Showing papers on "Slip ratio published in 2005"

Hydrodynamics of Taylor Flow in Vertical Capillaries: Flow Regimes, Bubble Rise Velocity, Liquid Slug Length, and Pressure Drop

Abstract: Two-phase flow hydrodynamics in vertical capillaries of circular and square cross sections were experimentally studied, using air as the gas phase and water, ethanol, or an oil mixture as the liquid phase. The capillary hydraulic diameters ranged from 0.9 mm to 3 mm, with the superficial gas and liquid velocities covering a span of 0.008−1 m/s, which is typical of that obtained in monolith reactors. Using a high-speed video camera, four distinct flow regimes were observed within the range at which experiments were conducted: bubbly, slug-bubbly, Taylor, and churn flows. Annular flow was observed at excessively high gas and low liquid flow rates, well beyond those of interest to this study. Based on the definition of a two-class flow regime, the combination of two parametersthe slip ratio (S) and the ratio of the superficial gas velocity to two-phase superficial velocity (UG/UTP)was observed to be suitable for determining the transition from homogeneous flow to nonhomogeneous flow. The influence of capill...

Slip behavior in liquid films on surfaces of patterned wettability: Comparison between continuum and molecular dynamics simulations

Abstract: We investigate the behavior of the slip length in Newtonian liquids subject to planar shear bounded by substrates with mixed boundary conditions. The upper wall, consisting of a homogenous surface of finite or vanishing slip, moves at a constant speed parallel to a lower stationary wall, whose surface is patterned with an array of stripes representing alternating regions of no shear and finite or no slip. Velocity fields and effective slip lengths are computed both from molecular dynamics (MD) simulations and solution of the Stokes equation for flow configurations either parallel or perpendicular to the stripes. Excellent agreement between the hydrodynamic and MD results is obtained when the normalized width of the slip regions, a/sigma>~O(10), where sigma is the (fluid) molecular diameter characterizing the Lennard-Jones interaction. In this regime, the effective slip length increases monotonically with a/sigma to a saturation value. For a/sigma<~O(10) and transverse flow configurations, the nonuniform interaction potential at the lower wall constitutes a rough surface whose molecular scale corrugations strongly reduce the effective slip length below the hydrodynamic results. The translational symmetry for longitudinal flow eliminates the influence of molecular scale roughness; however, the reduced molecular ordering above the wetting regions of finite slip for small values of a/sigma increases the value of the effective slip length far above the hydrodynamic predictions. The strong correlation between the effective slip length and the liquid structure factor representative of the first fluid layer near the patterned wall illustrates the influence of molecular ordering effects on slip in noninertial flows.

Apparent slip and viscoplasticity of concentrated suspensions

Abstract: The apparent slip flows of incompressible and viscoplastic (Herschel–Bulkley) fluids in plane Couette, capillary, and rectangular slit dies under fully developed, isothermal, and creeping flow conditions were analyzed assuming that the apparent slip layer consists solely of the binder and its thickness is independent of the flow rate. Both the drag-induced (plane Couette) and pressure-induced (capillary and slit) flows generate the same dependencies of the wall-slip velocity on the wall shear stress. Navier’s slip coefficient, which relates the wall-slip velocity to the shear stress, is similar for all three flows and is a function of the thickness of the apparent slip layer and the shear viscosity of the binder. The assumed apparent slip mechanism provides methodologies for the determination of the slip velocity values that are consistent with the traditional Mooney method and furthermore allows the determination of the true shear rate of the suspension at the wall and the yield stress. The analysis of t...

