Multiple scattering in a trabecular bone: influence of the marrow viscosity on the effective properties

The Foldy and the Waterman and Truell approximations are used to determine the effective properties of the coherent wave that emerges after multiple scattering of a plane longitudinal fast wave by the largest pores in a trabecular bone. The unit scattering cell considered is either a single pore or two close cylindrical pores (cluster), at a fixed overall bone porosity. In the cluster case, the effective attenuation is about twice that obtained with one single pore per scatterer. It is shown that taking into account the marrow viscosity leads only to minor differences on the effective dispersion and attenuation.     

Including dispersion and attenuation directly in the time domain for wave propagation in isotropic media

When sound propagates in a lossy fluid, causality dictates that in most cases the presence of attenuation is accompanied by dispersion. The ability to incorporate attenuation and its causal companion, dispersion, directly in the time domain has received little attention. Szabo [J. Acoust. Soc. Am. 96, 491-500 (1994)] showed that attenuation and dispersion in a linear medium can be accounted for in the linear wave equation by the inclusion of a causal convolutional propagation operator that includes both phenomena. Szabo's work was restricted to media with a power-law attenuation. Waters et al. [J. Acoust. Soc. Am. 108, 2114-2119 (2000)] showed that Szabo's approach could be used in a broader class of media. Direct application of Szabo's formalism is still lacking. To evaluate the concept of the causal convolutional propagation operator as introduced by Szabo, the operator is applied to pulse propagation in an isotropic lossy medium directly in the time domain. The generalized linear wave equation containing the operator is solved via a finite-difference-time-domain scheme. Two functional forms for the attenuation often encountered in acoustics are examined. It is shown that the presence of the operator correctly incorporates both, attenuation and dispersion.     

Soft mode dispersion and ‘waterfall’ phenomenon in relaxors revisited

Results of recent inelastic neutron scattering studies of lead-based relaxor ferroelectrics by Gvasaliya et al. [J. Phys.: Condens. Matter 17, 4343 (2005); J. Phys.: Condens. Matter 19, 016219 (2007)] have put in question the existence of the “waterfall” anomaly–an apparent vertical dispersion segment joining the TA and TO branches–observed earlier in low-energy [ξ00] phonon dispersion curves of these materials. In the present article, we review the results of earlier experiments and model calculations together with the outcome of our recent measurements on PMN using the same instrumental set-up as Gvasaliya et al. to conclude that the “waterfall” feature is not an experimental artefact. We also give some hints on a possible explanation of the results of Gvasaliya et al., by exploring the fact that the reported dispersion of the underdamped transverse optic branch follows the longitudinal acoustic (LA) branch dispersion surprisingly closely.     

利用声衰减反演气泡群分布的方法研究

通过测量含气泡水的声衰减反演气泡群参数是获取水中气泡分布的重要方法,但是经典方法忽略了较高浓度气泡水中的强频散特性和气泡振动参数的改变,导致反演较高浓度气泡群分布时会产生巨大误差.为解决这个问题,本文基于等效媒质理论建立起了声衰减和相速度的联系,并考虑了含气泡水平均量对气泡阻尼系数和共振频率的影响.在此基础上,通过将反演气泡分布和修正相速度及气泡振动参数交替迭代的方法,有效地消除了高浓度气泡水中由频散和气泡振动特性改变引起的误差.与实验数据对比发现,气泡群孔隙率达到10^-5时,考虑含气泡水的频散特性会显著降低反演误差;而当气泡群孔隙率达到10^-3时,气泡阻尼系数和共振频率的修正会对反演结果变得重要.本文方法在反演孔隙率为10^-3-10^-2的高浓度气泡群时,仍有较好效果,这可为获取水下较高浓度气泡群分布提供方法借鉴.     

Multi-Party Quantum Key Agreement Protocol with Bell States and Single Particles

International Journal of Theoretical Physics - Liu et al. [Quantum Inf. Process. 12, 1797–1805 (2013)] proposed a multi-party quantum key agreement (QKA) protocol based on single particles....     

