the unsteady Navier–Stokes equations in a segregated manner, and can be implemented with only a single solution of the momentum/pressure equations being obtained at each time step, or with the momentum/pressure system being iterated until a convergence criterion is attained.The time accuracy of such methods can be determined by the accuracy of the momentum/pressure coupling, irrespective of the accuracy to which the momentum equations are solved. It is shown that the time accuracy of the basic projection method is first-order as a result of the momentum/pressure coupling, but that by modifying the coupling directly, or by modifying the intermediate velocity boundary conditions, it is possible to recover second-order behaviour. It is also shown that pressure correction methods, implemented in non-iterative or iterative form and without special boundary conditions, are second-order in time, and that a form of the non-iterative pressure correction method is the most efficient for the problems considered. Copyright © 2002 John Wiley & Sons, Ltd.
TL;DR: In this paper, it was shown that the recoil momentum spectrum is the Radon transform of the WIMP velocity distribution, which can be used to obtain analytic expressions for the recoil spectra of a variety of velocity distributions.
Abstract: Directional dark matter detectors will be able to record the recoil momentum spectrum of nuclei hit by dark matter WIMPs. We show that the recoil momentum spectrum is the Radon transform of the WIMP velocity distribution. This allows us to obtain analytic expressions for the recoil spectra of a variety of velocity distributions. We comment on the possibility of inverting the recoil momentum spectrum and obtaining the three-dimensional WIMP velocity distribution from data.
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TL;DR: In this paper, the effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed with a view to possible applications in nuclear physics and astrophysics.
Abstract: Brueckner calculations including a microscopic three-body force have been extended to isospin asymmetric nuclear matter. The effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed with a view to possible applications in nuclear physics and astrophysics. It is shown that, even in the presence of the three-body force, the empirical parabolic law of the energy per nucleon vs isospin asymmetry $\beta=(N-Z)/A$ is fulfilled in the whole asymmetry range $0\le\beta\le 1$ up to high densities. The three-body force provides a strong enhancement of symmetry energy increasing with the density in good agreement with relativistic approaches. The Lane's assumption that proton and neutron mean fields linearly vary vs the isospin parameter is violated at high density in the presence of the three-body force. Instead the momentum dependence of the mean fields is rather insensitive to three body force which brings about a linear isospin deviation of the neutron and proton effective masses. The isospin effects on multifragmentation events and collective flows in heavy-ion collisions are briefly discussed along with the conditions for direct URCA processes to occur in the neutron-star cooling.
TL;DR: In this article, the Navier-Stokes equation for compressible viscous fluid is considered on the half space in R3 under the zero-Dirichlet boundary condition for the momentum with initial data near an arbitrarily given equilibrium of positive constant density and zero momentum.
Abstract: The Navier-Stokes equation for compressible viscous fluid is considered on the half space in R3 under the zero-Dirichlet boundary condition for the momentum with initial data near an arbitrarily given equilibrium of positive constant density and zero momentum. Time decay properties in L2 norms for solutions of the linearized problem are investigated to obtain the rate of convergence in L2 norms of solutions to the equilibrium when initial data are sufficiently close to the equilibrium in \(\). Some lower bounds are derived for solutions to the linearized problem, one of which indicates a nonlinear phenomenon not appearing in the case of the Cauchy problem on the whole space.
TL;DR: In this paper, the mass, heat, and momentum fluxes are determined to first order in the spatial gradients of the hydrodynamic fields, and the associated transport coefficients are identified.
Abstract: Hydrodynamic equations for a binary mixture of inelastic hard spheres are derived from the Boltzmann kinetic theory. A normal solution is obtained via the Chapman–Enskog method for states near the local homogeneous cooling state. The mass, heat, and momentum fluxes are determined to first order in the spatial gradients of the hydrodynamic fields, and the associated transport coefficients are identified. In the same way as for binary mixtures with elastic collisions, these coefficients are determined from a set of coupled linear integral equations. Practical evaluation is possible using a Sonine polynomial approximation, and is illustrated here by explicit calculation of the relevant transport coefficients: the mutual diffusion, the pressure diffusion, the thermal diffusion, the shear viscosity, the Dufour coefficient, the thermal conductivity, and the pressure energy coefficient. All these coefficients are given in terms of the restitution coefficients and the ratios of mass, concentration, and particle s...
TL;DR: In this paper, the amplitude and frequency of the superimposed oscillations in terms of the slow roll parameter were derived and presented in a form that exhibits the amplitude of these small oscillations given the lower bound of the modifications of the power spectrum.
