引气混凝土气泡尺寸分布的三维重构

基于体视学和几何概率理论给出了引气混凝土三维气泡尺寸重构方法,由二维平面上气泡截面圆的直径分布计算气孔的实际尺寸分布,并生成了一个多尺度分布的立方体模型结构验证了该三维重构方法的合理性.然后,使用邻近粒子表面最近间距的解析解研究了气泡细度和混凝土含气量对邻近气泡表面最近间距平均值的影响,并与用传统方法得到的气泡间距因子进行了比较.结果表明,在含气量相同的条件下,用传统方法得到的气泡间距因子是邻近气泡表面最近间距平均值的3-4倍.该方法的给出,为从二维截面上获得的引气混凝土中的气泡截面圆信息获取实际气泡在三维空间中的气泡间距信息提供了依据.     

水中微气泡光散射偏振特性的研究

在Mie散射理论的基础上，建立气泡光散射模型，计算水中微小气泡与固体微粒在不同的粒径参数和散射角条件下的光散射偏振特性，并对两者进行比较．结果表明，水中气泡及微粒对入射光散射后偏振状态的改变十分复杂．总体上气泡的退偏振效应比固体微粒强25％，微小气泡与固体微粒对偏振状态的影响在粒径域和散射角上具有选择性．     

超声造影剂微泡尺寸分布及浓度测量方法

通过计算机采集光学显微图片,以Image-ProPlus和Photoshop两种图像分析软件实现了微泡的浓度及尺寸分析进行定性测定。结果表明,本文采用的方法快速、简便、重现性良好,可作为超声造影剂微泡的定性测定。     

树脂流动对气泡运动特性的影响

为了排除复合材料成型过程中的气泡, 建立了气泡运动可视化装置, 研究了树脂流动状态和流动速度对气泡运动速度的影响, 并在此基础上建立了气泡运动模型。研究结果表明: 树脂流动对气泡运动有明显的带动或阻碍作用。当树脂流动方向与气泡运动方向相同时, 随着树脂流速的增加, 气泡的运动速度明显增大; 而流动方向相反时, 随着树脂流速的增加, 气泡的运动速度呈明显下降的趋势。所建立的气泡运动模型与实验结果基本吻合。该研究结果将为热压成型过程中气泡运动模型的建立奠定基础。      

共振态气泡浓度和尺寸分布与声压谱的关系

文章给出声波在气－液混合物中的传播方程。根据声压的频谱导出相应的共振态气泡的份额。引入临界气泡份额和极大声压相应的气泡份额,推断出共振态气泡份额与相应声压之间存在线性关系。     

铁电体尺寸对其开关特性的影响

应用铁电体极化反转的Orihara-Ishibashi理论，讨论了圆形铁电薄膜和球形铁电体的开关电流以及开关时间对系统尺寸的依赖性。数值计算表明，不论是二维还是三维铁电系统，其铁电畴反转过程中产生的开关电流都随系统尺寸减少而下降，开关时间随系统尺寸减少而缩短。     

铁电体尺寸对其开关特性的影响

应用铁电体极化反转的Orihara Ishibashi理论 ,讨论了圆形铁电薄膜和球形铁电体的开关电流以及开关时间对系统尺寸的依赖性。数值计算表明 ,不论是二维还是三维铁电系统 ,其铁电畴反转过程中产生的开关电流都随系统尺寸减少而下降 ,开关时间随系统尺寸减少而缩短     

边界条件对水下爆炸气泡运动特性的影响分析

在近距爆炸条件下,受到边界的影响,水下爆炸气泡会出现非球状运动情况。为了得到球状气泡假设的适用条件,对深水自由场、近自由液面、近刚性壁和弹性边界等不同边界条件下水下爆炸气泡的动态特性进行了仿真计算。通过对气泡最大半径和脉动周期仿真结果的对比分析,可以得到如下结论:当爆心距离边界约等于气泡最大半径时,自由表面会使气泡的最大半径略有减小,周期缩短,刚性壁的影响与之相反,弹性边界的影响介于二者之间;在近自由边界条件下爆距大于2倍气泡半径,近弹性结构和刚性壁边界条件下爆距大于3倍气泡半径时,水下爆炸气泡可近似认为球状脉动。     

失超引起的气泡对超导体绝缘特性的影响

综述了超导设备在失超情况下液氦中气泡的形成,以及气泡对超导设备的绝缘特性的影响,分析了在均匀和不均匀电场中热传导率、压强及电极表面状况对击穿特性的影响,并探讨了提高失超情况下液氦击穿电压的方法.     

