共查询到20条相似文献,搜索用时 31 毫秒
1.
H. Wada 《The European physical journal. E, Soft matter》2009,28(1):11-16
The dynamics of a rotating elastic nano-ring driven in a viscous fluid by an externally applied torque about a specific axis
is studied using elasto-hydrodynamic simulations. We show that a helical deformation of the ring filament is excited, and
that this leads to directed propulsion which is independent of the direction of rotation. It is found that the propulsive
force and efficiency initially increase as the torque is increased, and then decrease discontinuously at a buckling transition
at a critical torque. This unique propulsive behavior at the shape transition arises due to its specific geometry, i.e., circularity of an elastic filament. The implications of the behavior for artificial microscopic devices are discussed. 相似文献
2.
We have constructed a dual trap optical tweezers set-up around an inverted microscope where both the traps can be independently
controlled and manipulated in all the three dimensions. Here we report our observations on rotation of red blood cells (RBCs)
in a linearly polarized optical trap. Red blood cells deform and become twisted in hypertonic phosphate buffer saline and
when trapped, experience an unbalanced radiation pressure force. The torque generated from the unbalanced force causes the
trapped RBC to rotate. Addition of Ca++ ions in the solution, keeping the osmolarity same, makes the cell membranes stiffer and the cells deform less. Thus the speed
of rotation of the red blood cells can be controlled, as less deformation and in turn less asymmetry in shape produces less
torque under the radiation pressure resulting in slower rotation at the same laser power 相似文献
3.
Involving the intrinsic power transmission torque/speed coupling characteristics of prime-movers, a rotating elastic connection disk subjected to periodically fluctuating transmitted torque and rotational speed generated by the fluctuation of external loads is investigated. Using Galerkin's method, the rotating elastic connection disk is modeled as a parametrically excited gyroscopic system. The effects of the torque/speed coupling, transmitted torque fluctuation amplitude and frequency, and constant parts of the transmitted torque and the rotational speed on the system dynamic stability are explored for the disk modes possessing different nodal diameters. The rotational speed, transmitted torque and their fluctuations can all result in system instability of the elastic connection disk. The instability can be suppressed or avoided by operating at small amplitude and low frequency of the transmitted torque fluctuation, and by operating in the weakly coupled torque/speed regime of the prime-movers. Low rotational speed avoids the instability in the case of a small transmitted torque, but medium rotational speed operation is valuable to suppress the instability induced by a large transmitted torque and its fluctuation. Instability parameter regions for the positive and negative torque/speed coupling coefficient are roughly similar in shape, but there are some differences in the value of the instability coefficient. 相似文献
4.
Many types of bacteria swim by rotating a bundle of helical filaments also called flagella. Each filament is driven by a rotary
motor and a very flexible hook transmits the motor torque to the filament. We model it by discretizing Kirchhoff’s elastic-rod
theory and develop a coarse-grained approach for driving the helical filament by a motor torque. A rotating flagellum generates
a thrust force, which pushes the cell body forward and which increases with the motor torque. We fix the rotating flagellum
in space and show that it buckles under the thrust force at a critical motor torque. Buckling becomes visible as a supercritical
Hopf bifurcation in the thrust force. A second buckling transition occurs at an even higher motor torque. We attach the flagellum
to a spherical cell body and also observe the first buckling transition during locomotion. By changing the size of the cell
body, we vary the necessary thrust force and thereby obtain a characteristic relation between the critical thrust force and
motor torque. We present a elaborate analytical model for the buckling transition based on a helical rod which quantitatively
reproduces the critical force-torque relation. Real values for motor torque, cell body size, and the geometry of the helical
filament suggest that buckling should occur in single bacterial flagella. We also find that the orientation of pulling flagella
along the driving torque is not stable and comment on the biological relevance for marine bacteria. 相似文献
5.
In the year 2002 publications of A.V.M. Khachatourian and A.O. Wistrom were released, in which the existence of an electrostatic torque has been claimed. This moment of force should act in a three sphere configuration, where one sphere is held at a constant electric potential. This claim was based on an observed rotation and was supported by a mathematical solution derived by Wistrom and Khachatourian. The theoretical work of Wistrom and Khachatourian as well as the interpretation of the observed rotation were criticized by several scientists who offered alternative explanations for the rotation. We therefore designed an experimental setup which enabled us to investigate the phenomenon. By performing numerous measurements, we showed that the rotation is due to asymmetric mass distribution within the sphere, which is dislocated due to electrostatic forces between the spheres. We were able to clear our measurements from this effect and observed a null result more than two orders of magnitude smaller than predicted by Khachatourian and Wistrom's theory. We therefore showed that the rotation doesn't occur in an electrostatic system within the resolution of our experiment. 相似文献
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7.
