Transient Friction Research Articles

Study on the transient performance of textured water-lubricated bearings considering different acceleration conditions

This study analyses the transient friction dynamics behavior of water-lubricated bearings (WLBs) with a textured structure, which explains the mechanism of texture structure influencing the hydrodynamic effect of WLB in the physical aspect. A comparison of experimental and numerical data is carried out to validate the proposed mixed lubrication model with a textured structure for WLBs. The effects of texture type, texture angle, acceleration mode, and acceleration time on the nonlinear friction dynamics properties of WLBs are investigated. The result shows that various texture structures exhibit distinct pumping effects and that the optimal friction dynamics performance of WLBs can be achieved by adopting the right herringbone texture and an acceptable texture angle. It is advisable to utilize the reverse S-shaped acceleration mode, as it may efficiently mitigate hydrodynamic shock, minimize frictional contact at the initial startup stage, and control the rotor's vibration in later stages. The brief acceleration time may result in a transient shock that hampers proper lubrication, consequently affecting the stable operation of WLBs. The study's findings offer helpful suggestions for the enhanced design of WLB structures and the mitigation of wear and vibration.

On the analytical solution of transient friction in channel flows

On the analytical solution of transient friction in channel flows

Transient Friction Analysis of Pressure Waves Propagating in Power-Law Non-Newtonian Fluids

Modulated pressure waves propagating in the drilling fluids inside the drill string are a reliable real-time communication technology that transmit data from downhole to the surface during oil and gas drilling. In the analysis of pressure waves’ propagation characteristics, the modeling of transient friction in non-Newtonian fluids remains a great challenge. This paper establishes a numerical model for transient pipe flow of power-law non-Newtonian fluids by using the weighted residual collocation method. Then, the Newton–Raphson method is applied to solve the nonlinear equations. The numerical method is validated by using the theoretical solution of Newtonian fluids and is proven to converge reliably with larger time steps. Finally, the influencing factors of the wall shear stress are analyzed using this numerical method. For shear-thinning fluids, the friction loss of periodic flow decreases with the increase in flow rate, which is opposite to the variation law of friction with the flow rate for stable pipe flow. Keeping the amplitude of pressure pulsation unchanged, an increase in frequency leads to a decrease in velocity fluctuations; therefore, the friction loss decreases with the increase in frequency.

Observing and Modeling Short‐Term Changes in Basal Friction During Rain‐Induced Speed‐Ups on an Alpine Glacier

AbstractBasal shear stress on hard‐bedded glaciers results from normal stress against bed roughness, which depends on basal water pressure and cavity size. These quantities are related in a steady state but are expected to behave differently under rapid changes in water input, which may lead to a transient frictional response not captured by existing friction laws. Here, we investigate transient friction using Global Positioning System vertical displacement and horizontal velocity observations, basal water pressure measurements, and cavitation model predictions during rain‐induced speed‐up events at Glacier d'Argentière, French Alps. We observe up to a threefold increase in horizontal surface velocity, spatially migrating at rates consistent with subglacial flow drainage, and associated with surface uplift and increased water pressure. We show that frictional changes are mainly driven by changes in water pressure at nearly constant cavity size. We propose a generalized friction law capable of capturing observations in both the transient and steady‐state regimes.

Lateral force controlled impact tribometer characterizing transient friction: Effect of contact pressure

A new Lateral Force Impact Tribometer (LFIT) is designed to quantify the dynamic friction of synthetic seat belt and D Ring Polymer (DRP) under transient conditions during the initial sliding period. LFIT simulates the sliding between two bodies subjected to a sudden lateral force applied, resulting in relative motion. Transient friction under suddenly applied initial acceleration conditions is obtained from the displacement response. LFIT overcomes stiffness concerns of most displacement-controlled tribometers in which the sensors and actuators influence the contact stiffness, nature of asperity interaction, and frictional characteristics. The displacement vs. time response showed the variation in displacement during the initial 0.3 mm sliding distance. LFIT is modelled as a spring-mass-Coulomb damper system applied for transient analysis. The effect of contact pressure on the transient Coefficient of Friction (CoF) is studied in the present work. This paper perhaps presents the first report that enables the capture of the friction transition within 10 ms from the start of sliding.

