声场作用下两空化泡相互作用的研究

建立了声场作用下两空化泡泡壁的运动方程,得出了双空化泡的共振频率,振动半径及空化噪声声压．由频率方程,振动半径和声压方程可以看出两气泡的运动情况与单气泡的运动情况有着明显的不同．共振频率,共振振幅及声压与两气泡之间的间距有关．在一定的简化条件下,运用MATLAB语言对共振频率,共振振幅及空化噪声声压进行了数值求解,发现共振频率和共振振幅随空泡间距的增大而增大,空化噪声声压随距离增大先增大后减小． 关键词： 超声 空化 频率 声压     

基于边界元法的气泡同波浪相互作用研究

为研究水下爆炸气泡同波浪的相互作用,本文引入数值与解析相结合的方法来考虑远场对气泡的影响,并解决了开域自由面在边界积分中的奇异性问题．然后通过将流场速度势分解为波浪入射势和气泡扰动势,研究了波浪对近自由面气泡的动态特性的影响,并对不同特征参数的波浪下水下爆炸气泡进行模拟,分析了波长和初始相位角对气泡动态特性的影响．通过对结果的分析发现：对于气泡自身形态,在收缩阶段波浪的存在会使气泡上半部分以及自。由面诱导射流产生偏移;而对与水冢形态,除了会产生偏移,水冢高度和宽度也会受到明显的影响．这种影响程度随初始相位角周期性变化,而在本文所研究的波长范围内,随波长的增加递减．     

在频闪光照射下振动弦的视觉形象

当在弦中形成驻波时,由于振动过程进行得较快,而无法直接分辨驻波的某些细节.本文拟介绍振动弦在频闪光照射下的视觉形象,及根据视觉形象来分辨弦驻波的某些细节与确定弦共振频率的方法. 用频闪仪发出的闪光照射振动的弦,当闪光频率与弦的共振频率(即弦中驻波的频率)稍有不同时,将会观察到弦作“缓慢振     

二维气/液界面不稳定性数值模拟

 以多介质的体积分数方法和三阶PPM（Piecewise Parabolic Method）方法为基础,给出了适用于多介质流体动力学数值模拟的计算方法和程序MFPPM。利用MFPPM程序对在高压气体冲击作用下的气体/液体交界面的Richtmyer-Meshkov（RM）不稳定性及其引起的流体混合现象进行了数值模拟研究。主要研究在不同的初始扰动情况下流体混合区的发展,并细致研究了流体混合区的宽度、气泡和尖钉高度随时间的增长情况及不同初始扰动对它们的影响;同时还研究了网格尺度不同时混合区、气泡以及尖钉的构型和高度的增长情况。通过对计算结果的分析得出,流体混合区、气泡以及尖钉的发展与初始扰动有密切的关系,特别是在后期影响更为显著;混合区宽度的变化过程和尖钉相似,而气泡高度的变化基本上呈线性增长趋势,且受初始扰动的影响比较小,但是其构型却有明显差别;网格的影响也主要体现在对混合区、气泡和尖钉的构型上。     

超疏水表面液滴的振动特性及其与液滴体积的关系

超疏水表面液滴的振动特性与接触线的移动、液滴体积、基底振幅等因素密切相关.本文在基底振幅较小且恒定的条件下,研究了超疏水表面液滴的共振振幅、模式区间、共振频率等振动特性及其与液滴体积(20—500μL)的关系.此外,将基于一般性疏水表面建立的Noblin共振频率计算模型应用于超疏水表面,并提出“虚驻点”的概念,借此对模型进行了误差分析和修正.研究表明:1)共振时,液滴高度变化率即比振幅随体积增大而增大,随阶数增大而减小;2)各模式区间的起止频率首尾相接,其范围随体积增大而减小;3)液滴体积越大,共振频率越小,随着阶数增大,共振频率f与体积V的关系趋于f-V^–0.4,不同于一般性疏水表面上的f-V^–0.5;4)直接应用Noblin模型计算共振频率会产生较大误差,主要原因在于液滴表面波波段数量统计存在较大偏差,而修正后的模型可以准确计算超疏水表面大体积液滴的共振频率.     

