Variation Of Friction Coefficient Research Articles

Comparative study on heat transfer performance of γAl2O3−C2H6O2 and γAl2O3−H2O nanofluids via Prabhakar fractional derivative model for MHD channel flows

The prime focus of this work is to investigate the thermal behavior of ethylene glycol and water based γ Al 2 O 3 nanofluids for free convective flow through a stationary channel. The imposition of uniform magnetic effects is assumed in the normal direction of the vertical channel, which is nested in a porous material. The Fourier law for estimation of the thermal flux is generalized by dint of the Prabhakar fractional derivative. The viscosity and thermal conductivity of considered nanofluids are measured by employing experimentally evaluated relations of these quantities. Some dimension-free quantities are introduced and the resulting generalized unit-less fractional governing equations are solved analytically with the assistance of the Laplace transformation. Several tables and graphical illustrations are provided to anticipate the influence of various factors on flow distribution and temperature profile. A comprehensive study comprised of multiple parts such as anticipation of the thermal performance, variation in skin friction coefficient, and parametric impacts on thermal and velocity profiles is conducted to deeply analyze and compare the behavior of γ Al 2 O 3 − C 2 H 6 O 2 and γ Al 2 O 3 − H 2 O nanofluids. The outcomes suggest that there is a noteworthy enhancement in heat transfer rate due to an increase in fractional parameters. The enhancement in volume proportion of nanoparticles reduces the velocity of nanofluid. Moreover, the improvement in heat transfer rate of ethylene glycol and water due to the dispersion of γ Al 2 O 3 nanoparticles is 7.98% and 10.68% respectively.

Static and dynamic friction coefficient in low loads and sliding speed conditions

Using an unidirectional oscillator including a mass-spring system, the authors evidenced the important variations of the static friction coefficient both for very low contact pressure (0.002 MPa) and for high contact pressure (386 MPa).The experiments were realised by the CETR UMT-2 Tribometer with a variation of the sliding speed between 0.02 mm/s to 8 mm/s. The oscillator mass have 0.242 grams and the stiffness of the elastic spring had 77 N/m. The contact surface of the oscillator mass have a roughness Ra = 0.1 μm and the opposite plane surfaces have roughness of Ra = 0.1 μm and Ra = 2.5 μm. Two types of experiments were realized: the sliding between two plane surfaces with a nominal pressure of 0.002 MPa and the sliding between a plane surface and three steel balls of 6 mm with a maximum Hertz contact pressure of 386 MPa. For very low pressure the obtained static friction coefficient varied between 0.08 to 0.3 and the dynamic friction coefficient varied between 0.08 to 0.16. For high pressure the obtained friction coefficient varied between 0.2 to 0.4 and the dynamic friction coefficient varied between 0.15 to 0.2. To simulate the sliding speed the model of Zuleeg [5] for dynamic friction coefficient has been adapted to the experimental parameters and the dynamic equation of mass displacement as function on time has been numerical integrated.

Об одном подходе к аналитическому решению задачи экструзии труб при наличии внутреннего высокого давления

Based on the assumption that external v p and internal r p pressures acting on the pipe are equal: the circumferential stress of the pipe is determined. Simplified plasticity condition is obtained and the stress-strain state of the pipe is investigated by the analytical method. The problem of extrusion of pipes loaded with internal high pressure is firstly completely solved in the case of the presence of friction, and then by a fairly simplified method in the absence of friction. This is due to the fact that in the main case, in the denominator of the formula for determining the Ϭm meridional stress, there is a coefficient, that takes into account friction, and in the case of k = 0 it is impossible to immediately determine the corresponding formula for the case in the absence of friction. To implement this process, the formula of the meridional stress was transformed to obtain an exponential function and its representation in the form of a series. Dimensionless meridional, circumferential and average stresses are determined, depending on the degree of pipe deformation, which makes it possible to use the formulas of the deformation theory of plasticity of porous materials to study the process of compaction of the pipe material. Numerical calculations are carried out in the MS EXCEL software environment for various coefficients of friction at 10% of the initial porosity of the material. To obtain a fully compacted material of a sintered pipe, the process of extrusion of the pipe with various dimensions is modeled in a corresponding conical die.

