Acoustic radiation from the elastic impact of a sphere with a slab

The transient sound radiation from the impact of a spherical object with a slab is analysed theoretically and compared with experimental results. It is shown that the major source of sound radiation when a sphere impinges on a massive plate is due to the sudden change of velocity of the sphere. By using the method of images, an analytical expression predicting the sound pressure waveform is obtained. Experimental results confirm the theoretical analysis and provide further explanation of this rigid body type of acoustic radiation.     

散射条件下声源分离及参数获取的平行障板方法*

声纳系统或水声设备湿端不可避免存在某些散射体,克服散射体干扰对声源本身辐射特性进行测量目前还缺乏简捷有效的方法。依据刚性圆盘散射规律,通过有限元建模分析了与观察方向平行布设的刚性圆盘对声源辐射特性的影响规律,结果表明：在满足远场条件时,在较小的允许误差范围内观察轴线上声压幅值分布与引入平行障板无关。并在此基础上尝试提出一种简捷有效的平行障板方法,实现散射条件下声源分离与参数获取。通过有限元仿真分析,在1~20kHz频率范围内,半径0.4m厚8mm的钢板对脉动圆管声源级分离测量引入误差在±0.6dB以内。     

Mean force on a small sphere in a sound field in a viscous fluid

A mean force exerted on a small rigid sphere by a sound wave in a viscous fluid is calculated. The force is expressed as a sum of drag force coming from the external steady flow existing in the absence of the sphere and contributions that are cross products of velocity and velocity derivatives of the incident field. Because of the drag force and an acoustic streaming generated near the sphere, the mean force does not coincide with the acoustic radiation pressure, i.e., the mean momentum flux carried by the sound field through any surface enclosing the sphere. If the sphere radius R is considerably smaller than the viscous wave penetration depth delta, the drag force can give the leading-order contribution (in powers of delta/R) to the mean force and the latter can then be directed against the radiation pressure. In another limit, delta< or =R, the drag force and acoustic streaming play a minor role, and the mean force reduces to the radiation pressure, which can be expressed through source strengths of the scattered sound field. The effect of viscosity can then be significant only if the incident wave is locally plane traveling.     

On the sound field of a resilient disk in free space

Radiation characteristics are calculated for a circular planar sound source in free space with a uniform surface pressure distribution, which can be regarded as a freely suspended membrane with zero mass and stiffness. This idealized dipole source is shown to have closed form solutions for its far-field pressure response and radiation admittance. The latter is found to have a simple mathematical relationship with the radiation impedance of a rigid piston in an infinite baffle. Also, a single expansion is derived for the near-field pressure field, which degenerates to a closed form solution on the axis of symmetry. From the normal gradient of the surface pressure, the surface velocity is calculated. The near-field expression is then generalized to an arbitrary surface pressure distribution. It is shown how this can be used as a simplified solution for a rigid disk in free space or a more realistic sound source such as pre-tensioned membrane in free space with non-zero mass and a clamped rim.     

Transition radiation at the boundary of a black body

Transition radiation from a charged particle intersecting the boundary with a black body has been examined. It has been shown that the radiation intensities for the cases when the relativistic particle enters the black body and leaves it are strongly different.     

表面产生负氢离子引出MCC算法设计

本文深入研究负氢离子输运及引出物理机理,理论分析了交换电荷反应及库仑碰撞过程,并设计了相应数值计算模块.在此基础上,采用有限差分法计算负氢离子所受洛伦兹力,运用蒙特卡罗碰撞方法处理负氢离子与其他粒子间的碰撞,成功研制了表面产生负氢离子输运及引出的全三维MCC算法,结合国外热门离子源JAEA 10A离子源进行模拟验证,结果显示:随着过滤磁场增大,引出离子数越大,离子源空间生存离子总数越小;当过滤磁场较小时,气压越大引出离子数越多,当过滤磁场较大时,气压越小引出离子数越多.     

刚体定点运动时本体极面与空间极面间关系的证明

给出了刚体定点运动时本体极面与空间极面间关系的一种证明方法.     

