共查询到20条相似文献,搜索用时 31 毫秒
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Photoacoustic point source 总被引:6,自引:0,他引:6
We investigate the photoacoustic effect generated by heat deposition at a point in space in an inviscid fluid. Delta-function and long Gaussian optical pulses are used as sources in the wave equation for the displacement potential to determine the fluid motion. The linear sound-generation mechanism gives bipolar photoacoustic waves, whereas the nonlinear mechanism produces asymmetric tripolar waves. The salient features of the photoacoustic point source are that rapid heat deposition and nonlinear thermal expansion dominate the production of ultrasound. 相似文献
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为解决探测水下目标的电磁散射问题,提出大比例变换的总场-散射场源时域有限差分(FDTD)方法.该方法包含两次FDTD计算:第一次计算采用细网格得到激励源周围的近场值;第二次计算采用粗网格得到远距离的电磁场值.两次FDTD计算通过总场-散射场边界建立联系.实现细粗网格的大比例变换,例如变换比例N=10,大大节省了计算时间,降低了计算内存的消耗,提高了计算效率.通过算例验证该方法的正确性和有效性.最后,计算水下岩层中存在异常体时的电磁响应,指出当岩层中异常体电导率不同时,接收点处电磁场的幅值和相位均不相同. 相似文献
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针对水下近水面锥柱组合壳声固耦合多借助于数值方法求解的现状,本文提出一种半解析方法从机理上分析此类问题。首先基于能量泛函和Sanders壳体理论、虚拟弹簧法以及力与力矩平衡条件建立锥柱组合壳的结构模型;然后采用Legendre谱元法和二维傅里叶变换得到含自由液面的水下声场模型;最后由非线性迭代法和高斯积分求解耦合系统声振控制方程。通过与参考文献和数值方法结果的对比,验证了本文方法的收敛性、正确性和可靠性。研究结果表明,结构参数、浸没深度和激励频率与远场辐射声压密切相关。本文工作可推广到水下含内部结构的复杂旋转组合壳在不同结构边界及声边界下的声固耦合问题。 相似文献
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The effect of the thermal nonlinearity caused by the temperature dependence of the specific heat and thermal conductivity of a sample, substrate, and air on the temperature field in a photoacoustic cell is stud-ied theoretically. Exact solutions are obtained for a steady temperature field with allowance for thermal nonlin-earity and for a nonsteady field without allowance for this nonlinearity. The nonsteady nonlinear problem was solved numerically. It is shown that, due to thermal nonlinearity, the linear dependence of the temperature of the irradiated surface on the heating beam intensity gradually transforms into a power-law dependence as the beam intensity increases. 相似文献
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J. A. Baldera-López 《The European physical journal. Special topics》2008,153(1):167-170
The analytical solution for the one-dimensional heat diffusion problem, involving a harmonic heat source in a single layer,
is used to provide of photoacoustic self-normalized methodologies for thermal diffusivity measurements for highly opaque materials.