MHD CASSON FLUID STAGNATION POINT FLOW AND HEAT TRANSFER OVER AN EXPONENTIALLY STRETCHING SURFACE IN PRESENCE OF UNIFORM HEAT SOURCE AND SINK WITH SUCTION EFFECT

The present study reveals the analysis of steady mixed convection MHD stagnation point flow of Casson fluid of non-Newtonian nature and Heat transfer over an exponentially stretching surface where the consequence of uniform heat source and sink are taken in to consideration. The presiding Non-linear Partial differential equations and the corresponding boundary conditions are formulated and thus transformed into pair of non-linear ordinary differential equations. The equations thus obtained are deciphered using Runge-Kutta fourth - order method with the help of MATLAB software. The results obtained for Skin friction coefficient and heat transfer rate for the case of Newtonian fluid are determined, which are in good harmony with the previously proclaimed results of other researchers.The impact of physical quantities such as Casson parameter, buoyancy parameter, Hartmann number, Prandtl number, heat source and sink, Suction parameter, on the fluid velocity and temperature are discussed through graphs for both assisting and opposing flow. The variation in Skin friction coefficient and Nusselt number are tabulated for various values of Hartmann number. Divergence in the velocity profile is observed for increase in Suction for two different values of Velocity ratio parameter. As Skin friction coefficient escalates with suction parameter indicating the exertion of drag force by the surface on the fluid flow. Also, the study reveals that the impact of Hartmann number is to minimize the boundary layer separation.

Role of Testing Conditions in Formation of Tribological Layers at Line Contacts of Antifriction CF-Reinforced PI- and PEI-Based Composites.

High-strength PI and PEI polymers differ by chemical structure and flexibility of the polymer chains that ensure lower cost and higher manufacturability of the latter. The choice of a particular polymer matrix is of actuality at design of antifriction composites on their basis. In this study, a comparative analysis of tribological behavior of PI and PEI- based composites was carried out with linear contact rubbing. The neat materials, as well as the two- and three-component composites reinforced with chopped carbon fibers, were investigated. The third components were typically used, but were different in nature (polymeric and crystalline) being solid lubricant fillers (PTFE, graphite and MoS2) with characteristic dimensions of several microns. The variable parameters were both load and sliding speed, as well as the counterface material. It was shown that an improvement of the tribological properties could be achieved by the tribological layer formation, which protected their wear track surfaces from the cutting and plowing effects of asperities on the surfaces of the metal and ceramic counterparts. The tribological layers were not formed in both neat polymers, while disperse hardening by fractured CF was responsible for the tribological layer formation in both two- and three component PI- and PEI-based composites. The effect of polymer matrix in tribological behavior was mostly evident in two-component composites (PI/CF, PEI/CF) over the entire P⋅V product range, while extra loading with Gr and MoS2 leveled the regularities of tribological layer formation, as well as the time variation in friction coefficients.

The strange case of shear-thinning in non-Brownian suspensions

<h2>Abstract</h2> Shear thinning is generally reported in Brownian suspensions where it is understood to arise from a decrease in the relative contribution of entropic forces. However it has also been often reported in experiments with non-colloidal systems and nominally-Newtonian matrices at high volume fractions. In spite of the generality of the observations, its origin is still a matter of debate and the behaviour is difficult to reproduce in numerical simulations where shear thickening is typically observed instead. Several explanations have been proposed in recent years ranging from hidden non-Newtonian effects in the small interparticle gaps (Vázquez-Quesada et al., Phys. Rev. Lett. 117 (10), 108001 (2016), J. Non- Newt. Fluid Mech. 248, 1-7(2017)), excluded volume effects (Chatté et al. Soft Matter,14, 879-89 (2018)) and variable friction coefficients (Chatté et al.Soft Matter,14, 879-89 (2018); Tanner et al.Rheol. Acta 57:635–643 (2018); Lobry et al. J. Fluid Mech., 860, 682–710 (2019)). Another possibility that has emerged recently is related to slip occurring in the interparticle gaps at the liquid-solid interface (Kroupa et al. Phys. Chem. Chem. Phys., 19, 5979-5984 (2017); Vázquez-Quesada et al., Phys. Rev. Fluids 3 (12), 123302 (2018); Kumar et al.J. Non-Newt. Fluid Mech., 281, 104312 (2020)). In this talk I will review some of these issues and recent theories developed in this context.