Transient sealing characteristics of Glyd-ring in the high water-based piston pair under reciprocating pump conditions

The velocity of the piston rod in the reciprocating pump is sinusoidal with time, so as to provide a theoretical basis for the design of the piston seal in the reciprocating high water-based pump, a transient numerical model of Glyd-ring is proposed based on the theory of soft elastic flow by considering the variation of the rod velocity. The transient friction and leakage characteristics of the Glyd-ring, and the film squeeze effect are obtained by finite element and finite volume method, and their effects from the sealing pressure and rod velocity are discussed. Finally, the reciprocating seal test is set up to verify the reliability of the proposed model and results.

Friction of fluoroplastic with steel in a hydraulic fluid medium when transient mode

The article considers the issues of experimental research of the antifriction properties of fluoroplastic seals of piston pump hydraulic systems in a transient friction mode. The statement about the influence of frictional heating on the tribological properties of the seal is substantiated. Model experiments are used to study the friction coefficients at the moment of changing the direction of the contacting surfaces movement and the transient period from the moment of starting from a standstill to the steadystate friction mode. The experiments were carried out under conditions of heating the hydraulic fluid to the maximum temperature. Particular attention is paid to the change in the friction coefficient with increasing load-speed characteristics. The limiting contact pressures that ensure trouble-free operation of a seal made of radiation-modified fluoroplastic have been determined.

Numerical Simulation of Turbulent Flow and Friction Characteristics Through a Loop of Narrow Rectangular Channel Under Rolling Motions

Interaction of prevailing ocean waves and wind with the platforms containing the small modular reactors (SMRs) employed in marine environments may significantly alter the flow and friction characteristics inside these reactors. The present research is focused on the numerical study of the effects of rolling motions on the turbulent flow and frictional characteristics of a three-dimensional closed loop of narrow rectangular channels using Ansys Fluent. The computational results have been corroborated with experimental data present in literature. The results illustrate that flow and friction characteristics fluctuate sinusoidally as the loop undergoes rolling motion. Strong fluctuations were observed in the flow rate and time-dependent friction coefficient with an increase in rolling amplitude or reduction in the rolling period. These variations became more pronounced at low Reynolds numbers and diminished at higher Reynolds numbers. Increasing the fluid viscosity also subsided the rolling effects. The average flow velocity in the loop was found to decrease from 0.27 to 0.15 m/s in various phases of the rolling period. The relative Reynolds number was found to be reduced by 50% under rolling motions for the range of steady-state Reynolds numbers investigated in the present study. The transient friction coefficient was also found to oscillate under rolling motion with the same period as that of excitation. The transient friction coefficient’s oscillations also increased with rolling amplitude or reduction in the rolling period. However, the temporally averaged friction coefficient under rolling motions was found to be equal to the steady-state frictional coefficient in the loop.

Investigating Dynamic Weakening in Laboratory Faults Using Multi‐Scale Flash Heating Coupled With mm‐Scale Contact Evolution

AbstractFlash‐weakening models typically show good agreement with the total magnitude of weakening in high‐speed rock friction experiments, however deviations during the acceleration and deceleration phases, and at low and intermediate sliding velocities, remain unresolved. Here, we incorporate inhomogeneous mm‐scale normal stress evolution into a model for flash heating and weakening to resolve outstanding transient and hysteretic friction observed in laboratory experiments and to identify unique solutions to constitutive parameters. We conduced 37 rock friction experiments on Westerly granite using a high‐speed biaxial apparatus outfitted with a high‐speed infrared camera. We initiated velocity steps from quasi‐static rates of 1 mm/s to sliding velocities ranging from 300 to 900 mm/s and conducted both constant‐ and decreasing‐velocity tests following the velocity step. Two sliding surfaces geometries were used to control mm‐scale life‐times and rest‐times. Constant‐strength sliding is achieved within 2–3 mm of initiating the velocity step in all constant‐velocity experiments. Macroscopic surface temperature is inhomogeneous and increases with slip distance, velocity, and decreasing rest‐time. Weakening increases with sliding velocity and decreasing rest‐time. We combine thermal models with measured surface temperatures to constrain the evolution of local normal stress at the mm‐scale and incorporate this evolution into a flash‐weakening model that considers weakening at both the µm‐ and mm‐scale. The flash‐weakening model improves when the effects of mm‐scale wear processes are incorporated and multi‐scale weakening is considered, however some transient friction remains undescribed. Models will be advanced by further incorporating wear processes and by considering processes at the mm‐scale and above.

