Steady Friction Research Articles

Numerical study of laminar flow and friction characteristics in narrow channels under rolling conditions using MPS method

Abstract Modern small modular nuclear reactors can be built on a barge in ocean, therefore, their flow characteristics depend upon the ocean motions. In the present research, effect of rolling motion on flow and friction characteristics of laminar flow through vertical and horizontal narrow channels has been studied. A computer code has been developed using MPS method for two-dimensional Navier-Stokes equations with rolling motion force incorporated. Numerical results have been validated with the literature and have been found in good agreement. It has been found that the impact of rolling motions on flow characteristics weakens with increase in flow rate and fluid viscosity. For vertical narrow channels, the time averaged friction coefficient for vertical channels differed from steady friction coefficient. Furthermore, increasing the horizontal distance from rolling pivot enhanced the flow fluctuations but these stayed relatively unaffected by change in vertical distance of channel from the rolling axis. For horizontal narrow channels, the flow fluctuations were found to be sinusoidal in nature and their magnitude was found to be dependent mainly upon gravity fluctuations caused by rolling.

Tribological Properties of Aluminum and Silicon Borides at High Temperatures

In our present study, AlB12 and SiB6 plate specimens prepared by spark plasma sintering AlB12 and SiB6 powders, respectively, and their tribological properties were investigated in the temperature range of 25°C to 1000°C in air. Also, the micro Vickers hardness of the AlB12 and SiB6 plate specimens were evaluated in the temperature range of 25°C to 1000°C in an Ar gas atmosphere. It was found that both the steady friction coefficients of the AlB12 and SiB6 plate specimens were as low as 0.07 to 0.08 at 1000°C. According to XPS analyses, it was found that Al2O3 and B2O3 films were observed on the worn surfaces of the AlB12 plate specimens after sliding at 1000°C. On the other hand, SiO2 and B2O3 films were observed on the worn surfaces of the SiB6 plate specimens after sliding at the same temperature. It was also found that both the boride plate specimens exhibited micro Vickers hardness as high as 20 GPa in the temperature range of 25°C to 1000°C.

Dynamic friction modelling and parameter identification for electromagnetic valve actuator

A new modified LuGre friction model is presented for electromagnetic valve actuator system. The modification to the traditional LuGre friction model is made by adding an acceleration-dependent part and a nonlinear continuous switch function. The proposed new friction model solves the implementation problems with the traditional LuGre model at high speeds. An improved artificial fish swarm algorithm (IAFSA) method which combines the chaotic search and Gauss mutation operator into traditional artificial fish swarm algorithm is used to identify the parameters in the proposed modified LuGre friction model. The steady state response experiments and dynamic friction experiments are implemented to validate the effectiveness of IAFSA algorithm. The comparisons between the measured dynamic friction forces and the ones simulated with the established mathematic friction model at different frequencies and magnitudes demonstrate that the proposed modified LuGre friction model can give accurate simulation about the dynamic friction characteristics existing in the electromagnetic valve actuator system. The presented modelling and parameter identification methods are applicable for many other high-speed mechanical systems with friction.

The Modified One-Dimensional Hydrodynamic Model Based on the Extended Chezy Formula

Although steady uniform friction formulas have been introduced to the framework of a one-dimensional (1D) hydrodynamic model for centuries, the error of friction calculation inevitably undermines the performance of flood routing. Based on successful results of unsteady channel friction research studies, a newly proposed unsteady friction model is introduced to establish a modified 1D hydrodynamic model (namely, the modified SVN model). With the help of a carefully designed parameter calibration method, the performance of the modified SVN model was compared with that of the original SVN model in a simulation test for a hydraulic experiment. This study’s results revealed that compared with the original SVN model, the modified SVN model could achieve a better simulation in both the flow depth and the sectional averaged velocity simulations. Furthermore, it could reduce the peak value error and the time-at-peak error as well, indicating that the use of an unsteady friction model can efficiently improve the performance of a 1D hydrodynamic model.

