Detection of object resonances by vibro-acoustography and numerical vibrational mode identification

Chalk sphere and cylinder resonance frequencies related to compressional and bending modes were detected in water, using vibro-acoustography, a relatively new imaging technique. The variable (radiation) force of low-frequency excitation, produced by intersecting two primary focused ultrasound waves with slightly different frequencies, forces the object to vibrate. The low-frequency acoustic emission field, resulting from object vibration, was detected by a hydrophone. By fixing the object at the focus of the ultrasound beam and sweeping the frequency of one of the primary beams within a chosen bandwidth, it was possible to detect some of the resonance frequencies (those related to compressional and bending modes) via variations in acoustic emission amplitude. Experimental results showed excellent agreement with finite element calculations. This method can be used to characterize the presence of heterogeneities in various media, in the field of materials science or biology.     

Positron annihilation in molecules by capture into vibrational Feshbach resonances of infrared-active modes

Enhanced positron annihilation on polyatomic molecules is a long-standing and complex problem. We report the results of calculations of resonant positron annihilation on methyl halides. A free parameter of our theory is the positron binding energy. A comparison with energy-resolved annihilation rates measured for CH3F, CH3Cl, and CH3Br [L. D. Barnes, Phys. Rev. A 74, 012706 (2006)10.1103/PhysRevA.74.012706] shows good agreement and yields estimates of the binding energies.     

Determination of bound states and resonances of the Debye-potential by complex scaling

For a statically screened Coulomb-potential, two particle bound and resonant states are determined which correspond to complex poles of the S-matrix. The complex scaling method is used in order to calculate both the real eigenvalues of the Hamiltonian and the complex resonances in the same way. The eigenfunctions are described by complex power series; for the coefficients a recursion formula is given.     

Trapped modes and resonances for water waves over a slightly perturbed bottom

We construct exact solutions describing trapped water waves over an underwater ridge of small height in the shallow water approximation and in the complete formulation. Resonances (antibound states) in the case of an underwater trench are also constructed.     

Solitons and their resonances on two-dimensional superfluid films

The dynamics of saturated two-dimensional superfluid⁴He films is shown to be governed by the Kadomtsev-Petviashvili equation with negative dispersion. It is established that the phenomena of soliton resonance could be observed in such films. Under the lowest order nonlinearity, such resonance would happen only if two dimensional effects are taken into account. The amplitude and velocity of the resonant soliton are obtained.     

Spatial resonances and superposition patterns in a reaction-diffusion model with interacting Turing modes

Spatial resonances leading to superlattice hexagonal patterns, known as "black-eyes," and superposition patterns combining stripes and/or spots are studied in a reaction-diffusion model of two interacting Turing modes with different wavelengths. A three-phase oscillatory interlacing hexagonal lattice pattern is also found, and its appearance is attributed to resonance between a Turing mode and its subharmonic.     

Baryon resonances and their properties from the CLAS data

The study of the baryon spectrum is a fundamental part of the scientific program in Hall B at Jefferson Lab. By using the CLAS detector, reactions involving electromagnetic production of exclusive hadronic final states in the kinematic region of baryon resonances have been extensively measured, with the purpose of extracting information on non-strange baryon excited states. Some selected topics in the N ^* CLAS experimental program are outlined and corresponding new experimental results are reported and briefly discussed.     

Localization of a transverse elastic wave in a semibounded acoustic superlattice of ferrimagnetic and superconducting layers: 3. Leaky surface modes and associated resonances

In terms of the effective medium method, conditions for the existence of leaky shear surface acoustic waves in a semibounded fine-layered magnetic superlattice consisting of ferrimagnetic and superconducting layers are determined. On this basis, the possibility of a resonance interaction between a surface elastic SH wave propagating in the magnetic superlattice and a shear bulk wave propagating in the adjacent nonmagnetic medium is investigated.     

Comparison of continuous-wave (CW) and tone-burst (TB) excitation modes in vibro-acoustography: Application for the non-destructive imaging of flaws

This paper describes the application of continuous-wave (CW) and tone-burst (TB) vibro-acoustography (VA) experiments for imaging a flawed composite plate. For both modes, the ultrasound frequency is set at f₁ = 3 MHz and f₂ = 3 MHz + ∣Δf∣. The plate was placed at the focus of the transducer and scanned point-by-point over an area of 60 mm by 50 mm on its frontal face with an increment step equal to 0.25 mm/pixel. The resulting acoustic emission amplitude at ∣Δ f∣ is recorded. For the CW mode the difference frequency was set at ∣Δf∣ = 12.9 kHz. For the TB mode, the burst-emitted signal was 100 μs long at a pulse repetition frequency (PRF) of 100 Hz corresponding to bursts of 300 cycles at 3 MHz, and the difference frequency was set at ∣Δf∣ = 44 kHz. The resulting VA images readily show the shape of the flaws. The images also reveal considerable detail of internal substructures such as the fibers used to reinforce the plate. However, the CW VA image shows an artifact caused by the effect of ultrasound standing waves established between the plate and the concave surface of the transducer, resulting in masking some of the flaws. On the other hand, the TB-VA image is free from such artifact. Despite some advantages of using TB-VA, there are some limitations related to this mode. Advantages and limitations of using the two modes are discussed.     