压电陶质钛酸钡矩形薄板轮廓模耦合振动的研究

引言 一定形状和大小的压电元件,有它特定的若干个不同的振动模,这些模往往不独立,而是彼此发生耦合的,这使振动特性变得相当复杂。为了探索耦合振动的机理,我们研究了压电陶质钛酸钡矩形薄板轮廓模耦合振动的特性。对于这个问题,Hayashi和田中哲郎等人曾作过研究,但他们的理论只能分析耦合共振频率,而不能分析耦合反共振频率和耦合导纳的频率响应特性。为此,在他们的工作基础上我们提出了一个新的等效电路,它不但可以分析耦合共振频率,而且可以分析耦合反共振频率和耦合导纳的频率响应特性。     

双气泡振子系统的非线性声响应特性分析

对初始半径不同的双气泡振子系统在声波作用下的共振行为和声响应特征进行了分析.利用微扰法分析了双泡系统的非线性共振频率,由于气泡间耦合振动的非线性影响,双泡系统存在双非线性共振频率.倍频共振和分频共振现象的存在使得双泡系统振幅-频率响应曲线有多共振峰,且随着非线性增强,共振区向低频区移动.通过对气泡平衡半径、双泡平衡半径比以及气泡间距的分析发现,耦合作用较强的情形发生在系统共振频率附近、气泡半径比接近1以及气泡间距小于10R_(10)的范围内,同时观察到了此消彼长的现象,充分体现了气泡在声场中能量转换器的特征.     

径向复合圆柱压电超声换能器

提出一种三元径向复合圆柱压电超声换能器,并对其径向振动特性进行了研究。基于弹性力学理论及机电类比原理,导出了柱坐标系中分割处理径向极化压电陶瓷管准厚度模振动及薄壁短圆管径向振动的机电等效电路;利用径向力和振速连续的边界条件,得出了径向复合圆柱压电换能器系统的径向振动机电等效电路及其共振频率方程。探讨了换能器径向共振频率及有效机电耦合系数随其几何尺寸的变化关系。研究表明,换能器的径向共振频率及有效机电耦合系数随其内芯半径和预应力管壁厚度增大而降低。研制了一些径向复合圆柱压电换能器,并对其径向共振频率进行了测试。结果表明,理论与实验结果基本一致。      

温度和场冷间隙对高温超导磁悬浮系统弛豫的影响

悬浮力是高温超导磁悬浮列车运行中的一个至关重要的参数.悬浮力的弛豫特性是高温超导磁悬浮列车安全运行的一大阻碍.适度降低工作温度是有效改善悬浮力弛豫特性的有效手段之一.本文从数值计算和实验两个层面研究了不同冷却温度和不同场冷间隙下的悬浮力的弛豫现象.发现悬浮力的弛豫现象随着超导块冷却温度的降低而减弱，随着场冷间隙的增大而略微增大，但是随着冷却温度的降低，增大程度会有所减弱.该研究有助于通过合理设计高温超导材料工作温度，以提高磁悬浮系统性能表现.     

声波在含气泡液体中传播特性及产热效应*

该文对含气泡液体中的声波方程采用线性分析方法,研究了超声波在含气泡液体中的传播特性以及产热效应。当声波在含气泡液体中传播时,气泡的存在会影响声波的传播,在声波频率接近气泡共振频率的频段内,声信号在液体中传播时剧烈衰减,而在声波频率远远高于或低于气泡共振频率时,声波的传播基本不受影响。在接近气泡共振的频段内,声波耗散的能量最终转化为热能。同时液体中的气泡会在声波驱动下径向振动并辐射声波,伴随气泡壁在液体中的粘滞振动,热量随之产生。结果表明,两种产热机制分别在不同频段起主导作用。     

A study of vibration characteristics on a luxury wheelchair and a new prototype wheelchair

The transmission of wheelchair vibrations to the body will influence comfort, performance and the long-term health of the user. Improved knowledge of vibration transmissibility and its variability enhances our understanding of various human responses to vibration. In this study, an outdoor experiment and an experiment with vibration simulation using two wheelchairs (high-quality models of a new prototype wheelchair taken from two different stages of the iterative production procedure) were performed. The study confirms that the human body is very sensitive to the frequency range of 0.5-, as found in the literature. Both wheelchairs equipped with passive suspension system did not perform adequately in this frequency range and even amplified the input signal at the resonance frequency (3-). As the risk of physical damage is not likely to improve with these wheelchair suspension systems, the future depends on new designs with higher low-frequency comfort and affordable additional costs.     

