波导型声透镜实现空气中声聚焦

0引言本文实现了一种波导型声透镜结构,使频率在500Hz-2000Hz范围内的平面声波在空气中的可控区域内实现声聚焦。首先,通过几何声学理论,对声透镜的轮廓进行了初步设计;其次,通过COMSOL软件对波导型声透镜结构的各个参数进行了优化设计;进而,加工出实体,进行实验验证。     

气体声学温度计中声波导管的优化设计研究

声波导管的设计是影响声学信号信噪比的关键因素,导管内径越大、长度越短,越利于声波传输,但同时对声学共鸣腔产生更大的扰动。提出了采用变径声波导管降低声波的能量损耗和扰动方法,建立了变径声波导管的衰减和扰动模型,对比分析声学信号在不同尺寸声波导管内的能量衰减和导管对圆柱轴向非缔合声学共振频率和半宽的扰动,获得了优化的导管尺寸,在声波传输能量损失较小的情况下对内长为80 mm圆柱腔体首个轴向非缔合声学共振频率产生的相对扰动在3×10^-5以内,该声波导管的优化设计可为高温气体声学温度计的深入研究提供理论支持。     

均匀土中有限长桩瞬态横向动力响应

解析地研究了桩顶受到横向冲击荷载时桩的瞬态横向动力响应。把桩身当作Bernoulli-Euler梁,桩周土当作Winkler地基。由Laplace正变换和反变换分别得到桩土系统的传递函数和单位脉冲响应。进一步得到桩顶横向振动速度的频响函数、频域和时域表达式。对桩的横向动力响应和轴向动力响应进行了对比研究;研究了桩长、桩周土剪切波速和桩顶横向激振力作用时间对桩的瞬态横向动力响应的影响;还研究了桩中弯曲波的衰减。     

感应式衰减器——电阻匹配网络与T型电阻网络的等效原理和应用

一、前言我国于1978年建立了高准确度和高稳定度的感应式衰减器,应用在射频衰减标准装置(DZB-1)中,取得良好的效果,并已定型生产。在DZB-1型装置上,采用串联替代衰减测量法检定电阻式衰减器,这要求复杂的检定设备,造价较高。为此,我们考虑在音频下采用并联电路微差补偿方法,它具有对电源稳定度要求低,没有串入固定衰减匹配器,量程可以扩大等优点。但采用本方案检定电阻式衰减时,会遇到一些理论问题。例如,感应式衰减器大的输入阻抗和小的输出阻抗的特点与电阻式衰减器不符合;感应式衰减器与电阻式衰     

工程中三维有限长声屏障的优化设计及应用

声屏障因其经济有效,常作为主要的降噪措施,在工程中应用较广,但其结构尺寸设计多采用经验法和试探法,成本往往较高。根据已有的工程声屏障衰减算法,以实际工程中声屏障的综合成本最小为目标,建立声屏障结构尺寸的优化设计模型。利用MATLAB优化工具箱,对目标函数求解最优值。采用该方法,对江西某水泥厂立磨的声屏障进行优化设计,并使用Cadna/A噪声预测软件进行验证对比。结果表明,在达到相同降噪效果的情况下,声屏障的综合成本降低,验证了方法在实际工程应用中的可行性。     

工程中三维有限长声屏障的优化设计及应用

声屏障因其经济有效,常作为主要的降噪措施,在工程中应用较广,但其结构尺寸设计多采用经验法和试探法,成本往往较高。根据已有的工程声屏障衰减算法,以实际工程中声屏障的综合成本最小为目标,建立声屏障结构尺寸的优化设计模型。利用MATLAB优化工具箱,对目标函数求解最优值。采用该方法,对江西某水泥厂立磨的声屏障进行优化设计,并使用Cadna/A噪声预测软件进行验证对比。结果表明,在达到相同降噪效果的情况下,声屏障的综合成本降低,验证了方法在实际工程应用中的可行性。     

