Wall Slip Effects Research Articles

The Effects of Finite Ionic Sizes and Wall Slip on Entropy Generation in Electroosmotic Flows in a Soft Nanochannel

Abstract The combined effects of finite ionic sizes and boundary slip on the entropy generation in mixed pressure driven and electroosmotic flows (EOFs) in a soft nanochannel are investigated in this study. The soft nanochannel is represented by a rigid nanochannel covered by a charged polyelectrolyte layer (PEL) on its surface. The entropy generation analysis of EOFs in such a soft nanochannel is addressed for the first time. Under the assumption of high zeta potentials, the electric potential, velocity, and temperature distributions are obtained numerically by using the finite difference method. Subsequently, the thermal transport characteristic and the corresponding entropy generation analysis are discussed based on the obtained velocity and temperature distributions. Our results show that the soft nanochannel in the present model is not appropriate for cooling purposes. We also demonstrate that the steric factor v and the PEL thickness d can enhance the entropy generation rate. However, the slip boundary coefficient γ, the drag parameter α, and the equivalent electric double-layer (EDL) thickness λFCL can restrain this entropy generation rate. In addition, the contributions of Joule heating and viscous friction in the entropy generation rate are more prominent than the contribution due to heat transfer. The present theoretical research can be used to design the efficient thermofluidic devices.

Unsteady Three-Dimensional MHD Nanofluid Flow Over a Stretching Sheet with Variable Wall Thickness and Slip Effects

The stretching sheets with variable thickness may occur in engineering applications more frequently than a flat sheet. Due to its various applications, in the present analysis we considered a three dimensional unsteady MHD nanofluid flow over a stretching sheet with a variable wall thickness in a porous medium. The effects of radiation, viscous dissipation and slip boundary conditions are considered. Buongiorno’s model is incorporated to study the combined effects of thermophoresis and Brownian motion. The dimensionless governing equations are solved by using MATLAB bvp4c package. The impact of various important flow parameters is presented and analysed through graphs and tables. It is interesting to note that all the three boundary layer thicknesses are diminished by slip parameters. Further, the unsteady parameter decreases the hydromagnetic boundary layer thickness.

Film thickness build-up in zero entrainment velocity wide point contacts

Abstract In this work, elastohydrodynamic wide point contacts are studied under Zero Entrainment Velocity (ZEV) conditions. Contrary to classical rolling-sliding contacts, no hydrodynamic lift (oil wedge effect) is expected in stationary isothermal ZEV contacts. However, both experiments and numerical simulations taking into account temperature gradients in the thickness of the lubricant show the occurrence of a full-film lubrication regime over a large range of surface velocities, contact loads and external temperatures. Numerical simulations, including thermal effects but neither transient nor wall slip effects, are in good agreement with experimental film thickness measurements. They allow for both a qualitative description of the mechanisms at stake and to a first tentative minimum film thickness prediction under those specific conditions.

Emission of electromagnetic waves from walls of esophagus under domination of wall slip effect

Abstract On explanation of biomedical applications and its complexity, the contraction and relaxation of Eyring-Powell fluid in esophagus has turned out to be notable amongst the challenging research fields of fluid mechanics. Therefore, the main idea of this paper is to classify the simultaneous significances of compliant walls and emission of electromagnetic waves from walls of electrically conducting Eyring-Powell fluid. Conductivity of the fluid is increased by adding the Brownian motion and thermophoresis effects. Moreover, the velocity of the channel and the fluid particles near the surfaces are assumed to be different. Due to vertical contraction and relaxation, the body forces are taken in the form of mixed convection. The dimensionless variables are utilized to convert the required partial differential equations including mass, momentum and energy into dimensionless form. The solutions of the reduced partial differential equations subject to the given boundary conditions are calculated by using Mathematica. Moreover, the various solutions for dimensionless fluid concentration, velocity, and temperature distributions are captured for different values of embedded parameters. The different dimensionless parameters governing the flow, heat and mass transfer characteristics are the Schmidt number, radiation parameter, Hartmann number, local temperature Grashof number, slip parameters, local nanoparticles Grashof number, thermophoresis parameter, Brownian motion parameter, fluid parameters and Prandtl number. Maximum velocity is noted an increasing function of velocity slip parameter. Further maximum temperature enhances by increasing Brownian motion and thermophoresis parameters. More specifically the effects of slip parameters, Brownian motion and thermophoresis parameters on the flow stream have been examined. The velocity profile shows enhance behavior for large values of velocity parameter and mixed convection, whereas, reverse impact is noticed for magnetic field. Increasing concentration slip parameter causes reduction in nanoparticle volume fraction, while temperature of the fluid rises by increasing thermal slip parameter.

