A coupled edge-/face-based smoothed finite element method for structural-acoustic problems

The edge-based smoothed finite element method (ES-FEM) and the face-based smoothed finite element method (FS-FEM) developed recently have shown great efficiency in solving solid mechanics problems with triangular and tetrahedral meshes. In this paper, a coupled ES-/FS-FEM model is extended to solve the structural-acoustic problems consisting of a plate structure interacting with the fluid medium. Three-node triangular elements and four-node tetrahedral elements are used to discretize the two-dimensional (2D) plate and three-dimensional (3D) fluid, respectively, as they can be generated easily and even automatically for complicated geometries. The field variable in each element is approximated using the linear shape functions, which is exactly the same as that in the standard FEM. The gradient field of the problem is obtained particularly using the gradient smoothing operation over the edge-based and face-based smoothing domains in 2D and 3D, respectively. The gradient smoothing technique can provide a proper softening effect to the model, effectively solve the problems caused by the well-known “overly-stiff” phenomenon existing in the standard FEM, and hence significantly improve the accuracy of the solution for the coupled systems. Intensive numerical studies have been conducted to verify the effectiveness of the coupled ES-/FS-FEM for structural-acoustic problems.     

A hybrid SEA/modal technique for modeling structural-acoustic interior noise in rotorcraft

This paper describes a hybrid technique that combines Statistical Energy Analysis (SEA) predictions for structural vibration with acoustic modal summation techniques to predict interior noise levels in rotorcraft. The method was applied for predicting the sound field inside a mock-up of the interior panel system of the Sikorsky S-92 helicopter. The vibration amplitudes of the frame and panel systems were predicted using a detailed SEA model and these were used as inputs to the model of the interior acoustic space. The spatial distribution of the vibration field on individual panels, and their coupling to the acoustic space were modeled using stochastic techniques. Leakage and nonresonant transmission components were accounted for using space-averaged values obtained from a SEA model of the complete structural-acoustic system. Since the cabin geometry was quite simple, the modeling of the interior acoustic space was performed using a standard modal summation technique. Sound pressure levels predicted by this approach at specific microphone locations were compared with measured data. Agreement within 3 dB in one-third octave bands above 40 Hz was observed. A large discrepancy in the one-third octave band in which the first acoustic mode is resonant (31.5 Hz) was observed. Reasons for such a discrepancy are discussed in the paper. The developed technique provides a method for modeling helicopter cabin interior noise in the frequency mid-range where neither FEA nor SEA is individually effective or accurate.     

Multi-focus design of underwater noise control linings based on finite element analysis

Varied, counter-demanding objectives in designing the underwater noise control linings are addressed using a finite element model based methodology. Four different kinds of designs are proposed to attend to diverse and conflicting requirements concerning echo reduction (ER) and transmission loss (TL) performance of these linings. In this regard a slightly modified hybrid type finite element based on the Pian and Tong (PT) formulation has been used to make the computational efforts less demanding as compared to the original one. The adequacy of this formulation has been shown by comparing its results with the analytical, finite element analysis based, and experimental results. Different unit cell representations for different types of distributions of air cavities on the linings are discussed with respect to their limitations and applicability. Effect of static pressure is studied by using a simplified technique which can be used to simulate deep sea testing environment. Performance variation of different designs is investigated under different water depths to study their applicability in such situations.     

More on finite element modeling of damped composite systems

Finite element procedures are developed and verified for layered beams and rings having either continuously or discontinuously constrained viscoelastic damping layers. The two configurations considered are (1) a three-layered sandwich beam or ring (closed curved beam) consisting of two thin elastic layers with a viscoelastic core in between, and (2) a damped composite made of a thin-walled elastic structure having a finite number of mass segments or elastic segments adhered to it by a viscoelastic material. Viscoelastic material dependence on frequency and temperature is accounted for. Numerical predictions of transverse driving point impedances agree very well with available experimental data.     

