VISCOSITY EFFECTS ON THE BEHAVIOR OF A RISING BUBBLE

In the incompressible fluid flow regime, without taking consideration of surface tension effects, the viscosity effects on the behavior of an initially spherical buoyancy-driven bubble rising in an infinite and initially stationary liquid are investigated numerically by the Volume Of Fluid （VOF） method. The ratio of the gas density to the liquid density is taken as 0.001, and the gas viscosity to the liquid viscosity is 0.01, which is close to the case of an air bubble rising in water. It is found by numerical experiments that there exist two critical Reynolds numbers Re1 and Re2 , which are in between 30 and 50 and in between 10 and 20, respectively. As Re 〉 Re1 the bubble will have the transition to toroidal form, and the toroidal bubble will break down into two toroidal bubbles. In this case viscosity will damp the development of the liquid jet and delay the formation of the toroidal bubble. As Re〈Re1 the transition will not happen. As Re2 〈 Re 〈 Re1, the bubble will split from its rim into a toroidal bubble and a spherical cap-like bubble, and as Re〈Re2 the splitting will not occur and the bubble can finally reach a stationary shape. With the decrease of the Reynolds number, the stationary shape changes from spherical-cap bubble with skirt to dimpled peach-like bubble. Before the bubble reaches its stationary shape the vortex structure in the flow field varies with time. The vortex structure corresponding to bubble stationary shape varies with the Reynolds number. It is also found that there exists another critical Reynolds number Re^＊ which is in between Re1 and Re2 , and as Re 〈 Re^＊, after the bubble rises in an accelerating manner for a moment, it will rise with an almost constant speed, and the speed increases with increasing Reynolds number. As Re 〉 Re^＊, it will not rise with a constant speed. The mechanism of the above phenomena has been analyzed theoretically and numerically.

Modeling effective viscosity reduction behaviour of solid suspensions

Under a simple shearing flow, the effective viscosity of solid suspensions can be reduced by controlling the inclusion particle size or the number of inclusion particles in a unit volume. Based on the Stokes equation, the transformation field method is used to model the reduction behaviour of effective viscosity of solid suspensions theoretically by enlarging the particle size at a given high concentration of particles. With a lot of samples of random cubic particles in a unit cell, our statistical results show that at the same higher concentration, the effective viscosity of solid suspensions can be reduced by increasing the particle size or reducing the number of inclusion particles in a unit volume. This work discloses the viscosity reduction mechanism of increasing particle size, which is observed experimentally.

NEW METHOD FOR EFFECTIVE VISCOSITY OF COLLOIDAL DISPERSIONS WITH PERIODIC MICROSTRUCTURES

One of the central theoretical problems in the colloid field is to determine the rheological relation between the macroscopic properties of colloidal suspensions and the microstructures of the systems. In this work, the authors develop a method of transformation field by which one call calculate the effective viscosity of an incompressible: viscous fluid containing colloidal particles (either solid particles: or liquid drops) fixed at the points of a periodic lattice. The effective viscosity of a colloidal dispersion of spherical particles is calculated. The predictions of the theory are in good agreement with the Einstein's formula for suspensions and the Taylor's formula for emulsions at low particle concentrations. At higher particle concentrations, the theory reproduces the results of Nunan and Keller. The method is also applicable to the viscosity of colloidal systems with non-spherical particles.

Rheology of a Viscous-Plastic Liquid in a Porous Medium

The hydrodynamics of the capillary flow of a viscous-plastic liquid in cylindrical rectilinear pores is considered, as a result of which the structural velocity distribution over the pore cross section is obtained. Analytical solutions are proposed for the equations of hydraulic diffusion and nonlinear filtration for a non-Newtonian fluid in a cylindrical porous medium. It is noted that when a non-Newtonian fluid flows in a porous medium, the filtration equations take a nonlinear form due to the effective viscosity, shear, and yield stresses taken into account in its structure. The proposed solutions make it possible to evaluate the state of the porous medium and its main parameters (permeability, hydraulic diffusion, and effective viscosity coefficients). The obtained solutions are compared with existing experimental data for non-Newtonian oils.

On the structural features of fiber suspensions in converging channel flow

The structural features of fiber suspensions are dependent on the fiber alignment in the flows. In this work the orientation distribution function and orientation tensors for semi-concentrated fiber suspensions in converging channel flow were calculated, and the evolutions of the fiber alignment and the bulk effective vis-cosity were analyzed. The results showed that the bulk stress and the effective viscosity were functions of therate-of-strain tensor and the fiber orientation state ; and that the fiber suspensions evolved to steady alignment and tended to concentrate to some preferred directions close to but not same as the directions of local stream-lines. The bulk effective viscosity depended on the product of Reynolds number and time. The decrease of ef-fective viscosity near the boundary benefited the increase of the rate of flow. Finally when the fiber alignment went into steady state, the structural features of fiber suspensions were not dependent on the Reynolds numberbut on the converging channel angle.

