Theoretical relationship between sound velocity and the physical properties of submarine sediment

Density and elastic modulus change ratios are introduced to describe the sound velocity of submarine sediment.The density change ratio is a composite parameter describing the sound velocity.It is expressed by three physical parameters:porosity,solid phase density and seawater density.The elastic modulus change ratio is also a composite parameter of sound velocity.It is expressed by three physical parameters,including porosity,solid phase modulus and seawater bulk modulus.The sound velocity formula can be developed into a Taylor polynomial formula of these two composite parameters.The change in the two composite parameters constitutes the sound velocity surface,which contains the complete information regarding velocity properties and sediment characteristics.The one-parameter velocity formula is a curve on the velocity surface.Each porosity-velocity empirical formula,which represents various sea locations and conditions,is transformed to a standard form.This result is the product of a reference velocity and a modulation function.Comparisons of the numerical calculation and measurements show that the derived modulation functions yield similar results.The difference between the velocity formula derived in this paper and the Wood velocity formula is due to the elastic modulus models.     

Centrifugal instability in rotating shallow water and the problem of the spiral structure in galaxies

A new instability predicte by theory to occur in rotating shallow water in which the rotation velocity has a discontinuity, in a regime where the flow velocity exceeds the characteristics velocity of the waves, has been found experimentally. The instability develops when the radial gradient of the angular velocity across the discontinuity is negative; such an instability is likely to be responsible for the formation of the spiral structure in galaxies which have a similar rotational velocity profile.     

Potential of the test particle in the magnetic field I

The problem of the test particle potential in the external homogeneous magnetic field is solved in the unmagnetized plasma. It is shown that for the case when the parallel component velocity of the test particle is greater than the thermal velocity of the background particles, the potential has the Coulomb character while for the case where the parallel component velocity is less than the thermal velocity the potential is of Debye character. The Larmor radius of the test particle appears as a additional parameter in these potentials.     

入射速度对长杆弹垂直侵彻行为的影响规律

 以长杆弹垂直侵彻半无限厚靶板为研究对象,分析了弹体最大侵彻深度与入射速度的关系,研究了弹体入射速度对侵彻最大深度的影响规律。研究表明：靶板的强度和界面效应使弹体在侵彻过程中存在一个临界速度,当入射速度大于临界速度时,弹体的侵彻才能通过开坑阶段进入准稳定阶段,它是造成当入射速度较小时侵彻深度随入射速度的提高而几乎不变或缓慢增加的主要原因;准稳定侵彻过程中弹体速度和侵彻速度基本不变,并且两者存在线性关系,这种关系只与弹体和靶板的材料性能有关,是造成当入射速度较大时侵彻深度随入射速度的提高呈快速线性增大的主要原因。     

基于一维FFT频谱估计的移动物体速度估计法

提出了时间空间频率混合领域中基于一维FFT频谱估计的移动物体速度估计方法。结合移动物体的抽出,把具有两个移动物体的序列图像分离为分别包含一个移动物体的两个序列图像,当再次进行速度估计时会大大减少背景及噪声对速度估计结果的影响,可提高速度估计的精度。     

Splitting Scheme Integration of the Two-and-One-Half-Dimensional Vlasov Equation

A numerical splitting scheme technique is applied to the integration of the Vlasov equation in two-space and three velocity dimensions. Calculations are performed for a cold, stationary ion, mobile electron plasma in the presence of a constant external magnetic field. Results for Landau damping and the two-stream instability are presented in both one and two velocity dimensions. The Landau damping results show a plateau structure in the electron velocity distribution function in both the parallel and perpendicular directions formed as a result of the damping of a high initial electric field propagating at 45° to the parallel velocity axis. The two-stream velocity distribution function shows pitch angle scattering and velocity diffusion for an electron beam initially propagating at 45° to the magnetic field direction through a low-temperature equal density background. The two-stream distribution function evolves to a stable quasi-isotropic plateau structure similar to that found in auroral sounding rocket data.     

On the consistency of the postulates of special relativity

The special theory of relativity is based on two postulates: (I) All inertial frames are equivalent for the performance of physical experiments; and (II) the velocity of light is a universal constant. It is shown that if postulate I is true, then postulate II is not true, and if postulate II is true, then postulate I is not true. Further, it is shown that the only possible velocity consistent with postulate I follows the rule that the velocity of light with respect to the observer is equal to the velocity of light with respect to the source plus the velocity of the source with respect to the observer.     

