Coupling effects on a cantilever subjected to a follower force

In this investigation a solution methodology is presented for studying the stability of a uniform cantilever having a translational and rotational spring at its support, carrying two concentrated masses, one at the support and the other at its tip, and subjected to a follower compressive force at its free end. The analysis is based on Timoshenko's beam theory by considering the cantilever as a continuous elastic system. The coupling effects on the flutter load are fully assessed for a variety of parameters such as translational and rotational springs at the support, translational and rotational inertia of the concentrated masses, and cross-sectional shape, as well as transverse shear deformation and rotatory inertia of the mass of the column.     

Stability and free vibration analyses of cantilever shear buildings with semi-rigid support conditions and multiple masses

The stability and free vibration analyses of a cantilever shear building with generalized support conditions and with multiple masses (rotational and translational) rigidly attached at both ends and along its height are presented. The proposed model includes the simultaneous effects of: (1) lateral and rotational elastic restraints at the base support; (2) a uniform distributed mass and rotary inertia plus lumped rotary and translational masses rigidly attached at both extremes and along its height; (3) linearly distributed axial load plus the concentrated vertical axial loads caused by the lumped masses; and (4) shear deformations and shear forces induced by the applied axial forces. A parametric study is carried out that shows the importance of all variables included in this work on the stability and dynamic behavior of cantilever shear buildings, particularly the effects of the attached lumped masses and the rotational and translational constraints at the base support. A comparison with results presented by other researchers in previous studies shows that the proposed method and corresponding equations can be very useful in the assessment design of cantilever shear buildings. The main objective is to present readily solutions on the static stability and free vibration of cantilever shear buildings with generalized support conditions and multiple masses rigidly attached. The proposed method and corresponding expressions for the natural frequencies and modal shapes, buckling modes and axial critical loads are extensions of those presented recently by the senior author.     

On the rotational diffusion of rod-like macromolecules in lyotropic-mesomorphic phases

The rotational diffusion of rod-like macromolecules, idealized as monodisperse rods, in concentrated solution is considered. The Doi-Edwards model is modified to explain the experimentally observed increase of the rotational relaxation time with increasing concentration in the lyotropic-nematic phase of rigid rod-like molecules in solution. Our approach consists in (i) reconsidering the original Doi-Edwards model to eliminate unphysical behaviour of the rotational relaxation time at the limit of perfect order, and (ii) including concentration effects on the translational diffusivity of rods in the Doi-Edwards model. Predictions of the corrected model are compared with steady flow viscosity data for poly(n-alkylisocyanates). In the lyotropic-nematic phase a very good qualitative agreement between the theory and experiment is observed. Additionally, the model applied to a highly concentrated isotropic phase explains in a natural way the viscosity behaviour as the concentration is increased towards the critical value for formation of the lyotropic-nematic phase.     

水平管道中段塞流的跟踪模拟

本文对以unit cell模型为基础建立起来的段塞流跟踪模型进行了改进。利用改进后的模型对水平管道中空气-水段塞流的压降、液塞平移速度、液塞长度等特征参数进行了模拟研究,并将模型预测结果与相应的试验结果进行了比较。结果表明,改进后的模型可以有效地预测以上各参数的演变规律。模拟和试验结果均表明,水平管道中段塞流的液塞平移速度符合正态分布,液塞长度符合对数正态分布。     

脉动流化床内球柱形颗粒流动行为研究

本文结合离散单元模型和计算流体力学方法对脉动流化床内气固流动特性进行数值模拟.结果表明,脉动气流的波形和振幅对球柱形颗粒的流动行为有明显的影响.方波脉动气流作用下的时均颗粒总平动和旋转动能最大,其次是正弦波脉动和三角波脉动,恒定气流下的最小.功率谱分析结果可以发现方波气流下主频幅度最大,其次是正弦波、三角波和恒定气流....     

