基于无量纲分析的法向恢复系数模型研究

作为描述接触碰撞过程能量损失的重要参量,恢复系数的深入研究对于提升现有接触碰撞力模型预测性能、准确描述接触碰撞现象,并进一步探明其对机械系统整体动态特性影响规律方面具有重要作用.鉴于现有恢复系数模型计算精度的局限性,本文基于无量纲分析方法,提出了一种考虑材料特性与初始碰撞速度的新型恢复系数模型.具体实施过程如下:首先,利用有限元软件ABAQUS建立弹性球-理想弹塑性基底法向接触碰撞数值仿真模型,分别从最小网格尺寸设置与接触碰撞能量转换角度验证了所建模型的有效性;基于此模型开展多工况下的数值模拟研究,分析不同材料弹塑性参数与初始碰撞速度对接触碰撞响应的影响;在此基础上,引入无量纲化参数E*/(ρv_nc²)与σ_y/E*,寻找恢复系数与弹塑性参数及初始碰撞速度间的函数关系;进一步结合Johnson塑性碰撞理论,反向推算获取屈服速度与材料属性的映射关系,最终建立无量纲化恢复系数新模型;通过与低速试验数据、高速有限元模拟结果的对比,验证了新模型的预测精度和泛化性能.     

球体垂直入水空泡实验研究

针对球体垂直入水问题开展了实验研究,分析了入水空泡的形成、发展、闭合及溃灭过程。通过开展不同初始入水速度及表面沾湿状态的实验研究,得到了入水速度及表面沾湿状态对球体入水空泡流场的影响,同时分析了球体在垂直入水过程中的位移、速度、加速度以及阻力因数。结果表明,球体在水下的运动参数具有较强的非线性特性,速度较高、入水空泡深闭合的条件下,球体的运动参数及阻力因数曲线具有明显的波动。

     

法向载荷和滑动速度对GCr15钢干滑动摩擦的影响

本文研究了在干滑动摩擦情况下,GCr15钢与YG8硬质合金对磨时的摩擦系数与法向载荷及滑动速度的关系。结果表明,摩擦系数是随着法向载荷和滑动速度的增加而降低。根据磨损表面发生的变化和磨损表面温度计算,作者认为法向载荷和滑动速度的增加使闪温增高,从而导致了摩擦磨损表面的局部熔化,即形成了边界润滑,故此摩擦系数降低,而且在较高的载荷和滑动速度条件下,不同显微组织材料的摩擦系数之差减小。     

真三向应力作用下深部储层砂岩渗透率各向异性实验研究

渗透率各向异性是沉积岩层理结构中一个非常典型的现象,一方面它由原生沉积结构决定,即原生各向异性,另一方面它受到总应力和孔隙压力影响,即诱发各向异性.为研究真三向应力条件下储层砂岩渗透率原生与诱发各向异性,以中国东北部S6储气库储层砂岩为研究对象,采用东北大学自主研制的硬岩真三轴应力-渗流耦合装置对储层砂岩进行渗流实验,通过稳态法完成同一砂岩3个相互垂直方向的渗透率测试.实验结果表明:储层砂岩在施加的应力和孔隙压力范围内,平行层理方向的渗透率k x为100.94～113.98 m D,k y为98.34～111.41 mD,垂直层理方向的渗透率k z为54.98～63.29 mD;储层砂岩3个正交方向渗透率均随主应力增加而减少,随孔隙压力加载而递增;与气体渗流方向垂直的应力对渗透率的影响大于与气体渗流方向平行的应力对渗透率的影响;当外部应力方向都垂直于气体渗流方向时,与层理垂直的应力对渗透率的影响大于与层理平行的应力对渗透率的影响;孔隙压力对储层砂岩渗透率的线弹性响应并不是各向同性的,孔隙压力对水平层理方向所产生的渗透率增量超过了垂直层理方向.研究结论为地下储气库储层砂岩渗透率准确预测提...     

不同尺寸砂岩动态力学性质和应力平衡性的试验研究

采用大直径分离式霍普金森压杆系统获得的不同尺寸试样的实验冲击动态力学参数有差异,因此在直径100 mm压杆上进行了3种直径（50、75和100 mm）和5种长径比（0.4、0.5、0.6、0.8和1.0）的砂岩试样冲击试验,分析了不同尺寸试样应力-应变曲线和应变率曲线随尺寸的变化,提出了用于比较波形对齐重合度的波形叠加系数,并与应力平衡因子共同构建了动态应力平衡性研究体系,由此确定大直径霍普金森压杆试验的试样建议尺寸。同时,利用高速摄影机监测试样的动态破坏状况。结果表明:当长径比相同时,直径75与100 mm岩石试样的动态抗压强度测试值相近,直径50 mm试样具有更明显的长度效应;随着试样直径的增大,应变率曲线从单峰变为双峰;小尺寸试样更易发生轴向劈裂破坏,大尺寸试样受内部应力波叠加影响产生了较大的拉应力,易发生层裂拉伸和轴向劈裂的复合型破坏;对直径75 mm且长径比0.3~0.4的试样,波形对齐后重合度较高,在起始破坏前拥有充足的应力平衡时间,应变率加载效果较好。获得了砂岩试样冲击压缩试验的尺寸效应,可为大直径岩石试样的尺寸选择提供参考。     

