Analysis of the ultrasonic field radiated by time-delay cylindrically focused linear arrays

The ultrasonic field at the scanning central plane of a linear array whose driving delays are quadratically varied for cylindrical focusing is studied. An efficient and accurate algorithm based on the classical time domain convolution-impulse response methodology is applied for calculations. The method takes into account the full size of the array elements and does not use any paraxial or far-field approximations. The cases of linear and sector scanning are studied. Patterns of array impulse responses h(A)(x ,t) at selected field zones, including the main propagation axis and the focal line, are described for different steering conditions. In order to facilitate the analysis of h(A)(.) curves, a set of temporal and spatial parameters are defined. In particular, the concepts of "elements" times of arrival vectors" and "virtual aperture" are shown to be helpful in achieving a simple interpretation of array fields. Plots of pressure under wide-band and continuous-wave radiations are also presented and discussed. In particular, several discrepancies with classical approaches are mentioned, as well as other aspects of the focused held such as the location of the point of maximum amplitude, and the lateral deterioration and asymmetries caused by steering.     

Toward a reference ultrasonic cavitation vessel: Part 2--investigating the spatial variation and acoustic pressure threshold of inertial cavitation in a 25 kHz ultrasound field

As part of an ongoing project to establish a reference facility for acoustic cavitation at the National Physical Laboratory (NPL), carefully controlled studies on a 25 kHz, 1.8 kW cylindrical vessel are described. Using a patented high-frequency acoustic emission detection method and a sonar hydrophone, results are presented of the spatial variation of inertial acoustic cavitation with increasing peak-negative pressure. Results show that at low operating levels, inertial acoustic cavitation is restricted to, and is strongly localized on, the vessel axis. At intermediate power settings, inertial acoustic cavitation also occurs close to the vessel walls, and at higher settings, a complex spatial variation is seen that is not apparent in measurements of the 25 kHz driving field alone. At selected vessel locations, a systematic investigation of the inertial cavitation threshold is described. This was carried out by making simultaneous measurements of the peak-negative pressures leading to inertial cavitation and the resultant MHz-frequency emissions, and indicates an inertial cavitation threshold of 101 kPa +/- 14% (estimated expanded uncertainty). However, an intermediate threshold at 84 kPa +/- 14% (estimated expanded uncertainty) is also seen. The results are discussed alongside theoretical predictions and recent experimental findings.     

超声空化对喇曼散射的影响

1　引言超声空化的过程是集中声场能量并迅即释放的过程。空化泡崩溃时 ,极短时间在空化泡周围的极小空间内 ,产生 50 0 0 K以上的高温和大约 5× 10 7Pa的高压 ,温度变化率高达 10 9K/ s,并伴生强烈的冲击波和时速达 4 0 0 km的射流 [1]。在这样特殊的物理环境下 ,物质的物理性质和一些物理现象是否发生变化或受环境的影响呢 ?作者曾经设计了一种特殊形状的喇曼光谱样品池 ,它既能让激光束透过样品 ,又能在样品中引入超声空化场 ,从而可研究超声空化对喇曼散射光谱的影响 [2 ]。作为该研究工作的继续 ,本文选择了四氯化碳和乙二醇两种样…     

Fiber characterization in a stationary ultrasonic field

An instrument has been developed to characterize the mean dimensions of softwood fiber samples. It is based on the phenomenon that particles of cylindrical shape diluted in water and shorter than one fourth of the acoustic wavelength migrate to nodal planes of acoustic radiation pressure and reorient parallel to these planes when subjected to a stationary ultrasonic field. As the resonator operating frequency is 72 kHz, fibers up to 5 mm in length can be measured. The time evolution of the fiber suspension during ultrasonic excitation is monitored with a collimated beam of light. Scattered light signals collected off-axis in the plane perpendicular to the acoustic nodal planes are shown to be a function of the weighted average fiber length. Results are presented for pulp samples in the average fiber length range of 0.2 to 3 mm. It was found that there is a region where the scattered light is linearly related to concentration. Acoustooptical measurements obtained at initial concentration in this linear region, for all fractions, have shown that the longer the average length from screen classifier is, the faster the layer formation is. Since the fiber length the radius are proportional for a wood species, this observation is in agreement with the theoretical prediction.     

Sonoluminescence and subharmonic generation in a cavitation zone of aqueous sodium chloride solutions

The variation of the sonoluminescence and the subharmonic component of the cavitation noise spectrum is investigated in a solution of sodium chloride in water.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 51, No. 3, pp. 417–424, September, 1986.     

