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31.
变截面弹性直杆纵振动分析的小波--DQ法   总被引:2,自引:1,他引:1  
在经典微分求积(DQ)法基础上, 根据多分辨分析理论, 以尺度函数为基础构造插值基函数, 形成了新的微分方程边值问题的求解方法------小波--DQ法, 并应用该方法分析了变截面弹性直杆的纵振动问题, 给出了其频率方程, 计算出了不同参数下固支--固支, 自由--自由楔形直杆和锥形直杆的固有频率,数值结果表明该方法是一个简单易行高精度的方法,该方法可以推广应用于其他力学问题的数值分析.  相似文献   
32.
    
Here, the large amplitude free flexural vibration behaviors of thin laminated composite skew plates are investigated using finite element approach. The formulation includes the effects of shear deformation, in-plane and rotary inertia. The geometric non-linearity based on von Karman's assumptions is introduced. The non-linear governing equations obtained employing Lagrange's equations of motion are solved using the direct iteration technique. The variation of non-linear frequency ratios with amplitudes is brought out considering different parameters such as skew angle, number of layers, fiber orientation, boundary condition and aspect ratio. The influence of higher vibration modes on the non-linear dynamic behavior of laminated skew plates is also highlighted. The present study reveals the redistribution of vibrating mode shape at certain amplitude of vibration depending on geometric and lamination parameters of the plate. Also, the degree of hardening behavior increases with the skew angle and its rate of change depends on the level of amplitude of vibration.  相似文献   
33.
    
Chemical insecticide foggers have long been used to control mosquito populations and reduce the spread of malaria, West Nile virus, and dengue fever. In this study, we show that simple fogs of water droplets, devoid of chemicals, are effective at obstructing the flight of Anopheles freeborni mosquitoes. Using high-speed video, we film the flight of mosquitoes in denser-than-air fluids such as fogs produced by household humidifiers and heavy gas. In gases with more than twice the density of air, mosquitoes cannot maintain flight stability, but pitch and roll uncontrollably until they fall to the ground. We here show that the increased drag of denser-than-air flight environments is sufficient to render a mosquito׳s gyroscopic sensors and wingbeat controllers, their halteres, ineffective. Anomalous drag forces are estimated with steady and unsteady drag analyses. We glue masses of 10 ng to each haltere, equivalent to 0.0005% the mass of the mosquito. Mosquitoes with such a minor treatment executed flight patterns, similar to mosquitoes in fog, indicating a mosquito׳s haltere are highly tuned to aerodynamic properties of normal air, making them sensitive to contaminants. The failure of mosquito flight in fog reveals potential new methods for mosquito control that do not require insecticide-laced particles.  相似文献   
34.
    
We present a numerical study on vortex-induced vibration (VIV) of a freely vibrating two degree-of-freedom circular cylinder in close proximity to a stationary plane wall. Fully implicit combined field scheme based on Petrov–Galerkin formulation has been employed to analyze the nonlinear effects of wall proximity on the vibrational amplitudes and hydrodynamic forces. Two-dimensional simulations are performed as a function of decreasing gap to cylinder diameter ratio e/D[0.5,10] for reduced velocities U[2,10] at ReD=100 and ReL=2900, where ReD and ReL denote the Reynolds numbers based on the cylinder diameter and the upstream distance, respectively. We investigate the origin of enhanced streamwise oscillation of freely vibrating near-wall cylinder as compared to the isolated cylinder counterpart. For that purpose, detailed analysis of the amplitudes, frequency characteristics and the phase relations has been performed for the isolated and near-wall configurations. Initial and lower branches in the amplitude response are found from the gap ratios of 0.75 to 10, similar in nature to the isolated cylinder laminar VIV. A third response branch has been found between the initial and the lower branch at the gap ratio of e/D0.60. For near-wall cases, phase relation between drag force and streamwise displacement varies from close to 0° to 180°. Between e/D[5,7.5], the effect of wall proximity on the frequency response tends to disappear. The effect of mass-ratio is further investigated. Finally, we introduce new correlations for characterizing peak amplitudes and forces as a function of the gap ratio for a cylinder vibrating in the vicinity of a stationary plane wall.  相似文献   
35.
Rain–wind-induced vibration (RWIV) of stay cables has become a concern in bridge engineering over the past decades. The excitation mechanism of RWIV remains unclear. Many researchers believe that the upper rivulet is crucial to RWIV. However, experimental study on rivulet is challenging and limited. The current study designs and tests a cable model of 160 mm in diameter in an open jet wind tunnel. The upper rivulet movement and cable vibration are simultaneously measured. The importance of the upper rivulet in RWIV is directly demonstrated by alternately controlling the upper and lower rivulets. The characteristics of the upper rivulet movement and the effects of this movement on RWIV are investigated in detail. The experiment shows that the rivulet–cable system is coupled, which causes the cable and upper rivulet to vibrate at different amplitudes under the same wind speed. The upper rivulet harmonic movement changes the wind loading on the cable, causing the harmonic vibration of the cable, which in turn exerts a harmonic inertia force on the rivulet. A large vibration of the coupled system then develops.  相似文献   
36.
    
