Motion Of Suspended Particles Research Articles

多粒子分散系への適用のための格子ボルツマン法の基礎研究 : 2次元円形粒子まわりの一様流を対象とした検討(流体工学,流体機械)

We have conducted the basic study on the lattice Boltzmann method from the viewpoint of applying it to many particle suspensions, by considering a uniform flow past a two-dimensional circular particle (i.e., cylinder). Successful application of the lattice Boltzmann method to particle dispersions is strongly dependent on what rough lattice system can be used, since the motion of suspended particles and the flow field have to be solved simultaneously; unless a rough lattice system is used, suspensions with sufficient number of particles cannot be treated in simulations. Hence, the present study mainly discusses the dependence of the flow field on the fineness of the lattice system, and the validity of several boundary models, including a revised boundary one presented here. Even if a significantly rough lattice system with the lattice unit width of one-fifth as long as the particle diameter is used, the results give rise to physically reasonable agreement with the Navier-Stokes solutions. We may, therefore, conclude that the lattice Boltzmann method is a promising tool as a micro-simulation technique with multi-body hydrodynamic interactions for many particle suspensions.

Effect of viscosity of base fluid on thermal conductivity of nanofluids

This study aims to investigate the effect of viscosity of the base fluid on the thermal conductivity of nanofluids in which Fe3O4 nanoparticles are suspended in the base fluid composed of diesel oil and polydimethylsiloxane. Viscosity of the base fluid is varied by changing the volumetric fractions between both fluids. The measured thermal conductivity of nanofluids gradually approaches the value predicted by the Maxwell equation by increasing the viscosity. It demonstrates that the viscosity of nanofluids does affect the thermal conductivity of nanofluids, and the Brownian motion of suspended particles could be an important factor that enhances the thermal conductivity of nanofluids.

Continuous hydrophoretic separation and sizing of microparticles using slanted obstacles in a microchannel

We report a microfluidic separation and sizing method of microparticles with hydrophoresis--the movement of suspended particles under the influence of a microstructure-induced pressure field. By exploiting slanted obstacles in a microchannel, we can generate a lateral pressure gradient so that microparticles can be deflected and arranged along the lateral flows induced by the gradient. Using such movements of particles, we completely separated polystyrene microbeads with 9 and 12 microm diameters. Also, we discriminated polystyrene microbeads with diameter differences of approximately 7.3%. Additionally, we measured the diameter of 10.4 microm beads with high coefficient of variation and compared the result with a conventional laser diffraction method. The slanted obstacle as a microfluidic control element in a microchannel is analogous to the electric, magnetic, optical, or acoustic counterparts in that their function is to generate a field gradient. Since our method is based on intrinsic pressure fields, we could eliminate the need for external potential fields to induce the movement of particles. Therefore, our hydrophoretic method will offer a new opportunity for power-free and biocompatible particle control within integrated microfluidic devices.

Gas‐solid Two‐phase Mixtures Flowing Upward through a Confined Packed Bed

AbstractThis paper deals with flows of a gas‐solid two‐phase mixture through a confined packed bed. Both experimental work and numerical modelling are performed on the behaviour of suspended particles within the packed bed. The experimental work is carried out with a non‐intrusive Positron Emission Particle Tracking (PEPT) technique, which tracks particle motion at the single particle level for a prolonged period thus allows both the microscopic and macroscopic solids behaviour to be analysed under the steady‐state conditions. A continuous based model is used to simulate the flow behaviour. The model uses a newly proposed porosity model and treats the suspended and packed particles as a binary mixture with the packed particles being at zero velocity. The results show that the model captures the main features of solids behaviour in terms of the radial distributions of the suspended particle concentration and the axial solids velocity. Both the experiments and modelling suggest that the wall effect on the motion of suspended particles be limited to a small region close to the wall (∼0.5–1 packed particle diameter). However, deviations exist between the model predictions and experiments; more work is therefore proposed to improve the interaction terms in the model between the suspended and packed particles.

