Nonlinear disturbance evolution in a two-dimensional boundary-layer along an elastic plate and induced radiated sound

The interaction between a boundary-layer flow and an elastic plate is addressed by direct numerical simulation, taking into account the full coupling between the fluid flow and the flexible wall. The convectively unstable flow state is harmonically forced and two-dimensional nonlinearly saturated wavelike disturbances are computed along archetype-plates with respect to stiffness and natural frequencies. In the aim of determining the low-Mach-number radiated sound for the system, the simulation data are used to solve the Lighthill's equation in terms of a Green's function in the wavenumber-frequency space. Different degrees of fluid-structure coupling are implemented in the radiated sound model and the resulting acoustic pressure levels are compared. The sound radiation levels are shown to be increased in the presence of flexible walls with however significant differences in the radiated pressure levels for different coupling assumptions.     

平板湍流边界层的声辐射

基于平板湍流边界层的壁压起伏波数—频率谱 ,给出了一种湍流边界层声辐射的估算方法 ,并对光滑平板湍流边界层和平板表面粗糙度引起的湍流边界层声辐射进行了分析。结果表明 :湍流边界层声辐射是一种四极子声辐射 ,且其辐射声能集中于平板表面粗糙度引起的湍流边界层声辐射 ;光滑平板湍流边界层的声辐射也不可忽略。     

Topology optimization of a plate coupled with acoustic cavity

Optimization of the topology of a plate coupled with an acoustic cavity is presented in an attempt to minimize the fluid–structure interactions at different structural frequencies. A mathematical model is developed to simulate such fluid–structure interactions based on the theory of finite elements. The model is integrated with a topology optimization approach which utilizes the moving asymptotes method. The obtained results demonstrate the effectiveness of the proposed approach in simultaneously attenuating the structural vibration and the sound pressure inside the acoustic domain at several structural frequencies by proper redistribution of the plate material.Experimental verification is carried out by manufacturing topology optimized plates and monitoring their vibration and sound radiation into a rigid acoustic cavity. The measured sound pressure and plate vibration are found to be in good agreement with the predictions of the mathematical model.The presented theoretical and experimental techniques present valuable tools in the design of a wide variety of critical structures which must operate quietly when subjected to fluid loading.     

Discrete material optimization of vibrating laminated composite plates for minimum sound radiation

This paper deals with vibro-acoustic optimization of laminated composite plates. The vibration of the laminated plate is excited by time-harmonic external mechanical loading with prescribed frequency and amplitude, and the design objective is to minimize the total sound power radiated from the surface of the laminated plate to the surrounding acoustic medium. Instead of solving the Helmholtz equation for evaluation of the sound power, advantage is taken of the fact that the surface of the laminated plate is flat, which implies that Rayleigh’s integral approximation can be used to evaluate the sound power radiated from the surface of the plate. The novel Discrete Material Optimization (DMO) formulation has been applied to achieve the design optimization of fiber angles, stacking sequence and selection of material for laminated composite plates. Several numerical examples are presented in order to illustrate this approach.     

基于奇异分解和自适应BEM积分算法的水下航行体机械噪声预报

提出一种改进的声学边界元法（M-BEM）用于准确计算水下航行体发动机振动引起的近场辐射噪声。分别采用奇异分解技术和自适应边界元积分算法解决了Helmholtz积分方程在求解近场声压时出现的超奇异积分和奇异积分问题。采用一脉动球源的声辐射算例对方法进行验证,数值解与精确解误差小于1．5dB。结合有限元方法并考虑流固耦合作...     

Truss topology optimization under uncertain nodal locations with proportional topology optimization method

This paper presents an approach to solving truss topology optimization problem with small uncertainty in the locations of the structural nodes. The nodal locations in the truss are assumed to be random, and the probabilistic method is used here to deal with the uncertainty. The objective of the optimization problem is to minimize the mean compliance of the truss structure under nodal location uncertainty. It is a well-acknowledged barrier to compute the inverse of the structural stiffness matrix which involves variations in the optimization problem. In this paper, based on Neumann series expansion, this optimization problem can be recast into a simpler deterministic structural optimization problem. In order to avoid the sensitivity calculations for the objective function, the proportional topology optimization method which shows comparable e?ciency and accuracy with gradient-based method is used. The numerical examples demonstrate the effectiveness and high e?ciency of the proposed approach, and further illustrate that the optimal truss topology can be dramatically impacted by nodal location uncertainties.     

