穿孔管消声器声学特性的有限元分析

三维有限元法被发展用于预测和分析穿孔管消声器的声学性能。直通穿孔管消声器和三通穿孔管消声器传递损失的有限元计算结果与实验测量结果吻合良好,表明了三维有限元法预测穿孔管消声器声学性能的适用性和精度。进而有限元法被用于研究几何结构对三通穿孔管消声器声学性能的影响,结果表明,中间管插入端腔会使共振频率向低频偏移,在三通穿孔管消声器的右侧增加端部共振器能获得良好低频消声效果。     

穿孔管阻性消声器声学特性的有限元分析

将有限元法应用于预测穿孔管阻性消声器的声学性能。直通穿孔管阻性消声器传递损失的有限元计算结果与实验测量结果吻合良好,表明了有限元法预测穿孔管阻性消声器声学性能的适用性和精度。进而有限元法被用于研究吸声材料的填充密度(流阻率)、吸声材料的厚度和穿孔率对穿孔管阻性消声器声学性能的影响,结果表明,增加吸声材料的填充密度,可以改善中高频消声性能,并使峰值频率向低频方向移动;增加吸声材料厚度,可以改善阻性消声器的中高频消声性能,而对低频声学性能影响较小;膨胀腔包覆吸声材料可以改善中高频消声效果,同时消除通过频率;增加穿孔率,可以提高穿孔管阻性消声器的高频消声性能,并使共振峰向高频方向偏移;吸声材料背后增加空气腔,可以在较宽的频率范围内获得较为平坦的消声曲线。     

Diffraction efficiency of low-resolution Fresnel encoded lenses

A mathematical model to describe the behavior of low-resolution Fresnel lenses encoded in any low-resolution device (e.g., a spatial light modulator) is developed. From this model the diffraction efficiency is calculated in terms of all the parameters that characterize these lenses.     

Finite element analysis of engines

The basic principles are outlined by means of a simple example, some applications given and the way is explained in which finite element analysis is carried out.     

Finite element analysis of bonded repairs to edge cracks in panels subjected to acoustic excitation

The skin of an aircraft can vibrate as a result of pressure waves caused by engine and/or aerodynamic effects. In modern fighter aircraft such as the F/A-18, sound pressure levels have been recorded up to 170 dB over the surface of the skin. In the F/A-18 cracking has occurred in the lower nacelle, typically along the boundaries of the panel. These cracks often originate from a fastener line, grow along the boundary and then turn into the centre of the panel. In the case of the F/A-18, cracking was due to higher than expected pressure levels caused by an aerodynamic disturbance at the inlet lip. Attempts have been made to repair these panels with boron fibre patches, however the cracks have continued to grow. This paper aims at attempting to understand the mechanisms of cracking of the panels subjected to acoustic excitation and the influence of bonded repairs. Also, the analysis is extended to a feasibility study on the effects of enlarging the patch and increasing material damping on the stress intensity factor.     

穿孔管消声器横截面模态及消声特性的有限元分析

将有限元法应用于计算穿孔管消声器的横截面模态频率,推导了相应的有限元公式并编写了计算程序。对于圆形同轴结构的模态频率,有限元法计算结果与解析法计算结果吻合良好,表明了有限元法预测穿孔管消声器横截面模态的准确性。之后将有限元法应用于计算和分析孔径、穿孔率和穿孔管偏移对直通穿孔管消声器横截面模态和消声特性的影响。结果表明,穿孔率低于40%时,孔径减小或穿孔率增大均能使(0,1)阶模态频率升高,消声器中频消声效果变好;穿孔率高于40%后,孔径和穿孔率对(0,1)阶模态频率影响较小。对于非同轴结构,平面波截止频率为第2阶模态频率,对于给定的孔径和穿孔率,穿孔管偏移对第2阶模态频率影响较小。     

Subharmonic focal-length intensities formed by Fresnel lenses

Binary Fresnel lenses produce focused spots at subharmonics of the principal focal length of the lens. The intensities of these focal spots can be controlled by variation of the relative widths of the rings of the Fresnel lens compared with the spacings between the rings. Theory is presented and experimentalverification is provided with Fresnel lenses written onto the magneto-optic spatial light modulator.     

Diffraction theory of optimized low-resolution Fresnel encoded lenses

A mathematical model describing the behavior of low-resolution Fresnel encoded lenses (LRFEL's) encoded in any low-resolution device (e.g., a spatial light modulator) has recently been developed. From this model, an LRFEL with a short focal length was optimized by our imposing the maximum intensity of light onto the optical axis. With this model, analytical expressions for the light-amplitude distribution, the diffraction efficiency, and the frequency response of the optimized LRFEL's are derived.     