The role of electrode impedance and electrode geometry in the design of microelectrode systems

Microelectromechanical systems (MEMS) employing spatially and/or temporally nonuniform electric fields have been extensively employed to control the motion of suspended particles or fluid flow. Design and control of microelectromechanical processes require accurate calculations of the electric field distribution under varying electrolyte conditions. Polarization of electrodes under the application of an oscillating voltage difference produces dynamic electrical double layers. The capacitive nature of the double layers significantly inhibits the penetration of the electric field through the double layer and into the surrounding bulk electrolyte at low frequencies. This paper quantitatively discusses the effect of electrode impedance on the electric field distribution as a function of field frequency, electrolyte composition, and electrode zeta potential in microelectrode systems. The design principles for the electrode geometry and configuration are also discussed in terms of their effects on the electric field magnitude and nonuniformity.

Monitoring the Impact of Simulated Deep-sea Mining in Central Indian Basin

Monitoring of deep-sea disturbances, naturai or man-made, has gained significance due to the associated sediment transport and for the ensuing alterations in environmental conditions. During the Indian Deep-sea Environment Experiment (INDEX), resuspension of deep-sea sediment in the Central Indian Basin (CIB) resulted in an increase and lateral movement of suspended particles, vertical mixing of sediments, changes in sedimentological, biochemical, and geochemical conditions and an overall reduction in benthic biomass. Monitoring the conditions 44 months after the experiment has shown a partial recovery of the benthic ecosystem, with indications of restoration and recolonization.

Solids behaviour in a gas–solid two-phase mixture flowing through a packed particle bed

This paper reports the solids behaviour in a dilute gas–solid two-phase mixture flowing through a packed bed. The positron emission particle tracking (PEPT) technique was used in the work, which allowed investigation of three-dimensional solids motion at the single suspended particle level. Processing of the data gave solids velocity, the residence time of suspended particles, bed tortuosity in terms of solids motion, as well as solids occupancy in the cross-section of the packed bed. The results suggest that the wall effect on the motion of suspended particles is limited to approximately one packed particle diameter under the conditions of this work. Both the average axial and radial velocities of suspended particles, normalised by the superficial gas velocity, change periodically with radial position, but the periodicity does not correspond exactly to the packed particle diameter. The peak and trough values of the average axial velocity of the suspended particles in the bulk region are, respectively, ∼ 25 % and ∼ 15 % of the superficial gas velocity under the conditions of this work and the superficial gas velocity shows little effect. The peak and trough values of the average radial velocity of the suspended particles in the bulk region are, respectively, +5% (positive) and - 5 % (negative) of the superficial gas velocity. The results of the residence time and tortuosity of the suspended particles show an approximately Gaussian distribution with the peak residence time and tortuosity increasing with decreasing superficial gas velocity. The occupancy data suggest that particles spend more time in an annular region close to the wall, indicating a non-uniform particle distribution across the packed bed cross-section.

A CFD study of the deep bed filtration mechanism for submicron/nano-particle suspension

The mechanism of the deep bed filtration for submicron and nano particles suspension was conducted by means of a force analysis on the suspended particles flow path through order-packed granular filter beds. The flow fields through the filter beds were calculated by using the commercial available CFD software--Fluent. Various types of granular packing structures, such as the simple cubic packing, body-centered packing and face-centered packing structures were chosen for analysis. The motion of suspended particle of 2.967, 0.816, 0.460 and 0.050 microm in diameter, respectively, were tracked by considering the following forces including a net gravitational force, hydraulic drag force, lift force, Brownian force, van der Waals force and a double layer force. The effects of the granular bed packing structure, the porosity of these beds and the suspended particle diameter on the capture efficiency of a granular filter bed were examined. The force analysis depicts that the inertial effect and van der Waals force increased the capture probability of particles on the granular filter bed while the lift force and the Brownian force decreased the particle deposition. Simulated results show that among the chosen packing structures, the face-center packed granular bed gives the greatest pressure drop and capture efficiency of particles due to the lower packing porosity. The simple cubic packed filter bed showed the lowest pressure drop and capture efficiency of particles due to the greatest packing porosity among the chosen packing structures. It is mainly due to the simple cubic packing structure in which there exists the free vertical downward flowing path and thus exhibits a higher packing porosity. The comparisons of the simulated capture efficiency with experimental results depicted that the body-centre packed granular bed showed the best approximation of capture efficiency compared to that of the randomly packed granular bed.