异形截面管不均匀膨胀充液成形数值模拟

目的对低碳钢不均匀膨胀率异形截面弯管零件充液成形工艺参数进行研究。方法利用有限元方法对低碳钢异形截面管零件的充液成形过程进行有限元仿真,对影响圆角破裂的关键工艺参数进行分析及优化。结果液室压力过小,弯管圆角内侧不易贴模;压力过大,圆角内侧会因材料过度减薄而发生破裂。推头轴向进给量合适时,可以在圆角处形成"有益皱纹",防止材料不均匀膨胀发生破裂。结论成形压力在300 MPa,推头的轴向进给量为50 mm时,可以成形出合格零件。     

有限长圆柱壳结构水下声辐射有源控制实验

以水下有限长圆柱壳为对象,实验研究水下结构声辐射有源控制。实验中通过引入次级力源,以单个或两个误差点处的声压最小为目标函数,分析次级激振器和误差传感器布放（位置和数目）以及激励频率对控制效果的影响。结果表明：有源控制能够有效控制水下圆柱壳线谱噪声;增加次级激振器和误差传感器的数目,能够增大降噪区域;有源控制在3 000 Hz高频处对壳体噪声控制依然有效。有源控制能够对多线谱信号激励壳体辐射噪声产生抑制作用。     

波导中目标的声散射

1引言 在水平分层浅海波导内,目标的入射和散射特性都比在自由场中复杂,不仅边界面之间存在卢传播的多次反射,而且目标和界面之间也存在多次反射,所以浅海中目标散射场必然具有较强的相干起伏特征.从八十年代末期各国开始重视研究这个问题,如Ingenito1采用的本征函数展开法研究了在水平分层介质上均匀流体中刚性球的散射;Athanassoulis2等人研究了浅海中柱岛的散射.而大多数目标在波导中的散射问题没有准确解,通常采用数值方法求解.赵洪3采用边界积分及单一矩方法研究过浅海短柱的散射.     

渐变截面背腔微穿孔管消声器声传递特性分析

为提高微穿孔管消声器的性能,研究了背腔结构对消声器性能的影响。基于一维声传播理论和微穿孔结构吸声理论,推导了渐变截面背腔微穿孔管消声器的声传播理论模型,利用传递矩阵法求出声学传递损失,并将理论计算结果与有限元仿真分析结果进行了比较,在等容积条件下分析了结构参数对传递损失的影响。结果表明:微穿孔管消声器的传递损失曲线在背腔的轴向模态频率处有极小值;对于锥形体结构背腔,增加锥度能够拓宽吸声频带,提高背腔轴向共振频率处的极小值;对于弧形体结构背腔,减小弧形半径能够提高消声器的低频处的吸声效果。     

Circular-harmonic vector analysis of a dielectric waveguide with a cross-cut-circle cross section

Vector-wave solutions of dielectric waveguides with cross-cut-circle cross sections, which are commonly produced by reflow of the waveguide core on a planar substrate at high temperature, are rigorously analyzed by the use of the point-matching method. As a numerical example, the propagation constant, electromagnetic-field distribution, and convergence of mode solutions for a semicircular waveguide are presented, and the properties of these modes are discussed.     

Acoustic intensity for a long vessel with noncircular cross section

The acoustic intensity distribution around and within long vessels of noncircular cross section was investigated for parameters typical of biomedical ultrasound and blood vessels. We have developed a collocation method for finding the acoustic field when a uniform plane wave is obliquely incident on a long, not necessarily cylindrical, impedance interface. Results are presented for vessels of noncircular cross section and for vessels with thick walls of nonuniform thickness. The intensity in the vicinity of the vessel, throughout the lumen, and in the wall, is calculated for intermediate length scales, i.e., vessel radius and wall thickness in the range 1 to 10 wavelengths. The intensity distribution is an interference pattern, with complicated regions of increased and decreased intensity. These results are compared with approximate intensity obtained using ray theory. Effects not predicted by ray theory and intensity variations that will be significant in any close ultrasonic investigation of these vessels are revealed.     

Thermal resistance between two long parallel cylinders of elliptical cross section

It is shown that the problem of determining the thermal resistance between two long parallel cylinders of elliptical cross section can be reduced to the corresponding problem for two infinitely thin parallel bands. The thermal resistance of the system is plotted vs the geometrical dimensions.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 15, No. 5, pp. 899–905, November, 1968.     