Structure and dynamics of water at liquid–vapour interfaces covered by surfactant monolayers of neutral stearic acid and charged stearate ions

The behaviour of water molecules at liquid–vapour interfaces with a surfactant monolayer of either stearic acid molecules or anionic stearate ions is investigated by means of molecular dynamics simulations. The density and dipolar orientational profiles and also the dynamics of translational and rotational motion of interfacial water molecules are calculated in the present work and the results are compared with the bulk liquid water and also of liquid–vapour interface of surfactant-free water. The present simulation results are also compared with available experimental results of similar interfacial systems with a monolayer of either neutral or ionic surfactants.     

Model reduction for parametric instability analysis in shells conveying fluid

Flexible pipes conveying fluid are often subjected to parametric excitation due to time-periodic flow fluctuations. Such systems are known to exhibit complex instability phenomena such as divergence and coupled-mode flutter. Investigators have typically used weighted residual techniques, to reduce the continuous system model into a discrete model, based on approximation functions with global support, for carrying out stability analysis. While this approach is useful for straight pipes, modelling based on FEM is needed for the study of complicated piping systems, where the approximation functions used are local in support. However, the size of the problem is now significantly larger and for computationally efficient stability analysis, model reduction is necessary. In this paper, model reduction techniques are developed for the analysis of parametric instability in flexible pipes conveying fluids under a mean pressure. It is shown that only those linear transformations which leave the original eigenvalues of the linear time invariant system unchanged are admissible. The numerical technique developed by Friedmann and Hammond (Int. J. Numer. Methods Eng. Efficient 11 (1997) 1117) is used for the stability analysis. One of the key research issues is to establish criteria for deciding the basis vectors essential for an accurate stability analysis. This paper examines this issue in detail and proposes new guidelines for their selection.     

On steady and pulsatile motion of blood

A mathematical model has been developed to study the effect of magnetic field on steady and pulsatile motion of blood through a small, rigid, circular tube by microcontinuum approach. The numerical solutions for velocity and cell rotational velocity are obtained and their nature is shown graphically.     

Equipartition of rotational and translational energy in a dense granular gas

Experiments quantifying the rotational and translational motion of particles in a dense, driven, 2D granular gas floating on an air table reveal that kinetic energy is divided equally between the two translational and one rotational degrees of freedom. This equipartition persists when the particle properties, confining pressure, packing density, or spatial ordering are changed. While the translational velocity distributions are the same for both large and small particles, the angular velocity distributions scale with the particle radius. The probability distributions of all particle velocities have approximately exponential tails. Additionally, we find that the system can be described with a granular Boyle's law with a van?der?Waals-like equation of state. These results demonstrate ways in which conventional statistical mechanics can unexpectedly apply to nonequilibrium systems.     

Orientational correlation and velocity distributions in uniform shear flow of a dilute granular gas

Using particle simulations of the uniform shear flow of a rough dilute granular gas, we show that the translational and rotational velocities are strongly correlated in direction, but there is no orientational correlation-induced singularity at perfectly smooth (beta=-1) and rough (beta=1) limits for elastic collisions (e=1); both the translational and rotational velocity distribution functions remain close to a Gaussian for these two limiting cases. Away from these two limits, the orientational as well as spatial velocity correlations are responsible for the emergence of non-Gaussian high-velocity tails. The tails of both distribution functions follow stretched exponentials, with the exponents depending on normal (e) and tangential (beta) restitution coefficients.     

Dynamic simulation of a tilting-disc valve with clearance in pulsatile channel flow

Abstract  

In this study, a fully coupled fluid–structure interaction (FSI) software system for a pulsatile flow across a moving tilting-disc valve with clearance is developed. Unsteady pulsatile flow coupled with induced valve motion has been examined in details. State-of-the-art computational fluid dynamics (CFD) methods are adopted in the present flow solver development. To account for nonstationary valve motion, the meshes surrounding the valve are generated and updated in each time-marching step using hybrid grid method. A single-degree-of-freedom rotational valve model is integrated simultaneously with the CFD adaptive time-stepping. It is found in the present study that, on both side of the occluder, strong shedding vortices occur and persist in the valve closing phase. These closure vortices show great influence on the prediction of the regurgitate flow characteristics and the subsequent valve opening dynamics as well. Based on the present software system, the results obtained from quasi-steady simulations performed at various instants of interest with prescribed valve motion are critically evaluated to assess whether simplified flow and valve conditions may lead to erroneous conclusions.     