Shell waves and acoustic scattering from ultrasound contrast agents

Ultrasound contrast agents are encapsulated microbubbles, filled either with air or a higher weight molecular gas, ranging in size from 1 to 10 μm in diameter. The agents are modeled as air-filled spherical elastic shells of variable thickness and material properties. The scattered acoustic field is computed from a modal series solution, and reflectivity and angular scattering are then determined from the computed fields for agents of various properties. We show that contrast agents also support shell resonance responses in addition to the monopole response, which has been the focus of previous contrast agent studies. Lamb waves appear to be the source of these additional responses. A shell or curvature Lamb wave generates dipole peaks in the 1- to 40-MHz range for 2.5 to 3.5 μm radius agents with elastic properties approximating those of albumin protein. The inclusion of damping affects the lower frequency dipole peaks but is less important for responses occurring above approximately 30 MHz. Moreover, these responses hold untapped potential for clinical ultrasound applications such as tissue perfusion studies and high frequency contrast agent imaging     

High-frequency dynamics of ultrasound contrast agents

Ultrasound contrast agents enhance echoes from the microvasculature and enable the visualization of flow in smaller vessels. Here, we optically and acoustically investigate microbubble oscillation and echoes following insonation with a 10 MHz center frequency pulse. A high-speed camera system with a temporal resolution of 10 ns, which provides two-dimensional (2-D) frame images and streak images, is used in optical experiments. Two confocally aligned transducers, transmitting at 10 MHz and receiving at 5 MHz, are used in acoustical experiments in order to detect subharmonic components. Results of a numerical evaluation of the modified Rayleigh-Plesset equation are used to predict the dynamics of a microbubble and are compared to results of in vitro experiments. From the optical observations of a single microbubble, nonlinear oscillation, destruction, and radiation force are observed. The maximum bubble expansion, resulting from insonation with a 20-cycle, 10-MHz linear chirp with a peak negative pressure of 3.5 MPa, has been evaluated. For an initial diameter ranging from 1.5 to 5 microm, a maximum diameter less than 8 microm is produced during insonation. Optical and acoustical experiments provide insight into the mechanisms of destruction, including fragmentation and active diffusion. High-frequency pulse transmission may provide the opportunity to detect contrast echoes resulting from a single pulse, may be robust in the presence of tissue motion, and may provide the opportunity to incorporate high-frequency ultrasound into destruction-replenishment techniques.     

一种适用于实验室造影应用的高声强发生器

经声经后的造影剂能提高声学成象的清晰度，有可能提高超声临床诊断的质量。用实验室现有的超声设备制剂时其操作要十分小心且声强低而时间长，为克服这些缺点，研制了一台换能器工作在并联谐振点的高声强发生器。本文离电路的主要设计方法。初步实验的结果表明，效果良好，能满足实验室造影剂声振的要求．     

E1 超声造影剂的非线性参量研究

近年来对超声造影剂的研究兴趣日趋增加是由于它有可能改进超声临床诊断的效果。含有微气泡的液体可用作有效的超声造影剂。液体中气泡的存在可提高液体的非线性参量值。本文用检测二次谐波的方法实验上测定了５种超声造影剂的非线性参量：四种含气泡的液体（声振后）和半乳糖Ｅｃｈｏｖｉｓｔ３００。结果表明，含有微气泡的液体具有非常大的非线性参量值。对这种高非线性值的现象文中进行了可能的解释和讨论     

