Showing papers on "Slip ratio published in 2007"

Rate-dependent slip boundary conditions for simple fluids.

Abstract: The dynamic behavior of the slip length in a fluid flow confined between atomically smooth surfaces is investigated using molecular dynamics simulations. At weak wall-fluid interactions, the slip length increases nonlinearly with the shear rate provided that the liquid/solid interface forms incommensurable structures. A gradual transition to the linear rate dependence is observed upon increasing the wall-fluid interaction. We found that the slip length can be well described by a function of a single variable that in turn depends on the in-plane structure factor, contact density, and temperature of the first fluid layer near the solid wall. Extensive simulations show that this formula is valid in a wide range of shear rates and wall-fluid interactions.

Slip flow in non-circular microchannels

Abstract: Microscale fluid dynamics has received intensive interest due to the emergence of Micro-Electro-Mechanical Systems (MEMS) technology. When the mean free path of the gas is comparable to the channel’s characteristic dimension, the continuum assumption is no longer valid and a velocity slip may occur at the duct walls. Non-circular cross sections are common channel shapes that can be produced by microfabrication. The non-circular microchannels have extensive practical applications in MEMS. Slip flow in non-circular microchannels has been examined and a simple model is proposed to predict the friction factor and Reynolds product fRe for slip flow in most non-circular microchannels. Through the selection of a characteristic length scale, the square root of cross-sectional area, the effect of duct shape has been minimized. The developed model has an accuracy of 10% for most common duct shapes. The developed model may be used to predict mass flow rate and pressure distribution of slip flow in non-circular microchannels.

Traction Control based on Slip Ratio Estimation Without Detecting Vehicle Speed for Electric Vehicle

Abstract: In traction control systems, it is generally required to detect vehicle speed in order to obtain the slip ratio. However, it is very difficult to measure vehicle speed directly. In this paper, novel estimation method of slip ratio is proposed without detecting the vehicle speed. The simulations and experiments both of slip ratio estimation and control are carried out by an electric vehicle to verify the proposed algorithm.

Effect of surface roughness on rate-dependent slip in simple fluids.

Abstract: Molecular dynamics simulations are used to investigate the influence of molecular-scale surface roughness on the slip behavior in thin liquid films. The slip length increases almost linearly with the shear rate for atomically smooth rigid walls and incommensurate structures of the liquid/solid interface. The thermal fluctuations of the wall atoms lead to an effective surface roughness, which makes the slip length weakly dependent on the shear rate. With increasing the elastic stiffness of the wall, the surface roughness smoothes out and the strong rate dependence is restored again. Both periodically and randomly corrugated rigid surfaces reduce the slip length and its shear rate dependence.

Direct measurement of slip length in electrolyte solutions

Abstract: Electrokinetic effects and electrostatic repulsion between tracer particles and glass surface have both been proposed as possible sources that would lead to false slip results obtained from velocimetry-based measurements. Using a three-dimensional total internal reflection velocimetry technique, we address such a concern by comparing the measured slip lengths between nonionic solutions and electrolyte solutions whose ionic concentrations have been predicted to reduce the electricity-induced slip effect to a submolecular level. It is observed that the presence of electrolytes has no effect on the measured slip lengths, suggesting that the observed slip velocities are most likely not due to electrostatic and electrokinetic effects, but are consequences of true boundary slip.

A kinetic-theory based first order slip boundary condition for gas flow

Abstract: In this paper, a first-order slip boundary condition is derived using the Chapman-Enskog solution of the Boltzmann equation. In comparison with the existing slip models such as first, second, and 1.5-order slip models, the Poiseuille flow rate predicted by the new slip model shows better agreement with that calculated by the linearized Boltzmann equation. The slip boundary condition is also applied to predict the pressure field in gas lubrication problems.

Quantifying hydrodynamic slip: a comprehensive analysis of dewetting profiles.

Abstract: To characterize nontrivial boundary conditions of a liquid flowing past a solid, the slip length is commonly used as a measure. From the profile of a retracting liquid front (e.g., measured with atomic force microscopy), the slip length can be extracted with the help of a Stokes model for a thin liquid film dewetting from a solid substrate. Specifically, we use a lubrication model derived from the Stokes model for strong slippage and linearize the film profile around the flat, unperturbed film. For small slip lengths, we expand the linearized full Stokes model for small slopes up to third order. Using the respective model, we obtain, in addition to the slip length, the capillary number, from which we can estimate the viscosity of the fluid film. We compare numerical and experimental results, test the consistency and the validity of the models/approximations, and give an easy-to-follow guide of how they can be used to analyze experiments.