Ultrasonic visualization of dynamic behavior of red blood cells in flowing blood

It is well known that the scatter of ultrasound by blood is mainly attributed to red blood cells (RBCs) and RBC aggregation. In the present review, researches of hemodynamic influence on RBC aggregation and ultrasound backscatter from blood were overviewed. A mock flow loop and a cylindrical chamber were employed to produce various blood flows, such as pulsatile, oscillatory, and rotational flow. The “black hole” (BLH), a dark hole at the tube center surrounded by bright zone in the cross sectional B-mode image and “bright collapsing ring” (BRCR) phenomena, appearance of bright ring at the periphery and collapse of it at the center during a pulsatile cycle, were observed under pulsatile flow. The combined effects of shear rate and flow acceleration on RBC aggregation were suggested as a possible mechanism for these phenomena. The stroke volume-dependence of the “bright ring” phenomenon under oscillatory flow could also be explained by flow acceleration. The enveloped echo images from rotational flow in a compact blood chamber showed the spatial and temporal variations of RBC aggregation, which varied with the mammalian species. In the stenotic model, it was found that the echogenic variation increased locally at a distance of three tube diameters downstream from the stenosis during decelerating period, which was proposed to be mainly due to flow turbulence. The similar ‘bright ring’ was also observed fromin vivo human carotid artery in harmonic imaging.     

二分量胶体悬浮系统的短时间动力学

研究了二分量带电粒子悬浮系统的短时间平动和转动自扩散系数.由于存在静电相互作用和流体力学作用,扩散系数与两种粒子的尺寸比,它们的体积分数,以及所带的有效电荷都有关.计入了流体力学相互作用对扩散张量的二体贡献和首项三体贡献.计算结果表明,流体力学作用对于带电粒子系统的影响要小于它对硬球粒子系统的影响.扩散系数随两种粒子的尺寸比和它们的体积分数变化的关系可以用有效硬球模型来解释,而其定性结果与实验相符合.     

The effect of HF field on diffusion and convection in a plasma column

A cylindrical plasma model with both axial and azimuthal steady magnetic field is used. Nonlinear excitation of time averaged electric currents by the HF field is analyzed. Ponderomotive forces of the HF field also lead to translational and rotational plasma motions. Radial plasma diffusion can be modified by the presence of the HF field at sufficiently low frequencies. The analysis can be applied to problems of anomalous transport.     

The generation of partial and total vibrational inversion in colliding molecular systems initiated by IR-laser absorption

Some characteristics of energy flow in colliding molecular systems vibrationally excited by laser radiation are discussed. Among these is the fact that the rotational structure of the pumped vibrational level tends to thermalize via fast R-T transfer during the pump pulse. It can be shown that the amount of molecules being excited depends strongly on the translational temperature and the pump line chosen, an effect which is important for V - V pumping schemes. It is further possible to generate partial and total vibrational inversion in such systems. Chosing the hydrogen fluoride molecule as our model system we present computations and experiments which are in good agreement. The results obtained seem to be generalizable for other molecules as well.     