Hydrophobicity and tribological properties of Al2O3/PTFE composite coating

Al2O3/polytetrafluoroethylene (PTFE) composite coating was prepared on titanium alloy by cathode plasma electrolytic deposition (CPED) and impregnation method, to improve the hydrophobicity and tribological properties. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analysis of the coating indicate that PTFE penetrates into the interior of the coating and is well bonded to titanium alloy substrate by cross-linking with Al2O3 ceramic coating. The contact angles were measured by contact angle measurement, and the tribological properties of the composite coating were evaluated by sliding wear test. The surface of the composite coating is found to possess good hydrophobicity with a water contact angle of 140°. The results also indicate an improved tribological properties of Al2O3/PTFE composite coating at room temperature with a steady friction coefficient as low as 0.05. The self-lubricating anti-wear composite coating is expected to solve fouling problems and poor wear resistance of titanium alloys.

MoS2 additive to the MAO Al2O3 composite coatings with enhanced mechanical performances

MoS2-Al2O3 composite coatings were fabricated on aluminum alloy substrates by micro-arc oxidation in silicate electrolytes doped with different concentrations of MoS2 micro particles. MoS2 (3)-Al2O3 composite coating showed more compact and smooth structure and thus the least scattering distribution of the hardness values. It was found that the friction and wear resistance of composite coating could be improved with increasing MoS2 particles in a range of 1 ∼ 3 g l−1. MoS2 (3)-Al2O3 showed low and steady friction coefficient value of 0.655. The friction and wear resistance of composite coating was impaired with further addition of MoS2 particles with the appearance of obvious oscillation in the friction coefficients. The friction and wear mechanism for the MoS2 (3)-Al2O3 composite coating was thoroughly studied.

Hysteretic Behaviour of Asymmetrical Friction Connections Using Brake Pads of D3923

Asymmetrical Friction Connections (AFCs) can provide stable hysteretic behaviour to dissipate seismic energy. However, the metallic sliding surfaces used in such AFCs may develop metal to metal corrosion compromising performance and their frictional resistance may be low. Non-metallic sliding surfaces should therefore be considered. The quasi-static testing performance of 12 AFCs using bonded non-asbestos D3923 brake pads on the sliding surfaces is evaluated. Three configurations of brake pads bonded on the sliding surfaces of the cap and fixed plates were considered: i) brake pads bonded over the full sliding surfaces (A), ii) strips of brake pads bonded beside the bolt holes (B), and iii) full recessed brake pad with strips beside the bolt holes (C). The greatest AFC strength occurred in the initial sliding cycles, and became constant after the brake pad reached a steady wear state, at which time the hysteresis loop became symmetric, stable, and predictable. For the A, B, and C brake pad configurations, the steady wear state effective friction coefficient varied between 0.12–0.17, 0.10–0.14, and 0.12–0.17, respectively, with the average values being 0.15, 0.12, and 0.15. The peak values, obtained in the initial sliding cycles were 1.7, 2.2, and 1.9 times these average values, while the lower bound values, obtained after many sliding cycles were 0.71, 0.71, and 0.70 times these average values. The recessed brake pad Configuration C is the preferred configuration since it had the highest steady wear state friction coefficient and the least degradation.

Tribological Behavior of the 316L Stainless Steel with Heterogeneous Lamella Structure.

In this paper, the tribological behavior of 316L stainless steel with heterogeneous lamella structure (HLS), prepared through 85% cold rolling technology and subsequent annealing treatment (750 °C, 10 min), were conducted on a ball-on-disc tribometer under different normal loads in dry ambient air conditions. The morphologies, structures, and compositions of the raw and worn surfaces were analyzed by 3D surface profilometer, XRD, SEM, EDS and TEM. Based on this, the results showed that the HLS 316L stainless steel samples exhibited lower and more steady friction coefficients than coarse-grained samples, especially under higher loads, which can be attributed to the existence of numerous oxidative particles across sliding interfaces. However, the wear resistance of HLS 316L stainless steel sample was a little weakened compared to that of the coarse-grained sample under a normal load of 5 N. When the load increases up to 15 N, an obviously decreased wear resistance was found for the HLS of the 316L stainless steel sample, which was 50% lower than that of coarse-grained sample. This can be ascribed to the more severe oxidative and abrasive wear performance of HLS 316L stainless steel sample under dry sliding conditions.

