Comparative study of two fluid model and dense discrete phase model for simulations of gas-solid hydrodynamics in circulating fluidized beds

Computational fluid dynamics(CFD)has become a valuable tool to study the complex gas-solid hydrodynamics in the circulating fluidized bed(CFB).Based on the two fluid model(TFM)under the Eulerian-Eulerian framework and the dense discrete phase model(DDPM)under the Eulerian-Lagrangian framework,this work conducts the comparative study of the gas-solid hydrodynamics in a CFB riser by these two different models.Results show that DDPM could be used to predict gas-solid hydrodynamics in the circulating fluidized bed,and there are differences between TFM and DDPM,especially in the radial distribution profiles of solid phase.Sensitivity analysis results show that the gas-solid drag model exhibits significant effects on the results for both the two models.The specularity coefficient and the restitution coefficient in the TFM,as well as the reflection coefficient and the parcel number in the DDPM,exhibit less impact on the simulated results.     

Torsional static and vibration analysis of functionally graded nanotube with bi-Helmholtz kernel based stress-driven nonlocal integral model

A torsional static and free vibration analysis of the functionally graded nanotube(FGNT)composed of two materials varying continuously according to the power-law along the radial direction is performed using the bi-Helmholtz kernel based stress-driven nonlocal integral model.The differential governing equation and boundary conditions are deduced on the basis of Hamilton’s principle,and the constitutive relationship is expressed as an integral equation with the bi-Helmholtz kernel.Several nominal variables are introduced to simplify the differential governing equation,integral constitutive equation,and boundary conditions.Rather than transforming the constitutive equation from integral to differential forms,the Laplace transformation is used directly to solve the integro-differential equations.The explicit expression for nominal torsional rotation and torque contains four unknown constants,which can be determined with the help of two boundary conditions and two extra constraints from the integral constitutive relation.A few benchmarked examples are solved to illustrate the nonlocal influence on the static torsion of a clamped-clamped(CC)FGNT under torsional constraints and a clamped-free(CF)FGNT under concentrated and uniformly distributed torques as well as the torsional free vibration of an FGNT under different boundary conditions.     

Analysis on nonlinear effect of unsteady percolation in the inhomogeneous shale gas reservoir

The nonlinear effects of unsteady multi-scale shale gas percolation,such as desorption,slippage,diffusion,pressure-dependent viscosity,and compressibility,are investigated by numerical simulation.A new general mathematical model of the problem is built,in which the Gaussian distribution is used to describe the inhomogeneous intrinsic permeability.Based on the Boltzmann transformation,an efficient semi-analytical method is proposed.The problem is then converted into a nonlinear equation in an integral form for the pressure field,and a related explicit iteration scheme is constructed by numerical discretization.The validation examples show that the proposed method has good convergence,and the simulation results also agree well with the results obtained from both numerical and actual data of two vertical fractured test wells in the literature.Desorption,slippage,and diffusion have significant influence on shale gas flows.The accuracy of the usual technique that the product of viscosity and compressibility is approximated as its value at the average formation pressure is examined.     

Active control of multiscale features in wall-bounded turbulence

This study experimentally investigates the impact of a single piezoelectric(PZT)actuator on a turbulent boundary layer from a statistical viewpoint.The working conditions of the actuator include a range of frequencies and amplitudes.The streamwise velocity signals in the turbulent boundary layer flow are measured downstream of the actuator using a hot-wire anemometer.The mean velocity profiles and other basic parameters are reported.Spectra results obtained by discrete wavelet decomposition indicate that the PZT vibration primarily influences the near-wall region.The turbulent intensities at different scales suggest that the actuator redistributes the near-wall turbulent energy.The skewness and flatness distributions show that the actuator effectively alters the sweep events and reduces intermittency at smaller scales.Moreover,under the impact of the PZT actuator,the symmetry of vibration scales’velocity signals is promoted and the structural composition appears in an orderly manner.Probability distribution function results indicate that perturbation causes the fluctuations in vibration scales and smaller scales with high intensity and low intermittency.Based on the flatness factor,the bursting process is also detected.The vibrations reduce the relative intensities of the burst events,indicating that the streamwise vortices in the buffer layer experience direct interference due to the PZT control.     

Asymptotics for filtration of polydisperse suspension with small impurities

A model for deep bed filtration of a polydisperse suspension with small impurities in a porous medium is considered.Different suspended particles move with the same velocity as the carrier water and get blocked in the pore throats due to the size-exclusion mechanism of particle retention.A solution of the model in the form of a traveling wave is obtained.The global exact solution for a multiparticle filtration with one high concentration and several low concentrations of suspended particles is obtained in an explicit form.The analytic solutions for a bidisperse suspension with large and small particles are constructed.The profiles of the retained small particles change monotony with time.The global asymptotics for the filtration of a polydisperse suspension with small kinetic rates is constructed in the whole filtration zone.     

