共查询到20条相似文献,搜索用时 78 毫秒
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Le Floch JM Tobar ME Cros D Krupka J 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2007,54(12):2689-2695
The Bragg reflection technique improves the Q-factor of a resonator by reducing conductor and dielectric losses. This is achieved by designing a low-loss inner resonant region (usually free space) surrounded by an outer anti-resonant region made of distributed Bragg reflector layers. In this paper we develop a simple non-Maxwellian model and apply it to design three distinct cylindrical Bragg resonators based on the same set of single-crystal sapphire plates and rings by changing only the dimension of the cavity that supports the structure. To accomplish this, the simple model allows an arbitrary thickness for either the horizontal or the cylindrical dielectric reflectors by relaxing the condition that they must be lambda/4 thick. The model also allows for higher-order field variations in both the resonant and the anti-resonant regions. The resonators were constructed and experimental results were compared with the simple model and the rigorous method of lines analysis. For the fundamental mode, an unloaded Q-factor of 234,000 at 9.7 GHz was obtained. This is larger than that for a whispering gallery mode resonator. The resonator also exhibited a greatly reduced spurious mode density when compared to an overmoded whispering gallery mode resonator. 相似文献
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The Ritz method is one of the most elegant and useful approximate methods for obtaining solutions for the natural frequencies and vibration modes of elastic plates. It is simple to use and also straightforward to implement. In conventional Ritz method, the geometric boundary conditions are only satisfied and hence the Ritz method is known as a method that can produce upper bound solution results for the natural frequencies of elastic plates. On the other hand, the accuracy of the Ritz method for the solution of differential equations with mixed natural boundary conditions at the boundary lines is not very satisfactory. To overcome this difficulty, this paper presents a simple and accurate Ritz formulation in which the natural boundary conditions are exactly implemented. The versatility, accuracy, and efficiency of the proposed method for free vibration analysis of thick rectangular and skew plates are tested against other solution procedures. It is revealed that the proposed method to handle the mixed natural boundary conditions is simple to use and can produce highly accurate solutions for the natural frequencies of thick rectangular and skew plates involving free edges. 相似文献
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We propose a method to obtain long memory retention time and high contrast ratio in a tristate liquid crystal display device. The proposed device has three unique liquid crystal (LC) states that are known as splay, π twist, and bend, with different operating schemes for the two modes of operation, which are the memory (M) mode or dynamic (D) mode. A form of selective vertical or horizontal switching changes the optically compensated π twist LC as a common black state in both the M and D modes into a bend LC state for the D mode or into the splay LC state for the M mode, respectively. 相似文献
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《Advanced Powder Technology》2020,31(1):359-369
This paper presents a simple method for predicting particle attrition during pneumatic conveying. The model calculates the changes in the particle size during pneumatic conveying (as a result of the collisions between the particles and bend walls) by using empirical correlations for both the machine and material functions. The method does not require the use of complicated simulations such as DEM–CFD. Furthermore, the computational model was written in MATLAB, and the results agree well with the experimental results for salt particles. The computation time was very short: a few seconds for the first collision (particles passed through one bend), and below one minute for six collisions. The experimental results and parametric study show that higher bend radius ratios caused less damage to the conveyed material. Moreover, higher air velocities and larger pipe diameters caused more damage to the conveyed material. 相似文献