Mathematical modeling of the movement of suspended particles subjected to acoustic and flow fields

When particles are subjected to an acoustic field particle trajectories depend on the particle and fluid compressibility and density values. Hence a combination of acoustic and flow fields on particles can be used to deflect and trap, or to segregate and/or fractionate fine particles in fluid suspensions. Using particle physics in an acoustic field, a mathematical model was developed to calculate trajectories of deflected particles due to the application of acoustic standing waves. The resulting second order ordinary differential equation was quite stiff and hence difficult to solve numerically and did not have a closed form solution. The analysis of the above equation showed that the basic problems with numerical solutions could not be ameliorated through the use of standard rescaling techniques. A combination of phase space and asymptotic analysis turns out to be far more useful in obtaining approximate solutions. An approximate solution was derived which enabled the calculation of the particle trajectories and concentration at collection planes in the acoustic field. Analysis of the solution showed that all the particles move toward the pressure node to which the particles are supposed to move. Particles with 2 μm diameter took approximately 20 s to reach that node. Then at the bench scale, the above technology was implemented by building a flow chamber with two transducers at opposite ends to generate an acoustic standing wave. SiC particle trajectories were tracked using captured digital images from a high-resolution microscope. The displacements of SiC particles due to an acoustic force were compared with the mathematical model predictions. For input power levels between 3.0 and 5.0 W, the experimental data were comparable to mathematical model predictions. Hence it was concluded that the proposed approximate solution was both quantitatively and qualitatively closer to experimental results than the simplified form ignoring the second order term reported in the literature.

Numerical simulation of sediment transport in harbors

Numerical simulation of sediment transport is a coupled problem based on computation of profiles of water velocity and suspended sediment concentration. Effects of the water velocity profile and turbulent pulsation on vertical motion of suspended particles are emphasized in this study. Explanation of amplified sediment deposit in zones of separated flow near both natural and manmade bottom irregularities (like walls of shipping channels) is given. A semi-empirical approach to determination of long-term sediment deposit/erosion is suggested.

Oscillatory and chaotic thermocapillary convection in a half-zone liquid bridge

Thermocapillary-driven convection in a half-zone liquid bridge was investigated experimentally. The induced flows were categorized into several regimes mainly through the pattern of the suspended particle motion in the bridge and the surface temperature variation. Special attention was paid to the flow structures far beyond the critical condition. Chaotic and turbulent flows were realized in this configuration. They were distinguished from the periodic oscillatory flow by applying the pseudo-phase-space reconstruction from the time series of the surface temperature variation.

Various Flow Patterns Emerging in Oscillatory Thermocapillary Convection in a Liquid Bridge of High Prandtl Number Fluid

Thermocapillary-driven convection in a half-zone liquid bridge of high-Prandtl-number fluid was investigated experimentally. It is widely known that the flow field in the liquid bridge exhibits a transition from two-dimensional steady flow to three-dimensional 'oscillatory' one at a critical temperature difference. In this study, special attention was paid upon the flow structures far beyond the critical condition. Types of the induced flows were categorized into several regimes mainly according to the suspended particle motion in the bridge and the surface temperature variation. Chaotic and turbulent flows were realized in this configuration. The flow regimes were analyzed by applying the pseudo-phase-space reconstruction from the time series of the surface temperature, and the evaluations of the correlation dimension and the maximum Lyapunov exponent.

Numerical Simulation of Flow Fields and Particle Trajectories in Ciliary Suspension Feeding

A model describing the ciliary driven flow and motion of suspended particles in downstream suspension feeders is developed. The quasi-steady Stokes equations for creeping flow are solved numerically in an unbounded fluid domain around cylindrical bodies using a boundary integral formulation. The time-dependent flow is approximated with a continuous sequence of steady state creeping flow fields, where metachronously beating ciliary bands are modelled by linear combinations of singularity solutions to the Stokes equations. Generally, the computed flow fields can be divided into an unsteady region close to the driving ciliary bands and a steady region covering the remaining fluid domain. The size of the unsteady region appears to be comparable to the metachronal wavelength of the ciliary band. A systematic investigation is performed of trajectories of infinitely small (fluid) particles in the simulated unsteady ciliary driven flow. A fraction of particles appear to follow trajectories, that resemble experimentally observed particle capture events in the downstream feeding system of the polycheate Sabella penicillus, indicating that particles can be captured by ciliary systems without mechanical contact between particle and cilia. A local capture efficiency is defined and its value computed for various values of beat frequencies and other parameters. The results indicate that the simulated particle capture process is most effective when the flow field oscillates within timescales comparable to transit timescales of suspended particles passing the unsteady region near the ciliary bands. However, the computed retention efficiencies are found to be much lower than those obtained experimentally.