Heat and Mass Transfer of Free Convection Flow Over a Vertical Plate with Chemical Reaction Under Wall–Slip Effect

This research work deals with the influence of heat and mass transfer on the natural convection flow over an infinite vertical plate with constant concentration and ramped wall temperature under the influence of slip boundary condition. The effects of chemical molecular diffusivity along with chemical and heat transfer processes have been studied. Solutions for the fluid velocity, temperature and concentration are achieved in closed forms by applying Laplace transform technique. The influence of embedded physical parameters on the flow characteristics has been analyzed graphically.

Исследование фильтрационных свойств грунтов ограждающих сооружений шламохранилищ с учетом влияния эффекта пристенного скольжения

Установлена эмпирическая закономерность зависимости коэффициентов фильтрации песчаных грунтов от содержания в шламе полиакриламида в концентрации 0,17–0,90 мг/дм3 (эффект пристенного скольжения). Определен коэффициент фильтрации для грунтов ограждающих сооружений шламохранилищ Республики Беларусь с учетом присутствия в фильтрующемся шламе полиакриламида. Подтверждено результатами экспериментальных исследований и натурных наблюдений превышение скорости фильтрации шлама 0,045–0,047 см/с над скоростью фильтрации воды в 1,2–1,4 раза.

Influence of interfacial condition on rheological instability behavior of UHMWPE/HDPE/nano-SiO2 blends in capillary extrusion

In this study, rheological behaviors and capillary extrusion flow instabilities of ultra-high molecular weight polyethylene (UHMWPE)/high-density polyethylene (HDPE)/SiO2 composites containing modified nano-SiO2 or pure nano-SiO2 are investigated. Effects of interfacial conditions between the dispersed nano-SiO2 phase and PE matrix on rheological behaviors are analyzed. The results show that modified nano-SiO2 in the PE matrix has a relatively strong interfacial interaction with the polymer chains compared with pure nano-SiO2, thus causing a more pronounced shear thinning behavior and reducing extrudate swell during capillary extrusion. Nanocomposite extrudates experience the transition of smooth- sharkskin- oscillating distortion—overall melt fracture with the increase of shear rate (or shear stress). The interfacial interaction allows a more storage of elastic energy as the melt flow through the die, resulting in sharkskin distortion and oscillating distortion to occur prematurely at critical shear rates. It also restricts the elastic recovery of the melt after leaving the die, thus delaying sharkskin distortion under shear stress. At high shear rates, the modified nano-SiO2 particles begin to roll, and some of the unmodified particles move and embed to the wall. It is believed that the interfacial adsorption effect and the wall-slip effect of nanoparticles exist simultaneously in the whole extrusion process.

Online Rheometry Investigation of Flow/Slip Behavior of Powder Injection Molding Feedstocks.

Wall slip in the flow of powder injection molding (PIM) compounds can be the cause of unrealistically low viscosity values, and can lead to a failure of flow simulation approaches. Regardless of its importance, it has been considered only scarcely in the rheological models applied to PIM materials. In this paper, an online extrusion rheometer equipped with rectangular slit dies was used to evaluate the slip velocity of commercial as well as in-house-prepared PIM feedstocks based on metallic and ceramic powders at close-to-processing conditions. The tested slit dies varied in their dimensions and surface roughness. The wall-slip effect was quantified using the Mooney analysis of slip velocities. The smaller gap height (1 mm) supported the wall-slip effect. It was shown that both the binder composition and the powder characteristic affect slip velocity. Slip velocity can be reduced by tailoring a powder particle size distribution towards smaller particle fractions. The thickness of the polymer layer formed at the channel wall is higher for water-soluble feedstocks, while in the case of the catalytic polyacetal feedstocks the effect of surface roughness was manifested through lower viscosity at smooth surfaces.