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We present a three-dimensional (3D) analysis of a hybrid photonic crystal-conventional waveguide 90 degree bend proposed previously [Opt. Express 10, 1334 (2002)] as an ultracompact component for large-scale planar lightwave circuit integration. Both rigorous 3D finite-difference time-domain modeling and a simple perfect mirror model analysis were carried out for different Si post heights in the photonic crystal region. Results show that the bend efficiency increases rapidly with Si post height. For a post height of 6.5 microm, this structure yields a bend efficiency of 97.3% at a wavelength of 1.55 microm for 90 degree bends in 2 microm x 2 microm square channel conventional waveguides with a refractive index contrast of 3.55%, which is very close to the bend efficiency of 98.2% for the corresponding two-dimensional problem. Our 3D analysis permits the examination of issues such as out-of-plane scattering loss and the effects of finite Si post height that are not considered in two dimensions. 相似文献
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弹性中厚板模态特性的近似计算方法 总被引:4,自引:1,他引:3
文中介绍了求解中厚板振动模态特性的一个近似方法一模态摄动法。这一方法是在薄板模态子空间内实施,最后将复杂的偏微分方程的求解转化为一个代数方程组的求解。对于四过简支板,得到一个十分简单的计算公式。文中还对剪切变形和转动惯量以及剪切折合系数对四过简支厚板固有频率的影响进行了分析。算例表明,这一方法简单且具有较好的精度。 相似文献
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Brittle fracture in rounded-tip V-shaped notches 总被引:1,自引:0,他引:1
Two failure criteria are proposed in this paper for brittle fracture in rounded-tip V-shaped notches under pure mode I loading. One of these criteria is developed based on the mean stress criterion and the other based on the point stress criterion which both are well known failure criteria for investigating brittle fracture in elements containing a sharp crack or a sharp V-notch. To verify the validity of the proposed criteria, first the experimental data reported by other authors from three-point bend (TPB) and four-point bend (FPB) tests on PMMA at −60 °C and Alumina–7% Zirconia ceramic are used. Additionally, some new fracture tests are also carried out on the rounded-tip V-notched semi-circular bend (RV-SCB) specimens made of PMMA for various notch opening angles and different notch tip radii. A very good agreement is shown to exist between the results of the mean stress criterion and the experimental data. 相似文献
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矩形悬壁板自由振动精确解法 总被引:5,自引:4,他引:1
本文提出矩形薄板振动主振方向的概念。根据振形正交性的思想,在主振方向上矩形板振动波形是唯一的,由此建立了符合矩形悬臂板边界条件所激发出的振动曲线形态的振形函数表达式,并推导出矩形板的频率方程及相应的振形曲线。计算表明,这种方法符合板实际振动的规律性。 相似文献
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采用梁函数组合法对功能梯度复合材料矩形板进行动力特性分析,提出了适用于每边任取简支、固定、自由边界之一 (包括36种边界) 、材料功能梯度沿厚度任意分布的矩形板固有频率与振型的解析解一般表达式;在简化情况下,给出了各种边界条件功能梯度矩形板固有频率解的直接显式。所给出的固有频率与振型解的结果可用于功能梯度板的动力分析,工程上应用广泛,简明实用。 相似文献
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For cracked structural rectangular thin-walled tubes, an exact and very simple method to determine the stress intensity factors has been proposed based on a new concept of crack surface widening energy release rate. Unlike the classical crack extension energy release rate, the crack surface widening energy release rate can be expressed by the G*-integral and elementary strength theory of materials for slender cracked structures. From present discussions, a series of new and exact solutions of stress intensity factors are derived for cracked rectangular and square tubes. The present method can also be applied to cracked polygon thin-walled tubes. 相似文献
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The mode spectrum in an optical nanowaveguide consisting of a dielectric-core layer surrounded by two identical metal layers is investigated. A simple model based on mode matching to predict the properties of mode propagation in such optical nanowaveguides is proposed. It is shown that quasi-TM00 and quasi-TM10 modes supported by an optical microstrip line do not have a cutoff frequency, regardless of the size of the metal strips, the thickness of the dielectric slab, and the cross-sectional shape. The transverse size of the TM00 mode supported by a nanosized microstrip line was found to be approximately equal to the transverse dimension of the microstrip line. In closed rectangular and elliptical nanowaveguides, i.e., in which all dielectric surfaces are covered with metal films, the cross-sectional shape of the waveguide should be stretched along one side to produce propagation conditions for the fundamental mode. 相似文献
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A single three point bend specimen compliance method for determining JIC of high strength high fracture toughness steels is presented and a formula for calculatinff J-integral is proposed as follows.It is simple and valid for high strength high fracture toughness steels. The values of JIC and KIC measured by this method are in good agreement with those measured by standard test method. 相似文献
