Showing papers on "Slip ratio published in 2006"

Influence of periodic wall roughness on the slip behaviour at liquid/solid interfaces: molecular-scale simulations versus continuum predictions

Abstract: The influence of surface roughness on the slip behaviour of a Newtonian liquid in steady planar shear is investigated using three different approaches, namely Stokes flow calculations, molecular dynamics (MD) simulations and a statistical mechanical model for the friction coefficient between a corrugated wall and the first liquid layer. These approaches are used to probe the behaviour of the slip length as a function of the slope parameter ka = 2πa/λ, where a and λ represent the amplitude and wavelength characterizing the periodic corrugation of the bounding surface. The molecular and continuum approaches both confirm a monotonic decay in the slip length with increasing ka but the rate of decay as well as the magnitude of the slip length obtained from the Stokes flow solutions exceed the MD predictions as the wall feature sizes approach the liquid molecular dimensions. In the limit of molecular-scale wall corrugation, a Green–Kubo analysis based on the fluctuation–dissipation theorem accurately reproduces the MD results for the behaviour of the slip length as a function of a. In combination, these three approaches provide a detailed picture of the influence of periodic roughness on the slip length which spans multiple length scales ranging from molecular to macroscopic dimensions.

Momentum and Heat Transfer in a Laminar Boundary Layer with Slip Flow

Abstract: Flow in a laminar boundary layer is modeled using a slip boundary condition. The slip condition changes the boundary layer structure from a self-similar profile to a two-dimensional structure. Although the slip condition generally leads to decreased overall drag, two-dimensional effects cause local increases in skin friction. Other effects include thinner boundary layers, delayed transition to turbulence, and changes in the heat transfer at the wall. Without a thermal jump condition, slip will lead to increased heat transfer. When a thermal jump boundary condition is added to simulate real gases, the heat transfer decreases to below the no-slip values.

The flow of an elastico-viscous fluid past a stretching sheet with partial slip

Abstract: An analysis is carried out to study the flow characteristics in an elastico-viscous fluid (Walters' liquid-B model) over a stretching sheet with partial slip. The flow is generated due to linear stretching of the sheet. Using suitable similarity transformations on the highly non-linear partial differential equations we derive exact analytical solution with appropriate boundary conditions. The important finding in this communication is the effect of partial slip on the velocity and skin friction coefficient.

Direct measurement of slip velocities using three-dimensional total internal reflection velocimetry

Abstract: The existence and magnitude of slip velocities between deionized water and a smooth glass surface is studied experimentally. Sub-micron fluorescent particles are suspended in water and imaged using total internal reflection velocimetry (TIRV). For water flowing over a hydrophilic surface, the measurements are in agreement with previous experiments and indicate that slip, if present, is minimal at low shear rates, but increases slightly as the shear rate increases. Surface hydrophobicity is observed to induce a small slip velocity, with the slip length reaching a maximum of 96 nm at a shear rate of 1800 $\,{\rm s}^{-1}$ . Issues associated with the experimental technique and the interpretation of results are also discussed.

Does Shear Heating of Pore Fluid Contribute to Earthquake Nucleation

Abstract: [1] Earthquake nucleation requires reduction of frictional strength τ = μ (σ − p) with slip or slip rate, where μ, σn, and p are the friction coefficient, normal stress, and fluid pressure, respectively. For rate state μ at fixed (σ − p), instabilities can occur when d μss/dv 0 are linearly stable at all wavelengths to adiabatic perturbations when v is near a plate rate if the wall rock permeability exceeds a critical value that is orders of magnitude less than inferred. Thus shear heating alone cannot then nucleate unstable slip; frictional weakening is required. However, shear heating can produce inertial instability on velocity strengthening faults following strong stress perturbations. On faults with dμss/dv < 0, shear heating increases pore pressure faster than is dissipated by Darcy flow at slip speeds of order 1 mm s−1. For faults bounding half-spaces with uniform thermal and hydraulic properties, μ exceeds (σ − p) during nucleation for slip speeds in excess of 10−2 to 101 mm s−1, depending on parameters chosen. Thus thermal effects are likely to dominate late in the nucleation process, well before seismic waves are radiated, as well as during fast seismic slip. By the time shear heating effects dominate, inertial slip is imminent (∼10−1 s), so that time-to-failure calculations based on rate state friction are not biased by thermal pressurization.