用正弦波加载和超声实验研究饱和砂岩弛豫特征和衰减峰

采用Metravib分析仪在-50℃~100℃温度条件及300~400Hz的正弦波作用下进行泵油饱和砂岩应力应变实验,获得了虚模量温度峰与对应实模量的剧烈亏损规律,显示出饱和砂岩有别于由衰减峰引起的典型粘弹性行为。利用超声法对饱和大理岩与花岗岩样品进行波速和衰减特性测试,获得了大理岩的Biot衰减峰变化规律。该研究对岩石物理理论模型的发展具有重要意义,可以利用岩层的衰减和波速频散特性进行油气藏的预测,降低勘探风险。     

大理岩和砂岩动态本构的实验研究

探讨了在Ｈｏｐｋｉｎｓｏｎ压杆装置上进行岩石长杆冲击实验。用多组埋入式聚偏四氟乙烯薄膜应力计，进行了波衰减的测量，获得了大理岩、砂岩在干燥、饱水、饱油情况下的衰减系数。对实测波形运用拉格朗日分析和路径线方法获取了大理岩、砂岩动态本构关系。     

包含几何误差的机械结合面法向刚度研究

对包含几何误差的机械结合面进行离散化,离散后的微表面的基准平面高度满足结合面几何误差分布.每个微表面内,微凸体的高度只受粗糙度的影响.基于接触理论建立了微表面的法向刚度模型,通过对微表面模型集成获得了结合面的法向刚度模型.通过对所建模型的数值仿真,揭示了结合面法向刚度与间隙的非线性关系,几何误差的幅值和波长对法向刚度的影响以及非线性刚度对结合面振动特性的影响.计算结果表明:法向刚度随着间隙的减少而迅速增加,几何误差会导致结合面宏观上的局部接触和应力集中;在相同干涉量下,法向刚度随着几何误差幅值的增加而增加,但与结合面的波长没有关系;非线性刚度会导致结合面固有频率的下降和振动位移的不对称.     

粗糙表面法向接触刚度的分形模型

提出了以往有关粗糙表面法向接触刚度理论研究工作的缺陷与不足,并在一定的前提假设下,基于球体与平面的接触理论和粗糙表面的分形接触理论,从理论上给出了具有尺度独立性的粗糙表面法向接触刚度分形模型,并进行了数字仿真研究。     

Modeling of softsphere normal collisions with characteristic of coefficient of restitution dependent on impact velocity

This letter presents a theoretical model of the normal (head-on) collisions between two soft spheres for predicting the experimental characteristic of the coefficient of restitution dependent on impact velocity. After the contact force law between the contacted spheres during a collision is phenomenologically formulated in terms of the compression or overlap displacement under consideration of an elastic—plastic loading and a plastic unloading subprocesses, the coefficient of restitution is gained by the dynamic equation of the contact process once an initial impact velocity is input. It is found that the theoretical predictions of the coefficient of restitution varying with the impact velocity are well in agreement with the existing experimental characteristics which are fitted by the explicit formula.     

The coefficient of normal restitution for the Hertzian contact of two rough spheres colliding in a viscous fluid

We use previous theoretical results for the added mass, history and lubrication forces between two spheres colliding in a fluid with viscosity ν to investigate the effect of viscous dissipation on the coefficient of restitution during contact. We assume that the mechanical interaction is governed by Hertzian mechanical contact of small duration τ and that the minimum approach distance between particles is approximately equal to the height σ of surface micro-asperities. A non-dimensionalization of the equation of motion indicates that the contact dynamics is governed by two parameters – the ratio ϵ of the surface roughness σ and the sphere radius a, and a contact Stokes number defined as St_c = σ²/ντ. An asymptotic solution of the equation of motion in the limit of small ϵ/St_c is used to obtain an explicit expression for the coefficient of restitution during contact and the latter is compared with estimates based on numerical solutions of the non-linear equation of motion.     