An adaptive pneumatic suspension based on the estimation of the excitation frequency

A pneumatic suspension that can adapt itself to the incoming vibration is presented in this paper. A switching control strategy between two different configurations is proposed and studied. The objective is to avoid undesirable resonant frequencies. The control procedure is based on the pre-knowledge of the incoming vibration frequency, and when this frequency is unknown, a very efficient prediction technique is used. The results show that the adaptable suspension has improved performance as compared to any of its passive counterparts. The transient response when switching typically takes less than three cycles and does not hinder the suspension performance.     

Descriptive analysis of combine cabin vibrations and their effect on the human body

All on- and off-road vehicles are exposed to vibrations caused by unevenness of road or soil profile, moving elements within the machine or implements. A higher prevalence of low back pain is found in drivers of off-road machinery than in other drivers. In this study, significantly higher levels of low-frequency vibrations are found in the cabin of a combine, driving at high speed on a concrete surface, compared to driving slower on field road. Comfort values indicate that injury can result from long-term driving on the field as well as on a concrete road. As seats with suspension systems are the main transmission paths of vibration towards the spine of the driver, their vibration attenuating characteristics play an important role in comfort assessment. The resonant frequency of seats with passive suspension system, used in agricultural machinery, lies in the low-frequency range most excited in agricultural machinery. A seat with air suspension is found to attenuate better frequencies above and provide more comfort to the driver than a seat with a mechanical suspension.     

基于Polytec激光测振仪的光存储设备振动测试分析系统

针对光存储设备振动频率高的特点,采用Polytec激光测振仪拾取振动信号,并给出了基于Polytec激光测振仪的光存储设备振动测试分析系统。可用于硬盘盘片、磁头滑块、折臂组件、光盘盘片等振动测试,为光存储设备动态特性的测试和分析建立了良好的实验基础。     

On-resonance transmissibility methodology for quantifying the structure-borne road noise of an automotive suspension assembly

In this paper, a methodology is presented for the analysis of the structure-borne noise transmission paths for an automotive suspension assembly. First, a fully-instrumented test bench consisting of a wheel/suspension/lower suspension A-arm assembly was designed in order to identify the vibro-acoustic transmission paths (up to 250 Hz) for white noise excitation of the wheel. Second, frequency response function measurements between the excitation signal and each suspension/chassis linkages are used to characterize the different transmission paths that transmit energy through the chassis of the car. Finally, a synthesis of the major resonances of the suspension is drawn, with the objective of indicating which suspension transfer paths contribute the most to the structural forces transmitted to the chassis. On-resonance force transmissibility factors (ORTF) were calculated to provide an overall classification of the system resonances to the vibration transmission through the individual suspension linkages and in all axes.     

Ultrasonic Vibration Suspends Large Pendant Drops

A stationary substrate can suspend only small pendant drops even with excellent wetting ability because of gravity. We report the suspension of large pendant water drops by a copper substrate that vibrates ultrasonically with a frequency of 22 kHz. The mass of the largest pendant drop suspended by the vibrating substrate reaches 1.1 g, which is 9 times that by the same stationary substrate. The pendant drop deforms drasticaJly and quickly at both the beginning and the end of the vibration procedure. As the vibration power increases, the contact area between the drop and substrate expands and the drop height shrinks accordingly. Theoretical analysis indicates that the Bernoulli pressure induced by ultrasonic vibration may contribute strongly to enhancing the suspensibility of pendant drops.     

Dynamic stabilization of a bistable suspension system attached to a flexible host structure for operational safety enhancement

In engineering applications, a suspension system may be attached to a flexible host structure, e.g. spacecraft truss, to provide vibration isolation for sensitive instrumentation, where the suspension and host structure dynamics are strongly coupled. For linear suspensions, a resonance normally occurs adjacent to the roll-off frequency band, which significantly and detrimentally amplifies vibration transmission. To avoid the adverse resonance for operational safety enhancement, this research proposes a nonlinear bistable suspension and evaluates its performance when attached to a flexible host structure. Dynamic models of the bistable and comparable linear suspensions attached to the host structure are formulated, and steady-state responses are predicted using analytical and numerical methods. Results show that the bistable suspension can eliminate the harmful resonance via a dynamic stabilization phenomenon, and simultaneously retains the favorable isolation performance in the roll-off bandwidth as compared to the linear suspension. Series of experimental investigations support the analytical and numerical findings and help define design guidelines for operational safety improvement.     