Adsorption-induced transient friction of hydrogels on hydrophilic countersurfaces

Soft hydrated permeable surfaces of hydrogels exhibit unique lubrication behaviors, including frictional hysteresis found in tribo-rheometry measurements. A hydrogel lubrication model that describes the transient behavior was previously developed using the structure kinetics model in the field of rheology and rate-and-state friction model, where the friction change is described as a competition between buildup and breakdown rates. In this study, the model is further modified to include the effect of hydrophilicity of a countersurface. Ultraviolet (UV)/ozone treatment on an aluminum surface significantly removes organic materials, resulting in extremely hydrophilic surface. Friction response of a polyacrylamide hydrogel against untreated and UV/ozone-treated aluminum exhibited noteworthy difference in the trajectory of hysteresis. Model fits were conducted using the modified lubrication model on both hystereses, and the fitting parameters of both hystereses are compared with each other to identify a parameter addressing hydrophilicity. Based on the model fits, we suggest that the hydrophilicity of the countersurface initially prevents the adsorption on the hydrogel surface because it holds water better. However, once water goes out of the contact due to contact pressure, a stronger adsorption occurs, which increases friction and decreases the speed dependence of friction.

Tribology test design for friction measurements with application to oral medicines

In recent years tribology tests have been used to measure friction properties of oral consumables such as semi-solid foods and medicines. The tests aim to simulate thin-film mastication conditions and are intended to correlate with mouth feel or food texture properties. In this paper a new approach is proposed to better simulate shear conditions, fluid supply and friction data capture associated with mastication and swallowing. Two primary changes are suggested: these are the reduction of the inlet influence on lubricant film properties and the ability to measure transient and time-dependent friction. The new test was used to measure friction for a range of oral medicines including a viscous solution (cough syrup) and particulate suspensions (paediatric, calcium carbonate) in combination with an artificial saliva (mucin solution), The tongue-palate was replicated by a PCX glass lens loaded and reciprocating against a textured silicone surface. A short stroke length, comparable to the Hertzian diameter of the contact, was used so the contact operated in a partially replenished lubrication condition. This ensured the film in the contact region has the same composition as the bulk fluid. Friction was measured continuously during reciprocation for up to 5 cycles (comparable to mastication time) and data was sampled at 100 Hz to capture transient friction. Tests were run with and without a mucin layer present. The results showed that tests performed after 20 min adsorption of an artificial saliva solution reduced the friction coefficient from μ = 1 to μ = 0.2–0.3. Tests with the paracetamol suspensions, which contain hard particles, recorded transient friction spikes which were not recorded for the softer calcium carbonate suspensions. Key conclusions for the design of pertinent simulation tests are that the film properties in the oral cavity are not determined by the inlet as for classical lubrication. The (bulk) oral sample is captured in the tongue-palate contact and not continually replenished. When sheared it will exhibit time (transient, longer-term) and speed dependent friction responses which contribute to sensory and texture perception.

Effect of Instantaneous Starting Pump on Fluid Transient Flow in Managed Pressure Cementing

Abstract The fluctuating pressure generated by the instantaneous starting pump of cementing operation might easily cause formation fracture in narrow safety window formations. Accurate transient fluctuating pressure calculation and dynamically managed backpressure are required to achieve precise control of wellbore pressure in managed pressure cementing. Considering the transient flow characteristics of cementing fluids in the wellbore, unsteady transient friction, and the variation of high pressure-high temperature (HTHP) cementing fluid properties, a transient flow mathematical model of instantaneous starting pump during managed pressure cementing is established, and the method of characteristics is used for solution. Based on tht established model analyzes the magnitude and variation of wellbore pressure under different model factors, temperature conditions, pump start duration, and target pump flowrate, which can achieve more accurate analysis of transient flow. The pressure and flow fluctuations generated during the instantaneous starting pump of cementing are significant, and the dynamically managed wellhead backpressure can effectively control the wellbore pressure in the safe pressure window formations. This can reduce the risk of well leakage and provide reliable technical support for safe operations of instantaneous starting pumps during managed pressure cementing.