Friction Characteristics of Open Graded Asphalt Friction Courses with BOF and EAF Steel Slag Aggregates

Friction properties of open graded asphalt friction course (OGAFC) mixtures play an essential role in ensuring traffic safety, especially under wet weather conditions. This study evaluated the friction characteristics of basic oxygen furnace (BOF) and electric-arc furnace (EAF) steel slag aggregate incorporated OGAFC mixes. Nine sets of OGAFC mixes were fabricated using five percentage replacements (0%, 25%, 50%, 75%, and 100%) of the coarse natural aggregate fraction by BOF and EAF steel slags. The effect of modified binder type (polymer and crumb-rubber modified) on the frictional characteristics of OGAFC mixes was also examined. The principal friction test devices used were the British pendulum tester and dynamic friction tester. Multiple friction performance aspects were studied: (1) effect of surface condition (dry, wet, and ponding) on the measured friction coefficient; (2) separate evaluation of adhesion and hysteresis friction components; (3) variation of friction coefficient with varying slip speeds, and (4) friction performance later in the OGAFC service life using artificially polished aggregates, and comparison of the performance with unpolished aggregates. Results revealed that the use of steel slags in OGAFC mixes considerably enhanced the frictional characteristics and that the skid resistance of OGAFC-BOF mixes was better than that of OGAFC-EAF mixes. Frictional resistance of the mixes was found to increase with an increase in steel slag content. OGAFC mixes comprising polished steel slag aggregates also showed promising results with the increment in steel slag content. The findings also indicated that OGAFC-CRMB mixes perform better than OGAFC-PMB mixes.

Adaptive finite element simulation and experimental verification for fretting wear of PVDF piezoelectric thin films

An adaptive finite element method linked with user-defined subroutine UMESHMOTION is developed for simulating the tangential fretting wear of PVDF piezoelectric thin films. The three-dimensional (3D) sphere-on-flat contact model is established for the fretting wear simulation by using commercial software ABAQUS, where the Si3N4 ceramic ball is used as the punch in contact with PVDF piezoelectric thin films. The variable friction coefficient obtained from experiments is taking into consideration. The calculation of wear is based on the Archard's wear model. The tangential fretting motion with different loads and variable friction coefficients are applied in the finite element simulation. The fretting wear test are carried out to verify the numerical results under the same conditions. The results indicate that the present adaptive finite element model has a certain accuracy comparable to the experiment results for the fretting wear of PVDF piezoelectric thin films.

Numerical study of Williamson hybrid nanofluid flow with thermal characteristics past over an extending surface

AbstractThe energy and mass dissemination rate have been studied through Williamson hybrid nanofluid (NF) flow comprised of silver (Ag) and magnesium oxide (MgO) nanoparticles (NPs) past over an extending porous surface. The hybrid nanofluid has synthesized by dispersion of Ag and MgO nanoparticles in the base fluid (engine oil). The effects of the constant magnetic field, thermal dissipation, and heat source are also studied in the present analysis. The above scenario has been designed in the form of a nonlinear system of partial differential equations, which are processed through a similarity framework to the system of dimensionless ordinary differential equations. The results are obtained by the numerical computational approach parametric continuation method. It has been perceived that the velocity contour decreases with rising upshots of porosity parameter Kp and magnetic force M, while enhances with the variation of volume friction coefficient. The increment of Biot number Bi, heat source Q, and Eckert number Ec enhances the energy profile, respectively. Furthermore, the mass conversion rate decreases with the variation of thermophoretic parameters and Schmidt number.