Mechanisms for non-ideal flow in low-power arc-heated supersonic nozzles

The flow in a low-powered arc gas heater com- bined with a supersonic nozzle of throat diameter less than 1 mm is quite complicated and difficult to describe in quan- titative detail. Experiments on arc-heated supersonic jet thrusters of monatomic gases argon and helium have been carried out and their performance measured. The flow charac- teristics are analyzed with the help of numerical simulation. Results show that the viscous effect is the most important factor causing the large difference between ideal and real performance. A large outer section of the exit flow is slow- moving. This is especially pronounced in helium, where 70 % of the exit area of the nozzle might be in subsonic flow. Fric- tion forces can be much larger than the net thrust, reaching several times higher in helium, resulting in very low efficien- cies. Other factors causing the differences between ideal and real flow include： complex flow in the throat region, electric arc extending to the nozzle expansion section, heat transfer to the inlet gas and from the hot plasma, and environmen- tal pressure in the vacuum chamber. It is recognized that the ordinary concepts of supersonic nozzle flow must be greatly modified when dealing with such complicated situations. The general concepts presented in this paper could be helpful in guiding the design and operation of this equipment.

Possible dynamics of normal－fault earthquakes in the upper crust of the south part of the Qinghai－Xizang Plateau

A numerical model for generating normal fault earthquakes in the Qinghai-Xizang Plateau′S upper crust is constructed with 3-D elasto-viscous finite element method. Based on the numerical simulation calculation,some conclusions were got:If the effective viscosity of the upper crust material is less than that of lower strata of the crust in the Qinghai-Xizang Plateau, even under the strong push of India continent,the stress state of the upper crust can still be extensional in south part of the Qinghai-Xizang Plateau.Numerical simulations show that the stress state changes with the depth of the lithosphere,from extensional stress state in upper crust to compressive in the lower part.Extensional stress state may exist mainly in the upper crust of the south part of the Qinghai-Xizang Plateau.

Elastic-viscoplastic field at mixed-mode interface crack-tip under compression and shear

For a compression-shear mixed mode interface crack, it is difficult to solve the stress and strain fields considering the material viscosity, the crack-tip singularity, the frictional effect, and the mixed loading level. In this paper, a mechanical model of the dynamic propagation interface crack for the compression-shear mixed mode is proposed using an elastic-viscoplastic constitutive model. The governing equations of propagation crack interface at the crack-tip are given. The numerical analysis is performed for the interface crack of the compression-shear mixed mode by introducing a displacement function and some boundary conditions. The distributed regularities of stress field of the interface crack-tip are discussed with several special parameters. The final results show that the viscosity effect and the frictional contact effect on the crack surface and the mixed-load parameter are important factors in studying the mixed mode interface crack- tip fields. These fields are controlled by the viscosity coefficient, the Mach number, and the singularity exponent.

Limitations of Lattice Boltzmann Modeling of Micro-Flows in Complex Nanopores

The multiscale transport mechanism of methane in unconventional reservoirs is dominated by slip and transition flows resulting from the ultra-low permeability of micro/nano-scale pores,which requires consideration of the microscale and rarefaction effects.Traditional continuum-based computational fluid dynamics(CFD)becomes problematic when modeling micro-gaseous flow in these multiscale pore networks because of its disadvantages in the treatment of cases with a complicated boundary.As an alternative,the lattice Boltzmann method(LBM),a special discrete form of the Boltzmann equation,has been widely applied to model the multi-scale and multi-mechanism flows in unconventional reservoirs,considering its mesoscopic nature and advantages in simulating gas flows in complex porous media.Consequently,numerous LBM models and slip boundary schemes have been proposed and reported in the literature.This study investigates the predominately reported LBM models and kinetic boundary schemes.The results of these LBM models systematically compare to existing experimental results,analytical solutions of Navier-Stokes,solutions of the Boltzmann equation,direct simulation of Monte Carlo(DSMC)and information-preservation DSMC(IP_DSMC)results,as well as the numerical results of the linearized Boltzmann equation by the discrete velocity method(DVM).The results point out the challenges and limitations of existing multiple-relaxation-times LBM models in predicting micro-gaseous flow in unconventional reservoirs.

