Acoustic attenuation, phase and group velocities in liquid-filled pipes: Theory, experiment, and examples of water and mercury

Del Grosso's [Acustica 24, 299-311 (1971)] formulation, which predicts the phase speed of propagating axisymmetric modes inside a liquid-filled tube, is here extended to the complex domain in order to predict the attenuation, as well as the sound speed, of the modes as a function of frequency. Measurements of the sound speeds and the attenuations of the modes were performed in a water-filled Poly (methyl methacrylate) (PMMA) tube of internal radius, b=4.445 cm, in the range of the wavenumber-radius product, k(1)b, from 2 to 10. Parts of three or four modes were investigated and the measured sound speeds and the damping of the modes were compared with the theoretical predictions. The theory was then used to estimate the modal sound speeds and attenuations in a stainless-steel pipe filled with mercury having the same dimensions as are used in the Spallation Neutron Source at Oak Ridge National Laboratory, Tennessee.     

Hypersound attenuation and phase velocities in evaporated polycrystalline films of silver,copper and constantan between 1 and 24 GHz

A model for a recombination instability in a semiconductor far from equilibrium is analysed. It is based upon the simultaneous impact ionization of ground state and excited donors at low temperature. The number and the stability of spatially homogeneous steady states and their dependence upon external control parameters is investigated. For certain values of these parameters bistability andS-shaped current-voltage characteristics are found. Nonequilibrium phase transitions of first order between low and high conductivity states can be induced by varying the applied voltage. If certain generation coefficients are negligible, the transition changes from first to second order above some tricritical value of the other parameters.     

PaNdata: Open Data Infrastructure for Photon and Neutron Sources

Today's scientific research is conducted not just by single experiments, but rather by sequences of related experiments or projects linked by a common theme that lead to a greater understanding of the physical world. This is particularly true of research at large-scale facilities such as synchrotrons, neutron sources, and free-electron lasers. These provide a wide variety of techniques, instruments, and scientific applications as services to the user communities. The common denominator of the different kinds of experiments undertaken at facilities is the need to manage, visualize, and analyze vast amounts of data with high accuracy and reliability in near- to real time. This poses significant challenges on the underlying e-infrastructures at the facilities. The data volumes (up to 10 PB per year), peak data rates, the extremely large number of files of varying sizes and formats, and the unpredictable access patterns pose severe demands on the data infrastructures.     

Dispersion and attenuation by transmission,reflection, and mode conversion in welded pipes

Torsional wave dispersion and attenuation in an open empty welded pipe are determined from a multi-receiver position reflection experiment. The fundamental torsional wave is dominantly reflected at the free end and the converted non-axisymmetric flexural modes are naturally attenuated. The resulting phase velocity contours are in agreement with theoretical predictions. The transmission losses are quantified and compared to those reflective elements associated with end and weld reflection. At any reflective node, the incident wave is split between back and forward preserved mode scattering (“reflection/transmission”), conversion to other modes plus energy lost by absorption. The ratios for each element are quantified.     

Measurements of ultrasonic phase velocities and attenuation of slow waves in cellular aluminum foams as cancellous bone-mimicking phantoms

The water-saturated aluminum foams with an open network of interconnected ligaments were investigated by ultrasonic transmission technique for the suitability as cancellous bone-mimicking phantoms. The phase velocities and attenuation of nine samples covering three pores per inch (5, 10, and 20 PPI) and three aluminum volume fractions (5, 8, and 12% AVF) were measured over a frequency range of 0.7-1.3 MHz. The ligament thickness and pore sizes of the phantoms and low-density human cancellous bones are similar. A strong slow wave and a weak fast wave are observed for all samples while the latter is not visible without significant amplification (100x). This study reports the characteristics of slow wave, whose speeds are less than the sound speed of the saturating water and decrease mildly with AVF and PPI with an average 1469 m/s. Seven out of nine samples show positive dispersion and the rest show minor negative dispersion. Attenuation increases with AVF, PPI, and frequency except for the 20 PPI samples, which exhibit non-increasing attenuation level with fluctuations due to scattering. The phase velocities agree with Biot's porous medium theory. The RMSE is 16.0 m/s (1%) at n = 1.5. Below and above this value, the RMSE decreases mildly and rises sharply, respectively.     

Mixing rules for group velocities in nanocomposite materials and photonic crystals

Mixing rules for group velocities in nanocomposite materials with different architecture, including lamellarinhomogeneous nanotextures, Maxwell Garnett structures, and one-dimensional photonic crystals, are derived and analyzed. The group velocity can be controlled for such composite structures by changing nanocrystal sizes and varying the dielectric properties and the content of the constituent materials. The interference of scattered waves in structures with a spatial scale of optical inhomogeneities comparable to the radiation wavelength gives rise to new physical phenomena that cannot be described in terms of the effective-medium approximation.     