Hysteresis-induced bifurcation and chaos in a magneto-rheological suspension system under external excitation

The magneto-rheological damper(MRD) is a promising device used in vehicle semi-active suspension systems, for its continuous adjustable damping output. However, the innate nonlinear hysteresis characteristic of MRD may cause the nonlinear behaviors. In this work, a two-degree-of-freedom(2-DOF) MR suspension system was established first, by employing the modified Bouc–Wen force–velocity(F –v) hysteretic model. The nonlinear dynamic response of the system was investigated under the external excitation of single-frequency harmonic and bandwidth-limited stochastic road surface.The largest Lyapunov exponent(LLE) was used to detect the chaotic area of the frequency and amplitude of harmonic excitation, and the bifurcation diagrams, time histories, phase portraits, and power spectrum density(PSD) diagrams were used to reveal the dynamic evolution process in detail. Moreover, the LLE and Kolmogorov entropy(K entropy) were used to identify whether the system response was random or chaotic under stochastic road surface. The results demonstrated that the complex dynamical behaviors occur under different external excitation conditions. The oscillating mechanism of alternating periodic oscillations, quasi-periodic oscillations, and chaotic oscillations was observed in detail. The chaotic regions revealed that chaotic motions may appear in conditions of mid-low frequency and large amplitude, as well as small amplitude and all frequency. The obtained parameter regions where the chaotic motions may appear are useful for design of structural parameters of the vibration isolation, and the optimization of control strategy for MR suspension system.     

Simulation of periodic mono-sized hard sphere systems under different vibration conditions and resulting compaction

This paper presents a simulation study of confined periodic mono-sized hard sphere systems under different vibration conditions and their influence on the final compaction. An initial random loose packing is submitted to a series of vibration cycles allowing to transform it into a “suspension” in which a given proportion of particles have the possibility to vibrate. This “suspension” is then let to settle down and brought to a new denser packing. Different random local rules are used for the simulation of the displacements of particles during vibration.Firstly, a symmetric vibration is applied in which particles attempt to perform vertical upwards and downwards displacements of equal length. Shocks between particles are simply simulated by random upwards or downwards displacements. It has already been shown that in these conditions, and when the whole system is vibrating, the final packing density can be related to the initial density and to the vibration amplitude (“suspension model”). We show here that for a periodic packing, this model can be extended to partial “suspensions” in which only a proportion of particles is vibrating. An excellent agreement is found between this model and the simulation results as long as the packing is disordered. For large vibration amplitudes, an order appears amongst the system allowing to reach high densities (up to 0.66).Then, the symmetric vibration is replaced by a random vibration in which particles attempt to perform alternatively random upwards and downwards displacements whose inclination with the vertical axis follows a normal distribution. It is shown that in these conditions, the vibration still allows transforming the initial packing into a partial “suspension” but the proportion of vibrating particles is lower than for the symmetric vibration previously used. However, the “suspension model” can be extended with a reasonable agreement to this kind of vibration.     

Application of least mean square algorithm to suppression of maglev track-induced self-excited vibration

Track-induced self-excited vibration is commonly encountered in EMS (electromagnetic suspension) maglev systems, and a solution to this problem is important in enabling the commercial widespread implementation of maglev systems. Here, the coupled model of the steel track and the magnetic levitation system is developed, and its stability is investigated using the Nyquist criterion. The harmonic balance method is employed to investigate the stability and amplitude of the self-excited vibration, which provides an explanation of the phenomenon that track-induced self-excited vibration generally occurs at a specified amplitude and frequency. To eliminate the self-excited vibration, an improved LMS (Least Mean Square) cancellation algorithm with phase correction (C-LMS) is employed. The harmonic balance analysis shows that the C-LMS cancellation algorithm can completely suppress the self-excited vibration. To achieve adaptive cancellation, a frequency estimator similar to the tuner of a TV receiver is employed to provide the C-LMS algorithm with a roughly estimated reference frequency. Numerical simulation and experiments undertaken on the CMS-04 vehicle show that the proposed adaptive C-LMS algorithm can effectively eliminate the self-excited vibration over a wide frequency range, and that the robustness of the algorithm suggests excellent potential for application to EMS maglev systems.