A new approach for the estimation of the axial velocity using ultrasound

The most used estimation method for calculating the blood velocity in commercial scanners is the autocorrelation approach. The calculation of the mean velocity used in this method depends on the center frequency of the interacting ultrasound pulse which downshifts as a function of depth, introducing a bias. A new velocity estimator for the mean axial velocity is presented. The estimation principle is based on the 2D Fourier transform and the Radon transform. The input data are a sequence of RF data forming a 2D data input, one column for each pulse emission. A 2D segment is selected for a specific depth. This data segment is first transformed by a 2D Fourier transform, and the result is then transformed by a Radon transform. The center of gravity for the angles of the lines intersecting the origin of the R-theta coordinate system in the Radon domain gives the mean axial velocity for the data segment. The benefit of this method is an estimate of the mean axial velocity which is independent of the center frequency of the propagating ultrasound pulse. The estimate will only depend on f(s) and f(pr f). Results of the estimation method is presented based on both simple generated RF harmonic data for different signal/noise ratios and simulated acoustic RF responses from a 3D measurement situation with an array transducer and a tube with plug flow. The new method shows improvement with a factor of 1.5-4 on the standard deviation on the estimated mean velocity for the simulated case.     

Measurements of the velocity gradient using a laser Doppler phenomenon

This paper reports on a method for measuring the velocity gradient using a laser Doppler phenomenon. The velocity gradient is determined from the velocity difference between two different points of the probing object and is actually obtained from the optical heterodyning of two differently Doppler-shifted scattered light fields from two points of the object having different velocities. The properties of output beat signals are theoretically investigated from their spectral broadening including the effect of the detecting aperture. The preliminary experiment was performed by using a rotating glass disk whose velocity is different from its center to the outside. The experimental results shows the usefulness of the present method for measurements of the velocity gradient.     

On the dynamics of the clavichord: from tangent motion to sound

An experimental study of variations in the sound of clavichord notes at different dynamic levels is described. Radiated acoustic signal, tangent velocity and two tangent-string contact signals are synchronously measured for all 51 notes of an unfretted instrument. More than ten repeated measures are recorded in order to obtain as much variation in dynamic level as possible. The tangent motion, expressed in terms of velocity, is studied in the time and frequency domains. A model of the tangent-string contact point velocity is proposed. Then, three aspects of the sounded tones are analyzed: SPL and its relationship to tangent velocity, spectral slope, and pitch variations. These results indicate a linear relationship between sound pressure level and tangent peak log velocity. Spectral slope seems almost constant independent of tangent velocity and dynamic level. Both tangent velocity and finger pressure are shown to influence the fundamental frequency. In conclusion, controlling both finger velocity and finger pressure may prove challenging for the player, and this may explain why the sound quality of the clavichord depends so much on the players ability.     

On the possibility of natural formation of a transverse wave with a phase velocity lower than the velocity of light

The formation of a transverse wave with a phase velocity lower than the velocity of light, which can exist in an equilibrium plasma without a slow-wave structure in zero magnetic field, is described. It involves the transformation of a transverse wave with trapped electrons, traveling along the magnetic field, into a slow transverse wave after the removal of the magnetic field. During the evolution of the wave with trapped electrons, the magnetic induction decreases very slowly in the direction of the wave propagation. As a result, the velocity at which electrons are in resonant interaction with the wave increases; therefore, the electrons fall to the bottom of potential wells. Under the influence of the trapped electrons, the phase velocity of the wave decreases and becomes lower than the velocity of light. It becomes equal to the velocity at which the electrons are in resonance interaction with the wave at the instant when the magnetic field vanishes. It is demonstrated that a transverse wave with a velocity lower than the velocity of light can exist in an equilibrium plasma even after the magnetic field vanishes; in this case, the flow of trapped electrons serves as a slow-wave structure.     

Effects of the imperfect interface and piezoelectric/piezomagnetic stiffening on the SH wave in a multiferroic composite

Studied in the present work is the propagation of SH wave in a cylindrically multiferroic composite consisting of a piezoelectric layer and a piezomagnetic central cylinder. It is assumed that the interface in the composite is damaged mechanically, magnetically or electrically. The dispersion relations of SH wave are obtained for two kinds of electric-magnetic boundary conditions at the free surface, and then the phase velocity is calculated numerically. Parametric studies on the phase velocity yield three main conclusions. (a) The mechanical imperfection may remarkably reduce the phase velocity, depending on the combination of the values of the wavenumber and the thickness ratio. (b) The magneto-electrical imperfection has no obvious effect on the phase velocity in any cases. (c) The piezoelectric and piezomagnetic stiffening can increase the phase velocity.     

倾斜地层下的井孔声场分析及声速校正图版制作

理解声波在倾斜井孔下的传播规律,对于准确获得目的层真速度有着至关重要的作用。通过有限差分和几何声学分析方法对比了倾斜地层下纵横波首波速度的变化规律,旨在建立倾斜地层下的声速校正图版。研究结果表明,当倾斜地层速度"上低下高"时,从全波中得到的声波速度不再是地层的真实速度,均大于目的层真速度。尤其是当地层界面倾角大于45°时,即便是源距超过了4 m,仍对首波速度有影响,必须对其校正。相反,当倾斜地层速度"上高下低"时,任何源距和地层倾角下的视速度值都接近目的层真速度,可以不予校正。根据以上分析,建立了倾斜地层下的声速校正图版,并指出下层速度的变化对于校正图版有着较大影响,源距的变化对于图版影响很小,在不同源距下可以使用同一个图版。      

Measurements of the MHD dynamo in the quasi-single-helicity reversed-field pinch

The first experimental study of the MHD dynamo in a quasi-single-helicity (QSH) reversed-field pinch toroidal plasma is presented. In QSH plasmas, a dominant wave number appears in the velocity fluctuation spectrum. This velocity component extends throughout the plasma volume and couples with magnetic fluctuations to produce a significant MHD dynamo electric field. The narrowing of the velocity fluctuation spectrum and the single-mode character of the dynamo are features predicted by theory and computation, but only now are observed in experiment.     