Mesoscopic modelling of heterogeneous boundary conditions for microchannel flows

Abstract: We present a mesoscopic model of the fluid–wall interactions for flows in microchannel geometries. We define a suitable implementation of the boundary conditions for a discrete version of the Boltzmann equations describing a wall-bounded single-phase fluid. We distinguish different slippage properties on the surface by introducing a slip function, defining the local degree of slip for hydrodynamical fields at the boundaries. The slip function plays the role of a renormalizing factor which incorporates, with some degree of arbitrariness, the microscopic effects on the mesoscopic description. We discuss the mesoscopic slip properties in terms of slip length, slip velocity, pressure drop reduction (drag reduction), and mass flow rate in microchannels as a function of the degree of slippage and of its spatial distribution and localization, the latter parameter mimicking the degree of roughness of the ultra-hydrophobic material in real experiments. We also discuss the increment of the slip length in the transition regime, i.e. at ${O}(1)$ Knudsen numbers. Finally, we compare our results with molecular dynamics investigations of the dependence of the slip length on the mean channel pressure and local slip properties and with the experimental dependence of the pressure drop reduction on the percentage of hydrophobic material deposited on the surface.

Slip effects on the peristaltic flow of a non-Newtonian Maxwellian fluid

Abstract: In real systems there is always a certain amount of slip, which, however, is hard to detect experimentally because of the required space resolution. In this paper, we analyze the effect of slip boundary conditions on the dynamics of fluids in porous media by studying the flow of a Newtonian and non-Newtonian Maxwellian fluid in an axisymmetric cylindrical tube (pore), in which the flow is induced by traveling transversal waves on the tube wall. Like in peristaltic pumping, the traveling transversal waves induce a net flow of the liquid inside the pore. The viscosity as well as the compressibility of the liquid is taken into account. This problem has numerous applications in various branches of science, including stimulation of fluid flow in porous media under the effect of elastic waves and studies of blood flow dynamics in living creatures. The Navier-Stokes equations for an axisymmetric cylindrical pore are solved by means of a perturbation analysis, in which the ratio of the wave amplitude to the radius of the pore is small parameter. In the second order approximation, a net flow induced by the traveling wave is calculated for various values of the compressibility of the liquid, relaxation time and Knudsen number. The calculations disclose that the compressibility of the liquid, Knudsen number of slip flow and non-Newtonian effects in presence of peristaltic transport have a strong influence of the net flow rate. The effects of all parameters of the problem are numerically discussed and graphically explained.

On the MHD and slip flow over a rotating disk with heat transfer

Abstract: Purpose – To study the steady magnetohydrodynamic (MHD) flow of a viscous, Newtonian and electrically conducting fluid over a rotating infinite disk with slip boundary conditionDesign/methodology/approach – The governing equations, which are partial and coupled, are transformed to ordinary ones by utilizing the similarity variables introduced by Karman and the resulting equation system is solved by using differential transform methodFindings – It is observed that both the slip factor and the magnetic flux decrease the velocity in all directions and thicken the thermal boundary layerOriginality/value – This paper studies the combined effects of slip and magnetic flux to the flow and thermal fields over a rotating single free disk in an ambient fluid, which were never studied together before

Wall slip of bubbles in foams

Abstract: We present a computational analysis of the flow of liquid foam along a smooth wall, as encountered in the transport of foams in vessels and pipes. We concentrate on the slip of the bubbles at the wall and present some novel finite element calculations of this motion for the case of fully mobile gas/liquid interfaces. Our two-dimensional simulations provide for the first time the bubble shapes and entire flow field, giving detailed insight into the distribution of stresses and dissipation in the system. In particular, we investigate the relationship between the drag force and the slip velocity of the bubble, which for small slip velocities obeys power laws, as predicted by previous semianalytical treatments.