Simultaneous measurement of the phase and group velocities of Lamb waves in a laser-generation based imaging method

This paper describes a novel approach to the simultaneous measurement of the phase and group velocities of Lamb waves based on images of their propagation. The laser-generation based imaging method was first introduced to obtain images of Lamb wave propagation. The time series of snapshot images is used to make a position-time diagram, and the velocities can be estimated based on the slopes of the position curves. Thus, the phase and group velocities can be obtained by measuring the phase advance and energy flow of the Lamb wave, respectively. Details of the principle of simultaneous measurement are presented herein. Experimental verification was also performed in the range of 0.2-3.0 MHz-mm using aluminum plates. The average errors between experiment and theory in the phase and group velocities were 3.31% and 5.68%, respectively.     

Application of the WKB method to calculating the group velocities and attenuation coefficients of normal waves in the arctic underwater waveguide

Algorithms based on the WKB approximation are proposed for the fast and accurate calculation of the group time delays and effective attenuation coefficients of normal waves in the deep-water sound channel of the Arctic Ocean. These characteristics of the modes are determined in the adiabatic approximation by integrating the local group velocity and attenuation coefficient over the horizontal distance between the ends of the propagation path. According to the WKB method, the local group velocity is the ratio of two quantities. The first one is the sum of the length of the ray corresponding to the mode and the side displacement of the ray at the reflection by the ice cover. The second one is the sum of the travel time of the sound signal along the ray cycle and the time delay caused by the side displacement. The grazing angle of the ray is determined from the condition of quantization for the phase integral. According to the WKB method, the local attenuation coefficient of the mode is specified as the ratio of the squared modulus of the coherent reflection coefficient at the lower boundary of the ice cover and the sum of the cycle length and the side displacement of the ray. Simple recurrent relations are proposed to estimate, with fair accuracy and short calculating time, the phase integral, the integral that describes the cycle length, and the related local group velocities and attenuation coefficients. The capacity and efficiency of the algorithms are confirmed by the comparison of the aforementioned mode characteristics calculated by using the proposed relations and the precise computer code. The calculations are performed with the sound speed profiles obtained from the temperature and salinity measurements during the SEVER and SCICEX-1995 expeditions.     

Statistical properties of shocks in Burgers turbulence,II: Tail probabilities for velocities,shock-strengths and rarefaction intervals

This paper studies the structure of the random sawtooth profile corresponding to the solution of the inviscid Burgers equation with white-noise initial data. This function consists of a countable sequence of rarefaction waves separated by shocks. We are concerned here with calculating the probabilities of rare events associated with the occurrence of very large values of the normalized velocity, shock-strength and rarefaction intervals. We find that these quantities have tail probabilities of the form exp{–Cx ³},x1. This cubic exponential decay of probabilities was conjectured in the companion paper [1]. The calculations are done using a representation of the shock-strength and length of rarefaction intervals in terms of the statistics of certain conditional diffusion processes.     

Optimizing long-range order, band gap, and group velocities for graphene on close-packed metal surfaces

We compare different growth methods with the aim of optimizing the long-range order of a graphene layer grown on Ru(0001). Combining chemical vapor deposition with carbon loading and segregation of the surface layer leads to autocorrelation lengths of 240 ?. We present several routes to band gap and charge carrier mobility engineering for the example of graphene on Ir(111). Ir cluster superlattices self-assembled onto the graphene moiré pattern produce a strong renormalization of the electron group velocity close to the Dirac point, leading to highly anisotropic Dirac cones and the enlargement of the gap from 140 to 340 meV. This gap can further be enhanced to 740 meV by Na co-adsorption onto the Ir cluster superlattice at room temperature. This value is close to that of Ge, and the high group velocity of the charge carriers is fully preserved. We also present data for Na adsorbed without the Ir clusters. In both cases we find that the Na is on top of the graphene layer.     

Magnetostructural phase transitions in NiO and MnO: Neutron diffraction data

Structural and magnetic phase transitions in NiO and MnO antiferromagnets have been studied by high-precision neutron diffraction. The experiments have been performed on a high-resolution Fourier diffractometer (pulsed reactor IBR-2), which has the record resolution for the interplanar distance and a high intensity in the region of large interplanar distances; as a result, the characteristics of both transitions have been determined simultaneously. It has been shown that the structural and magnetic transitions in MnO occur synchronously and their temperatures coincide within the experimental errors: T_str ≈ T_mag ≈ (119 ± 1) K. The measurements for NiO have been performed with powders with different average sizes of crystallites (~1500 nm and ~138 nm). It has been found that the transition temperatures differ by ~50 K: T_str = (471 ± 3) K, T_mag = (523 ± 2) K. It has been argued that a unified mechanism of the “unsplit” magnetic and structural phase transition at a temperature of T_mag is implemented in MnO and NiO. Deviation from this scenario in the behavior of NiO is explained by the quantitative difference—a weak coupling between the magnetic and secondary structural order parameters.     