Biological filaments such as DNA or bacterial flagella are typically curved in their natural states. To elucidate the interplay of viscous drag, twisting, and bending in the overdamped dynamics of such filaments, we compute the steady-state torsional stress and shape of a rotating rod with a kink. Drag deforms the rod, ultimately extending or folding it depending on the kink angle. For certain kink angles and kink locations, both states are possible at high rotation rates. The agreement between our macroscopic experiments and the theory is good, with no adjustable parameters. 相似文献
8.
发展了考虑法向接触力、切向接触力和力矩、以及滚动摩擦力矩的三维三方程线性弹性-阻尼离散单元模型及计算程序,对薄滚筒内二元S型颗粒体系进行了数值模拟,发现采用本文的数学模型可以准确地预测出滚筒内二元S型颗粒流的分层现象.分析了影响滚筒内颗粒分层的因素,讨论了滚筒转速、颗粒装载率等参数对分层的影响,当转速较高时,滚筒内形成大颗粒在外、小颗粒在内、具有圆形界面的月亮模式,当转速较低时形成具有波浪形界面的花瓣模式,并且随着滚筒转速的逐渐降低,花瓣的数量逐渐增加,数值模拟结果与实验完全符合.模拟还得到了花瓣模式的形
关键词:
分层
滚筒
模式形成
离散单元方法 相似文献
9.
发展了考虑法向接触力、切向接触力和力矩、以及滚动摩擦力矩的三维三方程线性弹性-阻尼离散单元模型及计算程序,对薄滚筒内二元S型颗粒体系进行了数值模拟,发现采用本文的数学模型可以准确地预测出滚筒内二元S型颗粒流的分层现象.分析了影响滚筒内颗粒分层的因素,讨论了滚筒转速、颗粒装载率等参数对分层的影响,当转速较高时,滚筒内形成大颗粒在外、小颗粒在内、具有圆形界面的月亮模式,当转速较低时形成具有波浪形界面的花瓣模式,并且随着滚筒转速的逐渐降低,花瓣的数量逐渐增加,数值模拟结果与实验完全符合.模拟还得到了花瓣模式的形 相似文献
10.
U.R. Kristiansen 《Journal of sound and vibration》1980,72(3):403-413
Effective acoustic source positions (observed from the far field) have been located for the broad band noise from a cylindrical rod rotated about its mid-point by measuring the cross spectral density function of two microphone signals on the axis of rotation. Local source position Strouhal numbers could thereby be calculated. On the basis of acoustic power measurements it was demonstrated that the noise may be normalized on a rod tip Strouhal number basis, and that the velocity exponent is nearly constant when plotted against this parameter. The results indicate that vortex shedding like that for stationary cylinders in a cross flow (occurring along the outer of the rod for a rotational speed of 1000 rpm) is responsible for the high levels of broad band noise in a major peak region. Sources influenced by harmonics of the rod passing frequency were found for frequencies lower than the vortex shedding ones. At higher frequencies broad band noise was found to be emitted from the rod tip area. 相似文献
11.
Quantitative force sensing reveals essential information for the study of biological systems. Forces on molecules, cells, and tissues uncover functioning conditions and pathways. To analyze such forces, spherical particles are trapped and controlled inside an optical tweezers (OT) trap. Although these spherical particles are well‐established sensors in biophysics, elongated probes are envisioned for remote force sensing reducing heat damage caused by OT. There is thus a growing demand for force metrology with OT using complexly shaped objects, e.g., sac‐like organelles or rod‐like bacteria. Here, the employment of Zeolite‐L crystals as cylindrical force sensing probes inside a single optical trap is investigated. It is shown that cylindrical objects can be used as force probes since existing calibration assays can be performed with suitable corrections. Forces of active driving assays are compared with passive calibration methods. Finally, the investigations are extended to direct force measurements based on momentum calibration, in which the influence of rotation due to torque in a single optical trap is unveiled. Simulations reveal the relation between torque and the position of equilibrium in the trap. The results highlight the functionality of Zeolite‐L crystals as probes for force sensing, while opening perspectives for enhanced, accurate force metrology in biophotonics. 相似文献
12.
基于Kirchhoff理论讨论圆截面弹性螺旋杆的动力学问题.以杆中心线的Frenet坐标系为参考系,建立用欧拉角描述的弹性杆动力学方程.讨论其在端部轴向力和扭矩作用下保持的无扭转螺旋线平衡状态.在静力学和动力学领域内讨论其平衡稳定性问题.还讨论了弹性杆平衡的Lyapunov稳定性和欧拉稳定性两种不同稳定性概念之间的区别和联系.在一次近似意义下证明了螺旋杆在空间域内的欧拉稳定性条件是时域内Lyapunov稳定性的必要条件.导出了解析形式螺旋杆三维弯曲振动的固有频率,为螺旋线倾角和受扰挠性线波数的函数.