Lubrication Models with Small to Large Slip Lengths

Abstract: A set of lubrication models for the thin film flow of incompressible fluids on solid substrates is derived and studied. The models are obtained as asymptotic limits of the Navier-Stokes equations with the Navier-slip boundary condition for different orders of magnitude for the slip-length parameter. Specifically, the influence of slip on the dewetting behavior of fluids on hydrophobic substrates is investigated here. Matched asymptotics are used to describe the dynamic profiles for dewetting films and comparison is given with computational simulations. The motion of the dewetting front shows transitions from being nearly linear in time for no-slip to t 2/3 as the slip is increased. For much larger slip lengths the front motion appears to become linear again. Correspondingly, the dewetting profiles undergo a transition from oscillatory to monotone decay into the uniform film layer for large slip. Increasing the slip further, to very large values, is associated with an increasing degree of asymmetry in the structure of the dewetting ridge profile.

Friction and Slip at Simple Fluid-Solid Interfaces: The Roles of the Molecular Shape and the Solid-Liquid Interaction

Abstract: the fluid in the immediate vicinity of the solid wall, and conducted to explore in a systematic way the role of the strength of the fluid-solid interactions on the slip length. Comparing the flow behavior of two different fluids, on the same surfaces, at fixed strength of the fluid-solid interactions, we were able for the first time to demonstrate that small changes in the shape of the molecules of the fluid deeply affect the liquid-solid friction and the local velocity at the wall. The technique used, called near field laser velocimetry (NFLV) is based on total internal reflection-fluorescence recovery after photobleaching and has been described before [2,14]. We briefly present here its principle and the improvements in the data analysis we have brought, in order to justify the accuracy in the determination of the slip length values. We shall then compare the slip behavior of two different liquids chosen to have the same surface tension, on a series of chemically modified surfaces, with gradually decreasing strength of solid-fluid interactions but constant roughness. The two liquids are squalane and hexadecane. Their liquid-vapor surface tension are, respectively, 27.4 and 27: 6m J=m 2 at 20 � C, as measured using the ring technique

A note on the stability of slip channel flows

Abstract: We consider the influence of slip boundary conditions on the modal and nonmodal stability of pressure-driven channel flows. In accordance with previous results by Gersting [“Hydrodynamic stability of plane porous slip flow,” Phys. Fluids 17, 2126 (1974)] but in contradiction with the recent investigation of Chu [“Instability of Navier slip flow of liquids,” C. R. Mec. 332, 895 (2004)], we show that the slip increases significantly the value of the critical Reynolds number for linear instability. The nonmodal stability analysis, however, reveals that slip has a very weak influence on the maximum transient energy growth of perturbations at subcritical Reynolds numbers. Slip boundary conditions are therefore not likely to have a significant effect on the transition to turbulence in channel flows.

Numerical Analysis of a Journal Bearing With a Heterogeneous Slip/No-Slip Surface

Abstract: The no-slip boundary condition is part of the foundation of the traditional lubrication theory. It states that fluid adjacent to a solid boundary has zero velocity relative to the solid surface. For most practical applications, the no-slip boundary condition is a good model for predicting fluid behavior. However, recent experimental research has found that for certain engineered surfaces the no-slip boundary condition is not valid. Measured velocity profiles show that slip occurs at the interface. In the present study, the effect of an engineered slip/no-slip surface on journal bearing performance is examined. A heterogeneous pattern, in which slip occurs in certain regions and is absent in others, is applied to the bearing surface. Fluid slip is assumed to occur according to the Navier relation. Analysis is performed numerically using a mass conserving algorithm for the solution of the Reynolds equation. Load carrying capacity, side leakage rate, and friction force are evaluated. In addition, results are presented in the form of Raimondi and Boyd graphs. It is found that the judicious application of slip to a journal bearing’s surface can lead to improved bearing performance.

Experimental and simulation results of wheel-soil interaction for planetary rovers

Abstract: The ability to predict rover locomotion performance is critical during the design, validation and operational phases of a planetary robotic mission. Predicting locomotion performance depends on the ability to accurately characterize the wheel-soil interactions. In this research, wheel-soil interaction experiments were carried out on a single-wheel testbed and the results were compared with a single-wheel dynamic computer simulator which was developed in Matlab and Simulink's SimMechanics toolbox using a commercially-available wheel-soil interaction computer model called AESCO Soft Soil Tire Model (AS/sup 2/TM). Two different tire treads were used and compared in this study. There is good agreement between experimental and simulation results for wheel sinkage as a function of slip ratio; however, more investigation is needed to understand the differences observed for the drawbar pull and motor torque results.

