Consolidation of concentrated suspensions – shear and irreversible floc structure rearrangement

A mathematical model for a consolidation process of a highly concentrated, flocculated suspension is developed. The suspension is treated as a mixture of a fluid and solid particles by an Eulerian two-phase fluid model. We characterize the suspension by constitutive relations correlating the stresses, interaction forces, and inter-particle forces to concentration and velocity gradients. Irreversibility of the permeability and yield stress is modeled by a memory function. A simulation program using finite difference methods in time and space is applied to a two dimensional test case. Numerical experiments are carried out on consolidation under shear and gravity in a 2D box with moving bottom. Received: 30 January 2001 / Accepted: 30 May 2001     

A Mixed Suspension System for a Half-Car Vehicle Model

Inthis paper we realize the design of a mixed suspension system(an actuator in tandem with a conventional passive suspension)for the axletree of a road vehicle based on a linear model withfour degrees of freedom. We propose an optimal control law thataims to optimize the suspension performance while ensuring thatthe magnitude of the forces generated by the two actuators andthe total forces applied between wheel and body never exceedgiven bounds. The solution we derive takes the form of an adaptivecontrol law that switches between different constant state feedbackgains. The results of our simulations show that the bound onthe active forces is a design parameter useful for establishinga trade-off between performance and power requirement.     

Theoretical analysis of a new, efficient microfluidic magnetic bead separator based on magnetic structures on multiple length scales

We present a theoretical analysis of a new design for microfluidic magnetic bead separation. It combines an external array of mm-sized permanent magnets with magnetization directions alternating between up and down with μm-sized soft magnetic structures integrated in the bottom of the separation channel. The concept is studied analytically for simple representative geometries and by numerical simulation of an experimentally realistic system geometry. The array of permanent magnets provides long-range magnetic forces that attract the beads to the channel bottom, while the soft magnetic elements provide strong local retaining forces that prevent captured beads from being torn loose by the fluid drag. The addition of the soft magnetic elements increases the maximum retaining force by two orders of magnitude. The design is scalable and provides an efficient and simple solution to the capture of large amounts of magnetic beads on a microsystem platform.     

Numerical simulation of electrorheological fluids based on an extended Bingham model

In the framework of the macroscopic simulation of electrorheological fluids, we present an extension of the classical Bingham model which goes beyond pure shear flows and thus enables the simulation of settings in more complex geometries. Emphasis is on the numerical solution of the resulting nonsmooth minimization problem. We propose the method of augmented Lagrangians combined with an operator-splitting technique which allows to confine the nonlinearity to local, low-dimensional problems. Numerical results are given that illustrate the electrorheological effects for various shear rates and electric field strengths in case of an electrorheological suspension rotating between two revolving cylinders. Received: 9 April 1999 / Accepted: 23 July 1999     

Dissipative particle dynamics simulations for fibre suspensions in newtonian and viscoelastic fluids

A versatile model of fibre suspensions in Newtonian and viscoelastic fluids has been developed using dissipative particle dynamics method. The viscoelastic fluid is modelled by linear chains with linear connector spring force (the Oldroyd-B model), which is known to be a reasonable model for the so-called Boger fluid (a dilute suspension of polymer in a highly viscous solvent). The numerical results are in excellent agreement with the analytical results of the Oldroyd-B model in simple shear flow. An effective meso-scale model of fibre in DPD is proposed and then incorporated with simple Newtonian fluid and our Boger fluid to enable entirely study rheological properties of fibre suspensions in both Newtonian and viscoelastic solvents. The numerical results are well compared with available experimental data and other numerical models.     

板式气体吸收塔干板开车动态模拟方法

提出板式气体吸收塔干板开车动态模拟法,编写了用于求解板式气体吸收塔干板开车常微分方程组的MATLAB程序,并举例说明。本数学模型作了理论板、等温操作、恒摩尔持液假定,忽略持气量和塔板上的流体流动动态行为;针对开车实际过程,塔板一面从上到下逐板充液,一面逐一扩展常微分方程数,并巧妙地赋以各板常微分方程的积分初值,使动态解亦得以随时更新,直至塔底最后一板充液完成并顺利得出稳态解。本模型对于化学工程师了解板式气体吸收塔开车过程中塔板上持液量、液相组成和气相组成动态变化的细节具有重要意义。     

How to lift a box that is too large to fit between the knees

Many studies compared lifting techniques such as stoop and squat lifting. Results thus far show that when lifting a wide load, high back loads result, irrespective of the lifting technique applied. This study compared four lifting techniques in 11 male subjects lifting wide loads. One of these techniques, denoted as the weight lifters' technique (WLT), is characterised by a wide foot placement, moderate knee flexion and a straight but not upright trunk. Net moments were calculated with a 3-D linked segment model and spinal forces with an electromyographic-driven trunk model. When lifting the wide box at handles that allow a high grip position, the WLT resulted in over 20% lower compression forces than the free, squat and stoop lifting technique, mainly due to a smaller horizontal distance between the l5S1 joint and the load. When lifting the wide box at the bottom, none of the lifting techniques was clearly superior to the others.