Axisymmetric stagnation-point flow over a lubricated surface

Abstract: Axisymmetric stagnation-point flow is considered. A Newtonian fluid impinges orthogonally on a plane surface lubricated by a thin non-Newtonian liquid film of variable thickness. A slip-flow boundary condition is deduced, which allows for partial slip at the surface. The amount of slip, from full slip to no-slip, is controlled by a dimensionless slip coefficient. Similarity solutions are generally prohibited by the slip-flow boundary condition, except for one particular value of the power-law index of the lubricant. Solutions are presented for this case in order to demonstrate the influence of partial slip on the stagnation point flow. With increasing slip and reduced surface stress, a thinning of the viscous boundary layer is observed. The classical Homann flow is recovered in the no-slip limit.

Slip ratio estimating device and slip ratio control device

Abstract: A slip ratio estimating device for measuring the slip ratio without using the speed of a vehicle body and a slip ratio control device using the slip ratio estimating device are provided. A motor torque measured by a torque measurer is input into a vehicle model (601) and an SRE (602) which calculates the time derivative of the slip ratio. The vehicle model (601) derives the rotational speed of a wheel, the rotational acceleration of the wheel, and the speed of the vehicle body, and outputs the rotational speed of the wheel and the rotational acceleration of the wheel to the SRE (602). The SRE (602) outputs the slip ratio according to the motor torque, the rotational speed of the wheel, and the rotational acceleration of the wheel.

A wet-road braking distance estimate utilizing the hydroplaning analysis of patterned tire

Abstract: A wet-road braking distance estimate for the vehicles equipped with ABS (anti-lock brake system) is presented in this paper. The entire speed interval at braking is divided into several uniform sub-intervals, and the energy conservation law is applied to individual sub-intervals to predict the intervalwise braking times and the resulting velocity profile at braking and the total braking distance. The proposed method is based on a numerical-analytical approach such that the frictional energy loss of the patterned tire is computed by 3-D hydroplaning analysis while the other at the disc pad is analytically derived. The operation of ABS is numerically implemented by controlling the tire angular velocity such that the preset tire slip ratio on the wet road is maintained. The tire hydroplaning is analysed by generally coupling an Eulerian finite volume method and an explicit Lagrangian finite element method. Illustrative numerical experiment is presented to support the validity of the proposed estimate. Copyright © 2006 John Wiley & Sons, Ltd.

Magnetic fluid lubrication of porous-pivoted slider bearings with slip velocity:

Abstract: A theoretical model of a magnetic fluid lubrication of porous-pivoted slider bearing with slip velocity has been considered to see the effect of various parameters. The expression of load capacity and centre of pressure has been obtained in terms of slip and permeability parameters. The dependence of load and centre of pressure on magnetic parameter, permeability parameter, and slip parameter has been studied graphically. Minimization of the slip parameter and permeability parameter has been discussed for the possible increase in the load capacity.

Skid Steering in 4-Wheel-Drive Electric Vehicle

Abstract: This paper discusses skid steering applied to four wheel drive electric vehicles In such vehicles, steering is achieved by differentially varying the speeds of the lines of wheels on different sides of the vehicle in order to induce yaw Skid steer wheeled vehicles require elaborate tire model, so author choose the unite semi-empirical tire model From this model, longitudinal and lateral tire force can be calculated by slip ratio directly The vehicle model has 3-DOF, longitudinal, lateral and yaw direction, irrespective of suspension Induction motor is chosen as the driven motor, and the control method is rotor flux field oriented vector control To satisfy the requirement of the turn radius, the longitudinal slip must be controlled, so a method of slip limitation feedback is used in the simulation When the vehicle is turning on a slippery surface, because of the drop at the coefficient of road adhesion, the drive wheels may slip The traction control system reduces the engine torque and brings the slipping wheels into the desirable skid range Some simulation results about the steering accuracy and maneuverability are given in the paper

Water flow and slip on NAPL-wetted surfaces of a parallel-walled fracture

Abstract: [1] The effects of various wetting conditions and aperture sizes on water flow and slip in a parallel-walled fracture were investigated. Water flow experiments showed that a larger slip occurred as the surface became more hydrophobic. For a creosote-wetted surface of the fracture with an aperture of 508 μm, the increase in the water flow rate due to the slip was as much as 10.0% compared to that in a water-wetted surface. Similarly to a water-wetted surface, no slip took place at a gasoline-wetted surface, which was weakly hydrophobic. The slip lengths, a notional distance from the wall to a point inside the wall where fluid velocity extrapolates to zero, for light oil- and creosote-wetted surfaces were constant over a range of flow velocities in the laminar flow regime. The study indicates that no slip boundary condition-based equations are not adequate for quantifying water flow through NAPL-wetted fractures.