The effect of material properties on the radiation of impact sound from cylinders

A number of studies have investigated engineering methods of reducing forge-hammer noise by means of enclosure, blow reduction and external application of damping materials. Another alternative, in which parts and units are manufactured from metals with high internal damping, has yet to be fully investigated. In a theoretical and experimental study of sound radiation from cylindrical samples, candidate hammer materials are evaluated for potential reduction of radiated energy and energy spectrum. It is shown that the quantity of radiated sound is strongly dependent on the stiffness and damping properties of the candidate tool-steel, forged-iron and cast-iron samples. The rate of attenuation of sound radiation is also shown to be affected in a predictable manner from the (tabulated) properties of the metals.     

基于拓扑优化的声学结构材料分布设计

本文针对结构的声学设计问题进行研究,通过优化两种不同的材料在结构设计域内的拓扑分 布来最小化谐振结构所产生的声场中指定参考面/参考域内的声压。在研究中假定结构为线弹性小变 形结构,材料阻尼为Rayleigh阻尼,声学介质为无粘、可压缩、小扰动流体。对结构响应采用有限 元格式进行计算,对声场采用基于Helmholtz积分的边界元格式进行计算,由于声场在无穷远自由边 界的无反射条件在边界积分中能自动得到满足,该格式特别适合于具有开放边界的声场计算。建立 了结构有限元-声场边界元格式的耦合系统拓扑优化模型,导出了耦合系统敏感度分析的一般格式及 伴随格式。数值算例验证了所提出的结构-声学耦合系统优化方法的有效性和可靠性,并揭示了基于 声学准则的拓扑优化结果的有关特性 关键词边界积分,结构声学耦合系统,拓扑优化,敏感度分析,伴随方法     

Design of an Active Noise Control System Using a Distributed Actuator

A distributed acoustic actuator for active noise control, consisting of a piezoelectric PVDF film, bonded at each side of a carrier structure, is simulated and built. The piezoelements are driven in anti-phase, resulting in a bending motion of the actuator, and thus in the necessary out-of-plane displacement for sound radiation. An analytical model for the acoustic actuator is derived, relating the actuator's displacement to the applied voltage, taking into account the influence of the piezoelectric film on the actuator's stiffness. The model is used to optimise the specifications for the piezoelectric film and the carrier structure, resulting in the highest sound power output in a frequency range from 30–500 Hz. An analytical model for the behaviour of a double panel partition is derived. The analytical model is combined with the model for the acoustic actuator, describing an actively controlled double panel partition with a distributed acoustic actuator integrated in the cavity. A controller is added to the system to control the sound power transmitted through the double panel partition. Simulation results show that a substantial increase of transmission loss can be achieved in the low frequency region (30–500 Hz) with this configuration. This revised version was published online in July 2006 with corrections to the Cover Date.     

瞬态声振耦合问题的边界元分析及实验研究

本文探求任意外形结构在外力作用下的瞬态声场特性,研究声辐射机理,用时域边界元技术讨论外力、结构振动响应和声辐射三者之间的关系,并用实验结果加以验证,为瞬态声振耦合研究提供一种新的数值分析方法。     

Harmonic radiation from a liquid-filled spherical acoustic lens with an internal eccentric spherical source

Radiation of sound from a modally vibrating shell-encapsulated (eccentric) spherical source is analyzed in an exact manner using the classical method of separation of variables. The proposed model is a realistic idealization of a spherical acoustic lens with focal point inside the lens when used as a sound projector. The analytical results are illustrated with a numerical example in which the modal acoustic radiation impedance load on the source and the radiated far-field pressure are evaluated for representative values of the parameters characterizing the system. Numerical results clearly illustrate that in addition to frequency, surface velocity distribution and eccentricity of the source, the dynamic interaction of the encapsulating shell can be of great consequence in sound radiation.     