Developing natural convection with thermal creep in a vertical microchannel

Abstract: Thermal creep occurs in anisothermal gas microflow. It is highly desirable to understand the creep effect on the flow and heat transfer characteristics for developing natural convective microflow. In this study, we investigate the steady developing natural convective flow in an open-ended vertical parallel-plate microchannel with asymmetric wall temperature distributions. The boundary-layer equations subject to the boundary conditions with respect to dynamic pressure at the channel entry as well as higher-order jump temperature and slip velocity with thermal creep along the channel surface are employed. The mathematical model and the numerical code are validated through available macroscale work. Numerical solutions of high-order slip coefficient, slip/jump, velocity, pressure, temperature, flow rate, flow drag and heat transfer rate are presented for air at the standard reference state with complete accommodation. It is found that thermal creep has significant effect on the high-order slip effect and the flow and thermal fields. The creep effect is to increase the flow rate; moreover, valuable reduced flow drag and enhanced heat transfer are obtained.

Three-dimensional simulation of gaseous slip flow in different aspect ratio microducts

Abstract: Three-dimensional lattice Boltzmann method based simulations of a microduct have been undertaken in this paper. The objectives are to understand the different physical phenomena occurring at these small scales and to investigate when the flow can be treated as two dimensional. Toward this end, the Knudsen number and aspect ratio (depth to width ratio) are varied for a fixed pressure ratio. The pressure in the microduct is nonlinear with the nonlinearity in pressure reducing with an increase in the Knudsen number. The pressure behaves somewhat similar to two-dimensional microchannels, even when the aspect ratio is unity. The slip velocity at the impenetrable wall has two components: along and perpendicular to the primary flow direction. Our results show that the streamwise velocity near the centerline is relatively invariant along the depth for an aspect ratio of more than three, suggesting that the microduct can be modeled as a two-dimensional microchannel. On the other hand, the velocity component along the depth is never identically zero, implying that the flow is not truly two dimensional, although for practical purposes a two-dimensional treatment might suffice. A curious change in the vector direction in a plane normal to the flow direction is observed around an aspect ratio of four. These three-dimensional results are significant because they will help in theoretical development and flow modeling at microscales.

Analytical solutions for squeeze flow of Bingham fluid with Navier slip condition

Abstract: Squeeze flow between closely spaced parallel disks of a plastic material is considered for a Bingham fluid described by a bi-viscosity model with Navier slip condition. The flow field is divided into a Newtonian part with high viscous Newtonian fluid and a bi-viscosity part with yielded/unyielded fluids. The radial velocities and pressure gradients in the two parts are obtained, respectively. The slip velocity at the disk is proportional to the pressure gradient, which is proportional to r in the Newtonian part and is approximately proportional to large r in the bi-viscosity part. Furthermore, the explicit expressions of the pressure and the squeeze force are acquired through the approximately linear relationship between the pressure gradient and r in the bi-viscosity part. Also, distributions of the radial velocity and shear stress, and the effects of the slip coefficient, plastic viscosity and viscosity ratio on the radial velocity, pressure gradient, squeeze force and yield surface are discussed.

Gas Flow in Microchannel of Arbitrary Shape in Slip Flow Regime

Abstract: A theoretical analysis for laminar flow in the microchannels of arbitrary shape in slip flow regime is presented in this article. The momentum equations with the first-order slip, the second-order slip, and the thermal creep flow boundary conditions are solved by applying a computation-oriented method of the orthonormal function analysis for the fully developed laminar flow of the incompressible fluid in the microchannels. The dimensionless velocity profile and the friction factor are theoretically predicted for a microchannel of arbitrary shape. To justify the methodology, the friction factor of gas flowing in the rectangular microchannel is calculated and compared with the experimental data. The good agreement between analytic solutions and experimental data shows that within a definite extension of Knudsen number, the traditional Navier–Stokes equations with the slip boundary conditions can govern the gaseous slip flow mechanisms in microchannels, and the orthonormal function method is applicable to so...