The dynamics of clusters of flexible cylinders in axial flow: Theory and experiments

This paper presents an outline of the theory for the dynamics of clusters of independently supported flexible cylinders in axial flow, and an extensive discussion of the behaviour of such systems with increasing flow velocity, with special emphasis placed on the modal forms of free coupled motions of the cylinders and on the onset of instabilities. Results of an experimental study of the problem are also presented, involving systems of two, three or four cylinders supported at both ends and positioned symmetrically in the cylindrical test section of a water tunnel; experiments were conducted with different inter-cylinder gaps and support conditions. Both theory and experiment show that with increasing flow the system loses stability by buckling in one of its coupled modes, commonly in a pattern where cylinders move towards one another symmetrically, maximum displacement occurring just downstream of their midpoints. With increasing flow, theory predicts that other buckling instabilities are superimposed on the first; in the experiments the system remains buckled, changing modal patterns constantly; some of them correspond to those predicted by theory. At sufficiently high flow, oscillatory motion is observed, corresponding to theoretical flutter. Theory and experiment agree qualitatively in most essential features of the dynamical behaviour of the system, and quantitative agreement in the critical flow velocities for the onset of the first buckling instability is remarkably good.     

Evaluation of steady and pulsatile flow with dynamic MRI using limited flip angles and gradient refocused echoes

Magnetic resonance imaging sequences utilizing limited flip angles and gradient echoes yield rapid (less than 2 min) dynamic images of the cardiovascular system. These images contain both accurate anatomical and functional information. Using a gradient refocused acquisition in the steady state (GRASS) in the CINE mode, we studied the relationship between gradient echo signal intensity and velocity of steady and pulsatile flow in a phantom simulating medium to large vessels. Images were acquired on a 1.5 Tesla system (repetition TIME = 21 ms, echo TIME = 12 ms, flip ANGLE = 30 degrees). Data from each pulse interval were sorted in 16 images. Signal intensities from flow tube lumina and surrounding stationary water jacket were used to calculate contrast ratios which were compared to velocity measurements made with electromagnetic (EM) flow probes outside the magnet room. During steady flow, signal intensity contrast ratios increased with increasing flow and in a 10 mm thick slice, reached a peak at 48 cm/s, and declined for velocities up to 90 cm/s. Changes in instantaneous velocity during pulsatile flow correlated well (r > .88) with signal intensity changes up to a maximum mean velocity of 17 cm/s. Total signal intensity from the lumen for an “R to R” interval correlated extremely well (r > .97) with mean pulsatile flow velocities up to 30 cm/s. The excellent correlation between gradient echo signal intensity and actual flow velocities suggests that this imaging sequence might be useful for evaluating normal and pathologic flow phenomena.     

Energy transmission through a double-wall structure with an acoustic enclosure: Rotational effect of mechanical links

This paper investigates the rotational effect of mechanical links on energy transmission of a double-wall structure with an enclosure. A criterion is proposed to identify energy transmission mechanisms and predict the dominant transmitting path. Studies in different frequency ranges show a more significant energy transmission due to the rotational effect of the link at higher frequencies compared with lower ones. Comparison between the translational effect and the rotational effect on energy transmission shows that although both effects are important for the transmission mechanism analysis, the rotational effect on energy transmission is more remarkable at high frequencies for a soft translational link; whereas is insensitive for a stiff one.     

Modal coupling effects in the free vibration of elastically interconnected beams

The problem of free vibration of a uniform beam elastically interconnected to a cantilevered beam, representing an idealized launch vehicle aeroelastic model in a wind tunnel, is studied. With elementary beam theory modelling, numerical results are obtained for the frequencies, mode shapes and the generalized modal mass of this elastically coupled system, for a range of values of the spring constants and cantilevered beam stiffness and inertia values. The study shows that when the linear springs are supported at the nodal points corresponding to the first free-free beam mode, the modal interaction comes primarily from the rotational spring stiffness. The effect of the linear spring stiffness on the higher model modes is also found to be marginal. However, the rotational stiffness has a significant effect on all the predominantly model modes as it couples the model deformations and the support rod deformations. The study also shows that through the variations in the stiffness or the inertia values of the cantilever beam affect only the predominantly cantilever modes, these variations become important because of the fact that the cantilevered support rod frequencies may come close to, or even cross over, the predominantly model mode frequencies. The results also bring out the fact that shifting of the support points away from the first mode nodal points has a maximum effect only on the first model mode.