颗粒流理论在建立NOPD能量耗散模型中的应用

为了研究颗粒进入对流状态时非阻塞性颗粒阻尼(NOPD)的能量耗散机理,引入颗粒流理论建立NOPD能量耗散的解析模型。借助离散单元法(DEM)初步研究了阻尼器内部颗粒的对流运动,引入普朗特混合长度理论对稠密颗粒流本构关系进行修正;借鉴振荡流理论最终得到NOPD的能量耗散解析模型。研究结果得到NOPD能量耗散率随颗粒参数变化的一般规律。在此基础上,搭建NOPD能量耗散功率测试实验台,对NOPD的能量耗散功率进行测试,验证了上述模型的正确性。研究结果进一步揭示了颗粒处于对流状态时NOPD能量耗散机理,为NOPD的应用提供了理论指导。     

电磁场下颗粒阻尼动态特性试验研究

利用稳态能量流法对直流电磁场作用下颗粒阻尼器的阻尼特性进行了试验研究。试验结果表明:在一定振动情况下,通过施加直流电磁场的方法,可以改变颗粒阻尼器阻尼性能;在较低振动加速度幅值(32.5 m/s~2)下,颗粒体产生的损耗功率和损耗因子随电流强度的增强而减小;当振动加速度增大到32.5 m/s~2后,损耗功率和损耗因子会随电流强度的增强而增大。阻尼颗粒的有效质量随激励幅值的增加呈先平缓再降低的趋势,且电流强度的变化对有效质量影响较小。     

非阻塞性微颗粒阻尼柱阻尼特性的实验研究

摘要：针对三个非阻塞性微颗粒阻尼空心柱,分别采用铁粉、铅粉和砂粒作为填充颗粒,通过自由振动实验,研究颗粒填充率、质量比、空腔结构形式、颗粒材料类型等因素对构件阻尼特性的影响,探索构件的合理空腔结构形式。实验结果表明：颗粒填充率和质量比是柱阻尼的显著影响因素,设计颗粒阻尼柱时选取合适的质量比或填充率可以获得最大程度的阻尼增大效果。另外,多空腔结构形式、多空腔填充颗粒方案以及选用合理的颗粒材料类型均有助于柱阻尼的提高。不管采用金属颗粒还是非金属颗粒,颗粒质量比宜控制在0.3~0.5的范围。当颗粒用量不大时,选用铅粉、铁粉等金属颗粒比采用非金属颗粒能获得更大的结构阻尼。实验结果可为非阻塞性微颗粒阻尼柱的合理设计提供依据。
     

冲击荷载作用下EPS混凝土动态性能研究

配置聚苯乙烯(Expanded Polystyrene,EPS)颗粒体积掺量分别为10%,20%,30%,40%,50%的EPS混凝土,采用Φ100 mm分离式霍普金森压杆(SHPB)试验装置,以动态抗压强度和临界应变为指标,研究EPS混凝土在冲击荷载作用下的动态性能,探索EPS颗粒对混凝土动态性能的改善机理。结果表明:由于应变率效应,相同体积掺量的EPS混凝土动态抗压强度与临界应变随应变率的增加而提高,具有显著的应变率相关性;以临界应变为变形性能指标,由于EPS颗粒的微结构效应,在EPS颗粒体积掺量0～40%范围内,其变形性能随EPS体积掺量的增加而提高,当EPS颗粒体积掺量达到50%时,其变形能力有所降低。EPS颗粒体积掺量为40%时对混凝土变形性能的改善效果最佳。     

颗粒物质流变学行为和材料参数对颗粒阻尼器能量耗散的影响

颗粒阻尼技术(NOPD),作为一种简单有效的振动控制手段被广泛应用于各种场合。基于离散单元法仿真,结合NOPD中颗粒系统的运动状态对阻尼效果进行分析,探究不同振动条件对阻尼性能的影响,从细观尺度阐述颗粒系统的能量耗散机理。并通过正交试验,考察颗粒的密度、剪切模量、恢复系数、静摩擦因数、滚动摩擦因数等材料参数对损耗因子的影响。研究结果表明,颗粒系统表现出不同运动状态时的流变学行为及细观结构导致了阻尼效果的变化,颗粒系统在浮力对流状态时表现出最优的阻尼效果。不同的颗粒材料参数在不同颗粒系统运动状态条件下对系统阻尼的影响程度不同。     