Theoretical Studies of the Laser Induced Plasma Profile Near the Critical Density Region for an Adiabatic Plasma Model

A theoretical study of the variation of density and flow velocity due to pondermotive force, near critical density region of a laser – produced plasma, obeying an adiabatic pressure law is reported. The ion-continuity, momentum and wave equations are solved in a steady state. The density and flow velocity as a function of incident laser intensity are calculated and plotted for both underdense and overdense regions for adiabatic exponent γ = 4/3, 5/3, 2. The velocity decreases and density increases in the overdense region and velocity increase and density decreases in the underdense region with the increase of larger power at sonic point. It is found that the velocity is subsonic in the overdense region and supersonic in the underdense region. The nature of variation is similar to the isothermal model.     

颗粒速度在颗粒流稀疏流-密集流转变中的作用

用实验和计算模拟的方法研究了颗粒流中的颗粒速度与颗粒流特性的关系.实验研究发现当入口流量固定时，在出口上方高速运动的颗粒会使颗粒流由稀疏流向密集流转变的临界出口尺寸变小.当颗粒流转变为密集流后，颗粒速度的作用被出口上方的颗粒堆积区所消耗，最终变得与颗粒速度无关.二维分子动力学模拟计算得到了与实验相同的结论.通过二维分子动力学模拟计算，还给出了不同颗粒速度下体系的密度和速率在空间的分布图.这些分布图显示随着颗粒到达出口上方的瞬间速度的不同，颗粒堆积区的密度和高度均会改变，并最终导致颗粒流流动状态的改变. 关键词： 颗粒流 颗粒气体 分子动力学模拟     

气液两相流中的声速研究

气液两相流在工业各领域中广泛存在,而声速是描述其声学性质的一个重要参数。本文从流体的体积弹性模量的定义出发,推导了气液两相流中的声速随含气率的变化关系式,即混合流体的Wood声速公式,将其声速的部分计算结果和其他作者的实验数据进行了比较,吻合良好。并通过COMSOL有限元模拟软件得到不同气体分布下圆管谐振腔最低阶模式的共振频率,间接数值模拟研究了含气率对声速的影响。模拟结果与理论计算结果一致,当气液两相流中含气率较低时,声速随含气率的增大急剧减小。本研究结果为确定声速与气液两相流中的含气率间的关系提供了参考依据。     

液体声速随温度变化的特性研究

集成温度传感器（Integration temperature sensor）简称（ITS）应用于声速测量系统。用实验的方法,研究了液体中声的传播速度与温度变化的关系,并对水进行了实际测量。实验证明了这一测试系统的实用性和可靠性,为液体声速的进一步研究,提供了理想的实验依据和测量方法。     

On the velocity of propagation of a signal in nonlinear optical media

The maximum velocity of propagation of a signal, which is defined as the velocity of propagation of the wave front, is considered for electromagnetic waves in nonlinear media. It is shown that the magnitude of velocity is determined to a considerable extent on the form of the constitutive equation defining the relation between the polarization of the medium with the radiation field strength. In the noninertial nonlinearity model, this velocity may be smaller (in media with self-focusing nonlinearity) or larger (defocusing nonlinearity) than the velocity of light in vacuum. For real nonlinear media, for which the inertia of their response is taken into account, the wave front velocity coincides with the velocity of light in vacuum.     

Influence of air pressure on the performance of plasma synthetic jet actuator

Plasma synthetic jet actuator(PSJA) has a wide application prospect in the high-speed flow control field for its high jet velocity.In this paper,the influence of the air pressure on the performance of a two-electrode PSJA is investigated by the schlieren method in a large range from 7 k Pa to 100 k Pa.The energy consumed by the PSJA is roughly the same for all the pressure levels.Traces of the precursor shock wave velocity and the jet front velocity vary a lot for different pressures.The precursor shock wave velocity first decreases gradually and then remains at 345 m/s as the air pressure increases.The peak jet front velocity always appears at the first appearance of a jet,and it decreases gradually with the increase of the air pressure.A maximum precursor shock wave velocity of 520 m/s and a maximum jet front velocity of 440 m/s are observed at the pressure of 7 k Pa.The averaged jet velocity in one period ranges from 44 m/s to 54 m/s for all air pressures,and it drops with the rising of the air pressure.High velocities of the precursor shock wave and the jet front indicate that this type of PSJA can still be used to influence the high-speed flow field at 7 k Pa.