A finite element approach to a coupled structural-acoustic radiation system with application to loudspeaker characteristic calculation

The finite element method is applied to an elastic shell which is vibrating to generate acoustic radiation. A vibrating shell of revolution backed by an enclosure in its rear side is to radiate sound waves into a semi-infinite space in front. As a numerical example, some characteristics of a direct radiator type loudspeaker model are calculated and discussed. The driving point impedance and the far field sound pressure frequency characteristics are shown, together with the effects of the radiation and the enclosure.     

Analytical research on structural-acoustic coupling of a cavity surrounded by flexible panel

The structural-acoustic coupling characteristics, mechanisms, effect of structural-acoustic coupling on natural mode and natural frequencies of the system are analyzed theoretically and numerically. Formulae for the natural frequencies of the coupled system are derived. Some new conclusions are obtained. Analytical results demonstrate that the strongly coupled system indicates obvious closed-loop feedback characteristics, whereas the weakly coupled system indicates obvious feedforward characteristics, and it is because of the presence of the feedback loop that the natural characteristics and natural frequencies are changed. Cluster coupling characteristic between the structural and acoustic modes for the regular cavity and panel system is found, which determines the coupling interaction between the flexible panel and cavity. Any mode in one mode cluster only interferes the modes and the modal natural frequencies in the same cluster independently. The modal cluster coupling changes not only the natural frequencies of the system but also the modal order and structural mode shape.     

BOR-FDTD subgridding based on finite element principles

In this paper a recently developed provably passive and stable 3D FDTD subgridding technique, based on finite elements principles, is extended to body-of-revolution (BOR) FDTD. First, a suitable choice of basis functions is presented together with the mechanism to assemble them into an overall mesh consisting of coarse and fine mesh cells. Invoking appropriate mass-lumping concepts then leads to an explicit leapfrog time stepping algorithm for the amplitudes of the basis functions. Attention is devoted to provide the reader with insight into the updating equations, in particular at a subgridding boundary. Stability, grid reflection and dispersion are also discussed. Finally, some numerical examples for toroidal and cylindrical cavities demonstrate the stability and accuracy of the method.     

Regularised finite element model updating using measured incomplete modal data

This paper presents an effective approach for directly updating finite element model from measured incomplete vibration modal data with regularised algorithms. The proposed method is based on the relationship between the perturbation of structural parameters such as stiffness change and the modal data measurements of the tested structure such as measured mode shape readings. In order to adjust structural parameters at detailed locations, structural updating parameters will be selected at critical point level to reflect the modelling errors at the connections of structural elements. These updating parameters are then evaluated by an iterative or a direct solution procedure, which gives optimised solutions in the least squares sense without requiring an optimisation technique. In order to reduce the influence of modal measurement uncertainty, the Tikhonov regularisation method incorporating the L-curve criterion is employed to produce reliable solutions for the chosen updating parameters. Numerical simulation investigations and experimental studies for the laboratory tested space steel frame structure are undertaken to verify the accuracy and effectiveness of the proposed methods for adjusting the stiffness at the joints of structural members. The results demonstrate that the proposed methods provide reliable estimates of finite element model updating using the measured incomplete modal data.     

Improved prediction of the turbulence-shear contribution to wind noise pressure spectra

In previous research [Raspet et al., J. Acoust. Soc. Am. 123(3), 1260-1269 (2008)], predictions of the low frequency turbulence-turbulence and turbulence-mean shear interaction pressure spectra measured by a large wind screen were developed and compared to the spectra measured using large spherical wind screens in the flow. The predictions and measurements agreed well except at very low frequencies where the turbulence-mean shear contribution dominated the turbulence-turbulence interaction pressure. In this region the predicted turbulence-mean shear interaction pressure did not show consistent agreement with microphone measurements. The predicted levels were often much larger than the measured results. This paper applies methods developed to predict the turbulence-shear interaction pressure measured at the ground [Yu et al., J. Acoust. Soc. Am. 129(2), 622-632 (2011)] to improve the prediction of the turbulence-shear interaction pressure above the ground surface by incorporating a realistic wind velocity profile and realistic turbulence anisotropy. The revised prediction of the turbulence-shear interaction pressure spectra compares favorably with wind-screen microphone measurements in large wind screens at low frequency.     