Extraction of optical scattering parameters and attenuation compensation in optical coherence tomography images of multilayered tissue structures

A recently developed analytical optical coherence tomography (OCT) model [Thrane et al., J. Opt. Soc. Am. A 17, 484 (2000)] allows the extraction of optical scattering parameters from OCT images, thereby permitting attenuation compensation in those images. By expanding this theoretical model, we have developed a new method for extracting optical scattering parameters from multilayered tissue structures in vivo. To verify this, we used a Monte Carlo (MC) OCT model as a numerical phantom to simulate the OCT signal for heterogeneous multilayered tissue. Excellent agreement between the extracted values of the optical scattering properties of the different layers and the corresponding input reference values of the MC simulation was obtained, which demonstrates the feasibility of the method for in vivo applications. This is to our knowledge the first time such verification has been obtained, and the results hold promise for expanding the functional imaging capabilities of OCT.     

Electron Dispersion in Liquid Alkali and Their Alloys

Ashcroft's local empty core (EMC) model pseudopotential in the second-orderperturbation theory is used to study the electron dispersion relation, theFermi energy, and deviation in the Fermi energy from free electron value forthe liquid alkali metals and their equiatomic binary alloys for the firsttime. In the present computation, the use of pseudo-alloy-atom model (PAA)is proposed and found successful. The influence of the six different formsof the local field correction functions proposed by Hartree (H), Vashishta-Singwi (VS), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F), andSarkar et al. (S) on the aforesaid electronic properties is examined explicitly,which reflects the varying effects of screening. The depth of the negativehump in the electron dispersion of liquid alkalis decreases in the orderLi →K, except for Rb and Cs, it increases. The results of alloys are in predictive nature.     

Collisions of solitons and vortex rings in cylindrical Bose-Einstein condensates

Interactions of solitary waves in a cylindrically confined Bose-Einstein condensate are investigated by simulating their head-on collisions. Slow vortex rings and fast solitons are found to collide elastically contrary to the situation in the three-dimensional homogeneous Bose gas. Strongly inelastic collisions are absent for low density condensates but occur at higher densities for intermediate velocities. The scattering behavior is rationalized by use of dispersion diagrams. During inelastic collisions, spherical shell-like structures of low density are formed and they eventually decay into depletion droplets with solitary-wave features. The relation to similar shells observed in a recent experiment by Ginsberg et al. [Phys. Rev. Lett. 94, 040403 (2005)] is discussed.     

Velocity dispersion and attenuation in granular marine sediments: comparison of measurements with predictions using acoustic models

The large velocity dispersion recently reported could be explained by a gap stiffness model incorporated into the Biot model (the BIMGS model) proposed by the author. However, at high frequencies, some measured results have been reported for negative velocity dispersion and attenuation proportional to the first to fourth power of frequency. In this study, first, it is shown that the results of velocity dispersion and attenuation calculated using the BIMGS model are consistent with the results measured in two kinds of water-saturated sands with different grain sizes, except in the high-frequency range. Then, the velocity dispersion and attenuation in six kinds of water-saturated glass beads and four kinds of water-saturated silica sands with different grain sizes are measured in the frequency ranges of 80-140 and 300-700 kHz. The measured results are compared with those calculated using the BIMGS model plus some acoustic models. It is shown that the velocity dispersion and attenuation are well predicted by using the BIMGS model in the range of kd ≤ 0.5 (k: wavenumber in water, d: grain diameter) and by using the BIMGS model plus multiple scattering effects in the range of kd ≥ 0.5 in which negative velocity dispersion appears.     

