Modeling of Nonlinear Propagation in Multi-layer Biological Tissues for Strong Focused Ultrasound

A theoretical model of the nonlinear propagation in multi-layered tissues for strong focused ultrasound is proposed. In this model, the spheroidal beam equation （SBE） is utilized to describe the nonlinear sound propagation in each layer tissue, and generalized oblique incidence theory is used to deal with the sound transmission between two layer tissues. Computer simulation is performed on a fat-muscle-liver tissue model under the irradiation of a 1 MHz focused transducer with a large aperture angle of 35°. The results demonstrate that the tissue layer would change the amplitude of sound pressure at the focal region and cause the increase of side petals.     

Nonlinear Effect on Focusing Gain of a Focusing Transducer with a Wide Aperture Angle

Nonlinear effect on focusing gain of acoustic field radiated from a 1-MHz focusing transmitter with a wide aperture angle of 35° is theoretically and experimentally investigated. With the enhancement of nonlinearity, the focusing gains of both intensity and peak positive pressure show non-monotonic behaviour. There exist the same saturated levels at which the maximum outputs are reached and their spatial distributions are more localized. In contrast, the peak negative pressure always decreases monotonically and its spatial distribution is less localized.     

Optimization of thickness uniformity of coatings on spherical substrates using shadow masks in a planetary rotation system

A model is developed to improve thickness uniformity of coatings on spherical substrates rapidly and au- tomatically using fixed shadow masks in a planetary rotation system. The coating thickness is accurately represented by a function composed of basic thickness, self-shadow effect, and shadow mask function. A type of mask with parabolic contours is proposed, and the thickness uniformity of coatings on spheri- cal substrates can be improved in a large range of ratios of clear aperture （CA） to radius of curvature （RoC） by optimizing shadow masks using a numerical optimization algorithm. Theoretically, the thick- ness uniformity improves to more than 97.5% of CA/RoC from -1.9 to 1.9. Experimentally, the thickness uniformities of coatings on a convex spherical substrate （CA/RoC = 1.53） and on a concave spherical substrate （CA/RoC=-1.65） improve to be better than 98.5% after corrected by the shadow masks.     

Bubble nonlinear dynamics and stimulated scattering process

A complete understanding of the bubble dynamics is deemed necessary in order to achieve their full potential applications in industry and medicine. For this purpose it is first needed to expand our knowledge of a single bubble behavior under different possible conditions including the frequency and pressure variations of the sound field. In addition, stimulated scattering of sound on a bubble is a special effect in sound field, and its characteristics are associated with bubble oscillation mode. A bubble in liquid can be considered as a representative example of nonlinear dynamical system theory with its resonance, and its dynamics characteristics can be described by the Keller–Miksis equation. The nonlinear dynamics of an acoustically excited gas bubble in water is investigated by using theoretical and numerical analysis methods. Our results show its strongly nonlinear behavior with respect to the pressure amplitude and excitation frequency as the control parameters, and give an intuitive insight into stimulated sound scattering on a bubble. It is seen that the stimulated sound scattering is different from common dynamical behaviors, such as bifurcation and chaos, which is the result of the nonlinear resonance of a bubble under the excitation of a high amplitude acoustic sound wave essentially. The numerical analysis results show that the threshold of stimulated sound scattering is smaller than those of bifurcation and chaos in the common condition.     

Sound field of an electromagnetic acoustic transducer

The characteristic of radiated sound field excitated within a non-ferromagnetic aluminum by an electromagnetic acoustic transducer consisting of circle-spiral coil and vertical bias magnetic field is studied.This article presents an improved force source model according to the principle of electromagnetic induction.Then the analytic displacement solution of sound field produced by that force source model is deduced.The tangential and normal directivities of shear and longitudinal sounds are calculated numerically and compared with experimental results.Experimental results conform well to the theoretical prediction,which not only shows that the electromagnetic acoustic transducer of this conformation has a good directivity of vertical shear-wave radiation,but also confirms the given force source model is effective to describe the actual physical behavior of the probe.Theoretical and experimental study lays a foundation for optimization design and engineering application of the electromagnetic acoustic probe.     

Error sensing strategy for active acoustic structure based on distributed displacement sensors

The nearfield error sensing approach, which is one of the key problems encountered in the implementation of active acoustic structure, is investigated theoretically. The basic idea involves a limited number of PVDF film pairs be boned to the surface of the primary panel and secondary panels for measuring the total radiated sound power. In this paper, first, a theoretical model associated with the active acoustic structure is established. Second, the formulae for the shape coefficient of the PVDF pairs are derived based on the acoustic radiation modes and a PVDF sensing model. Finally, a number of computer simulations are performed and the shape of the PVDF pairs and corresponding reduction in the radiated sound power are calculated. The results demonstrate the feasibility of the proposed nearfield error sensing strategy.     