特定方向"零膨胀"的最小柔顺性结构优化设计

工程中很多承载结构必须面对苛刻的温度变化工作环境,如卫星天线、太空照相机和电子器件等。剧烈的温度变化引起较大的热变形,造成仪器信号失真,精度下降;同时温度应力也会造成结构破坏甚至失效,因此零膨胀材料的研制备受关注。近年来国内外很多学者对此进行了研究,设计出具有特定等效膨胀系数的微结构,但考虑到制备工艺的限制,这类具有复杂微结构的材料制备起来比较困难,成本较高;同时这类材料一般不具备足够的刚度,难以满足承载性能的要求。本文基于结构优化设计技术,采用拓扑优化方法直接设计出具备较高的承载性能和特定方向变形较少受热载荷影响的结构。本文提出采用多目标优化的方法设计圆环结构,使其具有较高的刚度和在热载荷下圆环内表面具有较好的热几何稳定性。由于用单相材料无法同时满足高刚度和低热膨胀的要求,因此假设结构由两种不同的材料构成,用连续体拓扑优化的方法设计三相材料(两种实体材料MAT-I、MAT-II和空材料)在设计域上的最优分布,使结构满足设计要求。由对称性,设计域取为圆环的一个扇面,将设计域离散成有限元网格,每个单元具有两个设计变量:实体材料的体分比和MAT-I在实体材料中所占的体分比,采用伴随法进行灵敏度分析,用GCMMA方法求解此问题,采用体积守恒的Heaviside密度过滤函数保证获得清晰的最优拓扑构型以及避免棋盘格式的出现。通过两个数值算例,表明使用本文提出的多目标优化模型能够得到特定方向"零膨胀"同时具有一定刚度的结构设计,且这种宏观结构尺度上的两种材料组成的拓扑构型相对易于制造。     

Study on calculation methods for acoustic radiation of axisymmetric structures in finite water depth

In this paper, a kind of FEM–WSM (Finite Element Method–Wave​ Superposition Method) is used to calculate the acoustic radiation of axisymmetric structures in finite water depth. FEM is used to solve the dry modes of axisymmetric structures, and WSM is applied along with the dry mode method to consider fluid–structure interaction effects and calculate the acoustic radiated field. This method combines the advantages of FEM and WSM. On one hand, it is suitable for complex or large axisymmetric structures on the one hand. On the other hand, it has higher computational efficiency than the FEM, and the computational domain size for the water is not limited. As long as the Green’s function is tailored for the boundary condition, the acoustic radiated field of axisymmetric structures in more complex ocean acoustic environments can be calculated by using this method. Besides, a least-square method is used to reduce the distortion resulting from computational errors of the modal estimates. The influence of the number of source and field points and the finite element mesh density on the calculation accuracy are discussed, eliciting some disciplinary conclusions. Using a spherical shell and a capsule shell as models, the results from the present approach, a semi-analytical method, and the crude FEM are compared to verify the correctness and efficiency. Based on numerical examples, the influence of the sea surface and the seafloor on the acoustic radiated field of structures in finite water depth is also analyzed.     

Investigation of downstream and sideline subsonic jet noise using Large Eddy Simulation

The sound fields radiated by Mach number 0.6 and 0.9, circular jets with Reynolds numbers varying from 1.7×10³ to 4×10⁵ are investigated using Large Eddy Simulations. As the Reynolds number decreases, the properties of the sound radiation do not change significantly in the downstream direction, whereas they are modified in the sideline direction. At low Reynolds numbers, for large angles downstream from the jet axis, the acoustic levels are indeed remarkably lower and a large high-frequency part of the sound spectra vanishes. For all Reynolds numbers, the downstream and the sideline sound spectra both appear to scale in frequency with the Strouhal number. However their peak amplitudes vary following two different velocity exponents according to the radiation direction. The present observations suggest the presence of two sound sources: a Reynolds number-dependent source, predominant for large radiation angles, connected to the randomly-developing turbulence, and a deterministic source, radiating downstream, related to a mechanism intrinsic to the jet geometry, which is still to be comprehensively described. This view agrees well with the experimental results displaying two distinguishable components in turbulent mixing noise [1, 2].     