基于能量平衡的层间隔震结构地震响应预测

本文建立了层间隔震结构最大地震响应时刻的能量平衡方程,利用RSS法求得隔震层与下部子结构的最大变形比,并给出了层间隔震结构全体弹性振动能、隔震层阻尼器滞回耗能和下部子结构粘滞阻尼耗能的表达式。通过引入设计用能量谱的概念,推导出了基于能量平衡的层间隔震结构地震响应预测式。最后以隔震层设置位置变化的12层层间隔震结构为仿真对象,验证了响应预测式的精度。结果表明,对于不同的输入地震波,响应预测曲线包络了大多数时程分析的结果,吻合良好。     

圆柱壳体阻尼材料布局拓扑优化研究

采用渐进结构拓扑优化方法,以阻尼结构模态损耗因子最大化为目标,阻尼材料体积分数为约束条件,阻尼胞单元为设计变量,建立了圆柱壳体阻尼材料布局拓扑优化模型,对约束阻尼以及自由阻尼材料布局进行了拓扑优化。研究了阻尼结构模态损耗因子对阻尼胞单元位置的灵敏度,导出灵敏度计算表达式。根据渐进优化算法的优化准则,通过逐步删除利用率低的材料,使目标模态损耗因子达到最大化。给出了数值计算的例子,理论计算结果验证了拓扑优化设计方法的正确性和有效性     

一族无条件稳定的显式结构动力学算法

利用离散控制理论分析HHT-α算法,提出了一族具有可控数值阻尼的无条件稳定显式结构动力学算法—显式HHT-α法,用于线性和非线性结构动力学分析。新算法采用显式的位移、速度递推式。研究了所提算法的精度,稳定性,数值色散和能量耗散特性。研究表明该算法对于线弹型和刚度软化型非线性系统是无条件稳定的,算法数值阻尼由单个参数控制,对于特定的参数值,所提算法不会产生数值能量耗散。此外所提出的显式算法的数值色散和能量耗散特性与隐式HHT-α算法相同。数值算例验证了理论分析的正确性。     

粗糙结合面法向接触的能量耗散与阻尼特性研究

为了能够预测粗糙结合面法向接触的能量耗散和阻尼的变化规律,在Hertz和Abbott-Firestone经典接触模型的基础上,提出了一种加载-卸载混合弹塑性接触模型。首先,研究单个微凸体在完全弹性和塑性接触状态下的受力特性,推导出混合弹塑性状态下接触面积和力的数学表达式,进而研究能量变化规律和接触阻尼特性。然后,通过文献中的实验数据,并且和完全弹性、塑性状态下的接触模型进行对比,验证了该模型的有效性。在此基础上,分析能量耗散和法向接触阻尼与法向变形量、硬度以及硬度指数的关系。结果表明,结合面能量耗散随着法向变形量的增加而变大,而法向接触阻尼随之减小;在法向变形量一定时,接触阻尼随着材料硬度的增加而变大,而硬度指数的影响很小。     

磁性液体阻尼减振器的实验研究

针对要求结构紧凑和能量耗散较小的场合,提出一种新的磁性液体阻尼减振器。这种减振器利用了磁性液体的独特性质,依靠液体的粘性阻尼耗散能量,是一种新型的吸振装置。利用基于弹性悬臂梁的减振实验,研究多种实验参数对这种减振器加于悬臂梁后减振效果的影响。实验结果表明,磁性液体阻尼减振器在实验中所有频率上对悬臂梁的振动都具有减振作用,而且同一减振器在小于1Hz的振动频率范围内减振效果最好;实验中同种结构参数的减振器当使用饱和磁化强度为27.01kA/m的磁性液体时达到了最好的减振效果;此外,磁性液体阻尼减振器对弹性悬臂梁的减振作用分别随着其中永磁体半径和永磁体孔半径的增大而增大,而且永磁体与外壳间有一最佳间隙,使其在其它参数相同时对悬臂梁的减振作用达到最大。     

