水下高分子吸声材料的声学设计

通过固体力学的波动理论，利用材料的弹性模量、损耗因子和密度等基本弹性常数，推导了材料的声波速度和声阻抗的实部、虚部，以及衰减系数的表达式。通过数值计算，模拟了材料声反射系数和衰减系数随材料声学参数的变化规律，分析了材料声学性能的影响因素，提出了通过优化物理参数的方法对材料进行声学设计的方法。     

Heat transfer in YBaCuO thin film/sapphire substrate system

The thermal boundary resistance at the YBaCuO thin film/Al₂O₃ substrate interface was investigated. The transparency for thermal phonons incident on the interface as well as for phonons moving from the substrate was determined. We have measured a transient voltage response of current-biased films to continuously modulated radiation. The observed knee in the modulation frequency dependence of the response reflects the crossover from the diffusion regime to the contact resistance regime of the heat transfer across the interface. The values of transparency were independently deduced both from the phonon escape time and from the time of phonon return to the film which were identified with peculiarities in the frequency dependence. The results are much more consistent with the acoustic mismatch theory than the diffuse mismatch model.We are grateful to A. Elantev for helpful discussion. We acknowledge the financial support of the Russian Scientific Council on the HTS problem (Project No. 90462).     

伴流声场计算的有限元方法及其应用

推导了势流中的声波方程,并运用伽辽金加权余量法建立了相应的有限元(Finite Element Method,FEM)弱形式。对于管道声学问题的计算,给出了所需边界条件的处理方法,通过离散和装配得到有限元矩阵方程。使用自行编写的有限元程序计算分析了Herschel-Quincke(H-Q)管的消声特性。结果表明,在中低频段有限元计算结果与一维理论计算结果吻合良好,从而验证了该方法的正确性;频率较高时,两者出现明显差异,主要是由于主管和支管交接处的三维波和三维流效应所致。介质流动影响H-Q管的消声特性,特别是共振频率和通过频率,马赫数越大,影响越显著。结构形式的改变可以大大改善特定频率范围的消声性能。     

Linear viscoelastic behavior of porous media with non-uniform saturation

Heterogeneous moisture distribution is usually observed in partially saturated porous media in the form of saturation patches. Such local heterogeneity can significantly influence the macroscopic hydro-mechanical and acoustic behavior of porous media. In this paper, a linear viscoporoelastic model is presented that can be used to address the effects of local fluid flow in the heterogeneously saturated porous media subjected to a small perturbation. The effects of local flow are characterized using the notion of capillary relaxation. The complex, frequency-dependent material properties characterizing the viscoporoelastic behavior are derived. A rigorous procedure is presented to evaluate the material parameters. The proposed model describes well the effects of patchy saturation on the dispersion and attenuation of the compressional wave in partially saturated rocks. It is found that at high saturation the effects of local gas-pressure redistribution are not negligible. A procedure is proposed to determine the capillary relaxation times using acoustic data. The proposed model provides an alternative methodology to characterize the effects of patchy saturation on the acoustic behavior of partially saturated porous media.     

Forced Nonlinear Vibrations of a One-Dimensional Bar with Arbitrary Distributions of Hysteretic Damage

An analytical model is presented which predicts the forced, nonlinear response of a bar with arbitrarily distributed damage. Damage, which is either described by quadratic hysteresis, or due to dislocations interacting with point defects distributed along the dislocations’ glide planes, is considered. The wave equation is solved by means of a perturbation approach. Resonance frequency shift caused by damage-induced material softening, nonlinear attenuation, and higher harmonics’ generation are evaluated. For damage which is described by quadratic hysteresis, this model recovers the well-known dependence of the three acoustic quantities mentioned above on the source’s strength. On the other hand, for damage due to dislocations, both frequency shift and nonlinear attenuation present a distinctive nonlinear behavior the origin of which resides in the stress dependence of the fraction of dislocations breaking away from the point defects. Furthermore, different distributions of damage having the same integrated intensity are shown to generate nonlinear effects of increasing magnitude as their spatial extent decreases. Finally, it is suggested that, once the effect of the source’s strength is removed, spectral features may be used to assess the spatial extent of damage.     