The self-normalized procedure involves the photoacoustic phase lag between the rear and front configurations. Three methodologies
are described; two of them involving linear fits in the photoacoustic thermally thick and thermally thin regimes. Comparison
between the theoretical normalized equations and the corresponding normalized experimental data allows for the development
of criteria on the selection of an appropriate modulation frequency range where a reliable analysis can be done. 相似文献
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当一束受正弦调制的激光入射于光声腔中的固体样品上时,由于非线性的光声效应,在光声腔中不仅能接收到基频成份的声信号,还能接收到其二次谐波成份。本文提出一个非线性热波束方程及其相应的非线性边界条件。借助于逐步近似法在光源为高斯径向分布的情形下,求解这一方程。利用Hankel变换获得这一方程的一级与二级近似解。解析结果表明,二次谐波的热波束仍然维持高斯径向分布,而其高斯半径比基频成份小。分析还表明,二次谐波的振幅不仅与线性热参数,而且也与非线性热参数有关。后者或许能提供样品的更多的有意义的信息。综合其各种特点,
关键词: 相似文献
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N. V. Studenichnik 《Acoustical Physics》2003,49(2):207-216
Experimental data are presented on the intensity and space-time characteristics of the sound field generated by explosions in underwater and surface sound channels of the Black Sea. The fine field structure is studied as a function of distance and positions of the source and the receiver. The discreteness of the field structure governed by the deterministic nature of the waveguide is revealed, and the destruction of this structure under the effect of the instability of the waveguide parameters is demonstrated. The effect of the rough sea surface on the sound field in the surface channel is studied, and the diffraction-caused propagation loss is estimated. The mechanism of the forerunner formation is considered. The experimentally observed sound field features are compared with the calculations. A possibility for solving the inverse problem is indicated, and the main parameters that are used in the ray method of determining the source coordinates in the underwater channel (i.e., the method earlier proposed by the author on the basis of the intrinsic structure of the sound field) are pointed out. 相似文献
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针对近水面声源和水下声源的深度判别问题,根据近水面声源难以激发低阶模态的物理现象,研究利用声源波数谱结构和波数位置的不同来分辨近水面声源和水下声源。通过采用MVDR的谱估计方法进行模态域波束形成,补偿水平阵各阵元之间各号简正波的相位差,获得主瓣窄、旁瓣低的声源信号波数谱。波数谱的波数位置与频率呈近似线性关系,水中声速剖面、海底参数、海深都会影响波数谱的具体结构和位置。此外,声源信号的到达角估计误差同样也会影响波数谱主瓣的位置估计。数值仿真结果表明,在浅海负跃层声速剖面条件下,可利用水平阵模态域波束形成判别声源深度,区分近水面声源和水下声源。 相似文献
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R. Reinisch 《Surface science》1979,85(2):432-456
The purpose of this paper is to study the nonlinear excitation of surface polaritons taking fully into account the damping of the active medium and the finite cross-section of the nonlinear polarization on the interface. This problem is solved using the guided wave calculation techniques where the EM field at the surface polariton frequency is expanded over a complete set of normal modes of the unperturbed interface. Using a “table method” we find that this set includes one guided mode, which is the surface polariton mode, and two classes of radiation modes. The expressions of all these modes are derived and interpreted physically. We then get the expression of the EM field excited at the surface polariton frequency inside and outside the pumped region and show that, in the general case, it is a mixture of all these normal modes. The end of the paper is mainly devoted to the study of the surface term occurring in the expression of the EM field at the surface polariton frequency: we point out the existence of a resonance phenomenon with two kinds of surface polariton modes: the “spatial” one and the “temporal” one. The corresponding dispersion curves, or resonance curves, are given and it is explained how each of them can be obtained experimentally. 相似文献
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Coupling near‐infrared (NIR) nanoscale absorbing materials with microbubbles (MBs) can generate a multifunctional dual imaging contrast agent. A new approach is presented for a hybrid photoacoustic/ultrasound contrast enhancer where pristine graphene is stably tethered to poly(vinyl alcohol) (PVA)‐based MBs. The main advantages of this approach are i) the preservation of optical and mechanical properties of intact graphene for an efficient photoacoustic (PA) enhancement and ii) the echogenicity and biocompatibility due to the robust anchoring of graphene to the bioinert PVA shell. PVA MBs provide ideal platforms for drug loading and ligand tethering for specific tumor targeting. One of the crucial goals toward this direction is optimizing this system in terms of balance between favorable acoustic/photoacoustic properties, immune shielding, and cytotoxicity. Such a combination strongly depends on the bridging moieties between graphene and the microbubble surface and can be easily tuned by PEGylation. The optimized graphene PVA MBs as contrast agent provide an efficient enhancement in vivo both in ultrasound and photoacoustic modes. The spectrally separable absorbance profile allows to a first demonstration of performing real‐time in vivo multiplexed photoacoustic imaging of graphene PVA MBs, and assessment of their full body biodistribution using a Vevo LAZR‐X photoacoustic imaging system. 相似文献