内蕴时间理论用于NOPD板结构响应计算的研究

非阻塞性微颗粒阻尼（NOPD）是一种在传统颗粒阻尼和冲击阻尼技术基础上发展起来的适用于恶劣环境的复合阻尼新技术，具有良好的减振效果，本文利用内蕴时间理论对NOPD板结构响应进行了分析，并在此基础上进行了仿真计算和实验验证，结果表明，将内时理论应用于NOPD结构响应计算分析是可行的，为NOPD的工程应用提供了一种有效的分析方法。     

Investigation of particle damping mechanism via particle dynamics simulations

Damping systems using granular particles as the damping medium are promising for application in extreme temperature conditions. In particle-based damping systems, mechanical energy is dissipated through inelastic collisions and friction between particles. In this work, we use particle dynamics simulations to investigate and compare the damping mechanisms of a piston-type thrust damper and a box-type oscillation damper. The mechanisms of energy transfer and energy dissipation are investigated. The roles of friction and inelastic collisions, as well as the wall effects in energy dissipation, are examined. The simulation results provide better understanding of the particle damping mechanisms, which may help in the design of next generation particle damping devices.     

Effects of momentum transfer on particle dispersions of dense gas–particle two-phase turbulent flows

A modified momentum transfer coefficient of dense gas–particle two-phase turbulent flows is developed and its effect on particle dispersion characteristics in high particle concentration turbulent downer flows has been numerically simulated incorporating into a second-order moment (USM) two-phase turbulent model and the kinetic theory of granular flow (KTGF) to consider particle–particle collisions. The particle fractions, the time-averaged axial particle velocity, the particle velocities fluctuation, and their correlations between gas and particle phases based on the anisotropic behaviors and the particle collision frequency are obtained and compared using traditional momentum transfer coefficients proposed by Wen (1966), Difelice (1985), Lu (2003) and Beetstra (2007). Predicted results of presented model are in good agreement with experimental measurement by Wang et al. (1992). The particle fluctuation velocity and its fluctuation velocity correlations along axial–axial and radial–radial directions have stronger anisotropic behaviors. Furthermore, the presented model is in a better accordance with Lu’s model in light of particle axial velocity fluctuation, particle temperature, particle kinetic energy and correlations of particle–gas axial–axial velocity fluctuation. Also, they are larger than those of other models. Beetstra’s model is not suitable for this downer simulation due to the relative lower particle volume fraction, particle collision and particle kinetic energy.     

NOPD的椭球状散体元建模

提出了能模拟不同颗粒形状的微颗粒组合体的椭球状散体元模型,从理论上解决了椭球单元的接触判断及运动学、动力学问题。采用该模型对NOPD复合阻尼结构进行了动力学计算,计算结果与实验结果具有很好的一致性,为进一步研究NOPD的阻尼机理提供了一种理论分析方法。     

The Influence of the Gravity Force on Longwave Marangoni Patterns in Two-Layer Films

An Euler–Euler two-fluid model based on the second-order-moment closure approach and the granular kinetic theory of dense gas-particle flows was presented. Anisotropy of gas-solid two-phase stress and the interaction between two-phase stresses are fully considered by two-phase Reynolds stress model and the transport equation of two-phase stress correlation. Under the microgravity space environments, hydrodynamic characters and particle dispersion behaviors of dense gas-particle turbulence flows are numerically simulated. Simulation results of particle concentration and particle velocity are in good agreement with measurement data under earth gravity environment. Decreased gravity can decrease the particle dispersion and can weaken the particle–particle collision as well as it is in favor of producing isotropic flow structures. Moreover, axial–axial fluctuation velocity correlation of gas and particle in earth gravity is approximately 3.0 times greater than those of microgravity and it is smaller than axial particle velocity fluctuation due to larger particle inertia and the larger particle turbulence diffusions.     

A dynamic cluster structure-dependent drag coefficient model applied to the riser in high density circulating fluidized bed

A dynamic cluster structure dependent (DCSD) drag model based on energy dissipation minimization using the two-fluid model (TFM) and the kinetic theory of granular flow (KTGF) is applied to the riser in high density circulating fluidized bed (HDCFB). The characteristics of gas and solid are predicted by means of the DCSD drag model with the convective and local accelerations. The law of granular collision energy dissipation and drag energy dissipation are compared and the influence of granular collision energy dissipation to agglomerate structure is researched. The study shows that granular collision energy dissipation has great influence on heterogeneous structure of riser. Simulated solid volume fraction, axial pressure drop and solid mass flux are compared to experimental measurements, numerical analysis suggests that the temporal-spatial heterogeneous structure characteristics and particle collision affect the flow of the riser in the literature.     