Abstract: As a simple model for unknown Planck scale physics, we assume that the quantum modes responsible for producing primordial curvature perturbations during inflation are placed in their instantaneous adiabatic vacuum when their proper momentum reaches a fixed high energy scale M. The resulting power spectrum is derived and presented in a form that exhibits the amplitude and frequency of the superimposed oscillations in terms of $H/M$ and the slow roll parameter $\ensuremath{\epsilon}.$ The amplitude of the oscillations is proportional to the third power of $H/M.$ We argue that these small oscillations give the lower bound of the modifications of the power spectrum if the notion of free mode propagation ceases to exist above the critical energy scale M.
TL;DR: In this article, the authors investigated the reaction mechanism for nucleon knockout by electron scattering in complex nuclei and showed that the electron-nucleon scattering process can be divided into soft Coulomb distortion and hard-scattering events.
Abstract: Nucleon knockout by electron scattering provides a powerful probe of the electromagnetic properties of nucleons and of the momentum distributions in nuclei. Since the nucleus is transparent with respect to the electromagnetic interaction, the nuclear volume can be probed uniformly. The weakness of the electromagnetic interaction allows one to separate the soft Coulomb distortion of the electron scattering process from the hard-scattering event in which, to a very good approximation, a single virtual photon transfers its energy and momentum to the nuclear electromagnetic current. The kinematic flexibility of electron scattering permits the momentum and energy transfers to be varied independently, with various kinematical conditions emphasizing different aspects of the reaction mechanism. For example, under conditions emphasizing different aspects of the reaction mechanism. For example, under conditions in which a single high-energy nucleon receives most of the energy transfer, the quasifree electron-nucleon scattering process is emphasized. This report investigates the following topics to understand this knockout process: Open-Photon-Exchange Approximation; Nucleon Form Factors; Proton Knockout Experiments on Few-Body Systems; Distorted Wave Analysis of (e,e{prime}N) Reactions; Spectral Functions from (e,e{prime}p) on Complex Nuclei; and Studies of the Reaction Mechanism for Nucleon Knockout. 390 refs., 7 figs., 7 tabs.
TL;DR: In this paper, two new momentum interpolation methods, called MMIM1 and MMIM2, are proposed to achieve numerical solutions that are independent of both the underrelaxation factor and the time step size.
Abstract: Discussions are given of the different momentum interpolation methods to evaluate the interface velocity in the collocated grid system. It is pointed out that the interface velocity is used in three cases in the overall numerical procedure of the solution of Navier-Stokes equations by utilizing a collocated grid: in the continuity equation; in the interface flow rate computation for the determination of the coefficients in discretization equation; and in the mass residual in the coefficient Ap. Analysis shows that it is better to adopt the momentum interpolation method in the three cases. Two new momentum interpolation methods, called MMIM1 and MMIM2, are proposed. Analysis shows that the two new methods can achieve numerical solutions that are independent of both the underrelaxation factor and the time step size. Taking lid-driven cavity flow as an example, numerical computations are conducted for several Reynolds numbers and different mesh sizes using the SIMPLE algorithm, and the results are compared w...
TL;DR: In this article, the vertical fluxes of mass, momentum, and energy were derived from point measurements in a horizontally homogeneous atmosphere, and a formula for the mean vertical velocity was found that is accurate to the ideal gas approximation.
Abstract: In a horizontally homogeneous atmosphere, the calculation of thevertical fluxes of mass, momentum, and heat from point measurementsare to lowest order calculated as the standard eddy covariance.Additional terms primarily related to the mean vertical velocity areadded to improve the accuracy of the calculation. A review ofprevious work is undertaken. The fluxes are reformulated beginningwith the equations of conservation of mass, momentum, and energy. Aformula for the mean vertical velocity is found that is accurate tothe ideal gas approximation. Relations are developed between fluxesat different heights. In addition to the relations for fluxes ofmass, sensible heat, and latent heat, the fluxes of momentum andturbulent kinetic energy are also considered.
TL;DR: By treating the real Maxwell field and real linearized Einstein equations as being imbedded in complex Minkowski space, one can interpret magnetic moments and spin-angular momentum as arising from a charge and mass monopole source moving along a complex world line in the complex Minksowski space as discussed by the authors.
Abstract: By treating the real Maxwell field and real linearized Einstein equations as being imbedded in complex Minkowski space, one can interpret magnetic moments and spin-angular momentum as arising from a charge and mass monopole source moving along a complex world line in the complex Minkowski space. In the circumstances where the complex center of mass world line coincides with the complex center of charge world line, the gyromagnetic ratio is that of the Dirac electron.
