Analytical modeling of fluid loaded orthogonally rib-stiffened sandwich structures: Sound transmission

An analytic model is developed to investigate the wave propagation and sound transmission characteristics of an infinite sandwich structure reinforced by two sets of orthogonal rib-stiffeners when subjected to convective fluid-loaded pressure. The rib-stiffeners are assumed to be identical and uniformly spaced, which can exert not only tensional forces and bending moments but also torsional moments on the facesheets. Inertial terms of the tensional forces, bending moments and torsional moments are introduced to account for inertial effects arising from the mass of the rib-stiffeners. With the surrounding acoustic fluids restricted by the acoustic wave equation, fluid-structure coupling is considered by imposing velocity continuity condition at fluid-panel interfaces. By applying the Bloch theorem for periodic structures, the structural and acoustic responses are expressed in a superposition form of space harmonics for a given wavenumber. The application of the virtual work principle for one periodic element yields two infinite sets of simultaneous algebraic coupled equations, which are numerically solved by truncating them in a finite range insofar as the solution converges. The validity and feasibility of the analytic model is qualified by comparing model predictions with existing results, in which the necessity and advantage of the exact modeling of rib-stiffener motions are also demonstrated. Specifically, the influences of inertial effects arising from rib-stiffener mass, the periodicity spacing of rib-stiffeners, and the airborne as well as structure-borne paths on the transmission of sound across the sandwich structure are quantified and conclusions of significant practical implications are drawn.     

纵振驱动球面弯曲振动超声聚焦系统的聚焦特性*

为了将超声聚焦效应应用于工业加工中的冷却技术中,该文提出一种由夹心式换能器纵向振动驱动球面弯曲振动超声聚焦系统。基于基尔霍夫-亥姆霍兹声场理论分析了由换能器中心面纵向振动和球面弯曲振动组成的复合超声振动条件下的声场聚焦特性,并通过实验进行验证。研究结果表明,该聚焦系统具有显著的聚焦特性,球面弯曲振动将声能汇聚在声场焦区;当声场相位相同时,换能器中心面纵向振动和球面弯曲振动产生的声场在焦区发生叠加,可以进一步提高焦区声压;减小换能器中心面半径和球面曲率半径、增加球面开口半径可以增强复合超声振动的聚焦效果。     

Vibration control of nonlinear Absorber–Isolator-Combined structure with time-delayed coupling

A nonlinear combined structure consisted of isolator and absorber with time-delayed coupling active control is proposed in this study, whose vibration suppression effectiveness and control mechanism are investigated. The mathematical model of the combined structure is obtained and stability analysis for different structural parameters and time delay are firstly carried out, which provides a general guideline for the ranges of active control parameters. Then the combined effect of nonlinearity and time delay on vibration suppression and energy transfer is discussed in details based on the analysis of control mechanism by the method of multiple scales. Since the time-delayed nonlinear absorber can induce internal resonance between different modes, the vibration energy at low frequencies can be transferred to high frequency mode and the vibration of the fundamental frequency range is thus suppressed. This paper provides a novel application of internal resonance in vibration suppression of an Absorber–Isolator-Combined structure.     

Wave analysis and control of double cascade-connected damped mass-spring systems

This paper presents wave analysis and control for double cascade-connected damped mass-spring systems, whose mass is connected beyond the adjacent masses. The system is motivated by a cantilevered tensegrity beam supporting tensile and compressive forces. The wave solution is derived from a recurrent formula, and the properties of the propagation constants are precisely investigated. Elimination of reflected waves provides the impedance matching controller. We show that the impedance matching controller can be constructed from a similarity transformation of the characteristic impedance matrix by a matrix composed of the propagation constants. A numerical example of vibration control of a tensegrity beam is shown.     