Velocity slip in microscale cylindrical Couette flow: the Langmuir model

Abstract: The velocity slip on the solid surfaces of microscale cylindrical Couette flow is investigated using the Langmuir adsorption model for the gas-surface molecular interaction. The accommodation coefficient in the Maxwell model, which is a free parameter based on the concept of diffusive reflection, is replaced by a physical parameter of heat adsorption in the Langmuir model. The phenomenon of velocity inversion is then clearly explained by introducing a velocity polar on the hodograph plane. It is also shown that the quantity used to determine the momentum slip in a concentric cylindrical geometry should be based upon the angular velocity, not the velocity itself. Finally, and despite their totally independent considerations of the gas-surface molecular interaction, the Maxwell and Langmuir slip models are shown to be in qualitative agreement with direct simulation Monte Carlo data in capturing the general features of the flow field.

Viscosity and slip velocity in gas flow in microchannels

Abstract: It was shown that as a result of the interaction of part of the molecules with a wall, the viscosity in the Knudsen layer is lower in comparison with the viscosity in the bulk fluid. The slip condition of Maxwell depends on the normal gradient of the tangential velocity on the wall. The reduction of the viscosity increases the tangential velocity and its normal gradient, and, therefore, increases the slip. The correction factors for the Maxwell expression for one-dimensional plane and tubular isothermal and incompressible flows as a function of the Knudsen number and of the kind of interaction of molecules with the wall were derived.

The effect of slip in the flow of a branched PP melt: experiments and simulations

Abstract: In this paper visualisation and direct velocity profile measurement experiments for a branched polypropylene melt in a 10:1 axisymmetric contraction demonstrate the onset of wall slip. Video processing of the flow shows the formation of vortices and their diminution with increasing flow rate. Numerical simulations using a multimode K-BKZ viscoelastic and a purely viscous (Cross) model—both of them incorporating a nonlinear slip law—were used to predict the flow kinematics and dynamics as well as to deduce the slip velocity function by performing fitting to the velocity profiles. It was found that the numerical predictions agree well with the experimental results for the velocity profiles, and vortex formation, growth and reduction. It is suggested that such experiments (visualisation of entrance flow and direct velocity profile measurement) can be useful in evaluating the validity of constitutive equations and slip laws in the flow of polymer melts through processing equipment.

The wrinkle‐like slip pulse is not important in earthquake dynamics

Abstract: [1] A particular solution for slip on an interface between different elastic materials, the wrinkle-like slip pulse, propagates in only one direction with reduced normal compressive stress. More general solutions, and natural earthquakes, need not share those properties. In a 3D dynamic model with a drop in friction and heterogeneous initial stress, the wrinkle-like slip pulse is only a small part of the solution. Rupture propagation is determined primarily by the potential stress drop, not by the wrinkle-like slip pulse. A 2D calculation with much finer resolution shows that energy loss to friction might not be significantly reduced in the wrinkle-like slip pulse.

A semiphysical tyre model for vehicle dynamics analysis of handling and braking

Abstract: A semiphysical model presented in this paper first includes the transient steer effect characterized by both the time delay of steering application and the stress relaxation of friction due to the tyre’s viscoelastic property. The model experimentally formulates the vertical force as a function of the tyre deflection, camber angle and lateral force. In order to describe the longitudinal and lateral forces as functions of the tyre’s characteristic parameters such as stiffness and friction, the basic formulations of the UA tyre model are reused. For self-aligning and overturning moments, first the application points of three forces are experimentally formulated, and then the moments are expressed as functions of the three forces and their application points. All characteristic parameters are experimentally determined as functions of the load, camber angle, slip angle, slip angle sweep rate and/or slip ratio for cornering and/or driving–braking. For the purpose of validation, the model is implemented into AD...