基于接触有限元方法的镜面面形仿真技术研究

为考察在外载作用下某镜体镜面面形的变化是否满足设计要求,研究在预紧力及重力作用下镜面面形的变化规律,使用接触非线性有限元方法对镜体组件进行了仿真,采用基于Zernike多项式的光机集成仿真接口程序对面形数据进行了处理,得到了消除刚体位移后表征面形变化的参数和表示刚体位移系数的分布曲线。结果表明在预紧力和重力作用下刚体位移明显,镜面平移占据了刚体位移的主要方面,基于集成仿真技术的面形处理程序可有效地消除刚体位移;预紧力存在在不同方向重力作用下对面形的影响略有不同,与预紧力相比,重力作用对面形的影响较小。在预紧力作用下面形仿真数据对于镜体设计、装调具有一定指导意义,也说明基于接触方法面形仿真的工程适用性。     

Full vectorial finite element modeling of novel polarization rotators

In this paper, the numerically efficient finite element based full vectorial modal and propagation approaches are used in order to analyze and design single and multiple sectioned passive polarization rotators (PRs). The effects of different waveguide parameters, such as the waveguide width, etching depth, sidewall slant angle and refractive index contrast on the PR performance in terms of polarization conversion efficiency, section length and losses are investigated in detail in order to obtain optimum PR designs. Moreover, a thorough sensitivity study of the fabrication tolerances and the operating wavelength on the performance of the PRs has also been carried out.     

基于薄层单元与弹簧单元的滚动直线导轨副动力学建模

针对某大型光机装置中使用的LM型直线导轨-滑块的结合面,采用薄层单元与弹簧单元模拟其接触面,建立了大行程传输结构模拟件的有限元模型。通过模型修正方法,根据模态试验结果对薄层单元弹性模量以及弹簧刚度进行了识别。模型修正后,结构前三阶模态频率计算结果与试验结果最大差别为2.23%,地脉动载荷下各测点位移响应计算结果与试验结果最大差别为7.61%。计算结果与试验结果具有较好的一致性,验证了模型的有效性。     

The influence of substructure modeling on the structural-acoustic response of a plate system

Changes in the vibro-acoustic response of a fluid-loaded plate due to variations in some of the modeling details associated with an attached substructure are examined. The attached substructure consists of a smaller plate supported by springs along each edge. To examine the important modeling issues, three studies are performed. In the first study, discrete changes in the system response due to discrete changes in the size of the region over which the spring elasticity is distributed are examined. In the second study, substructure modeling issues are examined by varying the number of degrees-of-freedom included in the substructure model. Finally, sensitivity relationships that express changes in the system response to changes in the scale of the spring elements are developed. These relationships are used to examine changes in the system response due to small variations in the scale of the distributed elasticity. Both the combined system response and acoustic radiation are computed using the Acoustic Surface Variational Principle and Hamilton's Principle. For the example cases considered, it is shown that details associated with the scale of the spring are only important for frequencies near or below the resonances of the isolated subsystem. Furthermore, only the dynamics of the substructure including rigid-body type motions are important.     

Time-domain prediction of impact noise from wheel flats based on measured profiles

Railway impact noise is caused by discrete rail or wheel irregularities, such as wheel flats, rail joints, switches and crossings. In order to investigate impact noise generation, a time-domain wheel/rail interaction model is needed to take account of nonlinearities in the contact zone. A nonlinear Hertzian contact spring is commonly used for wheel/rail interaction modelling but this is not sufficient to take account of actual surface defects which may include large geometry variations. A time-domain wheel/rail interaction model with a more detailed numerical non-Hertzian contact is developed here and used with surface roughness profiles from field measurements of a test wheel with a flat. The impact vibration response and noise due to the wheel flat are predicted using the numerical model and found to be in good agreement with the measurements. Moreover, compared with the Hertzian theory, a large improvement is found at high frequencies when using the detailed contact model.     

