共查询到20条相似文献,搜索用时 15 毫秒
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Nejib Hadda François Nicot Franck Bourrier Luc Sibille Farhang Radjai Félix Darve 《Granular Matter》2013,15(2):221-235
This paper examines instabilities in granular materials from a microscopic point of view through numerical simulations conducted using a discrete element method on two three-dimensional specimens. The detection and the tracking of grain scale deformation mechanisms constitute the key point for a better understanding the failure process and puzzling out what lies behind the vanishing of the macroscopic second order work. For this purpose, the second order work from microscopic variables, involving contact force and branch vector, was introduced and tracked numerically. Then, all contacts depicting negative values of the second order work were deeply investigated, especially their spatial distribution (homogeneity, agglomeration, dispersion $\ldots $ ) within the specimen according to the density of the granular assembly and to the loading direction. A set of comparisons has been considered in this context in order to highlight how a specimen is populated with such contacts whether it is loaded along a direction included within the plastic tensorial zone or along a direction for which the specimen is likely to behave elastically (elastic tensorial zone). Moreover, these comparisons concerned also loading directions within the cone of instability so that links between the vanishing of both microscopic and macroscopic second order works can be established and the local mechanisms responsible for failure occurrence may be figured out. 相似文献
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基于车道线斜率的车道偏离检测 总被引:1,自引:0,他引:1
针对结构化道路上运动车辆的车道偏离检测问题,采用单目视觉作为感知手段,分别从针孔模型下摄像机成像的空间几何关系和图像中车道线消失点的位置特点两个方面出发,推导出车辆直行情况下车道偏离率与两侧车道线斜率比之间的简单函数关系,该函数与摄像机内外参数无关。同时完成了在摄像机不同方向角下的车道偏离率测量试验,结果表明由于行车方向瞬时变化引起的摄像机方向角微小改变对车道偏离检测的影响可以忽略。道路现场试验结果表明,上述视觉测量方法得到的车道偏离率与手工实测结果相比,其相对误差小于5%。基于车道线斜率的单目视觉检测方法避开了繁琐的摄像机参数标定过程,达到了较高的车道偏离检测精度。 相似文献
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A more efficient camera calibration and optimisation method based on orthogonal vanishing points is proposed. After extracting corner points on the calibration images by using the Harris algorithm, some parallel lines are fitted via the least square method. The Levenberg–Marquard (L–M) algorithm is used to find out the optimum vanishing points. The intrinsic parameters are solved on the basis of the orthogonal vanishing point characters. The enhanced Tsai’s method is designed to solve the extrinsic parameters. The minimum residue optimisation function is introduced to calculate the distortion coefficients and optimise all parameters globally. The feasibility and efficiency of the proposed method is illustrated by an experimental study. The method can be used to obtain high precision calibration parameters without the need of sophisticated mechanical devices. It has great potential to calibration cameras in the field of computer vision, 3D measurement, etc. 相似文献
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Summary By appealing to the classical boundary-layer theory, the present paper investigates the effect of the freestream shear on the separation of the laminar boundary layer around a circular cylinder. It is shown that on the side of the cylinder with faster freestream velocity the location of the separation point (point of vanishing wall shear) is virtually unaffected by the freestream shear, while on the other side of the cylinder a critical shear rate is observed. Below this critical value, separation occurs typically at the rear surface of the cylinder and is found to shift towards the downstream direction with increasing freestream shear. Above the critical shear rate, the boundary layer separates from the windward side of the cylinder. Further increase of the freestream shear then causes the separation point to move towards the upstream direction. The present findings may have important implication on the issue regarding to the orientation of the lift force exerting on the cylinder. 相似文献
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提出了一种对摄像机进行自标定的方法.该方法是利用自然场景中三组两两正交的平行线所获得的消隐点之间具备的约束条件来实现标定的,并能够对获取的消隐点进行优化校正.利用该方法开展了实验研究,并与采用传统标定方法得到的结果进行了比较,结果表明本文所提出的算法具有较好的应用前景. 相似文献
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K. Eriksson 《International Journal of Fracture》2000,106(1):65-80
The expression for the J-integral at a point on a three-dimensional crack front, obtained from a surface independent integral, is in general a sum of a contour integral and an area integral. In this work a general expression of an area integral for a crack with a curved front is derived in curvilinear coordinates. In certain situations the area integral vanishes and previously known cases are a straight crack front in plane stress or plane strain. The general conditions for a vanishing area integral are studied. It is shown that the area integral is non-zero for cracks with a curved front in the direction of crack extension. Some examples of curved cracks are given, for which the area integral vanishes and that are of interest in practice. 相似文献
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A. M. Zenkour 《Journal of Materials Science》2007,42(23):9717-9724
An accurate solution for a rotating functionally graded annular disk is presented. Material properties of the present annular
disk are assumed to be graded in the radial direction according to a simple exponential-law distribution. The inner surface