一种Cartesian气动网格的自适应划分算法

提出了针对空间Cartesian结构化网格的自适应划分算法,以满足飞机总体设计过程中进行多学科设计优化时根据方案变化生成CFD计算网格的需要,使气动特性能够得以自动化分析,并且保证优化迭代过程的顺利进行.分析了Cartesian网格划分过程中的两个关键问题,并给出了解决方法.在此基础之上,从理论上推导了能够自动适应飞机布局形式以及几何尺寸任意变化的Cartesian网格生成算法和实际划分步骤,并给出了其中部分主要的计算公式,同时论述了自适应划分过程中的若干重要问题.阐述了自适应划分所需的飞机及其部件特征尺寸的获取途径.最后展示了几个由自适应划分算法生成的Cartesian网格实例以说明其实际应用.     

低温超导核磁共振陀螺仪模型

建立了单工作物质的三自由度3He低温超导核磁共振陀螺仪结构;利用了量子力学和经典动力学,经过严密的力学分析和数学演算,给出了三轴陀螺仪的工作原理和结构示意图;针对陀螺仪的交叉轴角速率耦合问题,给出了附加磁场线圈解耦法和冗余设计结构,最后建立了基于超导量子干涉仪探测磁矩,并采用最小二乘估计法来推导陀螺进动频率.陀螺仪测速范围可以达到10-9～103 rad/s,漂移为10-4 (°)/h.该陀螺仪结合低温超导技术具有高精度的前景.需要进一步对超导量子干涉磁矩检测仪的精度与陀螺性能进行研究.     

DEM investigation of macro-and micro-mechanical properties of rigid-grain and soft-chip mixtures

We investigated the macro-and micro-mechanical properties of rigid-grain and soft-chip mixtures(GCMs)through numerical simulations using the discrete element method.We present a novel framework for the discrete modeling of soft chips and rigid grains in conjunction with calibration processes.Several numerical triaxial tests were also performed on GCMs with 0%,10%,20%,and 30%volumetric chip contents,P.The simulation results demonstrate that increasing P leads to higher GCM toughness,higher deviatoric peak stress,and higher corresponding shear strain.Higher P also contributes to more volume contraction and less dilation.The friction angles at both the peak and residual state significantly increase with increasing P.In view of the micro-mechanical features,strong contact force chains develop along the loading direction,which results in considerable anisotropy in the peak and residual states.Both the formation of strong force chains and rotation of grains decrease with increasing P,whereas the grain sliding percentage increases.The tensile force is mobilized with shearing and higher P leads to less mobilization of the tensile force.These findings are useful for better understanding the internal structure of GCMs with different soft-chip contents,especially in granular mixture mechanics and geomechanics.     

Interaction effects on heat conduction and thermal stress in an infinite elastic plane with two circular holes

Numerous researches have focused on the physical behavior of an elastic material in the vicinity of a single hole under the assumption that the interaction effects arising from the introduction of multiple holes remain negligible if the holes are placed sufficiently far from each other.In an effort to understand hole interaction effects on heat conduction and thermal stress,we consider the case when two circular holes are embedded in an infinite elastic material and use complex variable methods together with numerical analysis to obtain solutions describing temperature and elastic fields in the vicinity of the two circular holes.The results indicate that the interaction effects on temperature distribution and stress strongly depend on the relative size of the two holes and the distance placed between them but not on the actual size of the holes.     

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编辑部的同志要我写一点回忆,以纪念<力学与实践>创刊30年.往事悠悠,使我又回到20世纪70年代那动荡不安,却又是激情四射的年代.     

向蔡树棠先生学习什么——写在送别蔡先生这一天

题记蔡树棠教授离开我们已近半年,他的音容笑貌还时不时地浮现在我的脑海里.我刚从中国科学技术大学回来,蔡先生曾在该校工作多年,而那里的几位年青学子居然不知道蔡树棠这个名字,这使我有点悲哀.这里公布我写于送别他那天的短文<向蔡树棠先生学习什么>,希望大家记住这位曾在力学园地耕耘了半个多世纪的知名学者.     

三维正交机织复合材料的力学性能研究

利用材料试验机装置开展了三维正交机织Kevlar/乙烯基树脂复合材料力学实验,实验表明面内经向拉伸和厚度z方向压缩时,材料具有一定的应变率效应,厚度z方向压缩强度远大于面内压缩强度,而面内经向和纬向的拉伸强度则相当.结合纤维的编织方式,对这些现象做了必要的定性分析.最后,本文基于三维正交机织复合材料的结构特点,推导了一组可用于估算三维正交机织复合材料弹性模量的公式,估算结果和实验值接近.     