Three-dimensional radiative transfer modeling using the control volume finite element method

In the present study, a three-dimensional algorithm for the treatment of radiative heat transfer in emitting, absorbing and scattering media is developed. The approach is based on the utilization of control volume finite element method (CVFEM) which, to the knowledge of the authors, is applied at the first time to 3D radiative heat transfer in participating media. The accuracy of the present algorithm is tested by comparing its predictions to other published works. Comparisons show that CVFEM produces good results. Moreover, this approach permits compatibility with other numerical methods used for computational fluids mechanics problems.     

自由枝晶生长相场模型的自适应有限元法模拟

采用自适应有限元方法求解了相场模型的控制方程,利用自适应有限元法在求解效率和精度上的优势,模拟了计算域较大,界面层厚度较薄的情况下镍过冷熔体中单个完整等轴晶的演化过程,使相场模型模拟结果更接近于真实物理模型,并探讨了二次枝晶臂的演化机理.模拟结果表明,二次枝晶臂的演化主要由热扩散控制,并受随机扰动影响,在四个象限内呈现出不对称生长.同时,受枝晶臂生长时排出的潜热积聚作用,同一侧的枝晶臂对新的二次枝晶臂的产生有促进作用.此外,二次枝晶臂出现合并、缩颈熔断、轴向熔化和径向熔化等四种粗化方式. 关键词： 自由枝晶生长 相场 自适应有限元法     

基于野外实测光谱统计分析的蚀变矿物填图

提出了一种新的基于野外实测光谱统计分析的蚀变矿物填图方法。该方法首先对野外测量得到的大量样本光谱数据进行聚类处理,从光谱的整体特征上将不同类型的样本区分开。第二步对各聚类结果中不同蚀变矿物分别建立其各自的判别函数。第三步在遥感影像上按照聚类得到的参考光谱分别进行大蚀变类的划分,最后在此基础上采用第二步得到的各蚀变类型的判别函数进行蚀变矿物的细化填图,得到最终的填图结果。该方法充分考虑了不同蚀变类型及蚀变组合的地区差异性,建立的蚀变矿物的判别函数更具有科学性,并且填图结果可在一定程度上进行可靠度评价。将该方法应用到新疆包古图地区的某一研究子区中,获得了较好的成果。     

基于地面实测光谱的湿地植物全氮含量估算研究

随着再生水越来越多的应用于城市湿地,湿地植物生长状态的大面积监测对于利用再生水的湿地恢复与重建具有重要意义。目前遥感技术已成为植物生长状态大面积监测的重要手段。本研究以北京市典型再生水城市湿地奥林匹克公园南园湿地为研究区,以反映植物生长状态的重要指标全氮(TN)为研究对象,在测定研究区湿地植物芦苇(Phragmites australis)和香蒲(Typha angustifolia)的叶片光谱及TN含量的基础上,对数据进行预处理并建立二者的关系模型,包括单变量模型(比值光谱指数(SR)模型和归一化差值光谱指数(ND)模型),与多变量模型(逐步多元线性回归(SMLR)模型和偏最小二乘回归(PLSR)模型),并利用交叉验证决定系数(R2CV)和均方根误差(RMSECV)对模型精度进行检验。结果表明,不同湿地植物类型相比,利用芦苇反射光谱建立的各种预测模型的精度都高于香蒲;不同回归模型相比,多变量回归模型的精度较高;多变量回归模型中,PLSR模型的精度高于SMLR模型,其R2CV可达0.80,RMSECV仅为0.24,是建立湿地植物光谱与TN含量关系的最优模型。研究成果不仅为湿地植物生长状态遥感探测提供参考借鉴,而且可以为利用再生水的城市湿地监测与管理提供有力的科学依据。