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This paper issues a simple criterion to characterize nonuniform domain switching. The criterion is obtained by minimizing switch-induced system energy change computationally. Combining the numerical result and experimental observations, we propose a quasi-analytic criterion defined by a piecewise function. The piecewise criterion is observed to coincide with the accurate solution very well. As examples, we apply the criterion to: (1) portray a nonhomogeneous switching zone near a stationary crack tip; (2) quantify the switch zone size; (3) predict the volume fractions of ferroelastic and ferroelectric switchings. These predictions all agree with experimental measurements. 相似文献
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矩形Mindlin板振动分析的DSC方法 总被引:1,自引:0,他引:1
提出了一种离散奇异卷积(DSC:Discrete Singular Convolution)方法来对基于Mindlin剪切变形理论的矩形厚板进行自由振动分析。此方法采用了Gauss delta序列核作为基函数并结合pb-2Rayleigh-Ritz方法(pb-2指的是atwo-dimensional polynomial function(p-2)and a boundary function(b))的边界函数得到了一种新型的Ritz方法。数值结果表明此方法相当精确有效。 相似文献
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王锦国 《理化检验(物理分册)》2002,38(1):37-40,36
根据小直径钢丝在反复弯曲试验中出现的试验数据不稳定情况,经过了对比试验,分析了拉紧力大小和反复弯曲次数的关系,提出了对反复弯曲试验方法和反复弯曲试验机,产品标准中规定最小反复弯曲次数改进的建议。 相似文献
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W. H. Prosser M. A. Hamstad J. Gary A. O'Gallagher 《Journal of Nondestructive Evaluation》1999,18(3):83-90
A comparison was made between two approaches to predict acoustic emission waveforms in thin plates. A normal mode solution method for Mindlin plate theory was used to predict the response of the flexural plate mode to a point source, step-function load, applied on the plate surface. The second approach used a dynamic finite element method to model the problem using equations of motion based on exact linear elasticity. Calculations were made using properties for both isotropic (aluminum) and anisotropic (unidirectional graphite/epoxy composite) materials. For simulations of anisotropic plates, propagation along multiple directions was evaluated. In general, agreement between the two theoretical approaches was good. Discrepancies in the waveforms at longer times were caused by differences in reflections from the lateral plate boundaries. These differences resulted from the fact that the two methods used different boundary conditions. At shorter times in the signals, before reflections, the slight discrepancies in the waveforms were attributed to limitations of Mindlin plate theory, which is an approximate plate theory. The advantages of the finite element method are that it used the exact linear elasticity solutions, and that it can be used to model real source conditions and complicated, finite specimen geometries as well as thick plates. These advantages come at a cost of increased computational difficulty, requiring lengthy calculations on workstations or supercomputers. The Mindlin plate theory solutions, meanwhile, can be quickly generated on personal computers. Specimens with finite geometry can also be modeled. However, only limited simple geometries such as circular or rectangular plates can easily be accommodated with the normal mode solution technique. Likewise, very limited source configurations can be modeled and plate theory is applicable only to thin plates. 相似文献
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In this paper, a simple oscillator-based biped walking method is described and a CORDIC-based FPGA hardware design method is proposed to effectively generate a walking gait in a biped robot. Based on the simple oscillator-based model, some equations represented by sinusoidal functions are proposed to describe a biped walking as a complete walking process with three modes (starting mode, gait cycle mode, and ending mode) and six phases. In these six phases, these oscillation parameters can be represented by the swing length, the step length, and the lifting height of the biped robot. Then an FPGA hardware structure based on the CORDIC operator named circular rotation is proposed and implemented on an FPGA chip. Finally, some comparison of the proposed CORDIC-based FPGA hardware method and the software method are presented. We can see that the proposed hardware method significantly reduces the processing time to generate gait trajectories of a biped robot. 相似文献