钢煤斗结构的有限元分析

通过对一个钢筋混凝土和钢煤斗的混和结构进行有限元分析,参照结构的实际破坏特征,找出破坏的原因,提出对该类结构设计有益的结论。采用了两种有限元软件,将分析结果进行了比较。讨论在兼顾准确和简洁的前提下如何建模和分析。同时也比较了几种建模方法的优劣,提出对有限元分析有参考价值的建议。     

Finite element analysis of membrane wrinkling

New results are presented for the finite element analysis of wrinkling in curved elastic membranes under‐going large deformation. Concise continuum level governing equations are derived in which singularities are eliminated. A simple and efficient algorithm with robust convergence properties is established to find the real strain and stress of the wrinkled membrane for Hookean materials. The continuum theory is implemented into a finite element code. Explicit formulas for the internal forces and the tangent stiffness matrix are derived. Numerical examples are presented that demonstrate the effectiveness of the new theory for predicting wrinkling in membranes undergoing large deformation. Copyright © 2001 John Wiley & Sons, Ltd.     

Finite element analysis of propagating buckles

A finite element technique is described for the analysis of propagating buckles in deep-water pipelines, accounting for large deformation, elastoplastic material behaviour and contact between regions of the interior wall of the pipe. Within the framework of a Lagrangian formulation and a convected co-ordinate system, nine-node isoparametric shell finite elements are employed along with contact elements. They are equipped with a so-called pressure node which facilitates the tracing of unstable equilibrium paths during the deformations of the pipe. For the analysis of a pipe surrounded by water, fluid-structure interaction is treated in an approximate manner. Numerical results are presented and comparisons with experimental data are made, demonstrating the excellent performance of the technique.     

Finite element analysis of anisotropic plates

Two aspects of the finite element analysis of mid-plane symmetrically laminated anisotropic plates are considered in this paper. The first pertains to exploiting the symmetries exhibited by anisotropic plates in their analysis. The second aspect pertains to the effects of anisotropy and shear deformation on the accuracy and convergence of shear-flexible displacement finite element models. Numerical results are presented which show the effects of increasing the order of approximating polynomials and of using derivatives of generalized displacements as nodal parameters.     

Finite element analysis of helicoidal waveguides

The purpose of the paper is to propose an efficient method to compute propagation modes in helicoidal waveguides. An helicoidal system of co-ordinates is introduced to define the structure and to set up the problem. These co-ordinates, albeit non-orthogonal, preserve the translational invariance in a way that allows a two-dimensional finite element model similar to that of classical straight waveguides     

Finite element analysis of wave motion

To estimate wave motion, the finite element method is presented based on the linear interpolation function and two-step explicit numerical integration in time. For the determination of free surface position, the two-step scheme based on the Eulerian technique is usefully employed. Travel and run-up of solitary wave have been analysed and compared with the analytical solution. Both results are reasonably well in agreement. This method is applied to estimate the wave force on the practical breakwater to show the validity of the method.     

Finite element post-buckling analysis for frames

Presented in this paper is a direct method of analysis of the elastic non-linear behaviour of frames. Emphasis is given to the method's capability of tracing the post-buckling path from a bifurcation point although the method can also trace the non-linear behaviour of frames with eccentricities. The method is proposed as an alternative to the main methods currently in use, the perturbation method and the incremental method. Conditions for equilibrium and stability are developed from a variational approach to the total potential. A finite element approximation is made and an efficient solution technique for the resulting non-linear equations is developed. Results for three frames are given demonstrating good agreement with solutions generated from other approaches.     

Finite element formulation of smart piezoelectric composite plates coupled with acoustic fluid

In the context of noise and vibration reduction by passive piezoelectric devices, this work presents the theoretical formulation and the finite element (FE) implementation of vibroacoustic problems with piezoelectric composite structures connected to electric shunt circuits. The originalities of this work concern (i) the formulation of the electro-mechanical-acoustic coupled system, (ii) the implementation of an accurate and inexpensive laminated composite plate FE with embedded piezoelectric layers connected to resonant shunt circuits, and (iii) the development of an efficient fluid-structure interface element. Various results are presented in order to validate and illustrate the performance of the proposed fully coupled numerical approach.     

Finite element analysis of perturbed compressible flow

The finite element representation of the linearized equations governing the steady compressible flow of an isentropic perfect gas is considered. The fully non-linear system of equations is linearized on the basis of small perturbation theory. The finite element matrix equations for an arbitrary polygonal element are generated by the method of weighted residuals: Galerkin's criterion is used. As an example, a triangular element in two-dimensional flow is treated in detail and numerical result for a sample problem are given.