Study on the multiple scattering effects of ultrasound contrast agents

In this paper, the multiple scattering of interacting encapsulated microbubbles in suspensions is calculated using two novel approaches--Kargl's effective medium approach and Ye and Ding's approach of 2nd-order correction. Two types of contrast agents with bubbles of different sizes and concentrations are chosen for our numerical simulations. One is Albunex, which is depicted by Church and has a size range of several microns, and the other is sodium laureate solution (before fractionation) described by Soetanto and Chan and has an average size of 35.5 microm. The numerical results from these two approaches are compared with that from the linear approximation. It is found that the multiple scattering effects on attenuation and dispersion of sound in suspensions are evident in the cases of high bubble volume fractions, basically greater than the order of 1 x 10(-4), and much more obvious for larger bubbles with average size of tens of microns.     

A general model for the absorption of ultrasound by biological tissues and experimental verification

In this paper, a closed-form expression is derived for the absorption of ultrasound by biological tissues. In this expression, the viscothermal and viscoelastic theories of relaxation processes are combined. Three relaxation time distribution functions are introduced, and it is assumed that each of these distributions can be described by an identical and simple hyperbolic function. Several simplifying assumptions had to be made to enable the experimental verification of the derived closed-form expression of the absorption coefficient. The simplified expression leaves two degrees of freedom and it was fitted to the experimental data obtained from homogenized beef liver. The model produced a considerably better fit to the data than other, more pragmatic models for the absorption coefficient as a function of frequency that could be found in the literature. Scattering in beef liver was estimated indirectly from the difference between attenuation in in vitro liver tissue as compared to absorption in a homogenate. The frequency dependence of the scattering coefficient could be described by a power law with a power of the order of 2. A comparable figure was found in direct backscattering measurements, performed at our laboratory with the same liver samples [Van den Aarssen et al., J. Acoust. Soc. Am. (to be published)]. A model for scattering recently proposed by Sehgal and Greenleaf [Ultrason. Imag. 6, 60-80 (1984)] was fitted to the scattering data as well. This latter model enabled the estimation of a maximum scatterer distance, which appeared to be of the order of 25 micron.     

Low frequency seabed scattering at low grazing angles

Low-frequency (LF) seabed scattering at low grazing angles (LGA) is almost impossible to directly measure in shallow water (SW), except through inversion from reverberation. The energy flux method for SW reverberation is briefly introduced in this paper. The closed-form expressions of reverberation in an isovelocity waveguide, derived from this method, indicate that in the three-halves law range interval multimode/ray sea bottom scattering with different incident and scattering angles in forming the reverberation may equivalently be represented by the bottom backscattering at a single range-dependent angle. This equivalent relationship is used to derive the bottom backscattering strength (BBS) as a function of angle and frequency. The LF&LGA BBS is derived in a frequency band of 200-2500 Hz and in a grazing angle range of 1.1°-14.0° from reverberation measurements at three sites with sandy bottoms. This is based on three previous works: (1) The closed-form expressions of SW reverberation [Zhou, (Chinese) Acta Acustica 5, 86-99 (1980)]; (2) the effective geo-acoustic model of sandy bottoms that follows the Biot model [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)] and (3) A quality database of wideband reverberation level normalized to source level [Zhou and Zhang, IEEE J. Oceanic Eng. 30, 832-842 (2005)].     

炸药爆炸作用下液体破碎后颗粒尺寸分布的研究

 对液体抛撒的液滴尺寸进行研究在军事和民用上是很重要的,国内刚开始使用激光散射仪开展此项研究工作。利用R. A. Dobbins等人的液体颗粒测量技术,研制了一套既简单又实用的测量液体抛撒过程中液滴尺寸的实验装置——激光散射仪。对于激光与液体微粒的相互作用,当微粒的反射与折射和吸收效应可被忽略时,可导出液体微粒对激光散射的光强公式。只要测量激光被微粒散射的光强,就可推算出微粒的Sauter平均直径。在使用激光散射仪测量液体抛撒液滴尺寸的实验中,用水代替爆炸抛撒液体,测量结果表明：液体抛撒二次破碎中,在固定位置测量到的云雾区液滴Sauter平均直径随测量时间的增加呈现出减小的趋势;而云雾区的宽度则随着与抛撒中心距离的增大而呈现出增加的趋势;云雾区前沿的液滴Sauter平均直径随着与抛撒中心距离的增加而呈现出先逐渐增大然后迅速减小的趋势。为便于比较,对燃料抛撒二次破碎进行了回收法测量和数值模拟计算,其测量与计算结果与用激光散射仪测量的结果有较好的一致性。     