Identification Method for Instantaneous Friction and Wear Energy Density Variation of High-Feed Milling Tool Flank

Instantaneous friction and wear energy density are important indicators of the friction and wear state of the tooth flank of the milling cutter. Existing methods for identifying the friction and wear on the tooth flank of milling cutters mainly focus on the overall level of friction energy consumption and the maximum width of the accumulated wear on the teeth flanks, ignoring the unevenness and variability of the friction energy consumption and wear volume distribution of the tool flank, and the dynamic relationship between the instantaneous friction and wear on each tooth flank of the milling cutter is to be revealed. Transient cutting position of a high feed milling cutter tooth under milling vibration, and the use of the micro-element method to construct a model for solving the transient wear volume on the tool flank. A method of transient friction energy consumption on the cutter flank was proposed. The variation property of the instantaneous friction and wear energy density on the flank was exposed. The identification method of the variation property of the energy density was proposed according to different analysis results of the distribution of friction and wear energy density and verified by experiments. The outcomes indicated the maximum and mean of the energy density at different positions on the flank had a high similarity with the wear depth distribution on the flank. Using this method, the influence property of the friction and wear state and the cutting parameters under the milling vibration of the milling cutter tooth can be identified.

Motion control of hydraulic actuators with nonlinear friction compensation: Applied to variable valve systems of diesel engine

Friction force plays an important role in motion control systems. Research shows that friction force is strongly nonlinear in most applications. And the nonlinear friction force has great influence on control performance, especially in motion control systems of hydraulic actuators. To improve control performance, many model-based controllers have been developed. In the past years, LuGre model has been widely used in most hydraulic actuators. However, for systems with one-way seals whose friction force is asymmetric between instroke and outstroke stage, LuGre model is no longer suitable. The paper developed a friction force test bench and studied the transient friction force of a hydraulic actuator which includes one-way seals. Experiment results show that friction force at instroke stage is bigger than that of outstroke stage for the variable valve actuator. Based on the experiment results, a bristle direction hypothesis is proposed. The bristle of the friction surface is thought to have an initial direction. For actuators with two-way seals or without seals, the bristle is thought to be perpendicular to the friction surface. And for actuators with one-way seals, the angle between bristles and friction surface is not 90° anymore. To apply the bristle direction hypothesis to controllers, a bristle direction coefficient is introduced to LuGre model to explain asymmetric friction behaviors. The simulation results have good agreements with experiment results. Based on the improved LuGre model, a backstepping controller is developed to compensate the nonlinear friction force. The friction force and system matched uncertainty are estimated by an extended state observer. Finally, some comparative experiments are implemented to verify the advantages of the improved LuGre model. The experiment results show that the improved LuGre model can help to reduce the engine valve lift tracking errors. At outstroke stage (engine valve close), the maximum absolute errors can be reduced by 40.6% to 65.0%, and the mean absolute errors can be reduced by 21.2% to 59.4%.

Mould heat transfer and friction behavior on the surface of wide and heavy slabs in the presence of longitudinal cracks

The longitudinal crack is a common and serious defect on the surface of wide and heavy slab. The mould heat transfer and friction characteristics of 28 surface longitudinal cracks are collected and investigated in order to reveal the rule of the surface longitudinal cracks. The results show that the decrease amplitude of temperature in the presence of longitudinal crack is between −4.0 and −22.6 °C, and the increase amplitude of temperature is between 3.4 and 28.2 °C. The decrease of temperature velocity is between −0.4 and −1.5 °C.s−1, while the increase of temperature velocity is between 0.32 °C.s−1 and 1.64 °C.s−1. When longitudinal cracks occur in mould, the heat flux of 21 cases has a decrease more than −0.02 MW · m−2. There are 18 abnormal fluctuations of mould’s transient friction during occurrence of the surface longitudinal crack, which is more than the stable casting. And there are 85.7% longitudinal cracks, which has an upward trend of friction tension or a downward trend of friction pressure.