Experimental Investigation of Effects of Slip Ratio on Elastohydrodynamic Lubrication Film Related to Temperature Distribution in Oil Films

Abstract: This article describes the mechanism of variations of elastohydrodynamic lubricated oil film under high slip ratio conditions. Experiments were conducted using a ball-on-disc apparatus. The thickness of the oil film was measured by optical interferometry. Temperature increase in the ball surface, disc surface, and oil film was determined by an improved infrared technique using a transparent disc without any coating and non-transparent disc with a comparatively thick Cr coating. The results reveal that the film thickness decreases with an increase in the slip ratio at constant entrainment velocities. The shape of the oil film indicates an increase in the thickness gradient at the inlet region and a slight increase in the oil film just behind the exit region. The temperature at the inlet region increases with the slip ratio and entrainment speed. Both the inlet region and the contact area exhibit a significant temperature difference between the surfaces. The variation of the shape of the oil film ca...

Numerical modeling of three-dimensional compressible gas flow in microchannnels

Abstract: Nitrogen gas flow in long microchannels with square cross-sections was simulated numerically with a three-dimensional continuum model with slip and no-slip boundary conditions. The governing equations of the model were solved by a control volume method. The numerical model was validated with the available experimental and numerical results. For incompressible flow, it was found that when Dh was less than 60 ?m, a slip boundary condition must be applied. An analytical expression for normalized friction coefficients, C*IC, i.e. the ratio of f Re (slip) to f Re (no-slip), was developed on the basis of incompressible flow behavior. For compressible flow, a parametric study was conducted for Dh = 1 ?m, L/Dh = 200 and with varying pressure ratios (PR = 1.5?5.0). It was found that as the pressure ratio increased from 1.5 to 5.0, compressibility effects increased while the rarefaction effects started diminishing. Slip effects also played an important role in the friction characteristics of microchannel flows. An analytical expression for normalized friction coefficients, C*C, i.e. the ratio of f Re (compressible) to f Re (incompressible), was developed on the basis of flow behavior for compressible flow. A comparative study of two-dimensional and three-dimensional flows was also conducted, and it was shown that the two-dimensional assumption for the compressible flow was not valid since it predicted 15?45% higher flow velocities, and 7?12% lower friction factors than those predicted by the 3D models.

Velocity slip of liquid flow in nanochannels

Abstract: Molecular dynamics simulations are carried out to investigate the velocity slip phenomenom of liquid flow in nanochannels. The liquid is argon, and the wall is taken to be platinum or its model solids. The effect of the surface wettability on the velocity slip is obtained through varying the potential interaction strength between the liquid and the wall. The simulation results show that the liquid adjacent to a hydrophilic surface is solid-like and has high density and orderliness. However, the liquid near a hydrophobic surface forms a gap of low density. The velocity slip of the liquid flow over solids decreases with the increasing interaction strength between the liquid and the channel surface. The slip, the no-slip and the negative slip are all possible to take place for different liquid-surface wettabilities. We find that the apparent slip of the liquid flow over solids is affected by the integrated factors of boundary slip, liquid sticking and liquid-internal slip.

Exact solutions of incompressible Couette flow with porous walls for slightly rarefied gases

Abstract: In this work, the transient incompressible Couette flow and steady-state temperature profiles between two porous parallel plates for slightly rarefied gases are solved exactly. The first-order approximation of slip velocity at the boundaries is used in the formulation. The solution is also applicable for Couette flow in micro-channels under certain circumstances. The influences of mass transfer and a nondimensional slip parameter on slip velocities are discussed. It is also found that the transient slip velocities at the walls are greatly different from the steady-state velocity slips. The influences of velocity slip and temperature slip parameters on the temperature distribution and heat transfer at the walls are analyzed and discussed. It is shown that the slip parameters can greatly change the temperature profiles and heat transfer characteristics at the walls.