Brownian motion of suspended particles in an anisotropic medium

Some experimental results in the case of anisotropic media are at variance with the Einstein-Stokes formula of Brownian motion and the Singwi-Sjolander model. The disagreement comes from the anchor effect. The concept of “pseudoparticle” is introduced in order to retain the Singwi-Sjolander formalism. Mossbauer spectroscopy can be used to estimate the thickness of the anchored liquid-crystalline molecular textures forming on the surface of Brownian particles.

Hydraulic Model of Siltation of Northern Channel in Hangzhou Bay

In southeast China, along the northern coast of the Hangzhou Bay from Ganpu to Jinshan, there is a stable deep channel, the Northern Channel, which is suitable for harbor construction and navigation. The possible siltation effect of several reclamation projects in Qiantang Estuary on the channel has been investigated in a hydraulic model of suspended load and local movable bed in order to ensure reliable operation of some large ports and intakes for cooling water of power plants along the channel. In conformity with the similarity principles for suspended particle motion and threshold of particle motion, an anti-rot sawdust is chosen as the model sand. Each tidal cycle in the test was controlled based on a time scale related to flow rate. The sediment concentration scale and the number of tidal cycles were determined by means of calibration tests. The model is a useful engineering method for developing and regulating of Qiantang Estuary. Experience gained with the model may be of use for similar investigations of other tidal estuaries.

Viscoelastic behavior of bimodal suspensions

The dynamic viscoelastic behavior of suspensions with bimodal particle radius distributions was examined varying the ratio of the radii of large and small particles (rL/rS) up to 3.3. Monodisperse silica spherical particles with radii ranging from 65 to 215 nm were used. The medium was ethyleneglycol, possessing a refractive index very close to that of the used silica particles. The medium reduced effectively interparticle potentials due to dispersion forces, and it provided bimodal suspensions with only the hard core interparticle potential. The total weight fraction of the suspended particles was kept at 52 wt %, while the weight composition of the small particles (XS) to the total particulate mass was increased from 0 to 1 with a 0.2 interval. The bimodal suspensions showed viscoelastic behavior with a frequency dependence similar to that of a unimodal (monodisperse) suspension, which was attributed to the contribution of Brownian motion of suspended particles. The zero-shear viscosities (η0) of the bi...

Layer formation in monodispersive suspensions and colloids

We present theoretical results on spontaneous stratification of sedimenting suspensions and colloids caused by a lateral temperature gradient. Fluid motion is treated in the Stokes approximation, and motion of suspended particles is described by Burgers equation with convection. The internal structure and interaction of shocks at convection roll boundaries is studied numerically using a reduced one-dimensional model based on a Galerkin approach. Qualitative comparison is made to experimental data.

Sedimentation of Clusters of Spheres

We describe a numerical method for calculating hydrodynamic interactions between spherical particles efficiently and accurately, both for particles immersed in an infinite liquid and for systems with periodic boundary conditions. Our method is based on a multipole expansion in Cartesian tensors. We then show how to solve the equations of motion for translational and rotational motion of suspended particles at large Peclet numbers. As an example we study the sedimentation of an array of spheres with and without periodic boundary conditions. We also study the effect of perturbations on the stability of the trajectories.

A Mössbauer study of the Brownian motion of suspended particles in a nematic liquid crystal: the effect of anchored molecular textures

The Brownian motion of suspended particles in EBBA liquid crystal is studied by the Mössbauer technique. Different values are obtained for both the radius of the particles and their mean square displacement in the nematic phase and the isotropic one. This behavior is attributed to anchored molecular textures on the particles' surfaces. The texture thickness is estimated from the Mössbauer data.

Theoretical study of the Brownian-like motion of suspended particles in nematic liquid crystals

We studied the Brownian-like motion of suspended particles in nematic liquid crystals. It is found that the Langevin equation approach is an effective theoretical tool and the results fit quite well with experiments. Employing the suspended particles as probes, this investigation provides a new method for the measurement of the viscosity coefficients of nematic liquid crystals.