Statement of Relevance: Lifting low-lying and large objects results in high back loads and may therefore result in a high risk of developing low back pain. This study compares the utility of a WLT, in terms of back load and lumbar flexion, to more familiar techniques in these high-risk lifting tasks.     

Lift and multiple equilibrium positions of a single particle in Newtonian and Oldroyd-B fluids

A heavy particle is lifted from the bottom of a channel in a plane Poiseuille flow when the Reynolds number is larger than a critical value. In this paper we obtain correlations for lift-off of particles in Oldroyd-B fluids. The fluid elasticity reduces the critical shear Reynolds number for lift-off. The effect of the gap size between the particle and the wall, on the lift force, is also studied. A particle lifted from the channel wall attains an equilibrium height at which its buoyant weight is balanced by the hydrodynamic lift force. Choi and Joseph [Choi HG, Joseph DD. Fluidization by lift of 300 circular particles in plane Poiseuille flow by direct numerical simulation. J Fluid Mech 2001;438:101-128] first observed multiple equilibrium positions for a particle in Newtonian fluids. We report several new results for the Newtonian fluid case based on a detailed study of the multiple equilibrium solutions, e.g. we find that at a given Reynolds number there are regions inside the channel where no particle, irrespective of its weight, can attain a stable equilibrium position. This would result in particle-depleted zones in channels with Poiseuille flows of a dilute suspension of particles of varying densities. Multiple equilibrium positions of particles are also found in Oldroyd-B fluids. All the results in this paper are based on 2D direct numerical simulations.     

公路清扫车集尘系统仿真设计与优化

运用计算流体力学技术对高速公路清扫车集尘系统的流场进行了仿真分析与结构改进。仿真采用有限体积法FVM，利用非结构化网格对集尘箱模型进行网格划分，采用k-ε湍流双方程模型模拟湍流流动。分析不同的集尘箱设计方案对灰尘沉降效率的影响，经过一系列优化，最后得到一个优化的设计方案。新集尘箱延长了气流在集尘箱内的停留时间，降低了靠近底部位置的气流速度，有利于垃圾的沉降与堆积，达到了实际设计要求，为开发具有自主知识产权高效率的公路清扫车做出了重要贡献。     

Parallel implementation of the fluid particle model for simulating complex fluids in the mesoscale

Dissipative particle dynamics (DPD) and its generalization—the fluid particle model (FPM)—represent the ‘fluid particle’ approach for simulating fluid‐like behavior in the mesoscale. Unlike particles from the molecular dynamics (MD) method, the ‘fluid particle’ can be viewed as a ‘droplet’ consisting of liquid molecules. In the FPM, ‘fluid particles’ interact by both central and non‐central, short‐range forces with conservative, dissipative and Brownian character. In comparison to MD, the FPM method in three dimensions requires two to three times more memory load and a three times greater communication overhead. Computational load per step per particle is comparable to MD due to the shorter interaction range allowed between ‘fluid particles’ than between MD atoms. The classical linked‐cells technique and decomposing the computational box into strips allow for rapid modifications of the code and for implementing non‐cubic computational boxes. We show that the efficiency of the FPM code depends strongly on the number of particles simulated, the geometry of the box and the computer architecture. We give a few examples from long FPM simulations involving up to 8 million fluid particles and 32 processors. Results from FPM simulations in three dimensions of the phase separation in binary fluid and dispersion of the colloidal slab are presented. A scaling law for symmetric quench in phase separation has been properly reconstructed. We also show that the microstructure of dispersed fluid depends strongly on the contrast between the kinematic viscosities of this fluid phase and the bulk phase. This FPM code can be applied for simulating mesoscopic flow dynamics in capillary pipes or critical flow phenomena in narrow blood vessels. Copyright © 2002 John Wiley & Sons, Ltd.