Wear behaviour and microstructural characteristics of cold sprayed nickel-alumina coatings on boiler steel

There is an excessive material loss in steels components due to sliding wear in different industrial applications. The SS 316 steel is extensively used in the power generation industry for boilers, induction fans, ducts, etc., and counter high sliding wear. The studies have shown that protective coatings deposited by thermal spray methods are successful in controlling the wear and enhancing the service life of steels. In this work, the nickel-alumina coating was deposited on SS 316 by cold spray technique to understand the effectiveness of coatings to resist wear. The wear behavior of coatings was analyzed by conducting the wear test on a pin-on-disc apparatus at different loads, i.e., 30, 40 and 50 N keeping the speed constant. The wear trends and variation in friction coefficient due to wear were observed. The mechanical and microstructural characterization was done by FE-SEM/EDS and XRD techniques. The coatings were found effective in resisting the wear on SS 316 steel. The results indicate that the wear rate of coatings increased with an increase in normal load.

Molecular Dynamics of Azobenzene Polymer with Photoreversible Glass Transition

The azobenzene polymer with photoswitchable glass transition temperatures and reversible solid-to-liquid transitions has attracted much attention in recent years. The different glass transition temperatures of the azopolymer in the trans-state and the cis-state introduce evident dynamical heterogeneity during cis–trans interconversion. The correlation between molecular dynamics and rheology in azopolymers remains elusive. In this work, we studied the molecular dynamics of an unentangled amorphous azopolymer without a liquid crystal phase. We found that the Lodge–Mcleish self-concentration (SC) model can quantitatively describe the composition dependence of the glass transition temperature, revealing the local environment of the cis (or trans) segments. The terminal relaxation behavior can be well predicted by the Rouse model with the homogeneous bead friction coefficient and the heterogeneous bead–spring (HBS) model with varying friction coefficients for cis or trans subchains. The difference between two models lies in the relaxation time at small length scales and its distribution. The HBS model predicts broadening the relaxation time spectrum, whose width varies with temperature and the cis content. This prediction is consistent with the broadening of the glass transition temperature in azopolymers. Then, the models were used to study the kinetics of thermal cis-to-trans isomerization of the azopolymer in the melt, proving to be a first-order kinetic process and further justified by the ultraviolet absorption spectrum.

Simple topographic parameter reveals the along-trench distribution of frictional properties on shallow plate boundary fault

The critical taper model best describes the first-order mechanics of subduction zone wedges. The wedge geometry, which is conventionally defined by two parameters, slope angle and basal dip angle, accounts for the strength of megathrust. By applying this theoretical model, fault frictional properties and earthquake occurrences can be compared among subduction zones, and within a single subduction zone, and the spatial distribution or temporal change of fault strength can be investigated. Slope angle can be accurately estimated from bathymetry data, but basal dip angle must be inferred from subsurface structure, which requires highly accurate depth-migrated seismic reflection profiles. Thus, application of the critical taper model is often limited by an insufficient number of highly accurate profiles, and the spatial distribution of frictional coefficients must be inferred from relatively few data. To improve this situation, we revisited the theoretical formula of the critical taper model. We found that the effect of basal dip angle on the critical taper model is small, and slope angle can be a proxy for the effective friction when the pore fluid pressure ratio is high, internal friction is small, or both. These conditions are met in many subduction zones. The validity of the approximation can be checked with a parameter newly introduced in this study. Therefore, this finding allows use of variations in slope angle, which could be obtained accurately from only the bathymetry as an approximation for relative variations in the effective coefficient of basal friction, if the targeted subduction meets the validity. We applied this approximation to the Japan Trench and estimated the variations in the friction coefficient distribution on the shallow plate boundary fault from 71 data points. We found that the area where the friction coefficient was smaller than the mean corresponded to a segment, where a large coseismic shallow rupture occurred during the 2011 Tohoku-Oki earthquake (Mw 9.0). Thus, by approximating tapered wedge geometry with a simple topographic parameter that can be obtained from existing global bathymetry, we can quickly estimate the distribution of frictional properties on a plate boundary fault along a trench and related seismic activity.Graphical