Effect of volume fraction and particle size on wall slip in flow of polymeric suspensions

Abstract: For especially highly concentrated suspensions, slip at the wall is the controlling phenomenon of their rheological behavior. Upon correction for slip at the wall, concentrated suspensions were observed to have non-Newtonian behavior. In this study, to determine the true rheological behavior of model concentrated suspensions, “multiple gap separation method” was applied using a parallel-disk rheometer. The model suspensions studied were polymethyl methacrylate particles having average particle sizes, in the range of 37–231 μm, in hydroxyl terminated polybutadiene. The effects of particle size and solid particle volume fraction on the wall slip and the true viscosity of model concentrated suspensions were investigated. It is observed that, as the volume fraction of particles increased, the wall slip velocity and the viscosity corrected for slip effects also increased. In addition, for model suspensions in which the solid volume fraction was ≥81% of the maximum packing fraction, non-Newtonian behavior was observed upon wall slip correction. On the other hand, as the particle size increased, the wall slip velocity was observed to increase and the true viscosity was observed to decrease. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 439–448, 2005

Slip Factor for Centrifugal Impellers Under Single and Two-Phase Flow Conditions

Abstract: Throughout the history of turbomachines investigators have tried to develop reliable methods for prediction of centrifugal pump behavior. Among the parameters available to estimate the performance of this kind of machine is the slip factor. In spite of being regarded as a variable of great significance in the analysis of turbomachinery, there seem to be a misconception regarding its concept and application. Indeed, empirical correlations have been widely used to estimate the slip factor, even in the case of two-phase flow applications, where it has not been investigated. Moreover, these correlations provide a constant value of the slip factor for a given impeller only at the best efficiency point, which is an important restriction to the pump performance prediction, considering that slip factor varies with the pump flow rate. In this study, three-dimensional computational fluid dynamics simulations were carried out on an impeller of known geometry (Ns = 1960) from which values of slip factor were calculated for both single- and two-phase flow (water and water-air). These results include curves of the slip factor as a function of the specific capacity and the gas-void fraction. Additionally, results for the slip factor in the case of single-phase flow (water) are given for various centrifugal impellers (N S = 1157, 1447, 1612, and 3513) in order to illustrate the influence of the flow rate on this parameter.

Effect of surface roughness on slip flows in hydrophobic and hydrophilic microchannels by molecular dynamics simulation

Abstract: The influences of surface roughness on the boundary conditions for a simple fluid flowing over hydrophobic and hydrophilic surfaces are investigated by molecular dynamics (MD) simulation. The degree of slip is found to decrease with surface roughness for both the hydrophobic and hydrophilic surfaces. The flow rates measured in hydrophobic channels are larger than those in hydrophilic channels with the presence of slip velocity at the walls. The simulation results of flow rate are correlated with the theoretical predictions according to the assumption of no slip boundary condition. The slip boundary condition also strongly depends on the shear rate near the surface. For hydrophobic surfaces, apparent fluid slips are observed on smooth and rough surfaces. For simple fluids flowing over a hydrophobic surface, the slip length increases linearly with shear rate for both the smooth and rough surfaces. Alternately, the slip length has a power law dependence on the shear rate for the cases of hydrophilic surfaces...

Direct Measurement of Slip Velocities Using Three-Dimensional Total Internal Reflection Velocimetry

Abstract: The possible existence of slip of liquids in close proximity to a smooth surface is studied experimentally via the dynamics of small particles suspended in a shear flow. Sub-micron fluorescent particles suspended in water are imaged and analyzed 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. Furthermore, surface hydrophobicity can be attributed for additional shear-rate dependent boundary slip. Issues associated with the experimental technique and the interpretation of results are also discussed.Copyright © 2005 by ASME