Experimental and Numerical Simulation of Water Hammer in Gravitational Pipe Flow with Continuous Air Entrainment

Water hammer is an undesired hydraulic shock phenomenon in water supply pipe systems. It is very important to simulate water hammer for preventing the hazard of over pressure. In order to predict the transient pressure caused by a valve closing in a gravitational pipe with continuous air entrainment, a numerical model based on the Lax-Wendroff format is established, and the matched boundary model is provided. Compared with the traditional methods, this study provides another access by considering the influence of the pipe flow velocity on the wave propagation to simulate transient processes. A corresponding experiment is conducted to optimize the numerical model. Based on the experimental result, an additional friction function is proposed to evaluate the influence of the air content on the attenuation. The result shows that the energy dissipation of the shock waves may be underestimated in air-water mixture flow using the common steady friction. By introducing the additional friction function, the improved model can more accurately simulate the attenuation of the water hammer in the gravitational pipe with continuous air entrainment. As there are plenty of practical water supply systems running with air content, the improved Lax-Wendroff Method (LWM) is valued in accurately predicting water hammer processes especially in those conditions.

High-temperature tribological properties of Mo-Si-B intermetallic alloy/Si3N4 tribopairs

In our present study, Mo-Si-B intermetallic alloy plate specimens of different compositions were prepared by spark plasma sintering, and their friction and wear properties were investigated by sliding against Si3N4 ball specimens in a temperature range of 298 K–1273 K in air. After the friction and wear tests, the worn surfaces of the plate specimens were investigated using a SEM with an EDS attachment and an XPS. The MoB-based alloy plate specimens exhibited steady friction coefficients as low as 0.1 at 1073 K due to the formation of low friction hexagonal BN and MoO3 films. The MoB-based alloy plate specimens exhibited similar low friction coefficients for 600 s from the start of the friction tests at 1273 K. The Mo5Si3- and MoSi2-based alloy plate specimens exhibited the highest steady friction coefficients (approximately 0.6) of all the plate specimens at 1073 K and 1273 K, respectively.

On the steady/quasi-steady dissipation term in the classic discrete vapour cavity model for simulating column separation

Different families of the Discrete Vapour Cavity Model (DVCM) are developed, including the frictionless, steady and quasi-steady friction models. A relaxation-dissipation approach is proposed to improve the timing of pressure pulses predicted by the classic DVCM. In this approach, a friction correction factor is introduced into the steady/quasi-steady friction term to reduce the local value of the dissipation term in regions facing with cavitation. The proposed approach is completely consistent with the classical water-hammer framework. The importance of the steady/quasi-steady friction term is investigated by comparing numerical results of different DVCMs with the experimental data for various cavitation problems. Based on a frictionless study, it is shown that there exists an unrealistic attenuation in pressure pulses of the classic DVCM. For problems with high-intensity cavitation, it is shown that the frictionless, steady and quasi-steady friction models generally produce different results, especially in terms of the pressure pulses timing. Within the range described in the manuscript, the timing of the classic DVCM pressure pulses can generally be improved by applying the proposed relaxation-dissipation approach on the steady/quasi-steady friction term.

Investigation on friction models for simulation of pipeline filling transients

ABSTRACTThe assumption of steady friction model is usually adopted for modelling of pipeline filling. To investigate the effect of different friction models on filling transients, the steady, quasi-steady and unsteady friction models are introduced when the filling processes are simulated in a reservoir-valve-pipeline system. The relevance of three friction models during pipeline filling is studied under different driving heads, valve opening times and pipe sizes. The numerical explorations show that the discrepancies in filling results between the quasi-steady friction model and the unsteady friction model are negligible but the discrepancies between the quasi-steady (or unsteady) friction model and steady friction model increase with the decrease in driving head and the increase in valve opening time in a small scale pipeline. Moreover, the importance of friction models would not be obvious when the pipe size increases gradually. This implies that one can use almost any reasonable friction model during rapid filling in a large scale pipeline.

Superlubricity of 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate Ionic Liquid Induced by Tribochemical Reactions.

The robust liquid superlubricity of a room-temperature ionic liquid induced by tribochemical reactions is explored in this study. Here, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM]TFS) could realize stable superlubricity (μ < 0.01) with water at the interfaces of Si3N4/SiO2. A superlow and steady friction coefficient of 0.002-0.004 could be achieved under neutral conditions (pH of 6.9 ± 0.1) after 600 s of running-in process. Various factors that could affect superlubricity were explored, including concentration of [EMIM]TFS, sliding speed, applied load, and volume of the lubricant. The results reveal that superlubricity can be achieved with [EMIM]TFS aqueous solution under a broad scope of conditions. The results of surface analysis show that a steady composite tribochemical layer comprising [EMIM]TFS, silica, ammonia-containing compounds, and sulfides was formed by tribochemical reactions between [EMIM]TFS and Si3N4 during the running-in period. The film thickness calculation reveals that the achieved superlubricity is in a mixed lubrication regime that comprises boundary lubrication and thin film lubrication. The superlubricity state is governed by a firm composite tribochemical layer, a molecular adsorption layer (electric double layer of [EMIM]TFS), and a fluid layer. The liquid superlubricity achieved by the ionic liquid is helpful for the development of new ionic liquids with superlubricity characteristics and is of great significance for scientific understanding as well as engineering applications.