Edelen’s dissipation potentials and the visco-plasticity of particulate media

Following is an elaboration on D. G. B. Edelen’s (1972–1973) nonlinear generalization of the classical Rayleigh-Onsager dissipation potentials and the implications for the models of viscoplasticity. A brief derivation is given via standard vector calculus of Edelen’s potentials and the associated non-dissipative or “gyroscopic” forces and fluxes. It is also shown that certain extensions of Edelen’s formulae can be obtained by means of a recently proposed source-flux relation or “inverse divergence,” a generalization of the classical Gauss-Maxwell construct. The Legendre–Fenchel duality of Edelen’s potentials is explored, with important consequences for rate-independent friction or plasticity. The use of dissipation potentials serves to facilitate the development of viscoplastic constitutive equations, a point illustrated here by the special cases of Stokesian fluid-particle suspensions and granular media. In particular, we consider inhomogeneous systems with particle migration coupled to gradients in particle concentration, strain rate, and fabric. Employing a mixture-theoretic treatment of Stokesian suspensions, one is able to identify particle stress as the work conjugate of the global deformation of the particle phase. However, in contrast to past treatments, this stress is not assumed to be a privileged driving force for particle migration. A comparison is made with models based on extremal dissipation or entropy production. It is shown that such models yield the correct dissipative components of force or flux but generally fail to capture certain non-dissipative, but mechanically relevant components. The significance of Edelen’s gyroscopic forces and their relation to reactive constraints or other reversible couplings is touched upon. When gyroscopic terms are absent, one obtains a class of strongly dissipative or hyperdissipative materials whose quasi-static mechanics are governed by variational principles based on dissipation potential. This provides an interesting analog to elastostatic variational principles based on strain energy for hyperelastic materials and to the associated material instabilities arising from loss of convexity.     

The role of contact friction in the dynamic breakage behavior of granular materials

Based on the discrete element method, a multi-scale model is employed to investigate the role of contact friction in the dynamic compression responses of brittle granular materials. Four numerical granular samples with different particle friction coefficients ranging from 0.0 to 2.0 are tested and the particle breakage extent is quantified with the Einav breakage index. It is observed that the relationship of the breakage extent with the axial stress is apparently non-monotonic concerning the particle friction coefficient. At the same stress level, the breakage extent exhibits a minimum when the particle friction coefficient is around 0.1 but increases significantly with the particle friction coefficient to both sides. The micro physical origin of this non-monotonic behavior is a distinct transition in dominant particle-breakage modes from tension to shear. Moreover, energy analyses also show non-monotonic evolution of the frictional and damping dissipation with the particle friction coefficient. The joint effect of these two dissipation terms contributes to the non-monotonic behavior of particle breakage. In addition, the accuracy and competence of two frequently-used micro quantities, fraction of sliding contacts and average coordination number, are discussed.

     

NOPD板减振特性的理论及实验研究

NOPD（Non-Obstructive Particla Damping)又称粉体阻尼技术是90年代发展起来的一种新的被动减振技术。本文以四边自由的等截面板为研究对象,从微小颗粒的摩擦和冲击两个方面建立其减振理论模型,实现对振动响应的数值仿真并进行了实验验证,对NOPD的减振特性进行了较深入的研究。     

紊流风场中开孔结构的孔口阻尼特性研究

利用伯努利方程导出了孔口粘滞阻尼比的表达式，并通过能量耗散原理得到了孔口阻尼比与开孔屋盖系统模态阻尼比的关系。设计制作了具有不同开孔率的两个平屋面气动弹性模型，进行了突然开孔后屋面风振稳态响应的风洞试验，利用经验模式分解(EMD)、随机减量技术(RDT)以及希尔伯特变换(HT)成功地提取了开孔屋盖系统第一模态的阻尼比，并与理论结果进行了比较，最终得出在紊流风场中孔口阻尼以粘滞阻尼为主的结论。论文最后根据粘滞阻尼的特性给出了开孔率的准确定义，并导出了临界开孔率的公式，为建筑抗风设计提供了重要的参考依据。