Short-wave instability due to wall slip and numerical observation of wall-slip instability for microchannel flows

Abstract: The stability of an incompressible parallel flow with a linear wall slip - where the amount of wall slip is proportional to the velocity gradient - is analysed. Numerical computations are performed for microchannel flows in the slip-flow region. The wall slip causes short-wave instability while the slip-flow model is stable for long waves. This instability disappears once the slip wall condition is replaced by the no-slip condition.

The Effect of Creep Flow on Two-Dimensional Isoflux Microchannels

Abstract: Micro channel convective heat transfer and friction loss characteristics are numerically evaluated for gaseous, two-dimensional, steady state, laminar, constant wall heat flux flows. The effects of Knudsen number, accommodation coefficients, second order slip boundary conditions, creep flow, and thermal/hydrodynamic developing flow are considered. These effects are compared through the Poisuelle number and Nusselt number. Numerical values for the Poisuelle and Nusselt numbers are obtained using a continuum based three-dimensional, unsteady, compressible computational fluid dynamics algorithm that has been modified with slip boundary conditions. To verify the numerical results, analytic solutions for the hydrodynamically and thermally fully developed Poisuelle and Nusselt numbers have been derived. The fully developed analytic Poisuelle and Nusselt numbers are given as a function of Knudsen number, the first and second order velocity slip and temperature jump coefficients, the Brinkman number, and the ratio of the thermal creep velocity to the mean velocity. Excellent agreement between the numerical and analytical data is demonstrated. Second order slip terms and creep velocity are shown to have significant effects on the Poisuelle and Nusselt numbers.Copyright © 2006 by ASME

Use of Fuzzy Controller for Hybrid Traction Control System in Hybrid Electric Vehicles

Abstract: In the normal condition, the front wheels follow the control trace of the driver and rear wheels follow the direction of the vehicle. The vehicle will spin and lose the control trace of the driver if the traction force is greater than the friction force. Therefore, a vehicle should maintain an adequate slip ratio of the tires and follow the control trace of the driver. This paper described a fuzzy Controller for Hybrid Traction Control System in Hybrid Electric Vehicles (HEVs) that prevented the spinning of the drive wheels during take-off and acceleration through targeted, brief brake impulses in motor torque. The task is to have the fuzzy supervisory controller generate the electric brake torque, for motor of a HEV. The electric brake torque is treated as reference input regenerative braking torque, for lower level control modules. When these lower level motor controller tracks its reference input, the desired slip ratio, can be reduced. Emergency lane change, tire slip ratio change simulations and experimental results were performed to show the effectiveness of the control. The efficiency and easy implementation of the Fuzzy Controller lead to the conclusion that Fuzzy Logic is an adequate and promising framework for Hybrid Traction Control System in Hybrid Electric Vehicles.

Effects of Slip on the Flow Characteristics of Laminar Flat Plate Boundary-Layer

Abstract: A uniform stream of viscous fluid flowing past a flat plate with slip at the fluid-plate interface is considered. The aim of this investigation is to examine the effects of slip at the wall on the laminar boundary-layer flow characteristics. For this purpose, an analytical and a computational study were conducted. The analytical study is based on the Blasius similarity solution for laminar boundary-layer flow past a flat plate. In the computational study, the flow over a flat plate is modeled and solved (using FLUENT) by dividing the computational domain into small control volumes and discretizing and solving the governing equations around these control volumes. The slip at the fluid-solid interface is accounted for by the Navier boundary condition (NBC). Three cases are considered, wherein the slip boundary condition is incorporated in three different ways, i.e., by specifying the slip length, or the slip velocity, or by assuming the slip length to be a function of shear rate. The flow characteristics are evaluated for different amounts of slip. The wall shear stress, the skin-friction coefficient, and the drag coefficient decrease by about 38% when the nondimensional slip length is increased from zero (noslip) to 2. The boundary-layer thickness, the displacement thickness, and the momentum thickness also decrease with increase in slip. The reduction in displacement thickness is much greater (about 68%) than the boundary-layer thickness or the momentum thickness. An excellent agreement between the analytical and computational results is noticed.© 2006 ASME