Normal Stress‐Dependent Frictional Weakening of Large Rock Avalanche Basal Facies: Implications for the Rock Avalanche Volume Effect

AbstractTo explore the mechanism behind the volume effect of rock avalanches observed in field data, a series of rotary shear tests were conducted at a shearing velocity of 0.87 m/s and varying normal stress levels (from 0.29 to 1.85 MPa) on soil collected from the basal facies of the Yigong rock avalanche in Tibet, China. This experimental study reveals that (1) as normal stress increases, the steady state apparent friction coefficient (μss) of the soil decreases, indicating a normal stress‐dependent feature of μss; (2) the higher the normal stress, the lower the decay rate of μss; (3) water vaporization induced by frictional heating is commonly observed in the tests accompanied by large decreases in μss, and excess pore pressure is generated in the shearing zones of the samples due to vapor accumulation; and (4) with increases in normal stress and decreases in soil permeability, an increasing feature of excess pore pressure is reached. Based on the experimental results, we propose that the coupled effects of water vaporization induced by frictional heating in avalanche basal facies and depth‐dependent permeability of avalanche basal facies should be contributed to the volume effect of rock avalanches.

Pressure Transient Model of Water-Hydraulic Pipelines with Cavitation

Transient pressure investigation of water-hydraulic pipelines is a challenge in the fluid transmission field, since the flow continuity equation and momentum equation are partial differential, and the vaporous cavitation has high dynamics; the frictional force caused by fluid viscosity is especially uncertain. In this study, due to the different transient pressure dynamics in upstream and downstream pipelines, the finite difference method (FDM) is adopted to handle pressure transients with and without cavitation, as well as steady friction and frequency-dependent unsteady friction. Different from the traditional method of characteristics (MOC), the FDM is advantageous in terms of the simple and convenient computation. Furthermore, the mechanism of cavitation growth and collapse are captured both upstream and downstream of the water-hydraulic pipeline, i.e., the cavitation start time, the end time, the duration, the maximum volume, and the corresponding time points. By referring to the experimental results of two previous works, the comparative simulation results of two computation methods are verified in experimental water-hydraulic pipelines, which indicates that the finite difference method shows better data consistency than the MOC.

Investigation on Tribological Properties of the Pre-oxidized Ti2AlN/TiAl Composite

Different oxidation layers on the Ti2AlN/TiAl substrate which was fabricated by in situ synthesis were prepared through thermal oxidation process. The microstructure, phase identification and elements distribution of the oxidation layers were analyzed. The tribological performance of pre-oxidized composites against Si3N4 ball at 25 and 600 °C, as well as the effect of pre-oxidation layers on tribological performance was systematically investigated. The results show that, compared to Ti2AlN/TiAl, the pre-oxidized composites present more excellent tribological properties, especially the wear resistance at 600 °C. It is a significant finding that, different from severe abrasive wear and plastic deformation of Ti2AlN/TiAl, the tribo-films formed by the pre-oxidation layers on the worn surface of pre-oxidized composites weaken abrasive wear and suppress the development of plastic deformation to protect the underlying composite substrate from wear. Moreover, the stable cooperation on the interface between tribo-films and Si3N4 ball results in the relatively steady friction coefficient.

Liquid Superlubricity of Polyethylene Glycol Aqueous Solution Achieved with Boric Acid Additive.

