Stewart平台的运动奇异性与力奇异性研究

Stewart平台运动时 ,平台可能发生运动奇异性和力奇异性 .通过对平台运动方程Jacobian矩阵和力平衡方程Jacobian矩阵的分析 ,给出了两Jacobian矩阵的元素、行列式和条件数之间的联系 ,并对比了两种奇异面方程 .数学分析与实例说明 。

冗余度大型射电望远镜广义Stewart平台

鉴于新一代大型射电望远镜的悬索舱体系统与Ｓｔｅｗａｒｔ平台机构在工作原理上 的相似性，将悬索舱体系统视为广义Ｓｔｅｗａｒｔ平台。为了消除系统的力奇异性，通过增加冗 余悬索，构造了冗余度大型射电望远镜广义Ｓｔｅｗａｒｔ平台。利用作用于舱体的悬索张力与载 荷之间的Ｊａｃｏｂｉａｎ变换矩阵的行列式与条件数，描述了系统工作空间内力奇异性的特点，以 及冗余驱动力消除系统力奇异性的显著效果。

Formation of Singularities in One—Dimensional Hydromagnetic Flow

Two results on the formation of singularities in solutions to the system of one-dimensional hydromagnetic dynamics are presented.In particular,it is shown that shocks form from a smooth spatial periodic flow in a finite time if the initial amounts of entropy and the “magnetic field” in each period are smaller than those of sound waves.A quantitative estimate of blow-up time is also given.

Wave Interaction with Dual Circular Porous Plates

In this paper we have investigated the reflection and the transmission of a system of two symmetric circular-arc-shaped thin porous plates submerged in deep water within the context of linear theory. The hypersingular integral equation technique has been used to analyze the problem mathematically. The integral equations are formulated by applying Green＇s integral theorem to the fundamental potential function and the scattered potential function into a suitable fluid region, and then using the boundary condition on the porous plate surface. These are solved approximately using an expansion-cure-collocation method where the behaviour of the potential functions at the tips of the plates have been used. This method ultimately produces a very good numerical approximation for the reflection and the transmission coefficients and hydrodynamic force components. The numerical results are depicted graphically against the wave number for a variety of layouts of the arc. Some results are compared with known results for similar configurations of dual rigid plate systems available in the literature with good agreement.

Stress field near an interface edge of linear hardening materials

The elastic-plastic singular stress field near an interface edge of bounded linear hardening material is substantially as same as that of bonded elastic materials whose Young' s modulus and Poisson ratio are substituted by equivalent values, respectively. Further investigation by the elasto-plastic boundary element method (BEM) on the stress field near the interface edge showed that the stress field there can be divided into three regions: the domain region of the elastic-plastic singular stress field, the transitional region and the elastic region. The domain region of the elastic-plastic singular stress becomes larger with the increasing of the linear hardening coefficient. When the linear hardening coefficient decreases to a certain value, the effective stress in most of the yield zone equals approximately the yield stress. The stress distribution in the elastic region under small-scale yielding condition was also investigated.

Uncertainty and universality in the power-law singularity as a precursor of catastrophic rupture

The prediction of catastrophic rupture of heterogeneous brittle material has been investigated by researchers in the past.In this work,the acoustic emission generated by marble samples under compression was analyzed to verify a power law singularity of index 1/2 as a catastrophe precursor.It is found that prior to catastrophe,the variation of the system response to the controlling parameter follows a power law of negative index,which proves the power law singularity as a common precursor of catastrophe.However,the power indexes vary with variables and samples.The uncertainty reflects sample specificity of an evolution induced catastrophe (EIC) process.

Fracture analysis of a plane crack problem under chemo-mechanical loading

Fracture analysis of a plane crack problem under chemo-mechanical loading is presented based on a linear chemo-elasticity model.The flux conductivity is introduced to characterize the influence of the crack defect on the diffusion process.Using Fourier transform and the dislocation density functions,the crack problem is reduced to a set of singular integral equations,which are solved numerically by the Lobatto-Chebyshev method.Parametric studies are conducted to reveal the effects of flux conductivity,geometric configuration,chemical and mechanical loads on the crack tip field.The numerical results show that the stress singularity at the crack tip is usually a mixture of mode Ⅰ and mode Ⅱ types.

EXCHANGE RATE FORECASTING WITH OPTIMUM SINGULAR SPECTRUM ANALYSIS

Forecasting exchange rate is undoubtedly an attractive and challenging issue that has been of interest in different domains for many years. The singular spectrum analysis （SSA） technique has been used as a promising technique for time series forecasting including exchange rate series. The SSA technique is based upon two main choices： Window length, L, and the number of singular values, r. These values are very important for the reconstruction stage and forecasting purposes. Here the authors consider an optimum version of the SSA technique for forecasting exchange rates. The forecasting performances of the SSA technique for one-step-ahead forecast of six exchange rate series are used to find the best L and r.