Equilibrium Points in the Restricted Four-Body Problem with Radiation Pressure

This paper studies numerically the photogravitational version of the restricted four-body problem, where an infinitesimal particle is moving under the gravitational attraction and radiation pressure of three bodies much bigger than the particle, the primaries. The fourth body does not affect the motion of the three bodies. These bodies are always at the vertices of an equilateral triangle (Lagrange configuration). We consider all the primary bodies (m ₁, m ₂, m ₃) as radiation sources with radiation factors of the two bodies (m ₂ and m ₃) equal. In this paper we suppose the masses of the three primary bodies are equal. It is found that the involved parameters influenced the positions of the equilibrium points. The linear stability of the relative equilibrium solutions is also studied and all these points are unstable.     

刚体定点转动的张量描述

通过引入转动张量来描述刚体的定点转动,避免了在用角位移描述刚体定点转动时所遇到的问题,即角位移在它是有限大小和无限小时属性发生了变化.验证了对于刚体定点无限小转动,可以分别采用角位移矢量和转动张量描述,两者是等价的.     

On acoustic radiation by a rigid object in a fluid flow

A problem of sound radiation by an absolutely rigid object, moving with respect to the surrounding fluid, is considered on the basis of the Lighthill's equation for aerodynamic sound. An integral representation of the radiated acoustic field is utilized, where the field is characterized as the sum of three fields, generated by a volume distribution of monopoles and by distributions of monopoles and dipoles on the surface of the rigid object. It is shown that, due to a discontinuity of Lighthill's stress tensor on the rigid boundary, a layer of surface divergence of hydrodynamic stresses on the boundary must be taken into account when evaluating the volume integral over Lighthill's quadrupole sources. When the contribution of the surface divergence is included in the solution of Lighthill's equation, amplitudes of the monopole and dipole sound radiated by the rigid object are shown to depend on the potential components of the normal velocity and the pressure on the rigid surface. The obtained solution is compared with Curle's solution for this problem, which establishes that the sound radiation by a rigid object is determined by the force exerted by the object upon the fluid. Both solutions are applied to two known problems of sound scattering and radiation by a rigid sphere in variable pressure and velocity fields. It is shown that predictions based on the obtained solution are equivalent to the results known from literature, whereas Curle's solution gives predictions contradicting the known results. It is also shown that the Ffowcs Williams and Hawkings equation, which coincides with Curle's equation for an immoveable rigid object, does not lead to the correct predictions as well.     

Radiation efficiency of unbaffled and perforated plates near a rigid reflecting surface

The accurate prediction of sound radiation from plate-like structures remains a challenging problem. Although the case of a plate set in a rigid baffle can be solved analytically, when the plate radiates sound into free space the problem is more difficult to solve; nevertheless, several approaches have been proposed to determine the sound radiation from an unbaffled plate. The present study extends the consideration to the situation of an unbaffled plate which is located close to a rigid reflecting surface. For this purpose, Laulagnet's model for the radiation efficiency of an unbaffled plate is extended by modifying the Green's function to include an image source due to the reflecting surface. The results show that, depending on the distance between the plate and the rigid surface, the radiation efficiency is considerably reduced at low frequencies. Additional reduction of sound radiation can be achieved by introducing perforation to the plate. However, at higher frequencies, the radiation efficiency is amplified relative to that for the plate in the absence of the rigid surface, both with and without perforation. These results have also been validated experimentally.     

A least-squares method to cancel rigid body displacements in a hole drilling and DSPI system for measuring residual stresses

This paper presents the evaluation of a method to cancel rigid body displacements that can be introduced when a hole drilling and digital speckle pattern interferometry (DSPI) combined system is used to measure residual stresses. The proposed method is based on a least-square calculation of three correction parameters determined from two evaluation lines located near the edge of the phase map where the displacement field generated by the drilling process is supposed to be negligible. The errors introduced by the method for different residual stress levels and rigid body displacements are analysed using a numerical simulation. An application of the method to experimental data is also presented.     

Hydroelastic dynamic characteristics of a slender axis-symmetric body

The slender axis-symmetric submarine body moving in the vertical plane is the object of our investigation.A coupling model is developed where displacements of a solid body as a Euler beam(consisting of rigid motions and elastic deformations) and fluid pressures are employed as basic independent variables,including the interaction between hydrodynamic forces and structure dynamic forces.Firstly the hydrodynamic forces,depending on and conversely influencing body motions,are taken into account as the governing equations.The expressions of fluid pressure are derived based on the potential theory.The characteristics of fluid pressure,including its components,distribution and effect on structure dynamics,are analyzed.Then the coupling model is solved numerically by means of a finite element method(FEM).This avoids the complicacy,combining CFD(fluid) and FEM(structure),of direct numerical simulation,and allows the body with a non-strict ideal shape so as to be more suitable for practical engineering.An illustrative example is given in which the hydroelastic dynamic characteristics,natural frequencies and modes of a submarine body are analyzed and compared with experimental results.Satisfactory agreement is observed and the model presented in this paper is shown to be valid.     