Effect of coefficient of restitution in Euler-Euler CFD simulation of fluidized-bed hydrodynamics

Collision between particles plays an important role in determining the hydrodynamic characteristics of gas-solid flow in a fluidized bed.In the present work,earlier work(Loha,Chattopadhyay. Chatterjee,2013) was extended to study the effect of the elasticity of particle collision on the hydrodynamic behavior of a bubbling fluidized bed filled with 530-μm particles.The Eulerian-Eulerian two-fluid model was used to simulate the hydrodynamics of the bubbling fluidized bed.where the solid-phase properties were calculated by applying the kinetic theory of granular flow.To investigate the effect of the elasticity of particle collision,different values of the coefficient of restitution were applied in the simulation and their effects were studied in detail.Simulations were performed for two different solid-phase wall boundary conditions.No bubble formation was observed for perfectly elastic collision.The bubble formation started as soon as the coefficient of restitution was set below 1.0,and the space occupied by bubbles in the bed increased with a decrease in the coefficient of restitution.Simulation results were also compared with experimental data available in the literature,and good agreement was found for coefficients of restitution of 0.95 and 0.99.     

Predicting the coefficient of restitution of impacting elastic-perfectly plastic spheres

The current work presents a different methodology for modeling the impact between elasto-plastic spheres. Recent finite element results modeling the static deformation of an elasto-plastic sphere are used in conjunction with equations for the variation of kinetic energy to obtain predictions for the coefficient of restitution. A model is also needed to predict the residual deformation of the sphere during rebound, or unloading, of which several are available and compared in this work. The model predicts that a significant amount of energy will be dissipated in the form of plastic deformation such that as the speed at initial impact increases, the coefficient of restitution decreases. This work also derives a new equation for the initial critical speed which causes initial plastic deformation in the sphere that is different than that shown in previously derived equations and is strongly dependant on Poisson’s Ratio. For impacts occurring above this speed, the coefficient of restitution will be less than a value of one. This work also compares the predictions between several models that make significantly different predictions. The results of the current model also compare well with some existing experimental data. Empirical fits to the results are provided for use as a tool to predict the coefficient of restitution.     

The oseen flow of finite clusters of spheres and the wake effect on the drag factor

In this paper, the wake effect on drag factor in the axisymmetric Oseen flow of the finite clusters of equally spaced spheres with same size is studied. Putting the Oseen lets on the centres of all the spheres, the series solution of the problem is obtained. By truncating the infinite series and applying the collocation method to solve a set of the linear algebraic equations, the approximate solution of the Oseen flow of finite clusters of spheres and the drag factor for each sphere are presented. The effect of the sphere number and spacing on the drag factor of each sphere under different Reynolds numbers are calculated and the wake effect as well as the shielding effect and the end effect are revealed. The influence of various parameters on the effects is considered and compared with the corresponding results of the Stokes flow. The convergence of the method is also studied numerically in this paper.     

The prediction of the coefficient of restitution between impacting spheres and finite thickness plates undergoing elastoplastic deformations and wave propagation

Nonlinear Dynamics - The coefficient of restitution (COR) is a pragmatic analytical tool needed to solve impact problems. The coefficient is customarily obtained empirically by executing...     

Anomalous rolling of spheres down an inclined plane

A sphere in air will roll down a plane that is tilted away from the vertical. The only couple acting about the point of contact between the sphere and the plane is due to the component of the weight of the sphere along the plane, provided that air friction is negligible. If on the other hand the sphere is immersed in a liquid, hydrodynamic forces will enter into the couples that turn the sphere, and the rotation of the sphere can be anomalous, i.e., as if rolling up the plane while it falls. In this paper we shall show that anomalous rolling is a characteristic phenomenon that can be observed in every viscoelastic liquid tested so far. Anomalous rolling is normal for hydrodynamically levitated spheres, both in Newtonian and viscoelastic liquids. Normal and anomalous rolling are different names for dry and hydrodynamic rolling. Spheres dropped at a vertical wall in Newtonian liquids are forced into anomalous rotation and are pushed away from the wall while in viscoelastic liquids, they are forced into anomalous rotation, but are pushed toward the wall. If the wall is inclined and the fluid is Newtonian, the spheres will rotate normally for dry rolling, but the same spheres rotate anomalously in viscoelastic liquids when the angle of inclination from the vertical is less than some critical value. The hydrodynamic mechanisms underway in the settling of circular particles in a Newtonian fluid at a vertical wall are revealed by an exact numerical simulation based on a finite-element solution of the Navier-Stokes equations and Newton's equations of motion for a rigid body.     

Drift of spheres in a rotating fluid

The drift of spheres in a rotating fluid is investigated. The problem is studied experimentally and numerically using the Galerkin method. It is shown that for small angular velocities of the fluid Ω the drift velocity of the spheres is almost independent of Ω, but once a certain threshold value Ω_* is attained the drift velocity rapidly decreases. The experimental dependence of the translational velocity of the sphere on the fluid angular velocity is explained on the basis of a theoretical analysis.