Simulation of Natural Convection Heat Transfer Enhancement by Nanoparticles in an Open-Enclosure Using Lattice Boltzmann Method

A numerical analysis is performed to investigate the laminar, free convection flow in an Open Enclosure Using Lattice Boltzmann Method (LBM) in the presence of Carbon nanotube and Cu nanoparticles. The problem is studied for different volume fractions of nanoparticles, and aspect ratio of the cavity for various Rayligh numbers. The volume fraction of added nanoparticles to water (as base fluid) is lower than 1% to make dilute suspensions. The study presents a numerical treatment based on LBM to model convection heat transfer of Carbon nanotube based nanofluids. Results show that adding a low value of Carbon nanotube to the base fluid led to significant enhancement of convection rate. Results show that adding nanoparticles to the base fluid enhances the rate of natural convection in a cavity. Make a comparison between Carbon nanotube and Cu-nanoparticles shows that the Carbon nanotube-nano- particle has better performance to enhance convection rate at comparison with Cu- nanoparticles.

Hydromagnetic peristaltic transport of copper-water nanofluid with temperature-dependent effective viscosity

The unique chemical mechanical, and thermodynamic properties of nanofluids make them a subject of great interest for scientists from all domains. Such fluids are of particular significance in biomedical engineering owing to their vast and novel applications in modern drug delivery systems; for example, mixed convective peristaltic flow of water-based nanofluids under the influence of an externally applied magnetic field is of particular significance. Hence, a lot of research has focused on peristalsis in the presence of velocity and thermal slip effects. An empirical relation for the effective viscosity of the nanofluid is proposed here for the first time. The viscosity of the nanofluid varies with temperature and nanoparticle volume fraction. Numerical simulation of the resulting nonlinear system of equations is presented for different quantities of interest. The results indicate that the maximum velocity and temperature of the copper-water nanofluid increase for larger variable viscosity parameter. The pressure gradient in the wider part of the channel is also found to increase as a function of the variable viscosity parameter. The variable viscosity parameter also influences the size of the trapped bolus. An increase in the nanoparticle volume fraction reduces the reflux phenomenon in a peristaltic flow.

A mean free path approach to the micro/nanochannel gas flows

We investigate the gas flows near to solid surfaces in terms of the local spatial variation in the molecular mean free path(MFP).Molecular dynamics(MD)is the appropriate scientific tool for obtaining molecularly-accurate dynamic information in micro and nano-scale gas flows,and has been used to evaluate the molecular mean free path of gases.In the calibration procedure,the viscosity of a gas in the homogeneous case can be recovered in our MD simulations and reach good agreement with the theoretical prediction and data from NIST.In surface-bounded gas flows,if the collisions between gas molecules and walls are counted,a spatially-varying mean free path is presented,and for the first time we have observed that the distribution of the free paths deviates from the exponential one and spikes appear in their distributions at larger Kn,i.e.in the transition flow regime.Based on elementary kinetic theory,the effective viscosity of the gas derived from the mean free path has been incorporated into the framework of the continuum-fluid dynamics equations,and micro-Couette flows are performed to demonstrate this potential application.

Plasma Pulse Technology: An uprising EOR technique

Conventionally oil recovery factor is too low,which leaves great prospects for the application of enhanced oil recovery(EOR)methods to increase recovery factor.EOR methods are capital intensive and few are environmentally hazardous.So the paper discusses on the alternate enhanced oil recovery technique which has tremendous potential to curb the challenges of conventional EOR methods.Plasma pulse technology(PPT)aided EOR treatment is administered with an electric wireline conveyed plasma pulse generator tool that is run in the well and positioned alongside the perforations.Using energy stored in the generator's capacitors,a plasma arc is created that emits a tremendous amount of heat and pressure for a fraction of a second.This in turn creates a broad band of hydraulic impulse acoustic waves that are powerful enough to clean perforations and near wellbore damage.These waves continue to resonate deep into the reservoir,exciting the fluid molecules and increasing the reservoirs natural resonance to the degree that it can break larger hydrocarbon molecules to smaller one and simultaneously reducing adhesion tension which results in increased mobility of hydrocarbons.The plasma pulse technology has been successfully used on production as well as injection wells.It has been used often as a remedial procedure to increase well's productivity that has been on production for a period of time.This paper throws light on fundamentals of this advancing plasma pulse technology,contrasting it with recent EOR techniques.Effectiveness of treatment in increasing oil recovery,it's applicability to different reservoir types and results achieved so far are also covered in the paper.