Fiber Bragg gratings for dispersion compensation in optical communication systems

Dispersion compensating fibers (DCF) are the most widely used technology for dispersion compensation. A DCF without Raman amplification introduces extra loss in the system, thus increasing the need for gain in the discrete amplifiers and degrading the noise performance. The idea to additionally use the DCF as a Raman gain medium was originally proposed by Hansen et al. in 1998. [1] This was quickly followed by Emori et al., who demonstrated a broadband, loss less DCF using multiple-wavelength Raman pumping. [2] DCF is a good Raman gain medium, due to a relatively high germanium doping level and a small effective area. To get sufficient gain with a reasonable pump power, a discrete Raman amplifier has to contain several kilometers of fiber, adding extra dispersion to the system that must be handled in the overall dispersion management. Dispersion compensating Raman amplifiers integrates two key functions: dispersion compensation and discrete Raman amplification into a single component.     

Sound propagation in saturated gas-vapor-droplet suspensions with droplet evaporation and nonlinear relaxation

The Sound attenuation and dispersion in saturated gas-vapor-droplet mixture in the presence of evaporation has been investigated theoretically. The theory is based on an extension of the work of Davidson [J. Atmos. Sci. 32(11), 2201-2205 (1975)] to accommodate the effects of nonlinear particle relaxation processes of mass, momentum and energy transfer on sound attenuation and dispersion. The results indicate the existence of a spectral broadening effect in the attenuation coefficient (scaled with respect to the peak value) with a decrease in droplet mass concentration. It is further shown that for large values of the droplet concentration the scaled attenuation coefficient is characterized by a universal spectrum independent of droplet mass concentration.     

Improved non-empirical absorption potential for electron scattering at intermediate and high energies: 30–10000 eV

The non-empirical model proposed by Staszewka et al. in 1983 for the imaginary part of the electron scattering optical potential is revised, improving the treatment used for the e⁻ −e⁻ dispersion. New cross-section calculations employing the revised potential are in agreement with available experimental data for elastic and inelastic processes.     

Relativistic and close-coupling effects in the spin polarization of low-energy electrons scattered elastically from cadmium

The measurements of the Sherman function in elastic electron-cadmium scattering by Bartsch et al. [J. Phys. B 25, 1511 (1992)] have been in serious disagreement with scattering theories for nearly two decades. The recently developed relativistic convergent close-coupling method is applied to the problem and found to be in excellent agreement with experiment over the complete energy range measured. The unusually rapid variation in the spin asymmetry parameter in the vicinity of 4 eV projectile energy is now explained in terms of unitarity of the close-coupling formalism.     

Conductance of inhomogeneous Luttinger liquids with a finite bandwidth

The finite-bandwidth conductance of a Luttinger liquid (LL) with a cluster of impurities is studied and its variation with respect to temperature is shown. The calculations are done using the correlation functions obtained using the powerful non-chiral bosonization technique (NCBT). The results are compared with those obtained by Matveev, Yue and Glazman [K. Matveev et al., Phys. Rev. Lett. 71, 3351 (1993)] who deal with a weakly interacting LL. By contrast, NCBT correctly provides the conductance for all values of the interaction strength (as well as the sign). In addition to finding perfect agreement with the results of Matveev et al. for both weakly repulsive and weakly attractive mutual interactions, we are also able to probe novel physics seen when the repulsion is strong - in the form of a weakly temperature dependent conductance when there is a definite relationship between the transmission amplitude of the non-interacting system and the holon velocity. Secondly, an unusual high conductance for strongly repulsive mutual interactions is observed for a weak barrier at low temperatures. Lastly, inclusion of backward scattering leads to the non-monotonic temperature dependence of conductance when dealing with fermions with spin. This work is also important as a validation of the NCBT itself.