Shear viscosity and wall slip behavior of dense suspensions of polydisperse particles

The significant problem of wall slip and shear viscosity of dense suspensions is addressed using steady torsional and capillary flows of a silicone polymer, incorporated with polydisperse particles with low aspect ratios, to achieve a relatively high maximum packing fraction, ϕm=0.86. Such a high ϕm allowed the preparation of well-mixed suspensions with a wide range of solid volume fractions, ϕ, i.e., 0.62 ≤ ϕ ≤ 0.82. It is demonstrated that the characterization of the relative viscosity and yield stresses of the particulate suspensions requires a proper treatment of wall slip effects. The wall slip velocity versus the shear stress relationship is governed by the apparent slip mechanism and is predictable using the shear viscosity of the binder and the thickness of the apparent slip layer. At shear stresses which are significantly above the yield stress, the relative shear viscosity of the suspensions depends solely on ϕ/ϕm. However, at lower shear stresses that are in the vicinity of the yield stresses, the relative shear viscosity becomes functions of both ϕ/ϕm and the shear stress.

End loss for Stokes flow through a slippery circular pore in a barrier of finite thickness

An analytical model based on the fluid cylinder approximation and eigenfunction expansions is developed for Stokes flow through a slippery circular pore in a barrier of finite thickness. The hydraulic resistance, which comprises the end resistance and Poiseuille resistance, is determined as a function of the pore thickness, slip length of the pore wall, and proximity of pores. The results are presented to reveal how wall slip may change, quantitatively and qualitatively, the effect of the pore thickness on the end resistance. It is shown, in particular, that the use of Sampson’s formula may underestimate the end loss under the effect of wall slip. Velocity slip on the wall will cause a greater departure of the velocity profile at the inlet from that of the fully developed flow, and therefore, a longer entrance length is required for the flow to attain its final state. Empirical formulas are proposed to facilitate quick calculation of the end resistance as a function of the controlling parameters.

Linear stability of a contaminated fluid flow down a slippery inclined plane

The linear stability analysis of a fluid flow down a slippery inclined plane is carried out when the free surface of the fluid is contaminated by a monolayer of insoluble surfactant. The aim is to extend the earlier study [Samanta et al., J. Fluid Mech. 684, 353 (2011)] for low to high values of the Reynolds number in the presence of an insoluble surfactant. The Orr-Sommerfeld equation (OSE) is derived for infinitesimal disturbances of arbitrary wave numbers. At low Reynolds number, the OSE is solved analytically by using the long-wave analysis, which shows that the critical Reynolds number decreases in the presence of a slippery plane but increases in the presence of an insoluble surfactant. This fact ensures a destabilizing effect of wall slip and a stabilizing effect of insoluble surfactant on the long-wave surface mode. Further, the Chebyshev spectral collocation method is implemented to tackle the OSE equation numerically for an arbitrary value of the Reynolds number, or equivalently, for an arbitrary value of the wave number. At moderate Reynolds number, wall slip exhibits a stabilizing effect on the surface mode as opposed to the result in the long-wave regime, while the insoluble surfactant exhibits a stabilizing effect on the surface mode as in the result of the long-wave regime. On the other hand, at high Reynolds number, both wall slip and insoluble surfactant exhibit a stabilizing effect on the shear mode. Further, it is shown that both surface and shear modes compete with each other to dominate the primary instability once the inclination angle is sufficiently small. In addition, new phase boundaries are identified to differentiate the regimes of surface and shear modes.

Slip and Hall Current Effects on Jeffrey Fluid Suspension Flow in a Peristaltic Hydromagnetic Blood Micropump