关键词:
弹性螺旋杆
Kirchhoff动力学比拟
Lyapunov稳定性
欧拉稳定性 相似文献
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14.
An analysis is presented of the secondary effects due to deformation as a result of the shearing force, to the kinetic energy of rotation, and to the gyroscopic coupling, on the linearized equation of a straight shaft turning at a constant angular velocity and which is subjected to an axial torque and a compressive force. The local equations of motion of a section of shaft are derived and the magnitudes of the coupling terms are studied, non-dimensional variables being used. Results obtained by different authors in certain particular cases are rediscovered, and, in particular, the case of a shaft on short bearings is studied. Some comparisons of the theoretical results with those from experiments are given. 相似文献
15.
G. P. Cherepanov 《Physical Mesomechanics》2016,19(1):1-5
In his main book “Discorsie Dimostrazioni Matematiche, Intorno a Due Nuove Scienze” published in 1638 by Elsevier in Leiden, Galileo Galilei, “the Father” of modern science, put the material science and strength of materials on the first place. He introduced the notions of stress and strength that have been fundamental since then. Moreover, in unison with Plato’s theory of forms he found out the perfect shape of a force-bent beam we call today equistrong. This discovery laid the foundation for search of other perfect elastic bodies as a continuation of Galilei’s work. There are no theorems of existence for equistrong bodies so that the quest for them is like a gold-digging. In what follows, the shapes of the following heavy, equistrong beams were found out: a) beam of constant thickness and of variable width, simply supported at both ends, b) beam clamped at one end and loaded at the other end while having either constant thickness and variable width, or constant width and variable thickness, and c) equistrong shape of the profile of aircraft wings accounting for gravity and lift loads. The shape of equistrong rod at buckling under a compressive force is found in the Euler’s problem. Equistrong structures possess minimum weight for given safety factor or maximum safety factor for given weight. 相似文献
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在实际的声操控中,由于声辐射力、表面张力和重力的共同作用,液滴往往呈现出椭球的形状,在螺旋声场中会受到力矩的作用而发生转动。从声波的散射理论出发,根据部分波展开法求解得到了椭球形液滴在Bessel驻波场中的声散射系数,并给出了其受到的声辐射转矩的解析式。在此基础上,对椭球形不可压缩液滴和椭球形可压缩液滴分别进行数值计算。仿真结果表明,不可压缩液滴的声辐射转矩与声束半锥角的关系更密切,而可压缩液滴则更依赖于特定的频率;提升Bessel驻波场的阶数有利于增强声辐射转矩的峰值,但在中低频处较难对可压缩液滴产生明显的力矩。该研究结果预期对利用螺旋声场进行液滴的操控具有理论指导作用。 相似文献
18.
针对激光主动成像的图像特性,提出一种基于快速轮廓转动力矩的目标识别方法。将转动力矩的概念引入目标识别中,提出的快速轮廓转动力矩特征(FCTF)不仅包含了轮廓的尺寸、位置、规则度以及目标的亮暗等信息,同时对于旋转、尺度缩放等变换具有不变性。采用转动力矩的快速计算方法,提高了识别算法的计算效率。识别算法首先使用最大稳定极值区域(MSER)算法检测出目标特征区域,并将其变换为圆形区域,然后结合快速转动力矩特征算法提取出目标区域的局部不变特征,最后输入训练好的支持向量机分类器进行识别。实验结果表明相比于已有的激光主动成像目标识别方法,所提算法对于旋转、仿射变换均具有更高的识别率,同时单帧平均运算时间为9.68 ms,满足激光主动成像目标识别系统实时性的要求。 相似文献
19.
The dynamics of a rotating elastic filament is investigated using Stokesian simulations. The filament, straight and tilted with respect to its rotation axis for small driving torques, undergoes at a critical torque a strongly discontinuous shape bifurcation to a helical state. It induces a substantial forward propulsion whatever the sense of rotation: a nanomechanical force-rectification device is established. 相似文献
20.
Quantitative predictions are presented of a depletion-induced torque and force acting on a single colloidal hard rod immersed in a solvent of hard spheres close to a planar hard wall. This torque and force, which are entirely of entropic origin, may play an important role for the key-lock principle, according to which a biological macromolecule (the key) is functional only in a particular orientation with respect to a cavity (the lock). 相似文献