Study of dry wear behavior of Novel ferrous samples prepared by powder metallurgy method

The sliding wear behavior of newly developed ferrous powder metallurgy samples was investigated under dry sliding conditions. Wear tests were conducted by varying the normal loads of 30 N, 40 N, and 50 N at a constant velocity of 1 m s−1 and then at a constant load of 30 N by varying the sliding velocities by 0.5 m s−1, 1 m s−1, and 2 m s−1 at room temperature. Furthermore, the variations in the wear rate and coefficient of friction at a constant sliding distance of 5400 m were studied at different normal loads keeping sliding velocity constant at 1 m s−1 as well as at different sliding velocities and maintaining normal load at 30 N. The wear characteristics were evaluated using weight loss measurements. The mechanism of wear was observed using a scanning electron microscope and compositional analysis using energy-dispersive X-ray spectroscopy (EDX). It was found that increased normal load had a dominant effect on the wear loss in comparison to increased sliding speed during the tribological testing of the powder metallurgy samples.

A 4D-Printed Structure With Reversible Deformation for the Soft Crawling Robot

Reversible deformations of the 4D-printed structures are attractive and promising for various application fields. In this study, the principle of reversible deformations for the bilayer structure consisting of SMP and elastic material is illustrated. By exploring the influence of printing parameters on deformation and resistance, a low-cost reversible bilayer structure with rational resistance distribution is designed to realize reversible deformation. Subsequently, the bilayer structure is employed to design a soft crawling robot with asymmetrical variable friction coefficient feet. By revealing the principle of locomotion by force analysis and deformation process analysis, a wave-like strategy is proposed to actuate the robot. Experiments verify the effectiveness of the designed structures.

Power-Optimal Control of a Stirling Engine's Frictional Piston Motion.

The power output of Stirling engines can be optimized by several means. In this study, the focus is on potential performance improvements that can be achieved by optimizing the piston motion of an alpha-Stirling engine in the presence of dissipative processes, in particular mechanical friction. We use a low-effort endoreversible Stirling engine model, which allows for the incorporation of finite heat and mass transfer as well as the friction caused by the piston motion. Instead of performing a parameterization of the piston motion and optimizing these parameters, we here use an indirect iterative gradient method that is based on Pontryagin’s maximum principle. For the varying friction coefficient, the optimization results are compared to both, a harmonic piston motion and optimization results found in a previous study, where a parameterized piston motion had been used. Thus we show how much performance can be improved by using the more sophisticated and numerically more expensive iterative gradient method.

Effects of Reynolds number and average angle of attack on the laminar scaling of oscillating foils

The variation in thrust generation with respect to Reynolds number was numerically evaluated for an oscillating foil with combined pitching and heaving motion at a range of reduced frequencies, amplitudes, and phase offsets. Laminar scaling (Re−0.5) was found accurate for a reasonable range of average angle of attack (α¯&amp;lt;20°). However, quantitative evaluation of laminar scaling using statistical measures indicates that its capability in predicting thrust variation weakens at higher reduced frequencies and amplitudes. This coincides with an increase in α¯ above 20°. Evaluation of the pressure and viscous forces revealed a dominance of the former toward total thrust generated at high frequencies for all cases, which also coincided with lower coefficient of determination (R2) for laminar scaling. The chordwise variation of pressure and skin friction coefficient provided further evidence indicating that pressure, in contrast to the skin friction, did not achieve an asymptotic trend with increasing Reynolds number, especially at higher frequencies and for all phase offsets. Qualitative evaluation of the developing leading edge vortex structure at increasing reduced frequencies and Reynolds numbers also supported the quantitative assessment of chordwise pressure variations. Empirical incorporation of Reynolds number into the complete scaling model was hence completed, which further validates the laminar scaling (Re−0.5) of propulsive thrust generation in oscillating foils with a coupled motion.