The magnetic properties of blood allow it to be manipulated with an electromagnetic field. Electromagnetic blood flow pumps are a robust technology which provide more elegant and sustainable performance compared with conventional medical pumps. Blood is a complex multi-phase suspension with non-Newtonian characteristics which are significant in microscale transport. Motivated by such applications, in the present article, a mathematical model is developed for magnetohydrodynamic pumping of blood in a deformable channel with peristaltic waves. A Jeffery’s viscoelastic formulation is employed for the rheology of blood. A two-phase fluid-particle (“dusty”) model is utilized to better simulate suspension characteristics (plasma and erythrocytes). Hall current and wall slip effects are incorporated to achieve more realistic representation of actual systems. A two-dimensional asymmetric channel with dissimilar peristaltic wave trains propagating along the walls is considered. The governing conservation equations for mass, fluid and particle momentum are formulated with appropriate boundary conditions. The model is simplified using long wavelength and creeping flow approximations. The model is also transformed from the fixed frame to the wave frame and rendered non-dimensional. Analytical solutions are derived. The resulting boundary value problem is solved analytically, and exact expressions are derived for the fluid velocity, particulate velocity, fluid/particle fluid and particulate volumetric flow rates, axial pressure gradient, pressure rise and skin friction distributions are evaluated in detail. Increasing Hall current parameter reduces bolus growth in the channel, particle-phase velocity and pressure difference in the augmented pumping region, whereas it increases fluid phase velocity, axial pressure gradient and pressure difference in the pumping region. Increasing the hydrodynamic slip parameter accelerates both particulate and fluid phase flow at and close to the channel walls, enhances wall skin friction, boosts pressure difference in the augmented pumping region and increases bolus magnitudes. Increasing viscoelastic parameter (stress relaxation time to retardation time ratio) decelerates the fluid phase flow, accelerates the particle-phase flow, decreases axial pressure gradient, elevates pressure difference in the augmented pumping region, and reduces pressure difference in the pumping region. Increasing drag particulate suspension parameter decelerates the particle-phase velocity, accelerates the fluid phase velocity, strongly elevates axial pressure gradient and reduces pressure difference (across one wavelength) in the augmented pumping region. Increasing particulate volume fraction density enhances bolus magnitudes in both the upper and lower zones of the channel and elevates pressure rise in the augmented pumping region.

Rheological and wall-slip behaviour of composite propellant suspension containing Al-nanopowder

ABSTRACTHydroxyl terminated polybutadiene composite propellant suspension mainly containing ammonium perchlorate (70%) and aluminium nanoparticles (ANP) (0–6%) was evaluated rheologically to determine the effect of wall slip occurring due to the formation of an apparent slip layer. True rheological properties have been obtained from gap-dependent steady-shear data using Tikhonov regularization method for the composite suspension. The advantage of this method is that it converts the gap-dependent steady-shear data into true rheological properties. The two-stage method can successfully establish both the true shear stress vs. shear rate behaviour and wall-slip parameters. The errors during rheological experimentation are analysed by determining the slip velocities and slip layer thickness. Slip velocity is observed to increase linearly with shear stress. Also, the slip layer thickness decreases with the increase in ANP content in the composite suspension. The maximum slip layer thickness of 2.13 µm is obtained for composition in which ANP is absent, and the same decreased to 0.24 µm for the composition containing 6% ANP. The rheological measurements show least deviation from gap-independent values as the amount of ANP in the propellant increases. Finally, a correlation of apparent slip layer thickness with normalized filler fraction is investigated to check the effect on wall-slip behaviour.

Influence on Journal Bearing Considering Wall-Slip in EHL

Based on the line contact Elastohydrodynamic model and the limiting shear stress model of the journal bearing, the limiting shear stress at the contact interface of bearing, shaft and lubricant is discussed. And the effects of limiting shear stress on the film thickness, the pressure profile and friction coefficient are analyzed. The effect of wall-slip on elastohydrodynamic lubrication of journal bearings is discussed when the shear stress at the interface of solids and liquids reaches its limiting value. The results show that different wall-slip types will produce different lubrication states and the wall-slip of shaft interface will increase the oil film thickness in the sliding region. On the contrary, the wall-slip of the bearing interface will decrease the oil film thickness. The friction coefficient will decrease when the wall-slip occurs and the degree is greater at the interface of shaft than bearing.

Effects of wall slip on ZrO2 rheological behavior in micro powder injection molding

The effects of wall slip on the rheological behavior of ZrO2 feedstock flowing through various channels were studied. As compared to conventional no-slip condition, a power-law wall slip model was established to simulate the filling behavior of ZrO2 feedstock flowing through the mold. The effects of wall slip on pressure distribution of the micro spool mandrils during filling was compared with the injection molded micro components. Experimental results verified that the simulation including wall slip yielded better prediction for the pressure gradient as well as crack propagation for the micro components. Increasing mold temperature not only enhanced the feedstock temperature flowing through the micro channels, but also reduced the temperature difference of the micro components and likely the ensuing thermal stress as well as cracks. Powder-binder separation is more sensitive to the mold temperature variation than melt temperature variation. Defect-free micro spool mandrils were injection molded using the optimized parameters.