Numerical Analysis of 3‐D Flow Past a Stationary Sphere with Slip Condition at Low and Moderate Reynolds Numbers

Abstract: Computations are performed to determine the steady 3‐D viscous fluid flow forces acting on the stationary spherical suspended particle at low and moderate Reynolds numbers in the range of 0.1≤Re≤200. A slip is supposed on the boundary so that the slip velocity becomes proportional to the shear stress. This model possesses a single parameter to account for the slip coefficient λ (Pa.s/m), which is made dimensionless and is called Trostel number (Tr=λ a/μ). Decreasing slip, increases drag in all Reynolds limits, but slip has smaller effects on drag coefficient at lower Reynolds number regimes. Increasing slip at known Reynolds number causes to delay of flow separation and inflect point creation in velocity profiles. At full slip conditions, shear drag coefficient will be zero and radial drag coefficient reaches to its maximum values. Flow around of sphere at full‐slip condition is not equal to potential flow around a sphere. Present numerical results corresponding to full slip (Tr→0) are in complete accord ...

Dynamic Modelling of Wheel-Terrain Interaction of a UGV

Abstract: Understanding the vehicle-terrain interaction is essential for autonomous and safe operations of skid-steering unmanned ground vehicles (UGVs). This paper presents a comprehensive analysis of the dynamic processes involved in this interaction, using the vehicle kinetics and the theory of terramechanics to derive systematically shear displacement, reaction force, and load distribution for a wheel. The new model is then summarized in the form of an algorithm to allow for computation of characteristic performance of the interaction such as slip ratios, rolling resistance, and moment of turning resistance for a number of terrain types. Given the current state of the vehicle and terrain parameters, the model can be used to estimate its next states and to predict the vehicle running path. The development is illustrated by simulation and verified with experimental data.

Control system and vehicle including the same

Abstract: A control system for a vehicle includes an ECU, a front wheel speed sensor, a rear wheel speed sensor, and an engine. The ECU detects an actual slip speed and an actual slip ratio of a rear wheel on the basis of respective detected values of a front wheel speed sensor and a rear wheel speed sensor. Slip speed traction control is started when the actual slip speed exceeds a threshold value of a slip speed when a vehicle is at a low speed, and slip ratio traction control is started when the actual slip ratio exceeds a threshold value of a slip ratio when it is at an intermediate or high speed. An output of the engine is adjusted depending on the actual slip speed in the slip speed traction control, while being adjusted depending on the actual slip ratio in the slip ratio traction control.

Application of LBM in Simulation of Flow in Simple Micro-Geometries and Micro Porous Media

Abstract: Isothermal gas flow is simulated in micro-Couette, micro-channel and micro porous media using the lattice Boltzmann model (LBM). To consider compressibility and rarefaction effects, two relaxation time models related t o Knudsen number Kn and local density are used. Diffuse-scattering boun dary condition (DSBC) and a combination of bounce back and specular boundary conditions are used to obtain slip velocit y at the wall. For micro-Couette flow, the slip vel ocity and the slip length as a function of Kn are calculated and compared with that of MD, DSMC and Maxwell theorem. For micro-Couette, the velocity profile along with the slip velocity for d ifferent values of Kn and the slip length as a function of Knudson numbe r are studied. It is shown that the flow structures are c hanged by changing the values of Kn . The nonlinear pressure drop and the velocity distribution along the streamwise directio n in the micro-channel flow are obtained and compared with available data, which resulted in good agreement. The Knudsen minimum phenomenon is successfully predicted within the LBM framework. Finally, gas flow characteristics in mic ro porous media for different Kn numbers, porosity and inlet to outlet pressure ratio are studied. The effects of compress ibility and rarefaction are considered.

Perturbation analysis on gas flow in a straight microchannel

Abstract: In the present paper, a steady subsonic gas flow either in a circular micropipe or in a planar microchannel driven by pressure within the slip flow regime is studied theoretically by using a perturbation expansion method to solve compressible Navier-Stokes equations The isothermal flow assumption used in previous theoretical studies is given up High-order boundary conditions of velocity slip and temperature jump are adopted at the wall The set of dimensionless governing equations with two small similarity parameters, namely, the ratio of height to length e, and the Knudsen number Kn, is approximated successively by using the perturbation expansions The various cases such as e≪Kn2, e∼Kn2, and e∼Kn15 are studied in detail Explicit analytical solutions for pressure, density, velocity, temperature, and mass flow rate are obtained up to order of O(Kn2) It is shown that the solution formulas for long channels (e≪Kn2) in lower order are in exact agreement with previous theoretical results In particular,

Traction and Yaw-rate Control of Electric Vehicle with Slip-ratio and Cornering Stiffness Estimation

Abstract: In traction control systems of vehicles, it is generally required to detect vehicle speed in order to obtain the slip-ratio. However, it is hard to measure vehicle speed directly. In the first part of this paper, novel estimation method of slip-ratio is proposed without detecting the vehicle speed. Based on this estimation, a slip-ratio controller is developed with feedback linearization. In the second part, a novel DYC is proposed with the adaptive observer which can identify the front and rear cornering stiffness independently as well as it can estimate the body side-slip angle. The advantages of proposed methods are verified by simulations and experiments with an electric vehicle which has two in-wheel motors.