Simulation of transient friction modes of fluoroplastic seals in hydraulic piston pumps

The article considers the issues associated with the friction of radiation-hardened fluoroplastic on steel in a hydraulic fluid environment during the transition period at the beginning of movement. The friction coefficients are studied when starting from a standstill, at the beginning and in steady motion. Changes in the value of the friction coefficient and changes in the amplitude of oscillation of the friction moment are analyzed. Particular attention is paid to the influence of the restart of motion in a friction coupling. Quantitative relationships of tribological properties are established depending on the time of relative sliding of the samples. It is noted that the friction coefficients in different modes are differ a lot, however, the running-in process proceeds quickly and in the practice of operating hydraulic devices, the running-in process of fluoroplastic seals can be ignored

Elastohydrodynamic Simulation of Pneumatic Sealing Friction Considering 3D Surface Topography

AbstractThis contribution presents an elastohydrodynamic lubrication (EHL) model for pneumatic spool valves. For an accurate estimation of the transient friction of this tribological sealing system, the surface topography of the cylindrical sealing counterfaces of the valve housings are measured and analyzed with an optical surface measurement instrument. Based on the surface topography data, tribological properties and flow factors of the system are derived. It has been found that the consideration of the surface topography has a significant influence on the simulation results of the EHL model, lowering the calculated friction force by up to 20 %.

Contribution of meniscus force to friction of multi-asperity sliding contact

Capillary adhesion contributes to friction of sliding multi-asperity contact in humid environment by increasing the solid-solid contact area; whereas the increased adhesion from capillary meniscus is hard to measure due to elastic recovery of contacting asperities. Here, we report the first direct measurement of increased interfacial adhesion from mesoscopic menisci in a macroscale multi-asperity contact comprising soft elastomer and glass surfaces with a diminishing water film. Results also found capillary adhesion and its contribution to a high transient friction coefficient decrease with increased interfacial roughness, which is consistent with prior literature hypotheses.

A Consistent Framework for Coupling Basal Friction With Subglacial Hydrology on Hard-Bedded Glaciers.

Below hard‐bedded glaciers, both basal friction and distributed subglacial drainage are thought to be controlled by a network of cavities. Previous coupled hydro‐mechanical models, however, describe cavity‐driven friction and hydraulic transmissivity independently, resulting in a physically inconsistent cavity evolution between the two components of the models. Here, we overcome this issue by describing the hydro‐mechanical system using a common cavity‐evolution description, that governs both transient friction and hydraulic transmissivity. We show that our coupling approach is superior to previous formulations in explaining a unique observation record of glacier sliding speed from the French Alps. We find that, at multi‐day to multi‐decadal timescales, sliding speed can be expressed as a direct function of basal shear stress and water discharge, without accounting for water pressure, which simply adjusts to maintain the cavitation ratio needed to accommodate the water supply.

Computational analysis of the transient mixed convective flow of nanofluid in the plume regions

Computer microchips and many other electrical devices that use microfluidics use nanofluids as coolants. Nanofluids have a higher thermal conductivity due to the volumetric proportion of nanoparticles. Thus, as the nanofluids in engineering and industrial process have a significant role, the current study aims to focus on the transient mixed convective flow of nanofluid in the plume regions. The numerical solutions for the transient mixed convection flow in the form of amplitude and phase angle are performed reasonably. The desired nonlinear mathematical model is created by incorporating periodic behavior into conservation equations and appropriate boundary conditions. Using appropriate scaling variables, the dimensioned equations are transformed into dimensionless ones. Solutions for the values of Prandtl number Pr, Schmidt number Sc, thermophoretic parameter Nt, Brownian motion parameter Nb and mixed convection parameters and show that the coupling between the fluid and heat transfer can have dramatic effects on the transient shear stress heat and mass transfer. The buoyancy force acting on the heated boundary layer of nanofluid can also have a significant impact on transient skin friction, heat and mass transfer. The governing process is accelerating or depending upon the position of the sphere.