Effects of velocity and thermal wall slip on magnetohydrodynamics (MHD) boundary layer viscous flow and heat transfer of a nanofluid over a non-linearly-stretching sheet: a numerical study

In this analysis, the boundary layer viscous flow of nanofluids and heat transfer over a non-linearly-stretching sheet in the presence of a magnetic field is presented. Velocity and thermal slip conditions are considered instead of no slip conditions at the boundary. A similarity transformation set is used to transform the governing partial differential equations into non-linear ordinary differential equations. The reduced equations are solved numerically using the Keller box method. The influence of the governing parameters on the dimensionless velocity, temperature, nanoparticle concentration as well as the skin friction coefficient, Nusselt number, and local Sherwood number are analyzed. It is found that as the velocity slip parameter increases, the velocity profile is decreased and the skin friction and heat transfer decreased while the mass transfer is increased. Increasing the thermal slip parameter causes decreases in the heat and mass transfer rates. The results are presented in both graphical and tabular forms.

Thermal analysis of MHD electro-osmotic peristaltic pumping of Casson fluid through a rotating asymmetric micro-channel

In this paper, we investigate the combined effects of wall slip, viscous dissipation, and Joule heating on MHD electro-osmotic peristaltic motion of Casson fluid with heat transfer through a rotating asymmetric micro-channel. Using long wavelength and small Reynolds number assumptions, the governing equations of momentum and energy balance are obtained and tackled analytically. The effects of various embedding parameters on the stream function, velocity, temperature, skin friction, Nusselt number and trapping phenomenon are displayed graphically and discussed. It is found that Casson fluid velocity, temperature, and heat transfer rate are enhanced with a boost in electro-osmotic force.

Numerical Simulation of Extrusion Molding of Single - Hole Propellant

In order to improve the simulation accuracy of single hole nitroguanidine propellant extrusion molding process, the influence of wall slip on the extrusion process was studied. The finite element method was used to simulate the extrusion forming of single pore nitroguanidine. The power law constitutive equation considering the effect of wall slip was constructed to calculate the slip factor and non-Newtonian index , the influence of the slip coefficient on the pressure field, velocity field and shear velocity field in the flow channel were analyzed. compared with the actual results, the error of the middle aperture is 1.37%, the arc thickness error is 0.96% and the outer diameter error is 1.12%.Those errors are less then 1.5%. So, the simulation results basically meet the production needs.

Rheological behavior of aqueous foams at high pressure

Aqueous foams are used in many oil field applications including drilling, fracturing, and enhanced oil recovery operations. The success of all these operations strongly depends on viscosity and stability of foam. Rheology of foams is a function of foam quality (i.e. gas volume fraction), liquid phase viscosity, pressure, and temperature. Besides, foam generation method and stability are factors that influence its rheology.This article presents results of an experimental study performed to investigate the effects of foam quality, pressure, and wall slip on aqueous-foam rheology. Extensive tests were conducted using a foam recirculating flow loop consisting of three pipe viscometers. Experiments were performed at ambient temperature, and varying pressure (6.89–20.68 MPa), foam quality (40–80%), and pipe diameter (3.06, 6.22 and 12.67 mm). The foam was generated by passing a mixture of water containing 2% surfactant solution and gas phase (nitrogen) through a needle valve. To minimize degradation while testing, the foam was regenerated by circulating at the maximum flow rate (0.55 L/min) before each flow measurement was made.Results indicate strong non-Newtonian behavior of foam, which closely fits the power law model. For foams with more than 55% quality, measured viscosities were higher than the ones reported in the literature. Noticeable wall slip was not observed. Foam viscosity change because of pressure variation at a constant foam quality was negligible. Foams with quality higher than 70% exhibited yield pseudoplastic behavior. Their rheological behavior can be described better by Herschel–Bulkley model than power law model when the shear rate is below 20 1/s.

Cavitation and slip performance analysis of dynamically loaded sleeve bearing

The generalized Reynolds equation of dynamically loaded sleeve bearing is established based on mass conservation boundary condition and wall-slip effect. The cavitation migration and flow velocity of spiral oil wedge bearing for different times are computed, using the finite difference method by solving the Reynolds equation of four slip states with cavitation effect. The results show that the cavitation area with cavitation effect and wall slip decreases compared to the cavitation area without regard to cavitation effect and wall slip. As time goes, the slip location at the surface of axis and sleeve moves generally along the rotation direction of axis, and the region of slip changes.