Brake Device For A Two-Wheeled Motor Vehicle, And Method Of Using Same

Abstract: In a braking device of a motorcycle, in which a rear wheel braking mechanism is operated in conjunction with a front wheel braking operation, accurate determination of a reduction in rear wheel load is provided, and additional reduction of the rear-wheel load is prevented. Determination of a reduction in rear-wheel load is based on vehicle speed in the front wheel braking operation, hydraulic pressure of a brake caliper on the front wheel side, and respective slip ratios of the front and rear wheels. When the rear-wheel load is reducing, hydraulic pressure of a brake caliper on the rear wheel side is controlled so as to keep the slip ratio of the rear wheel at or below a set value. A hydraulic modulator supplies/evacuates the hydraulic pressure to/from the brake caliper on the rear wheel side, and the slip ratio of the rear wheel is controlled by electrically controlling the hydraulic modulator.

On the boundary slip of fluid flow

Abstract: This paper presents a stress controlled boundary slip model and predicts the fluid-solid interface slip in a system of parallel sliding plates or a sphere approaching a smooth plane. The numerical simulation results are in striking agreement with the existing experimental observations. This model assumes that there is a limiting shear stress. No slip occurs if the surface shear stress is smaller than the limiting shear stress, and slip occurs when the surface shear stress equals it. It is found that boundary slip dramatically decreases the hydrodynamic pressure if the two squeezed surfaces have the same slip property. Finally, the hydrodynamic force reaches a saturation status and almost does not decrease any more. Compared with the no-slip solution, hydrodynamic force is found to decrease by over two orders in the case of boundary slip. When the squeezed surfaces have different slip properties, however, the hydrodynamic pressure is mainly controlled by the surface having a smaller surface limiting shear stress, and reduces more slowly compared with the case of two surfaces having the same slip property. Even when one of the surfaces has a zero surface limiting shear stress, a considerable hydrodynamic force still exists.

Enhanced melt strength and stretching of linear low-density polyethylene extruded under strong slip conditions

Abstract: The influence of extrusion under strong slip conditions on the extensional properties of linear low-density polyethylene was studied in this work. The material was extruded at two different temperatures under strong slip and no slip conditions, and was subsequently subjected to uniaxial elongational flow by means of a Rheotens device. Strong slip was evident through the elimination of sharkskin distortions and the stick-slip instability, as well as by the electrification of the extrudates. The extrudate swell was smaller in the presence of slip when comparing with no slip conditions at constant apparent shear rate, but it was found to be a unique function of the shear stress if comparison was performed at constant stress. The draw ratio and melt strength of the filaments obtained under slip conditions were larger compared to those without slip. In addition, draw resonance was postponed to higher draw ratios during the extrusion with strong slip at constant apparent shear rate. It is suggested that slip of the polymer at the die wall decreases the shear stress in the bulk, and therefore, restricts the disentanglement and orientation of macromolecules during flow, which subsequently produces the increase in draw ratio and melt strength during stretching.

Temperature jump and slip velocity calculations from an anisotropic scattering kernel

Abstract: This article deals with the problem of temperature jump and slip velocity at the wall in gas/surface interaction. A consistent modelling of an impermeable surface involving an anisotropic scattering kernel developed in previous works is used to establish boundary conditions in unstructured molecule gas flows. Thus, a temperature jump relation is derived in which the gas viscous effects at the wall and the mean velocity gradients appear. Likewise, a slip velocity relation is obtained in which both the slip coefficient and the thermal creep coefficient depend on the wall-to-gas temperature ratio. Moreover, both the temperature jump and the slip velocity relations involve not only one accommodation coefficient as usual, but also the gas/surface information through the various (notably normal and tangential) accommodation coefficients of the momentum components.