Drop-weight impact fragmentation of gas-containing coal particles

Research on coal fragmentation can play an important role in understanding coal and gas outbursts.The study discussed in this paper explored the fragmentation of gas-containing coal particles using the drop-weight impact method.The effects of equilibrium gas pressures and type of adsorbate gas on particle size distributions and fragmentation energy were investigated in detail.We found that the Fractal particle size distribution model can most effectively describe the crushed coal particle sizes.The equilibrium pressure and type of gas can influence the Fractal distribution parameter.The crushing energy is composed of energy to create new surfaces and other forms of energy that are dissipated but the equilibrium gas pressure and type of adsorption gas can affect energy consumption and crushing efficiency.This research will be of guiding significance to the intensity evaluation and mechanism understanding of coal and gas outbursts.     

Magnetic field effects on the nonlinear vibration of a rotor

The nonlinear vibration of a rotor operated in a magnetic field with geometric and inertia nonlinearity is investigated. An asymmetric magnetic flux density is generated,resulting in the production of a load on the rotor since the air-gap distribution between the rotor and the stator is not uniform. This electromagnetic load is a nonlinear function of the distance between the geometric centers of the rotor and the stator. The nonlinear equation of motion is obtained by the inclusion of the nonlinearity in the inertia, the curvature, and the electromagnetic load. After discretization of the governing partial differential equations by the Galerkin method, the multiple-scale perturbation method is used to derive the approximate solutions to the equations. In the numerical results, the effects of the electromagnetic parameter load, the damping coefficient, the amplitude of the initial displacement, the mass moment of inertia, and the rotation speed on the linear and nonlinear backward and forward frequencies are investigated. The results show that the magnetic field has significant effects on the nonlinear frequency of oscillation.     

A micromechanical model based on hypersingular integro-differential equations for analyzing micro-crazed interfaces between dissimilar elastic materials

The current work models a weak(soft) interface between two elastic materials as containing a periodic array of micro-crazes. The boundary conditions on the interfacial micro-crazes are formulated in terms of a system of hypersingular integro-differential equations with unknown functions given by the displacement jumps across opposite faces of the micro-crazes. Once the displacement jumps are obtained by approximately solving the integro-differential equations, the effective stiffness of the micro-crazed interface can be readily computed. The effective stiffness is an important quantity needed for expressing the interfacial conditions in the spring-like macro-model of soft interfaces. Specific case studies are conducted to gain physical insights into how the effective stiffness of the interface may be influenced by the details of the interfacial micro-crazes.     

Dynamic analysis of geared transmission system for wind turbines with mixed aleatory and epistemic uncertainties

This paper deals with the co-existence of mixed aleatory and epistemic uncertainties in a wind turbine geared system for more reliable and robust vibration analyses. To this end, the regression-based polynomial chaos expansion(PCE) is used to track aleatory uncertainties, and the polynomial surrogate approach(PSA) is developed to treat the epistemic uncertainties. This non-intrusive dual-layer framework shares the same collocation pool, which is extracted from the Legendre series. Moreover, the ...     

Inclusion of the tunneling phase shift for interferometric particle imaging for bubble sizing

The interferometric particle imaging technique makes use of the angular oscillations of the scattered light in the forward direction for droplet or bubble sizing.The out-of-focus image consists of fringes,the spacing of which reflects the interference between the surface-reflected light and the twofold-refracted light.Total internal reflection occurs when the incident light hits the bubble at a large incident angle.The tunneling phase shift is not included in the geometric optics approximation,which leads to a deviation from Mie theory.In this work,we modified the formula for describing the fringe spacing by including the tunneling phase shift of total internal reflection.Numerical analysis and experiments showed that the modification is effective for the measurement of bubbles smaller than 60μm.     

Charge transfer and bipolar charging of particles in a bubbling fluidized bed

Particle-particle and particle-wall collisions in gas-solid fluidized beds lead to charge accumulation on particles.This work evaluated the effect of fluidization time on charge transfer and bipolar charging(charge separation)and their influence on hydrodynamic structures in a fluidized bed.Experiments were performed with glass beads and polyethylene particles in a glass column.The pressure fluctuations and net electrostatic charge of particles were measured during fluidization.Wavelet and short-time Fourier transforms were used to analyze pressure fluctuations.The results revealed that bipolar charging is the dominant tribocharging mechanism in a bed of glass beads.Bipolar charging in a bed of particles with a narrow size distribution does not affect either hydrodynamic structures or the transition velocity to the turbulent regime.A large difference between the work functions of the wall and particle in the bed of polyethylene particles leads to high charge transfer.Formation of a stagnant particle layer on the wall eventually causes the energy of macro-structures to increase to its maximum.At longer fluidization times,the macro-structural energy decreases and bubbles shrink until the electrostatic charge reaches the equilibrium level.These results well describe the effect of fluidization time on hydrodynamic structures.     

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<力学与实践>走过了它30个年头了.我是一直陪伴着它走过来的. 第一届编委会的时候,我是编委.在主编卞荫贵,副主编朱照宣领导下做些具体工作.