On the sound fields of infinitely long strips

Exact solutions are derived for sound radiation from four kinds of infinitely-long strips: namely a rigid strip in a baffle of finite width, a resilient strip in free space, and a resilient or rigid strip in an infinite baffle. In one limit, the strip in a finite baffle becomes a rigid strip in free space and in the other, a line source in a finite baffle. Here "rigid" means that the surface velocity is uniform, whereas "resilient" means that the surface pressure is uniform, and the strip is assumed to have zero mass or stiffness, as if a force were driving the acoustic medium directly. According to the Babinet-Bouwkamp principle, radiation from a resilient strip in an infinite baffle is equivalent to diffraction of a plane wave through a slit in the same. Plots are shown for the radiation impedances, far-field directivity patterns, and on-axis pressure responses of the four kinds of strip. A simple relationship between the radiation admittance of the rigid strip in an infinite baffle and the resilient strip in free space is presented. The two-dimensional rectangular wave functions developed in this paper can be applied to related problems.     

Application of local negative feedbacks for active suppression of scattering and intrinsic radiation fields of a body

We theoretically show the possibility of reducing the scattering and intrinsic radiation fields of a body when using an active suppression system that includes a layer of monopole radiators that cover its surface and to which a local negative velocity (acceleration) or pressure feedback on the outer surface is applied. The radiators are controlled by signals from the receivers located on an additional outer surface. We have carried out an experiment that confirms the derived relations for the one-dimensional case of intrinsic radiation field suppression.     

基于蒙特卡罗方法的全身计数器设计研究

全身计数器用于对人体内照射污染的监测,对最小探测活度(MDA)有较高的要求。为使全身计数器具备较好的灵敏度,需要降低本底辐射和提高射线探测效率。基于MCNP5模拟了全身计数器中NaI探测器在不同几何位置时的能量响应,对探测器位置及准直器角度进行研究,同时对不同厚度的屏蔽结构进行了仿真计算分析。研究表明,当屏蔽钢板厚度为14.9 cm时,能够较大程度地降低本底辐射。在源探结构上探测腔准直器角度在26以上,探测器与体模的距离为20 cm时,能够得到理想的能谱响应和探测效率。     

Relative equilibria for the generalized rigid body

This paper gives necessary and sufficient conditions for the (n-dimensional) generalized free rigid body to be in a state of relative equilibrium. The conditions generalize those for the case of the three-dimensional free rigid body, namely that the body is in relative equilibrium if and only if its angular velocity and angular momentum align, that is, if the body rotates about one of its principal axes. For the n-dimensional rigid body in the Manakov formulation, these conditions have a similar interpretation. We use this result to state and prove a generalized Saari’s Conjecture (usually stated for the N-body problem) for the special case of the generalized rigid body.     

均质并有对称面的刚体转动惯量的一个定理

提出并证明了一个关于刚体转动惯量的新定理.该定理指出具有对称面的均质刚体,当位于与对称面垂直的某个平面上的转轴满足特定条件时,转动惯量大小与位于该平面上的转轴方向无关.     

Radiation loss in a compound plasma system with high and low temperature regions

A code has been developed for calculating the non-coronal radiation of impurity in a compound plasma system which consists of high and low temperature regions. The radiation loss of impurity carbon in this system is calculated and analysed. The cooling rate in this system is smaller than that in a homogeneous plasma system due to the particle exchange between the two regions of the system. The volumetric radiation is much bigger in the low temperature region than in the high temperature region because of the relatively low temperature and high electron density, despite the much smaller volume.     

刚体转动惯量实验误差分析

本文指出,在单通道计时法测量刚体转动惯量的实验中,当刚体系轴上摩擦力矩较小时,可通过取较大的予置数Ｎ,来减少由于测量时间的涨落引起的转动惯量的测量误差;同时指出,外力矩中不考虑物体下落的加速度ａ时所引起的转动惯量测量偏差不能忽略;前者使测量结果偏小,后者使测量结果偏大,ａ的计入使总的测量误差变小。