A Conceptual Study of Cavity Aeroacoustics Control Using Porous Media Inserts

In this study, an integrated flow simulation and aeroacoustics prediction methodology is applied to testing a sound control technique using porous inserts in an open cavity. Large eddy simulation (LES) combined with a three-dimensional Ffowcs Williams–Hawkings (FW–H) acoustic analogy is employed to predict the flow field, the acoustic sources and the sound radiation. The Darcy pressure – velocity law is applied to conceptually mimic the effect of porous media placed on the cavity floor and/or rear wall. Consequently, flow in the cavity could locally move in or out through these porous walls, depending on the local pressure differences. LES with “standard” subgrid-scale models for compressible flow is carried out to simulate the flow field covering the sound source and near fields, and the fully three-dimensional FW–H acoustic analogy is used to predict the sound field. The numerical results show that applying the conceptual porous media on cavity floor and/or rear wall could decrease the pressure fluctuations in the cavity and the sound pressure level in the far field. The amplitudes of the dominant oscillations (Rossiter modes) are suppressed and their frequencies are slightly modified. The dominant sound source is the transverse dipole term, which is significantly reduced due to the porous walls. As a result, the sound pressure in the far field is also suppressed. The preliminary study reveals that using porous-inserts is a promising technology for flow and sound radiation control.     

Large-eddy simulation of circular cylinder flow at subcritical Reynolds number: Turbulent wake and sound radiation

The flows past a circular cylinder at Reynolds number 3900 are simulated using large-eddy simulation(LES) and the far-field sound is calculated from the LES results. A low dissipation energy-conserving finite volume scheme is used to discretize the incompressible Navier–Stokes equations. The dynamic global coefficient version of the Vreman's subgrid scale(SGS) model is used to compute the sub-grid stresses. Curle's integral of Lighthill's acoustic analogy is used to extract the sound radiated from the cylinder. The profiles of mean velocity and turbulent fluctuations obtained are consistent with the previous experimental and computational results. The sound radiation at far field exhibits the characteristic of a dipole and directivity. The sound spectra display the-5/3 power law. It is shown that Vreman's SGS model in company with dynamic procedure is suitable for LES of turbulence generated noise.     

考虑时变刚度特性的复合材料微结构拓扑优化设计方法

理想的骨折内固定植入物在组织愈合或修复的过程中,其结构性能需要满足不同愈合阶段对生物力学的需求.提出一种对生物可降解复合材料微结构的时变刚度特性进行调控设计的拓扑优化方法,以达到理想的骨折内固定植入物特殊的时变刚度特性需求.使用具有不同降解速率和刚度的两种可降解材料,以相对密度作为设计变量来描述不同材料的分布,以特定降...     

Radiation and scattering of sound from a vortex ring

The mechanisms of generation and scattering of sound by a vortex ring are investigated on the basis of fluid dynamics. The vortex ring can serve as a simple dynamic model of the large-scale structures observed in shear flows. Moreover, it is probably the most easily studied vortex element that can be created experimentally. The sound scattering investigation also served to determine the extent to which the vortex is affected by sound, its selectivity with respect to such parameters as the acoustic frequency, the angle of incidence of the wave, etc. The perturbed motion is considered against the background of the steady-state motion of the ring. The perturbed motion in the vortex core is determined on the basis of linear incompressible fluid dynamics. Two terms of the expansion in the M number of the far acoustic field generated by the perturbations in the core are found in accordance with Lighthill's theory. The acoustic power and directivity of the radiation and the acoustic instability growth rate are calculated. It is shown that the scattering of sound by the vortex ring is a resonance effect, and the scattering amplitude near resonance is determined. The acoustic action on the hydrodynamic structure of the flow in the core of the ring is especially intense near the resonances and extends over a period short as compared with the characteristic time of the acoustic instability.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 83–95, May–June, 1987.     

应力约束全局化策略下的连续体结构拓扑优化

利用Mises强度理论，提出了应力约束全局化策略，将局 部的应力约束问题转化为结构整体的应变能约束问题. 基于ICM(独立、连续、映射)方法， 引入了独立、连续的拓扑变量，对单元重量、单元刚度和单元许用应力的过滤函数进行了选 择，建立了以重量为目标，以结构应变能代替应力约束的多工况下连续体结构拓扑优化模型， 寻找到了多工况下的最佳传力路径. 运用对偶二次规划方法对上述优化模型进行了求解. 另 外，利用PCL语言，在MSC/PATRAN的开发平台上，实现了应用应力约束全局化策略进行连 续体结构拓扑优化的模块化处理. 数值算例表明了该方法的可行性和有效性.