Many composite materials, including biological tissues, are modeled as non-linear elastic materials reinforced with elastic fibers. In the current paper, the full set of dynamic equations for finite deformations of incompressible hyperelastic solids containing a single fiber family are considered. Finite-amplitude wave propagation ansätze compatible with the incompressibility condition are employed for a generic fiber family orientation. Corresponding non-linear and linear wave equations are derived. It is shown that for a certain class of constitutive relations, the fiber contribution vanishes when the displacement is independent of the fiber direction.Point symmetries of the derived wave models are classified with respect to the material parameters and the angle between the fibers and the wave propagation direction. For planar shear waves in materials with a strong fiber contribution, a special wave propagation direction is found for which the non-linear wave equations admit an additional symmetry group. Examples of exact time-dependent solutions are provided in several physical situations, including the evolution of pre-strained configurations and traveling waves.  相似文献   
37.
    
In order to study cross flow induced vibration of heat exchanger tube bundles, a new fluid–structure interaction model based on surface vorticity method is proposed. With this model, the vibration of a flexible cylinder is simulated at Re=2.67 × 104, the computational results of the cylinder response, the fluid force, the vibration frequency, and the vorticity map are presented. The numerical results reproduce the amplitude‐limiting and non‐linear (lock‐in) characteristics of flow‐induced vibration. The maximum vibration amplitude as well as its corresponding lock‐in frequency is in good agreement with experimental results. The amplitude of vibration can be as high as 0.88D for the case investigated. As vibration amplitude increases, the amplitude of the lift force also increases. With enhancement of vibration amplitude, the vortex pattern in the near wake changes significantly. This fluid–structure interaction model is further applied to simulate flow‐induced vibration of two tandem cylinders and two side‐by‐side cylinders at similar Reynolds number. Promising and reasonable results and predictions are obtained. It is hopeful that with this relatively simple and computer time saving method, flow induced vibration of a large number of flexible tube bundles can be successfully simulated. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
38.
采用压电和粘弹性材料的杂交控制的研究进展   总被引:5,自引:0,他引:5  
杂交阻尼控制技术是许多工程结构减振和改善性能的重要方法之一.本文主要从杂交阻尼结构的几个特征方面:阻尼结构形式、建模假设、黏弹性材料建模、响应分析方法、控制方法、参数研究和结构选择来研究杂交阻尼结构的研究进展和主要研究成果.研究发现绝大多数的文献只局限于研究梁板的问题.因此很有必要加强对壳体等复杂结构的研究.最后文中指出了有关杂交阻尼结构继续发展的有待深入研究的几个方面.图0参72  相似文献   
39.
    
Mechanical vibration often occurs during protein crystallization; however, it is seldom considered as one of the factors influencing the crystallization process. This paper reports an investigation of the crystallization of five proteins using various crystallization conditions in a temperature‐controlled chamber on the table of a mechanical vibrator. The results show that mechanical vibration can reduce the number of crystals and improve their optical perfection. During screening of the crystallization conditions it was found that mechanical vibration could help to obtain crystals in a highly supersaturated solution in which amorphous precipitates often normally appear. It is concluded that mechanical vibration can serve as a tool for growing optically perfect crystals or for obtaining more crystallization conditions during crystallization screening.  相似文献   
40.
    
It is normally assumed that a commercial gaseous nitrogen cold‐stream provides a sample environment near 100 K and that the force of the cold‐stream does not induce movement in the sample. As might be expected, the reality is much more complex. Here, an investigation of one cold‐stream, starting with the temperature profile, is presented. Using silicon single crystals and flexible mounting loops, an approximate force/vibration profile of the cold‐stream is obtained. Results indicate that the center of the temperature profile is offset from the position suggested by the manufacturer‐supplied alignment tool and coincides with the area within the cold‐stream that has the most consistent force profile. Tests indicate that this region is only about one‐third of the width of the cold‐stream nozzle opening. To verify that the results were relevant to protein crystallographic data collection, the impact of cold‐stream position on the final data quality for lysozyme crystals was analyzed. On the basis of the observations it is recommended that users perform a temperature profile of their cold‐streams to ensure proper alignment instead of relying only on the alignment tool for setup. In addition, suggestions are made on what users can look for in data processing to identify problems with loop movement and what users can do to minimize the impact of these problems on their experiments.  相似文献   
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