Asymmetric oscillation of adherent targeted ultrasound contrast agents

With a lipid shell containing biotin, micron-sized bubbles bound to avidin on a porous and flexible cellulose boundary were insonified by ultrasound. The oscillation of these targeted microbubbles was observed by high-speed photography and compared to the oscillation of free-floating microbubbles. Adherent microbubbles were observed to oscillate asymmetrically in the plane normal to the boundary, and nearly symmetrically in the plane parallel to the boundary, with a significantly smaller maximum expansion in each dimension for bound than free bubbles. With sufficient transmitted pressure, a jet was produced traveling toward the boundary.     

Predicting backscatter characteristics from micron- and submicron-scale ultrasound contrast agents using a size-integration technique

In this paper, a computationally, inexpensive, size-integration method based on a modified Rayleigh-Plesset model is developed to predict backscatter spectra from groups of bubbles with various size distributions, incident acoustic amplitudes, and driving frequencies. The method was validated using experimentally measured spectra from contrast bubbles of various sizes: Optison, Levovist, ST68 microbubbles, and submicron bubbles. This method provides a computationally inexpensive means of examining backscatter spectrum from multiple bubbles, especially in predicting occurrence and relative amplitude of subharmonics and second harmonics.     

Doppler spectra from contrast agents crossing an ultrasound field

When contrast agents are injected in a fluid, it is implicitly assumed that they move at the same velocity as the fluid itself. However, a series of in vitro tests performed by using air-filled microbubbles suspended in distilled water, have shown that the Doppler spectrum generated in this case may be notably different from that obtained from non-resonating scatterers. In this paper, we show, through a simple simulation model, that the actual movement of microbubbles may be predicted as the result of the complex balance between two forces: the ultrasound radiation force, which tends to move the particles along the sound beam direction, and the fluid drag force, which tends to move the particles along the fluid stream. The contrast agents turn out to be displaced only during the passage of the ultrasound burst; during the remaining time, they are maintained at the fluid velocity by the drag force. Based on the total particle displacement estimated between consecutive pulses, a series of Doppler spectra corresponding to different intensity levels was computed. This series was shown to be in excellent agreement with the experimental spectra obtained in vitro using Levovist(R) (Schering AG, Berlin, Germany) particles suspended in distilled water flowing at a steady rate.     

Optical and acoustical observations of the effects of ultrasound on contrast agents

Optimal use of encapsulated microbubbles for ultrasound contrast agents and drug delivery requires an understanding of the complex set of phenomena that affect the contrast agent echo and persistence. With the use of a video microscopy system coupled to either an ultrasound flow phantom or a chamber for insonifying stationary bubbles, we show that ultrasound has significant effects on encapsulated microbubbles. In vitro studies show that a train of ultrasound pulses can alter the structure of an albumin-shelled bubble, initiate various mechanisms of bubble destruction or produce aggregation that changes the echo spectrum. In this analysis, changes observed optically are compared with those observed acoustically for both albumin and lipid-shelled agents. We show that, when insonified with a narrowband pulse at an acoustic pressure of several hundred kPa, a phospholipid-shelled bubble can undergo net radius fluctuations of at least 15%; and an albumin-shelled bubble initially demonstrates constrained expansion and contraction. If the albumin shell contains air, the shell may not initially experience surface tension; therefore, the echo changes more significantly with repeated pulsing. A set of observations of contrast agent destruction is presented, which includes the slow diffusion of gas through the shell and formation of a shell defect followed by rapid diffusion of gas into the surrounding liquid. These observations demonstrate that the low-solubility gas used in these agents can persist for several hundred milliseconds in solution. With the transmission of a high-pulse repetition rate and a low pressure, the echoes from, contrast agents can be affected by secondary radiation force. Secondary radiation force is an attractive force for these experimental conditions, creating aggregates with distinct echo characteristics and extended persistence. The scattered echo from an aggregate is several times stronger and more narrowband than echoes from individual bubbles.