Dielectric properties of perovskite crystals

The soft mode dynamical model has been used to study the dielectric properties of Perovskite-type crystals. The model Hamiltonian proposed by Pytte has been modified and designed in terms of creation and annihilation operators. The correlations appearing in the dynamical equation have been evaluated using double time thermal retarded Green’s function and Dyson’s equation. Without any decoupling the higher order correlations have been evaluated using the renormalized Hamiltonian and thus, all possible interactions among phonons have been taken into account. The expressions for phonon frequencies and widths have beenMcalculated. Using appropriate parameters the softening of different modes at different transition temperatures give rise to a series of transitions from cubic to tetragonal, orthorhombic or trigonal phases. The significantly temperature-dependent modes are considered responsible for damping constant, dielectric constant, tangent loss and attenuation constant for these crystals. The dielectric properties are directly related to the optical phonon frequencies and widths and acoustic attenuation to the acoustic and optical phonon widths. Using suitable approximations, the model explains the experimental results on dielectric properties and acoustic attenuation reported for LiNbO₃, SrTiO₃, BaTiO₃ and LaAlO₃.     

Determination of the thermal boundary resistance in the transport approach

We present a new theoretical determination of the thermal boundary resistance at a metal-liquid helium interface. The phonon temperature drops and heat flux densities at the interface are deduced from the numerical solution of the phonon Boltzmann equation inside the metal, with only electron-phonon scattering considered. A calculation of the thermal boundary resistance is performed and a comparison with the Khalatnikov theory is made ; the results differ considerably, the transport approach giving a far smaller resistance, though the phonon boundary conditions in our work are also determined by the classical acoustic theory.Laboratoire associé au Centre National de la Recherche Scientifique.     

Acoustic reflection from the boundary of anisotropic thermoviscoelastic solid with fluid

Vertical slownesses of waves at a boundary of an anisotropic thermoviscoelastic medium are calculated as roots of a polynomial equation of degree eight. Out of the corresponding eight waves, the four, which travel towards the boundary are identified as upgoing waves. Remaining four waves travel away from the boundary and are termed as downgoing waves. Reflection and refraction of plane harmonic acoustic waves are studied at a plane boundary between anisotropic thermoviscoelastic solid and a non-viscous fluid. At this fluid-solid interface, an incident acoustic wave through the fluid reflects back as an attenuated acoustic wave and refracts as four attenuating waves into the anisotropic base. Slowness vectors of all the waves in two media differ only in vertical components. Complex values of vertical slowness define inhomogeneous refracted waves with a fixed direction of attenuation, i.e. perpendicular to the interface. Energy partition is calculated at the interface to find energy shares of reflected and refracted waves. A part of incident energy dissipates due to interaction among the attenuated refracted waves. Numerical examples are considered to study the variations in energy shares with the direction of incident wave. For each incidence, the conservation of incident energy is verified in the presence of interaction energy. Energy partition at the interface seems to be changing very slightly with the azimuthal variations of the incident direction. Effects of anisotropy, elastic relaxation and thermal parameters on the variations in energy partition are discussed. The acoustic wave reflected from isothermal interface is much significant for incidence around some critical directions, which are analogous to the critical angles in a non-dissipative medium. The changes in thermal relaxation times and uniform temperature of the thermoviscoelastic medium do not show any significant effect on the reflected energy.     

Kapitza resistance: Axiomatic acoustic mismatch theory with loss

We examine the anatomy of the quantitative properties of thermal transport across a solid-liquid boundary as it is described by acoustic mismatch theory. The single parameter, the Kapitza resistance, is a function of four loss parameters and one thermal transport parameter, the thermal diffusivity of the fluid. The loss parameters are to be determined from the dispersion relations for phonons at the peak of the thermal excitation in the material. The temperature dependence of the Kapitza resistance depends on the variation of the phonon excitation in the material with temperature, the familiar temperature-cubed factor, and the variation of the loss factors with temperature and frequency for phonons at the thermal peak, and the variation of the diffusivity with temperature. Since these parameters are undetermined and experimentally rather inaccessible, we conclude that for the present the Kapitza resistance must be viewed as a technological heat transport parameter. Some discussion is given of the part played by second sound in helium II in the surface heat transport process.     