Sloshing and energy dissipation in an egg: SPH simulations and experiments

Tall structures, such as towers and bridges, can oscillate at excessive magnitudes when subjected to wind and earthquake loads. Liquid sloshing absorbers can be used to suppress these excessive oscillations by tuning the frequency of the sloshing to the critical frequency of the structure. Sloshing absorbers are simple structures consisting of a partially full container of liquid with a free surface. Tuning ensures that significant amounts of harmful energy can be extracted from the structure to the sloshing liquid. However, there needs to be a rapid means of dissipating this energy to avoid its returning back to the structure (then back to the liquid periodically).A hen׳s egg seems to have evolved to efficiently dissipate energy to protect its embryo using sloshing of its liquid content. Hence, the potential to implement the egg׳s unique properties as a sloshing absorber for structural control, is the main focus of this study. Numerical simulations, using Smoothed Particle Hydrodynamics (SPH), and experimental comparisons are presented in this paper. One objective is to demonstrate the ability of SPH to simulate complex free surface behaviour in three dimensions. Such a tool is then useful to identify different dissipation modes. Effects of fill volume and viscosity on the rate of dissipation, are also investigated.     

拉曼光谱的16种多环芳烃（PAHs）特征振动光谱辨识

借助密度泛函理论中B3LYP/6-311++G(d, p)方法对美国EPA优先控制污染物中的16种多环芳烃(PAHs)：萘、苊烯、苊、芴、菲、蒽、荧蒽、芘、苯并[a]蒽、稠二萘、苯并[b]荧蒽、苯并[k]荧蒽、苯并[a]芘、二苯并(a, h)蒽、二苯并[g, h, i]芘以及茚苯(1, 2, 3-cd)芘进行结构优化,并计算拉曼光谱振动频率和去偏振度,在此基础上辨识多环芳烃的拉曼特征光谱。研究显示,16种PAHs的拉曼振动主要分布在3个频区：200～1 000 cm-1（指纹区）、1 000～1 700和3 000～3 200 cm-1（基团频率区）,3个频区主要振动归属分别为环变形(ring def),碳碳伸缩(CCStr)、碳氢摇摆(CHw)及其耦合振动(CCStrCCw),碳氢伸缩(CHStr)。进一步分析显示,指纹区16种PAHs的去偏振度随苯环变形振动对称性增强而降低,在该频区去偏振度最小的频移处苯环呼吸振动的对称性最强,指纹区的峰强也在此处出现最大值。任意PAHs在指纹区的最强峰之间的波数差较大,在显微拉曼光谱的可分辨范围内,因而利用指纹区的去偏振度和最强峰可将16种PAHs逐一识别。烷烃、烯烃、炔烃、醇类和酚类、脂肪醚、芳基烷基醚、醛类、酮类、羧酸、酯类、胺类、腈类、酰胺类、酸酐、芳烃的振动频率和峰强分布不完全一致,利用PAHs与这几类物质拉曼频率和峰强分布的差异可以逐一排出干扰。     

Non-probabilistic reliability method and reliability-based optimal LQR design for vibration control of structures with uncertain-but-bounded parameters

Uncertainty is inherent and unavoidable in almost all engineering systems. It is of essential significance to deal with uncertainties by means of reliability approach and to achieve a reasonable balance between reliability against uncertainties and system performance in the control design of uncertain systems. Nevertheless, reliability methods which can be used directly for analysis and synthesis of active control of structures in the presence of uncertainties remain to be developed, especially in non-probabilistic uncertainty situations. In the present paper, the issue of vibration con- trol of uncertain structures using linear quadratic regulator （LQR） approach is studied from the viewpoint of reliabil- ity. An efficient non-probabilistic robust reliability method for LQR-based static output feedback robust control of un- certain structures is presented by treating bounded uncertain parameters as interval variables. The optimal vibration con- troller design for uncertain structures is carried out by solv- ing a robust reliability-based optimization problem with the objective to minimize the quadratic performance index. The controller obtained may possess optimum performance un- der the condition that the controlled structure is robustly re- liable with respect to admissible uncertainties. The proposed method provides an essential basis for achieving a balance between robustness and performance in controller design ot uncertain structures. The presented formulations are in the framework of linear matrix inequality and can be carried out conveniently. Two numerical examples are provided to illustrate the effectiveness and feasibility of the present method.     