Numerical Simulation of Fluid Flow in Microchannels With Superhydrophobic Walls

Abstract: The three-dimensional flow of a Newtonian fluid in a microchannel with superhydrophobic walls is computed using a finite element analysis. Calculations of the fully-developed laminar flow of water under a pressure gradient of 1 psi/cm in an 80 μm high channel with superhydrophobic upper and lower surfaces containing a 2 μm pitch array of 0.2 μm square posts shows a 40 percent flow enhancement relative to the smooth, non-patterned surface case, and an apparent slip length of 5.4 μm. A sharp gradient is observed in the axial velocity field within 0.5 μm of the post surface and normal to the post center. The calculated axial velocity field away from the superhydrophobic surface agrees well with the analytical solution for two-dimensional channel flow with Navier’s slip condition applying at the channel wall. Mesh refinement studies indicate the important role that adequate resolution of the sharp gradient in the velocity field adjacent to the post surface plays in obtaining accurate flow enhancement predictions. Decreasing the relative contact area of the fluid with the solid portion of the channel surface, either by increasing the post-to-post spacing or decreasing the post size, results in a monotonic increase in the flow enhancement. Wetting of the fluid into the post structure is shown to dramatically decrease the calculated flow enhancement. Calculations of the flow enhancement for fixed surface properties and varying channel heights result in apparent slip lengths that agree to within 1 percent, suggesting that the macroscopic flow behavior is adequately characterized in terms of an apparent slip model, with the magnitude of the slip length a function of the post size, post spacing and wetting behavior that characterize the local flow field.© 2005 ASME

The effect of frequency of fluctuating driving force on basic gaseous micro-flows

Abstract: The effect of frequency of fluctuation of the driving force on velocity slip and temperature jump at the wall(s) is theoretically investigated in four cases of basic gas micro-flow problems. The cases considered are the transient Couette flow, the pulsating Poiseuille flow, the Stoke's second problem flow and the transient natural convection flow. The formulation of the problem revealed that the controlling parameter of the problem is a combination of Knudsen number (Kn) and frequency of fluctuation (ω) in the form of an effective Knudsen number, that replaced the usual Knudsen number (Kn), and consequently the slip flow regime is found to be valid when 10-3≤ Kneff≤ 10-1. It is found that when the frequency is small the velocity and temperature profiles are similar to the corresponding classical macro-flow profiles at zero frequency. Also, the slip in velocity and the jump in temperature increase as the Knudsen number and/or the frequency of the driving force increases. In addition, the slip in velocity and the jump in temperature are found to be negligible when the frequency and/or Kn are sufficiently small.

Positioning device and positioning method

Abstract: PROBLEM TO BE SOLVED: To provide a positioning device and method capable of correcting a position update error of a measured value of a wheel speed sensor unit resulting from a slip, and updating the position of a moving body with high precision SOLUTION: When the position of the moving body can be updated by a positioning result from a GPS receiver 2, the relation between a slip ratio Sr and an acceleration Aw determined from information of the sensor is predetermined from information acquired from the wheel speed sensor 1 and the receiver 2 When the position of the moving body can not be updated by the positioning result from the receiver 2, a slip ratio Sr corresponding to a found acceleration Aw' is calculated from the above relation By using this slip ratio Sr and correcting the measured value of the wheel speed sensor unit, and performing a position update by using this corrected value, the precision of updating of the position of the moving body can be raised COPYRIGHT: (C)2007,JPO&INPIT

Advanced braking system for aircraft

Abstract: An advanced braking system for an aircraft is disclosed. A controller calculates the braking required from each wheel in terms of force. A constant deceleration is achieved throughout a braking run by calculating the braking from other sources, principally aerodynamic drag, and commanding a complementary total level of braking from the wheel brakes. The performance of each wheel and brake are monitored during the braking run to determine whether their braking performance is limited by the brake discs or by the tyre-ground interaction and to see whether the wheel is approaching the maximum slip ratio after which a skid occurs. The controller uses this information to distribute the total demand for braking amongst the wheels. In doing this, it also aims to keep the braking demand symmetrical across the aircraft and not to overheat the brakes. The controller further measures the braking force provided by a wheel and controls its brake pressure accordingly to achieve the force desired.