Experimentally verified prediction of friction coefficient and wear rate during running-in dry contact

When pristine solid surfaces experience sliding contact, the surface roughness undergoes various changes and deformations in a transient fashion giving rise to a nonlinear wear rate. During the first stage of the service life of the mechanical elements called running-in, this phenomenon occurs where many intertwined parameters interact in a complicated fashion. It is believed that the operating conditions of running-in largely impact the performance of a tribo-system. Thus, the ability to predict the variation of wear rate, friction coefficient, and surface asperities during running-in is important. To address this need, a model capable of predicting the variation of the aforementioned parameters is developed. To verify the predictions, the results are compared to pin-on-disk experiments. The comparison shows an acceptable agreement in the predicted and experimental data.

Numerical Analysis of Newtonian Heating Convective Flow by Way of Two Different Surfaces

In this paper, we offer a numerical study on heated non-Newtonian fluid with a Newtonian heating effect towards thermally stable stretching surfaces. A comparative analysis for two stretched surfaces, namely, plate and cylinder, is offered. The Casson fluid model is considered to be a non-Newtonian fluid model. By applying the suitable set of transformations, the non-linear coupled PDEs are transformed into non-linear ODEs. It is difficult to obtain the exact solution of such non-linear differential equations; therefore, we used the shooting method along with Runge–Kutta scheme. The influence of pertinent flow variables on velocity and temperature is presented through graphs. Notably from the results, heat generation parameters, Newtonian heating, and magnetic parameters enhanced the temperature profile, whereas Casson fluid and magnetic field parameters reduced the fluid velocity. It is also observed that increases in fluid temperature were more influenced at the cylindrical surface as compared with the flat plate. Moreover, we obtained remarkable results for the heat transfer rate by imposing Newtonian heating conditions at the surface; tables are used to present variations in the skin friction coefficient and Nusselt number at the thermally stable surfaces.

Adaptive slip engagement control of a wet clutch in vehicle powertrain based on transmitted torque estimation

In the slip engagement of a wet clutch, the solenoid current-to-actuator piston pressure model (CPM) and the piston pressure-to-clutch transmitted torque model (PTM) are utilized for the clutch torque control. However, the CPM is changed depending on the amount of hysteresis, which is influenced by the hydraulic fluid temperature, and the PTM is changed due to the variation of the clutch friction coefficient and the touch point. Thus, the CPM and PTM should be adapted using the measurement information of the solenoid current and piston pressure for precise control of the clutch torque. In this study, an adaptive control method of the clutch torque in the slip engagement of a wet clutch is proposed which. The purpose of this study is to propose a clutch torque control method that does not use a piston pressure sensor when controlling the clutch torque of a wet clutch, so the suggested method does not require measurement of the piston pressure. In the method, the adaptation of the target piston pressure-to-clutch torque model (TPTM) is performed instead of the adaptation of the CPM and PTM, and the clutch torque is controlled using the compensated TPTM and nominal CPM instead of the compensated CPM and PTM. The proposed control method is verified experimentally by applying it to the slip engagement control of an engine clutch equipped in a production parallel hybrid vehicle.

Coupled Thermo-Structural Analysis Model of Solid Rocket Motor Nozzle considering the Variation of Friction Coefficient under Operating Conditions

Multi-interface is a typical feature of solid rocket motor nozzles, and the interface evolution law is an important aspect to analyze the thermo-structural response of solid rocket motor nozzles. In this paper, based on the friction coefficient test at different temperatures, a coupled thermo-structural analysis model of solid rocket motor nozzle considering the variation of friction coefficient under operating conditions was established. Firstly, the friction coefficient was tested to model the variation at different temperatures. Then, by adopting structure gap, variable friction coefficient, thermal contact resistance, and friction heat production, a strongly coupled non-linear model was established. Simulations using the non-linear model and traditional model were performed, in which the stress of the throat insert was within the required stress range of the material. The ground firing test result demonstrated the validity of the analysis model, and the non-linear model was in a better agreement with the firing test than the traditional model. Therefore, it can be concluded that with a more specific friction coefficient to represent the friction behaviour of the different parts of the nozzle, the strongly coupled non-linear model established in this paper can reflect the essence of thermo-structural response of solid rocket motor nozzle.