The partition of unity finite element method for short wave acoustic propagation on non‐uniform potential flows

A novel numerical method is proposed for modelling time‐harmonic acoustic propagation of short wavelength disturbances on non‐uniform potential flows. The method is based on the partition of unity finite element method in which a local basis of discrete plane waves is used to enrich the conventional finite element approximation space. The basis functions are local solutions of the governing equations. They are able to represent accurately the highly oscillatory behaviour of the solution within each element while taking into account the convective effect of the flow and the spatial variation in local sound speed when the flow is non‐uniform. Many wavelengths can be included within a single element leading to ultra‐sparse meshes. Results presented in this article will demonstrate that accurate solutions can be obtained in this way for a greatly reduced number of degrees of freedom when compared to conventional element or grid‐based schemes. Numerical results for lined uniform two‐dimensional ducts and for non‐uniform axisymmetric ducts are presented to indicate the accuracy and performance which can be achieved. Numerical studies indicate that the ‘pollution’ effect associated with cumulative dispersion error in conventional finite element schemes is largely eliminated. Copyright © 2005 John Wiley & Sons, Ltd.     

Injection from a finite section of a flat plate placed parallel to a uniform stream

Summary A study is made of the effects on the fluid motion past a semi-infinite flat plate of introducing a uniform injection of extra fluid from a finite porous section of the plate. Steady laminar flow is considered with a uniform oncoming stream parallel to the plate in an incompressible fluid. A local analysis valid near the start and finish of the injection is found to produce unusually firm predictions for the singular behaviour of the solution of the Navier-Stokes equations there and in particular shows that separation ahead of the injection is inevitable for all positive Reynolds numbers R. Then a numerical treatment of these singular points, and of the Navier-Stokes equations and boundary conditions, is described and the results are presented for a range of values of R. The calculation method used is based on the accurate and efficient centred differencing technique suggested by Dennis [1] and developed recently by Dennis and Hudson [2]. Checks on the influences of mesh spacing, of the placing for the outer boundaries of the computational domain, and of the treatments of the singular points are given. The agreement found with the previous local analysis near the ends of the injection proves especially encouraging. In addition the results provide some guidance to the asymptotic structure of the flow at high Reynolds numbers and to the questions surrounding the occurrence of large-scale separations.     

The capture cross section of a dipole trap

An active distance of attraction model is developed for the capture cross section σ_s of a dipole trap, yielding a simple analytical form. A first order analysis of the field dependence shows that the result is valid up to moderately high field strengths, e.g. up to 5 × 10⁶ V m^-1 for the example quoted in the text. It should be borne in mind, however, that this marks the limit of the analysis rather than the actual onset of a pronounced field dependence of σ_s.The assumption of certainty of energy loss by the electron on entering the ?2kT equipotential surface limits the applicability of the result to low mobility solids and certain liquids; fortuitously these happen to encompass the substances predicted to be most suitable as bulk media in dipole capture investigations.     

Efficient dynamic finite element method for 3D objects with uniform cross‐section

An efficient implicit dynamic finite element method (FEM) for elastic 3D objects with uniform cross‐sections was developed. In this method, the finite element mesh is generated in such a way that the object to be analysed is at first sliced into layers with the same thickness along its generatrix and then each layer is discretized into finite elements of the same pattern. This way of discretization makes the mass, viscosity, and stiffness matrices into the repetitive block tridiagonal matrices. The repetitive block tridiagonal matrix has the characteristic, that the sequence of matrices which appears in the Gaussian elimination for the repetitive block tridiagonal matrix is a rapid convergent sequence. The process of the Gaussian elimination can be terminated when the sequence converges. The rest of the sequence is not necessary to be stored. The present method can save the computational time and memory by utilising this characteristic of the repetitive block tridiagonal matrix. A few examples of analyses including whole Hopkinson‐bar analysis were performed to demonstrate the effectiveness of the present method. The present method is applicable not only to the elasto‐dynamics but also to many other problems, such as thermal problems, electrical problems, and plastic problems without geometric non‐linearity. Copyright © 2005 John Wiley & Sons, Ltd.