活塞聚焦换能器的二次谐波声场

1　引　言　　研究聚焦换能器的非线性声场是非线性声学中受到关注的问题。这是由于聚焦活塞换能器在焦点附近能产生很强的非线性且圆型活塞聚焦换能器由于能有效地提高分辨率而被广泛应用于超声医学诊断及超声显微镜。对于有限振幅的活塞聚焦声场 ,分析时必须同时考虑衍射效应和非线性波型畸变效应 ,因此在数学分析上相当复杂 ,Lucas等人 [1] 在Parabloic近似下 ,导出了活塞聚焦换能器的基波 ,二次谐波积分型式的解 ,尽管这种解的形式是解析的 ,但由于解的形式是多重积分 ,对于实际的声场分析计算不够方便。由于高斯函数的积分具有相对简…     

Application of an ultrasonic technique to creep cavitation in silicon nitride

A non-destructive method based on measurements of ultrasonic wave velocities and Young's modulus is proposed for quantification of creep cavitation in silicon nitride. Tensile creep tests of silicon nitride were conducted at 1400°C in air and the tests were periodically interrupted to measure the longitudinal and transverse ultrasonic wave velocities and Young's modulus. The velocities and Young's modulus decreased linearly with tensile creep strain. The volume fraction of cavities was estimated from the values of the ultrasonic wave velocities and Young's modulus, and compared with the cavity volume predicted from tensile creep strain. The dependence of Young's modulus on volume fraction of cavities is discussed.     

超声空化对溴化锂溶液气泡运动的影响

根据溴化锂溶液的物性参数方程,利用试验数据和超声作用于均相液体中空化气泡运动的动力学模型,推导出溴化锂溶液超声空化气泡运动方程。利用MATLAB对溴化锂溶液超声空化气泡的运动特性进行数值模拟,对过冷沸腾和饱和沸腾条件下溴化锂溶液沸腾气泡的运动变化特性进行研究,得到超声空化效应对气泡运动特性变化影响的模拟结果。为溴化锂溶液传热性能和吸收式制冷机组制冷效率的提高提供一定的理论依据。     

The prediction of dynamic fracture evolution in PMMA using a cohesive zone model

A cohesive zone model was used in conjunction with the finite volume method to model the dynamic fracture of single edge notched tensile specimens of PMMA under essentially static loading conditions. In this study, the influence of the shape of the cohesive law was investigated, whilst keeping the cohesive strength and separation energy constant. Cohesive cells were adaptively inserted between adjacent continuum cells when the normal traction across that face exceeded the cohesive strength of the material. The cohesive constitutive law was therefore initially rigid, and the effective elasticity of the material was unaltered prior to insertion of the cohesive cells. Notch depths ranging from 2.0 to 0.1 mm were considered. The numerical predictions were compared with experimental observations for each notch depth and excellent qualitative and quantitative agreement was achieved in most cases. Following an initial period of rapid crack tip acceleration up to terminal velocities well below the Rayleigh wave speed, subsequent propagation took place at a constant rate under conditions of increasing energy flux to an expanding process region. In addition, attempted and successful branching was predicted for the shorter notches. It was found that the shape of the cohesive law had a significant influence on the dynamic fracture behaviour. In particular, the value of the initial slope of the softening function was found to be an important parameter. As the slope became steeper, the predicted terminal crack speed increased and the extent of the damage decreased.     

Optical properties of a thin-film stack illuminated by a focused field

Reflectance (R), transmittance (T), and absorptance (A) are calculated for a thin-film stack illuminated by a focused field. Based on Debye's integral representation, the electric and magnetic fields near focus are obtained, and the formulas for R, T, and A are represented as integrals of Poynting vectors. This formulation is applied to the case of a numerical aperture (N.A.) greater than 1.0 as well as to the case of a N.A. less than 1.0, and the corresponding numerical results are presented. They reveal that R, T, and A vary with N.A. and that the amount of variation increases with layer thickness.     