Investigation of sound field for a standing wave tube system with flow and with lateral Helmholtz resonator Ⅰ. Theoretical analysis

A lateral Helmholtz resonator added to a standing wave tube without flow has been validated as a method of noise reduction for combustion noise radiated from combustion channel of rockets or turbines. But in fact there is a flow with low velocity in the combustion channel. Therefore the theoretical analysis carried out is aimed at sound field of standing wave tube with flow and with lateral Helmholtz resonator. Certainly a relevant math-physical model should first be formulated. Here three key problems need to be solved: (1) To formulate the discontinuity condition at the joint between the standing wave tube and Helmholtz resonator in the case of flow. (2) To determine the acoustic impedance of Helmholtz resonator, considering the effects of flow, viscous and multihole. (3) To formulate the reflection condition at the end of the standing wave tube. Some formulas for analysis of the sound field in the tube with flow and with lateral Helmholtz resonator are deduced. These theoretical works have been validated by experiments.     

Recovery of the free field in noisy environment by using the spherical wave superposition method

To remove the scattering effect of the disturbing sound on the target source when implementing nearfield acoustic holography in a non-free field, a free field recovery technique based on the spherical wave superposition method is proposed. In the method, the sound field separation technique based on the spherical wave superposition method is first used to separate the incoming and outgoing fields, and a further step for separating the radiated and scattered fields is performed by utilizing the surface admittance of the target source as the boundary condition. The technique makes it possible to correctly identify noise sources in a non-free sound field. The basic principle of the technique is described firstly, a method for choosing the optimal number of spherical wave expansion terms is given, and two numerical simulations are used to demonstrate the validity of this technique. It is shown that, for the lower frequency, the scattering effect can be neglected, and the radiated field of the target source can be obtained by the sound field separation technique, however, as the increasing of the frequency, the scattering effect cannot be neglected, and the free field recovery technique has to be used to obtain the radiated field of the target source.     

Calculation of radiation acoustical fields from phased arrays with nonparaxial multi-Gaussian beam model

A nonparaxial multi-Gaussian beam model based on the rectangular aperture is proposed in order to overcome the limitation of paraxial Gaussian beam model which losing accuracy in off-axis beam fields. With the method, acoustical field generated by an ultrasonic linear phased array transducer is calculated and compared with the corresponding field obtained by Rayleigh-Sommerfeld integral, paraxial multi-Gaussian beam model, and Fraunhoffer approximation method. Simulation examples show that nonparaxial multi-Gaussian beam model is not limited by the paraxial approximation condition and can predict efficiently and accurately the acoustical field radiated by a linear phased array transducer over a wide range of steering angles.     

Down-looking synthetic aperture imaging ladar demonstrator and its experiments over 1.2 km outdoor

We present a down-looking synthetic aperture imaging ladar demonstrator and its experiments over 1.2 km in the field. The achieved imaging resolution is in agreement with the theoretical design, and the resulting two-dimensional image is satisfied. The capability to eliminate the influence from atmospheric turbulence is fully proven by the experiments.     

Rigorous solution of a problem for a cylindrical cavity containing a ferrite and a dielectric

A rigorous solution is given for modes of TM_nmo type. The complex resonant frequency _n is expressed via a transcendental characteristic equation. Some particular cases are discussed.Read at the Third All-Union Conference on Ferrites, Leningrad, 23 October 1963.     

Solving a relativistic quasipotential equation for a superposition of a nonlocal separable and a local quasipotential

Within the relativistic quasipotential approach to quantum field theory, a method is developed for solving a finite-difference quasipotential equation for the case where a total quasipotential describing the interaction of two relativistic spinless particles of unequal masses is a superposition of a nonlocal separable and a local quasipotential. The cases are investigated where the local component of the total interaction—it is assumed to be known—either admits or does not admit the existence of bound states. This makes it possible to obtain an exact expression for the increment of the phase shift, to determine the conditions of the existence of bound states, and to give a generalization of the Levinson theorem.     

Tunnelling process between a semiconductor or a metal and a polymer

The tunnelling lifetime of an electron lying in a p-type orbital localised at a given distance from a semiconductor or a metal is calculated by using Bardeen's method. It is then shown that even in the absence of broad bands, the hole injection process from semiconductors and metals into polymers should follow a Fowler-Nordheim dependence, provided that the current is not bulk-limited. In the semiconductor case, the current can be expressed by a fully analytical formula, and by an approximate one in the case of a metal. It is demonstrated that the effective Fowler-Nordheim barrier is not the mere difference between the metal work function or the semiconductor electron affinity and the HOMO level of the polymer, but a simple function of both levels. Received 6 April 2001 and Received in final form 29 May 2001     

Formation of a bistructure of a solid in a computer experiment

Molecular-dynamics was used to investigate the structural changes occurring in a three-dimensional solid when the solid is transferred from an amorphous into a crystalline state. Crystal cells of a new type — pentadecahedrons with five square lateral faces and ten regular triangular faces at the vertices of a cell — were found for the first time in a computer experiment. It is shown that a bistructure consisting of crystal cells of different types, including cells with five-fold symmetry axes, are stable in the solid. Fiz. Tverd. Tela (St. Petersburg) 40, 1919–1924 (October 1998)     

Modeling a waveguide with a nonlinear insert with a quadratic nonlinearity

The scalar problem of the scattering of a wave from a nonlinear insertion lying in the interior of a waveguide is reduced by the incomplete Galerkin method to the boundary value problem for a Hamiltonian system. The cases in which this problem admits a solution in finite terms are indicated. Examples are given to illustrate specific phenomena due to the nonlinearity of the problem.