Lattice dynamics of layered ferroelectric semiconductor compound TlGaSe2

We present the first-principles calculation of the lattice dynamics of the TlGaSe₂ ternary semiconductor having highly anisotropic crystal structure. Calculations have been performed using open-source code ABINIT on the basis of the density functional perturbation theory within the plane-wave pseudopotential approach. Results on the frequencies of phonon modes in the centre of Brilloin zone and the dispersion of transverse shear acoustic branch of the phonon spectra agree well with the experimental data on Raman scattering, infrared reflectivity and ultrasound wave propagation in TlGaSe₂. The calculated and experimental temperature dependencies of heat capacity are in a good agreement up to the room temperature. Along the layer, the low-frequency acoustic branch displays the bending wave behavior which is characteristic of the layer crystal structures.     

On a model for thermo-poroacoustic waves

We present a model for acoustic wave propagation in a porous material which also allows for propagation of a thermal wave. We provide a thermodynamic development of a Darcy law theory which is consistent with a relaxation time for the heat flux. Nonlinear acceleration wave evolution is considered in detail.     

Modelling of nonlinear dilatation response of fluids containing columns plastic and shear relaxation considered

Summary The non-linear wave equation governing the propagation and scatter of dilatation waves in discontinuous media is presented and its Eulerian numerical analogue is used to study scatter of acoustic dilatation waves by colums and ducts in elastic and visco-elastic fluids. Plastic and visco-elastic relaxation mechanisms are considered. The spectral form of the wave equation is developed and used to discuss dispersion. Selected applications of the numerical analogue to simulation of scatter, propagation and echo attenuation presented.     

基于机械品质因数的全波压电超声换能器设计

针对全波压电超声换能器常规设计方法存在的尺寸参数较多、计算较复杂等问题,研究了一种利用机械品质因数设计全波压电超声换能器的方法.基于压电超声理论推导了全波压电超声换能器的频率方程,利用电学理论推导了全波压电超声换能器各组成部分的等效电路,利用等效电路求取了在任意等效截面处的等效机械阻抗,进而推导出全波压电超声换能器各部...     

PSpice simulation of ultrasonic systems

The usage of electrical analogies for the simulation of wave generation and propagation in ultrasound transducers is well established. In this paper a PSpice approach that includes the temperature and frequency dependency of the transducer performance is proposed. The analogy between acoustic wave propagation and wave propagation in an electric transmission line is given. Further ways to deduce temperature and frequency dependencies are discussed. The simulation approach is applied to a pulse-echo setup for the determination of speed of sound and attenuation in liquids and solids. Experiments and simulations are made for three temperatures and in the frequency range 1-12 MHz using water, glycerine, and polymers (PMMA and PEEK) as test samples. Comparison shows a good agreement between simulation and experiments. Results for glycerine indicates that the available attenuation models for high viscosity liquids is inappropriate.     

聚合物的水声声衰减能力与动态力学性能的关系

以描述高分子材料粘弹行为的三元模型为出发点,以声波在高分子介质中的传播理论为依据,推导出了材料的水声声衰减能力与材料的动态力学性能参数包括损耗因子、松弛前的剪切模量、松弛后的剪切模量以及材料的密度和厚度之间的关系式。为实验验证所推导的关系式,设计合成了一系列阻尼性能不同的聚合物,分别测试了它们的动态力学性能和声衰减能力。用材料的动态力学性能参数计算得到的水声声衰减系数与实验测得的声衰减能力相符合。该数学模型为找出吸声系数与材料的动态力学性能之间的关系、指导水声材料设计奠定了基础。     

非周期型理想钻柱系统声传播特性研究

基于一维声传播理论推导出了非周期型理想钻柱系统的频散方程和透射波声衰减方程,数值计算了非周期型钻柱系统几何参量与其频散特性、声衰减特性之间的关系,并利用有限元ANSYS对非周期型理想钻柱系统的声传播特性的结果进行了验证。研究结果表明：非周期型理想钻柱系统的频散特性与多个周期性结构系统频散特性相“与”的结果一致,具有通带较窄而阻带较宽,且频率在阻带范围内的声波沿钻柱传播时声衰减较大的特性。     

Electron–Phonon Interaction in Doped Fullerides

We propose a theory of interaction of long wave molecular phonons with electrons in fullerides in the presence of disorder. Phonon relaxation rate and frequency renormalization are discussed. Finite electronic bandwidth reduces phonon relaxation rate at q=0. Electron–phonon coupling constants with molecular modes in fullerides are estimated. The results are in good agreement with photoemission experiments.