Improvement of immunodetection of bacterial spore antigen by ultrasonic cavitation

Ultrasonic cavitation was employed to enhance sensitivity of bacterial spore immunoassay detection, specifically, enzyme-linked immunosorbent assay (ELISA) and resonant mirror (RM) sensing. Bacillus spore suspensions were exposed to high-power ultrasound in a tubular sonicator operated at 267 kHz in both batch and flow modes. The sonicator was designed to deliver high output power and is in a form that can be cooled efficiently to avoid thermal denaturation of antigen. The 30-s batch and cooled flow (0.3 mL/min) sonication achieved an approximately 20-fold increase in ELISA sensitivity compared to unsonicated spores by ELISA. RM sensing of sonicated spores achieved detection sensitivity of approximately 10(6) spores/mL, whereas unsonicated spores were undetectable at the highest concentration tested. Improvements in detection were associated with antigen released from the spores. Equilibrium temperature increase in the tubular sonicator was limited to 14 K after 30 min and was maintained for 6 h with cooling and flow (0.3 mL/min). The work described here demonstrates the utility of the tubular sonicator for the improvement in the sensitivity of the detection of spores and its suitability as an in-line component of a rapid detection system.     

Controlled effect of ultrasonic cavitation on hydrophobic/hydrophilic surfaces

Controlling cavitation at the solid surface is of increasing interest, as it plays a major role in many physical and chemical processes related to the modification of solid surfaces and formation of multicomponent nanoparticles. Here, we show a selective control of ultrasonic cavitation on metal surfaces with different hydrophobicity. By applying a microcontact printing technique we successfully formed hydrophobic/hydrophilic alternating well-defined microstructures on aluminium surfaces. Fabrication of patterned surfaces provides the unique opportunity to verify a model of heterogeneous nucleation of cavitation bubbles near the solid/water interface by varying the wettability of the surface, temperature and ultrasonic power. At the initial stage of sonication (up to 30 min), microjets and shock waves resulting from the collapsing bubbles preferably impact the hydrophobic surface, whereas the hydrophilic areas of the patterned Al remain unchanged. Longer sonication periods affect both surfaces. These findings confirm the expectation that higher contact angle causes a lower energy barrier, thus cavitation dominates at the hydrophobic surfaces. Experimental results are in good agreement with expectations from nucleation theory. This paper illustrates a new approach to ultrasound induced modification of solid surfaces resulting in the formation of foam-structured metal surfaces.     

Tensile strength and visible ultrasonic cavitation of superfluid4He

Motion picture photographs of cavitation in He II revealed new characteristics pertinent to the liquid's tensile strength and bubble dynamics. A cylindrical acoustic standing wave with a frequency of 50.58 kHz induced the cavitation in He II at a temperature of 2.09 K. Analysis of light diffracted by the sound gave measurements of the acoustic pressure amplitude which were used both for selecting the best drive frequency and for obtaining the tensile strength. Bubbles appeared to originate on pressure antinodes, expanded to a diameter of 0.5–1.0 mm in about 0.3 msec, and eventually fragmented into smaller bubbles. They originated where the negative pressure extremum was as small as –0.6 bar (+0, –50%), a tensile strength much smaller than the predictions of theories developed for the homogeneous nucleation of bubbles in classical liquids. The bubble fragments were frequently nonspherical and had widths of 0.1–0.2 mm. Small bubbles also displayed an unexpected preference to originate on the surface of a stainless steel tube inserted in the sound field. Subsequent to nucleation, bubbles were frequently attracted to acoustic pressure nodes in agreement with a theory of vibrations and forces originally developed for bubbles in normal liquids. Attempts to detect first and second sound radiated by cavitation are described.Work supported by the National Science Foundation Grant DMR75-15628.NSF Predoctoral Fellow.     

Features of cavitation and cavitation erosion of waveguides of powerful ultrasonic units at higher-than-average pressure of the media

The features of the acoustic effects of cavitation created by the rod waveguides of powerful ultrasonic magnetostriction units in different liquid media at a pressure to 5 MPa have been investigated. It has been established that, under the conditions of maximum acoustical activity of cavitation realized at the medium's higher-than-average pressure, the erosion damage to the waveguides is of an unusual structurized character and develops from the central part of the end to its periphery. A result of the erosion damage may be the total loss of operating capacity by the waveguides manufactured from the most erosion-resistant materials (stainless steel and titanium alloys) even after a few hours of tests. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 4, pp. 690–696, July–August, 2008.     

Electromagnetic field for a focused light sheet incident on a plane surface

A solution procedure is developed for the determination of the electromagnetic field that results from the interaction of a focused light sheet with a plane surface. The effects of angle of incidence, relative index of refraction, polarization, and incident light sheet profile on the resulting electromagnetic field distribution are demonstrated.