Interference of scattered waves and mixing rules for group velocities in nanocomposite materials

Mixing rules for group velocities in nanocomposite materials with different architecture, including lamellar-inhomogeneous nanotextures, Maxwell Garnett structures, and one-dimensional photonic crystals, are derived and analyzed. The group velocity can be controlled for such composite structures by changing nanocrystal sizes and varying the dielectric properties and the volume-filling fractions of the constituent materials. The interference of scattered waves in structures with a spatial scale of optical inhomogeneities comparable to the radiation wavelength gives rise to new physical phenomena that cannot be described in terms of the effective-medium approximation.     

The attenuation of lined plenum chambers in ducts,II: Measurements and comparison with theory

Experimental measurements of the acoustic performance of single and three-pass lined plenum chambers are compared to calculations based on theoretical models described in a companion paper [1]. Generally, quite good agreement is obtained, subject to the limitations of the theories. For the sake of completeness, comparison is made between the performance of a single plenum chamber and that of an equivalent splitter silencer. The two are seen to differ somewhat in their attenuation characteristics. The aerodynamic pressure losses of all three silencers are compared, and observations are made concerning the relative mass, construction time, et cetera, of the single chamber and the splitter. A tentative design procedure for plena is suggested.     

Electromagnetically-induced transparency,slow light,and negative group velocities in a room temperature vapor of 4He1

⁴He¹ at room temperature is a particularly interesting system as velocity changing collisions (VCCs) are necessary to observe ultra-narrow (less than 10 kHz) EIT windows for purely electronic spins in the presence of Doppler broadening. Such narrow resonances are known to be linked to a dramatic reduction of the group velocity of a probe pulse, although the medium is transparent. The evolution of the delay is recorded with respect to the coupling beam intensity and to small Raman detunings. We also demonstrate that it is possible to use optically detuned resonances (Fano-like profiles) to see a transition from slow light to negative group velocity. All these measurements are found to be in good agreement with a simple model based on an effective homogeneous linewidth. To cite this article: F. Goldfarb et al., C. R. Physique 10 (2009).     

Structural,elastic, and vibrational properties of phase H:A first-principles simulation

Phase H(MgSiO_4H_2), one of the dense hydrous magnesium silicates(DHMSs), is supposed to be vital to transporting water into the lower mantle. Here the crystal structure, elasticity and Raman vibrational properties of the two possible structures of phase H with Pm and P2/m symmetry under high pressures are evaluated by first-principles simulations. The cell parameters, elastic and Raman vibrational properties of the Pm symmetry become the same as the P2/m symmetry at～ 30 GPa. The symmetrization of hydrogen bonds of the Pm symmetry at ～ 30 GPa results in this structural transformation from Pm to P2/m. Seismic wave velocities of phase H are calculated in a range from 0 GPa to 100 GPa and the results testify the existence and stability of phase H in the lower mantle. The azimuthal anisotropies for phase H are A_(P0)= 14.7%,A_(S0)= 21.2%(P2/m symmetry) and A_(P0)= 16.4%, A_(S0)= 27.1%(Pm symmetry) at 0 GPa, and increase to A_(P30)= 17.9%,A_(S30)= 40.0%(P2/m symmetry) and A_(P30)= 19.2%, A_(S30)= 37.8%(Pm symmetry) at 30 GPa. The maximum V P direction for phase H is [101] and the minimum direction is [110]. The anisotropic results of seismic wave velocities imply that phase H might be a source of seismic anisotropy in the lower mantle. Furthermore, Raman vibrational modes are analyzed to figure out the effect of symmetrization of hydrogen bonds on Raman vibrational pattern and the dependence of Raman spectrum on pressure. Our results may lead to an in-depth understanding of the stability of phase H in the mantle.     

Energy contributions in magnetite nanoparticles: computation of magnetic phase diagram,theory, and simulation

In this study, we present a theoretical analysis of magnetization processes by considering energy contributions in magnetite fine particles. The focus is on the K _S-driven magnetic phase transition taking place between the low surface-anisotropy ferrimagnetic state and the hedgehog configuration obtained in the high surface-anisotropy limit. Analytical expressions of energy terms (exchange, magnetocrystalline anisotropy, surface-anisotropy) are presented and their magnitudes are computed for different particle sizes. Monte Carlo simulations were also carried out for comparison purposes. A core–shell model is implemented for simulating magnetite nanoparticles between 2 and 10 nm in diameter. Our simulation framework is based on a three-dimensional classical Heisenberg-like Hamiltonian with nearest magnetic neighbors interactions. It includes exchange coupling, cubic magnetocrystalline anisotropy for core ions, and single-ion site surface-anisotropy for those atoms belonging to the shell. The magnetic phase diagram and comparisons between analytical and numerical results are presented and discussed.