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A solution to the problem of nonlinear surface vibration of a charged ideal liquid drop is found in a third-order approximation in initial multimode deformation of the equilibrium spherical shape by the method of many scales. It is shown that the spectrum of modes that are responsible for the shape of the drop at an arbitrary time instant depends considerably on the spectrum of modes governing the initial deformation of the drop. The latter spectrum also has an effect on nonlinear corrections to the vibration frequencies and, consequently, on a nonlinear correction to the critical Rayleigh parameter, which specifies the stability of the drop against self-charge. 相似文献
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Stanic B.V. Drljaca P.M. Boskovic B.O. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1998,26(5):1514-1519
Electron plasma waves excitation in suddenly created isotropic plasma as a result of weak nonlinear interaction of linearly polarized plane electromagnetic (EM) wave and electrons has been considered. By the use of standard perturbation method the problem is solved in closed form for the case of a simple harmonic source EM wave. The appearance of the second harmonic and time independent modes have been demonstrated. The efficiency of excitation of these modes is possible to control by varying the frequency of the source wave 相似文献
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J. M. Rey C. Romer M. Gianella M. W. Sigrist 《Applied physics. B, Lasers and optics》2010,100(1):189-194
The simultaneous dual-frequency operation of a resonant photoacoustic gas sensor based on the differential mode excitation
photoacoustic (DME-PA) technique is presented. The DME-PA method uses the excitation of two different modes in a resonant
photoacoustic cell and the gas concentration is derived from the amplitude ratio of these acoustic modes. With the simultaneous
dual-frequency excitation, the amplitude ratio needed by the DME-PA technique is obtained instantaneously, in contrast to
the sequential modulation scheme where additional time delays are introduced by changing the modulation frequency. For a given
excitation power reaching the photoacoustic cell, and a total acquisition time longer than 7 s, the simultaneous modulation
scheme provides an improved measurement uncertainty compared to the sequential scheme. The proposed sensor allows measuring
water vapour with a ±150 ppmV uncertainty using current-modulated near-infrared LEDs and a 15 s total acquisition time. 相似文献
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针对浅海环境中低频宽带水声脉冲信号, 研究基于频散特征结合时频分析的单水听器距离和深度估计方法. 以简正波理论为依据, 将单水听器上的接收信号表示成一系列传播模式之和的形式, 分析了经典波导环境下的频散现象, 采用自适应径向高斯核函数的时频分析方法来表征接收信号的频散特征. 为提高时频分辨率, 采用自适应径向高斯核函数的时频分布来提取频散关系曲线中传播模式的到达时间差, 利用模式的到达时间差估计声源的距离. 采用多模式联合匹配的方式, 通过二值掩模滤波的时频滤波方法, 提取所需的模式. 通过计算实际提取出的模式能量与预测的模式能量之间的误差, 建立代价函数, 并通过模式能量匹配的方式, 确定声源的深度. 通过对基于Pekeris波导模型的浅海环境进行仿真验证, 结果表明: 自适应径向高斯核函数的时频分析方法能够很好地反映信号本身的频散特征, 具有较高的时频分辨率, 克服了传统短时傅里叶变换时频表征的限制, 使得模式在时频域更加容易辨识和分离; 从测距效果来看, 不同模式组合下的距离估计结果不同, 采用在时频面上具有较高能量的模式, 可得到较为准确的距离估计; 选用高能量的模式所得的距离估计的相对误差小于2%. 在定深方面, 参与联合匹配的模式个数越多, 代价函数的峰值更加地尖锐, 同时具有低的伪峰, 深度估计的性能会进一步有所提升. 该工作对于研究低频水声脉冲信号的分离和提取具有重大意义.
关键词:
频散信道
时频分析
单水听器
定位 相似文献
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The problem of nonlinear radial pulsations and surface vibrations of a charged bubble placed in an ideal incompressible dielectric liquid is asymptotically solved up to the second order of smallness by the method of many scales. It is shown that, in the case of nonlinear vibrations, resonance energy exchange may take place not only between surface modes but also between the radial mode and a surface mode. A new type of instability (other than Rayleigh instability against the self-charge), instability against the excess vapor pressure in the bubble, is discovered. The new type of instability shows up as energy transfer from the centrosymmetric pulsation mode to all initially excited surface vibration modes simultaneously. 相似文献