Theoretical and numerical studies of non-equilibrium slip effects on a catalytic surface

Abstract: A new concentration slip model to describe the rarefied gas effect on the species transport in microscale chemical reactors was derived from the approximate solution of the Boltzmann equation. The present model is more general and recovers the existing models in the limiting cases. The analytical results showed that the concentration slip is dominated by two different mechanisms, the reaction induced concentration slip (RIC) and the temperature slip induced concentration slip (TIC). The magnitude of RIC slip is proportional to the product of the Damkohler number and Knudsen number. The impact of the velocity, concentration and temperature slips on the coupling between the surface catalytic reactions and the homogeneous gas phase reactions was examined using the detailed chemistry of hydrogen and methane within a wide range of accommodation coefficients in a two-dimensional microscale chemical reactor. The results showed that the impact of reaction induced concentration slip (RIC) effects on catalytic reac...

Lattice Boltzmann study of three-dimensional gas microchannel flows

Abstract: The compressibility and rarefaction of three-dimensional (3D) pressure-driven gas microchannel flow is studied by the lattice Boltzmann equation method. The method employs a modified particle distribution function and a Knudsen-number–relaxation-time relation without use of any ad hoc treatment at the wall for the slip velocity. The effects of the aspect ratio of the channel width to height, Ar, and the outlet Knudsen number, Kno, on pressure nonlinearity, slip velocity and mass flow rate are investigated. As Ar increases, the slip velocity decreases. As a consequence, the nonlinearity of pressure increases. Their distributions, as expected, get closer to some two-dimensional (2D) limit lines. With Ar being greater than 1, one of the most interesting phenomena is the variation of the slip velocity along the wall due to variation in shear rate. As Kno decreases, the differences between slip velocities Us-z at the bottom and top walls and Us-y at the side walls decrease. The mass flow rate is also calculated and compared with 2D analytical solution. The results show that the mass flow rate through the gas microchannel decreases with decreasing Ar. On the other hand, as Ar increases, the computed mass flow rate gets closer to 2D analytical solution.The page numbers of this article were corrected on 24 July 2006. The corrected electronic version is identical to the print version.

Vehicle drive unit

Abstract: A vehicle drive unit, for quickly recovering a grip even if a wheel slips, in which in a slip state or at a time when the slip state is going to be generated, by pulsating a torque applied to the wheel, an angular velocity W of the wheel is changed so as to change the slip ratio S close to a slip ratio at which a friction coefficient becomes maximum, and thus, an average of a friction coefficient μ is enlarged and an average of a vehicle driving force F generated by a wheel is enlarged. Since the vehicle driving force is enlarged, the vehicle is accelerated and is easily gripped. Further, in a grip state or at a time of traveling at a high velocity, a vibration and an undesired sound are suppressed by doing away with the torque pulsation or making the fluctuation band of the torque pulsation small.

Ship maneuvering device

Abstract: An instruction-value conversion subsection 31 of an operational-target-value computation section 30 converts instruction signals received from a joystick 25. In order to achieve a mode of movement of a ship as intended by a ship operator, by use of the converted values, a target propeller rotational-speed computation subsection 32 computes target rotational speeds for left and right propellers 13 and a propeller 14b of a thruster 14, respectively. In a main-engine operation control section 40, in order to rotate the propellers 13 at the respective target rotational speeds, a target slip ratio determination subsection 41 computes a slip ratio U for clutch mechanisms 120 of marine gears 12. A drive control subsection 42 controls operations of main engines 11 and the clutch mechanisms 120. In a thruster operation control section 50, a drive control subsection 52 drives and controls the propeller 14b so as to rotate the propeller 14b in a direction determined by an operation determination subsection 51.