Efimov states and their Fano resonances in a neutron-rich nucleus

Asymmetric resonances in elastic n+19C scattering are attributed to Efimov states of such neutron-rich nuclei, that is, three-body bound states of the n+n+18C system when none of the pairs is bound or some of them are only weakly bound. By fitting to the general resonance shape described by Fano, we extract the resonance position, width, and the "Fano profile index." While Efimov states have been discussed extensively in many areas of physics, there is only one very recent experimental observation in trimers of cesium atoms. The conjunction that we present of the Efimov and Fano phenomena may lead to experimental realization in nuclei.     

Al-Cu approximants and associated B2 chemical-twinning modes

The Al3Cu4 alloy, with an e/a ratio of 1.86 being close to ternary Al-Cu-TM (transition metal) quasicrystals, has been chosen for the search of Al-Cu approximants. Phase structures and compositions were studied using TEM, X-ray diffraction and EPMA techniques. Two new phases were found: face-centered orthorhombic oF-Al43.2Cu56.8 (a = 0.816(6), b = 1.414(9), c = 0.999(5) nm) and body-centered orthorhombic oI-Al41.3Cu58.7 (oI, a = 0.408(3), b = 0.707(4), c = 0.999(5) nm). Their e/a ratios are the same as that of the Al-Cu-Fe icosahedral quasicrystal. Both are B2 superstructures and their unit cell components can be expressed approximately as oF-Al36Cu48vacancies12 and oI-Al8Cu12vacancies4. They both exist in twinning variants of the types 120 degrees/[001] and 180 degrees/[310]. Such twinning modes indicate that these orthorhombic phases are the decomposition products of a high-temperature parent phase epsilon2-Al2Cu3, the atomic structure of which shows pentagonal atomic arrangements. Further analysis on the twinning modes of oF and oI leads to the recognition of the chemical-twinning mode of the basic B2 structure as 180 degrees/(111)B2. This kind of chemical twinning mode is responsible for the pentagonal atomic configuration in the Al-Cu approximants as well as for the pseudo-5-fold B2 twinning.     

Localized surface modes and resonances for vicinal surfaces: The (117) face of fcc crystals

Considering the (117) face of an fcc crystal, the dispersion relations of localized surface modes and resonances have been calculated using the method of generating coefficients of Green functions. The force between atoms are limited to the central force between the nearest neighbours. In the direction of propagation orthogonal to a step, for each wavevector value, a localized surface mode and two resonances have been found. Their polarization amplitude and width have been calculated. It is shown that these results can be qualitatively understood by folding three times the dispersion relation of the (001) face which is the crystallographic plane of the terrace. In the direction parallel to the step, the dispersion relations are obtained by the superposition of the sagittal and transverse dispersion relation of the (001) face. The (117) configuration introduces a coupling between these two polarizations and some resonances appear. A general rule which enables one to predict qualitatively the shape of the dispersion relation for any vicinal surface of type (11m) is expressed.     

Determination of the dynamics of temperature variation in a model object by acoustic thermography

An experiment on monitoring the dynamics of internal temperature variation in a model object by the acoustic thermography method is carried out. The measurements were performed in a cell filled with an aqueous solution of glycerol, into which a plasticine object was placed. Thermal acoustic radiation of the object was measured in the course of its heating and cooling. Two bars of acoustic thermometers positioned on two sides of the object were used for this purpose. The results of measurements allowed the reconstruction of the dynamics of the varying two-dimensional distribution of in-depth temperature. The position of the heated region, its temperature, and its characteristic size are estimated. In addition, an estimate is obtained for the absorption coefficient.     

Neutron resonances in 45Sc and their relation to thermal scattering parameters

The total neutron cross section of ⁴⁵Sc from 400 eV to 22 keV has been measured. The results are shown to be incompatible with the postulate that a J = 4 bound state dominates thermal scattering.     

Tail resonances of Fermi-Pasta-Ulam q-breathers and their impact on the pathway to equipartition

Upon initial excitation of a few normal modes the energy distribution among all modes of a nonlinear atomic chain (the Fermi-Pasta-Ulam model) exhibits exponential localization on large time scales. At the same time, resonant anomalies (peaks) are observed in its weakly excited tail for long times preceding equipartition. We observe a similar resonant tail structure also for exact time-periodic Lyapunov orbits, coined q-breathers due to their exponential localization in modal space. We give a simple explanation for this structure in terms of superharmonic resonances. The resonance analysis agrees very well with numerical results and has predictive power. We extend a previously developed perturbation method, based essentially on a Poincare-Lindstedt scheme, in order to account for these resonances, and in order to treat more general model cases, including truncated Toda potentials. Our results give a qualitative and semiquantitative account for the superharmonic resonances of q-breathers and natural packets.     

光电轴角编码器的编码方式及其发展趋势

光电轴角编码器是以码盘或计量圆光栅为核心元件的测角传感器。近年来,为满足光电轴角编码器小型化、智能化、集成化,特别是小型化的要求,各国在缩小光电轴角编码器的尺寸方面做了大量的研究工作,尤其是对编码方式的研究。本文主要介绍了目前光电轴角编码器的编码方式及编码原理,在对现有编码方式详细解读的基础上对制约光电轴角编码器快速小型化的因素进行了分析,认为影响编码方式成功实施的主要因素是发光和接收元件的尺寸。文中展望了光电轴角编码器编码方式的发展趋势,提出编码方式多样化和码道数量少量化有助于缩小光电编码器尺寸,加速其小型化的发展。 