of the disk is pure metal whereas the outer surface of the disk is pure ceramic. The boundary condition of rigid casing is
considered herein, that is the vanishing of the radial displacement at the outer surface. The boundary condition at the inner
surface of the disk is taken to be vanishing either radial displacement or radial stress. Analytical solutions for the elastic
deformation of the rotating functionally graded annular disks subjected to these boundary conditions are obtained. Numerical
results for radial displacement, circumferential and radial stresses are presented. Comparisons between the different rotating
homogeneous and functionally graded annular disks are made at the same angular velocity. The results show that distributions
of stresses and displacement through the radial direction of the rotating annular disk vary with different parameters. 相似文献
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This paper presents a method for camera calibration based on the orthogonal vanishing point calibration using concentric circles grating and wedge grating. This method, which we believe is new, uses the high-precision characteristics of phase extraction to obtain the feature points, thus decreasing the calibration errors caused by the traditional marker extraction errors of gray pattern. According to the simulation experiment analysis results, the concentric circles grating was designed with seven periods and the wedge grating was designed with four periods. In the real measuring experiment, the grating target and the similar gray concentric circles target were used to calibrate the camera, respectively. Through comparing the reprojective errors of the two methods, the method proposed is proven to improve the calibration accuracy and robustness for the vanishing point calibration algorithm. 相似文献
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在手工绘制两点透视图时,都会碰到图幅过大造成灭点与视点落在图板之外而难以操作的难题.采用一个与视灭三角形相似的小三角形作为绘图辅助,同时结合量点法中的分量点概念进行作图,就可以很好地解决这个难题,而且能更加快速准确地绘制两点透视图. 相似文献
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基于视觉的驾驶机器人导航技术 总被引:1,自引:0,他引:1
对驾驶机器人的视觉导航技术进行了研究,提出了一种快速可靠的车辆引导线检测算法.该算法首先对感兴趣的移动窗口进行有条件的边缘检测,进而引入了梯度方向直方图的处理思想,制定了车辆引导线边缘候选点集与有效点集的筛选算法,最后,通过所提出的一种基于标量化处理Kalman滤波方法实现了引导线特征参数的快速准确提取.试验表明,本文提出的检测算法能够满足复杂路况下的导航需要,具有30ms/帧的实时处理速度. 相似文献
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针对基于视频的人体运动测量过程中,需要进行相机标定导致测量效率低的问题,提出一种单目视频未标定状态下的人体运动测量系统设计方案。该系统采用极大似然估计算法求解场景中的消失点和消失线,根据射影变换的交比不变量确定人体实时身高。在身高测量的基础上,提出一种左右脚部特征点的提取方法用于测量行人的步幅。通过跟踪质心的运动来估计行人的轨迹、实时速度和步频,并对步行速度测量值进行正弦拟合,验证本系统测量的速度值符合行走客观规律。实验结果表明:身高和平均速度测量结果的相对误差分别不超过0.84%和3.72%,证明本系统的精确性和鲁棒性较好。 相似文献
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F Gori 《Journal of the Optical Society of America. A, Optics, image science, and vision》2001,18(7):1612-1617
Attention is called to a polarization basis formed by four Jones vectors that is reducible to two basic structures. In these states the field is linearly polarized at any point, but the polarization direction changes with the angular coordinate. Very simple equations hold for the field lines of these basis vectors. The adoption of this basis is of interest for beams possessing a single vortex of any order, say, m, as well as for beams with two vortices of opposite charges, m and -m. As an example, the application to vectorial Bessel-Gauss beams is briefly discussed. 相似文献
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大视场线结构光视觉传感器的现场标定 总被引:2,自引:0,他引:2
针对大视场线结构光视觉传感器的现场标定,提出了一种基于自由组合一维靶标的标定方法。自由组合多个一维靶标并固定,其中每个一维靶标至少包含3个共线特征点。靶标在传感器测量空间内自由摆放至少两次。以一维靶标两两之间夹角恒定为约束求解摄像机内部参数;由一维靶标消影点性质和特征点之间距离约束,求解各特征点在摄像机坐标系下的空间坐标;根据交比不变计算光平面与各一维靶标交点的摄像机坐标,拟合出光平面方程。实验表明,该方法可获得与平面靶标相当的标定精度,具有靶标加工容易、精度高、摆放次数少的特点,适合大视场线结构光视觉传感器的现场标定。 相似文献
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We show quantitatively through a computer simulation the effect ofvector diffraction on the image in optical lithography. Thesimulation was made with a new program, and the variation ofdiffraction as the magnification was varied was calculated for variousnumerical apertures and degrees of defocus. The diffraction at themask improves the images of lines across the polarizing direction, andthe diffraction at the lens improves the images of the lines along thepolarizing direction. We investigated the difference between imagesof lines along and across the polarizing direction as the magnificationincreased. Our result shows that the effect of the lens is sodominant that the images of lines along the polarizing direction arealways better. 相似文献
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利用超声波断口图技术对不同类型的带单侧贯穿裂纹的PMMA试件进行了测试,得到试件断口上的超声波线。基于ANSYS软件的有限元分析,计算了试件的三维J积分,得到的试件在不同裂纹长度下的三维J积分与它们断口图上的超声波线的形状和凸凹方向相似,将试件扩展裂纹前缘由直线调整为特定曲线后,计算出的三维J积分趋向常量,证实了扩展裂纹前缘的任一点,J积分矢量的幅值都是相同常量的假设。 相似文献
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P. Van Esch J-F. Clergeau K. Medjoubi 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》2005,540(2-3):361-367
Cylindrical gas detectors are often used in fixed-target experiments, because they are free of parallax effects in the equatorial plane. However, there is a growing demand to increase the width of these detectors in order to be more efficient or to cover more solid angle, hence a parallax effect starts to limit the resolution in that direction. In this paper, we propose a hardware correction for this problem which reduces the parallax error thanks to an applied potential on the front window that makes the electrostatic field lines at the entrance window radially point to sample position. A detailed analytical analysis of the solution is also presented. 相似文献