Solvent Effect on Characteristic Vibration of IR Spectrum of 4,4'-Dibromodiphenyl Ether

The optimal geometry, IR spectrum and vibration assignment of 4,4＇-dibromodiphenyl ether（BDE-15） in gas phase were calculated via the density functional theory（DFT） at the level of B3LYP/6-3 l＋G（d）. Based on the vi- bration assignment, the calculation of vibration frequencies and intensities of 5 main vibration types of BDE-15 in 25 kinds of solvents was carried out by means of a self-consistent reaction field（SCRF） theoretical model at the level of B3LYP/6-31＋G（d） to analyze the solvent effect on the vibration of IR spectrum of BDE-15. To study the solvent ef- fect further, C--O asymmetric stretching vibration fluctuating, which is relatively acute in both vibration frequency and intensity, was selected as the characteristic vibration to establish different linear solvation energy relation（LSER） models for solvent categoring. Solvent parameters（a, /3, ~r＊）, acceptable number（AN） and quantitative structure- activity relationship（QSAR） models were established via chemical quantum parameters of solvent moleculer, which were first been introduced to investigate different solvent-solute interaction mechanisms in alcoholic and non-alcoholic solvents on molecular level. At last, a single solvent molecule was embedded in the framework of po- larizable continuum model（PCM） to validate the effect of hydrogen bonding on solvent-solute interaction in alcohol solvents. The obtained results show that 5 main vibration types of BDE-15 in different solvents have small variation range in frequency and intensity and all the vibration frequencies in solvents are lower than those in gas phase, de- creasing along with the increasing of the dielectric Constant（e） of solvents exponentially. In contrast, all the vibration intensities in solvents are greater than those in gas phase and present positive exponential trend. Twenty-five solvents were divided into two categories（non-alcoholic solvents and alcoholic solvents） by LSER. The CmO asymmetric stretching vibration was mainly reg     

Properties of acoustic resonance in double-actuator ultra-sonic gas nozzle: numerical study

The ultra-sonic gas atomization(USGA) nozzle is an important apparatus in the metal liquid air-blast atomization process.It can generate oscillating supersonic gas efflux,which is proved to be effective to enforce the atomization and produce narrow-band particle distributions.A double-actuator ultra-sonic gas nozzle is proposed in the present paper by joining up two active signals at the ends of the resonance tubes.Numerical simulations are adopted to study the effects of the flow development on the acoustic resonant properties inside the Hartmann resonance cavity with/without actuators.Comparisons show that the strength and the onset process of oscillation are enhanced remarkably with the actuators.The multiple oscillating amplitude peaks are found on the response curves,and two kinds of typical behaviors,i.e.,the Hartmann mode and the global mode,are discussed for the corresponding frequencies.The results for two driving actuators are also investigated.When the amplitudes,the frequencies,or the phase difference of the input signals of the actuators are changed,the oscillating amplitudes of gas efflux can be altered effectively.     

Wind induced deformation and vibration of a Platanus acerifolia leaf

Deformation and vibration of twig-connected single leaf in wind is investigated experimentally.Results showthat the Reynolds number based on wind speed and lengthof leaf blade is a key parameter to the aerodynamic problem.In case the front surface facing the wind and with an increase of Reynolds number,the leaf experiences static deformation,large amplitude and low frequency sway,reconfiguration to delta wing shape,flapping of tips,high frequencyvibration of whole leaf blade,recovery of delta wing shape,and twig-leaf coupling vibration.Abrupt changes from onestate to another occur at critical Reynolds numbers.In casethe back surface facing the wind,the large amplitude andlow frequency sway does not occur,the recovered delta wingshape is replaced by a conic shape,and the critical Reynoldsnumbers of vibrations are higher than the ones corresponding to the case with the front surface facing the wind.Apair of ram-horn vortex is observed behind the delta wingshaped leaf.A single vortex is found downstream of theconic shaped leaf.A lift is induced by the vortex,and thislift helps leaf to adjust position and posture,stabilize bladedistortion and reduce drag and vibration.