TL;DR: In this paper, a new analytical model is developed for the equilibrium range of the spectrum of wind-forced ocean surface gravity waves, which is based on the recent understanding of the wind forcing of waves.
Abstract: A new analytical model is developed for the equilibrium range of the spectrum of wind-forced ocean surface gravity waves. We first show that the existing model of Phillips (1985) does not satisfy overall momentum conservation at high winds. This constraint is satisfied by applying recent understanding of the wind forcing of waves. Waves exert a drag on the air flow so that they support a fraction of the applied wind stress, which thus leaves a smaller turbulent stress near the surface to force growth of shorter wavelength waves. Formulation of the momentum budget accounting for this sheltering constrains the overall conservation of momentum and leads to a local turbulent stress that reduces as the wavenumber increases. This local turbulent stress then forces wind-induced wave growth. Following Phillips (1985), the wind sea is taken to be a superposition of linear waves, and equilibrium is maintained by a balance between the three sources and sinks of wave action. These assumptions lead to analytical formulae for the local turbulent stress and the degree of saturation, B(k), of waves in the equilibrium range. We identify a sheltering wavenumber, k s , over which the local turbulent stress is significantly reduced by longer waves. At low wavenumbers or at low winds, when k « k s , the sheltering is weak and B(k) has a similar form to the model of Phillips (1985). At higher wavenumbers or at higher winds, k >> k s , B(k) makes a transition to being proportional to k°. The additional constraint of conservation of momentum also yields a formula for the coefficient that appears in the solution for B(k). The spectra for mature seas are calculated from the model and are shown to agree with field observations. In particular, our model predicts more realistic spectral levels toward the high wavenumber limit compared to the previous model of Phillips (1985). We suggest that the model may explain the overshoot phenomena observed in the spectral energy levels as the fetch increases.
TL;DR: In this article, the authors used boson-fermion mapping to show that the single-particle momentum distribution in a one-dimensional gas of hard point-like bosons (Tonks gas) inside a harmonic trap decays as p-4 at large momentum p.
TL;DR: In this paper, the stability of linear and angular momentum of the center of mass and the underlying coordination of body segments was investigated for a sit-to-stand task to better understand how the nervous system organizes the redundant degrees of freedom available to accomplish this task.
Abstract: Oscillatory flow in pulsating heat pipes (PHPs) with arbitrary numbers of turns is investigated numerically. The PHP is placed vertically with the evaporator sections at the top and the condenser sections at the bottom. The governing equations, obtained by analysis of the conservation of mass, momentum, and energy of the liquid and vapor plugs, are nondimensionalized, and the problem is described by eight nondimensional parameters. The numerical solution is obtained by employment of an implicit scheme. The effects of the number of turns, length of the heating and cooling sections, and charge ratio on the performance of the PHP were also investigated.
TL;DR: In this paper, Monte Carlo techniques are used to model nonlinear particle acceleration in parallel collisionless shocks of various speeds, including mildly relativistic ones, and the absolute normalization of the particle distribution follows directly from their model assumptions and is explicitly determined.
TL;DR: The wave function of the phonons was shown to be a superposition of +q and -q free particle momentum states, in agreement with the Bogoliubov quasiparticle picture.
Abstract: Phonons with wave vector $q/\ensuremath{\Elzxh}$ were optically imprinted into a Bose-Einstein condensate. Their momentum distribution was analyzed using Bragg spectroscopy with a high momentum transfer. The wave function of the phonons was shown to be a superposition of $+q$ and $\ensuremath{-}q$ free particle momentum states, in agreement with the Bogoliubov quasiparticle picture.
TL;DR: In this paper, the sputtering yield of a cylindrical thermal spike is calculated using a two-dimensional fluid-dynamics model which includes the transport of energy, momentum, and mass.
Abstract: The sputtering yield Y from a cylindrical thermal spike is calculated using a two-dimensional fluid-dynamics model which includes the transport of energy, momentum, and mass. The results show that the high pressure built up within the spike causes the hot core to perform a rapid expansion both laterally and upwards. This expansion appears to play a significant role in the sputtering process. It is responsible for the ejection of mass from the surface and causes fast cooling of the cascade. The competition between these effects accounts for the nearly linear dependence of Y with the deposited energy per unit depth that was observed in recent molecular-dynamics simulations. Based on this we describe the conditions for attaining a linear yield at high excitation densities and give a simple model for this yield.