Boric acid is a weak acid and has been used as a lubrication additive because of its special structure. In this study, we report that boric acid could achieve a robust superlubricity (μ < 0.01) as an additive in polyethylene glycol (PEG) aqueous solution at the Si3N4/SiO2 interfaces. The superlow and steady friction coefficient of approximately 0.004-0.006 could be achieved with boric acid under neutral conditions (pH of approximately 6.4), which is different from the acidic conditions leading to superlubricity. The influence of various factors, including boric acid concentration, sliding speed, applied load, PEG molecular weight, and the volume of lubricant on the superlubricity, were investigated. The results reveal that the PEG aqueous solution with the boric acid additive could achieve superlubricity under a wide range of conditions. The surface composition analysis shows that the synergy effect between boric acid and PEG provides sufficient H+ ions to realize the running-in process. Moreover, a composite tribochemical film composed of silica and ammonia-containing compounds were formed on the ball surface, contributing to the superlubricity. The film thickness calculation shows that superlubricity was achieved in a mixed lubrication region, and therefore, the superlubricity state was dominated by both the composite tribochemical film formed via the tribochemical reaction on the contact surfaces and the hydrodynamic lubricating film between the contact surfaces. Such a liquid superlubricity achieved under neutral conditions is of importance for both scientific understanding and engineering applications.

Oxidation and Tribological Behavior of Ti-B-C-N-Si Nanocomposite Films Deposited by Pulsed Unbalanced Magnetron Sputtering.

Quinary Ti-B-C-N-Si nanocomposite films were deposited onto AISI 304 substrates using a pulsed d.c. magnetron sputtering system. The quinary Ti-B-C-N-Si (5 at.%) film showed excellent tribological and wear properties compared with those of the Ti-B-C-N films. The steady friction coefficient of 0.151 and a wear rate of 2 × 10-6 mm3N-1m-1 were measured for the Ti-B-C-N-Si films. The oxidation behavior of Ti-B-C-N-Si nanocomposite films was systematically investigated using X-ray diffraction (XRD), and thermal analyzer with differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). It is concluded that the addition of Si into the Ti-B-C-N film improved the tribological properties and oxidation resistance of the Ti-B-C-N-Si films. The improvements are due to the formation of an amorphous SiOx phase, which plays a major role in the self-lubricant tribo-layers and oxidation barrier on the film surface or in the grain boundaries, respectively.

Semi-implicit staggered discontinuous Galerkin schemes for axially symmetric viscous compressible flows in elastic tubes

We propose a novel family of staggered semi-implicit discontinuous Galerkin (DG) finite element schemes for the simulation of axially symmetric, weakly compressible and laminar viscous flows in elastic pipes. The equation of state (EOS) of the fluid is assumed to be barotropic and two different mathematical models derived from the compressible Navier-Stokes equations are considered in this paper.The first model describes cross-sectionally averaged 1D flows, including steady and frequency-dependent wall friction effects. The novelty of our numerical method compared to standard DG schemes consists in the use of a staggered mesh, where the pressure is defined over a primary grid and the velocity field is defined on edge-based staggered dual control volumes. This approach is well known from classical semi-implicit finite difference schemes for the incompressible Navier–Stokes equations, but it is still quite unusual for high order DG schemes. The continuity equation is integrated over the control volumes that belong to the main grid, while the momentum equation is integrated over the elements of the edge-based staggered dual grid. The nonlinear convective terms are discretized explicitly, while the pressure gradient and the mass flux are discretized implicitly. Up to second order of accuracy in time can be achieved with the so-called θ-method. Inserting the discrete momentum equation in the discrete mass conservation equation leads to a mildly nonlinear algebraic system for the degrees of freedom of the pressure. Such mildly nonlinear systems can be very efficiently solved using the Newton algorithm of Brugnano and Casulli. We observe that the linear part of the mildly nonlinear system is symmetric and positive definite.The second model is derived from the compressible Navier-Stokes equations in cylindrical coordinates. Assuming hydrostatic flow with constant pressure inside each cross section as well as axial symmetry, only the terms in the axial and the radial direction need to be considered. Therefore, we call the second model the 2Dxr model. Also in this case we use a staggered mesh for pressure and velocity and thus the same philosophy as for the 1D model can be applied to obtain the discrete pressure system. For the 2Dxr model a staggered DG scheme is also applied for the computation of the viscous stress tensor in the discrete momentum equation. However, in radial direction the resulting linear system for the friction terms is not symmetric and is thus solved using the Thomas algorithm for block three-diagonal systems.The use of a semi-implicit DG scheme leads to a very mild CFL condition based only on the fluid velocity and not on the sound speed, which makes the method very efficient, in particular in the limit cases when the speed of sound of the fluid tends to infinity (incompressible fluid) and in the rigid case where the wall strain of the pipe tends to zero. In addition, at every Newton step a symmetric positive definite and well conditioned block three-diagonal linear system is solved for the pressure, using a matrix-free conjugate gradient method. Moreover, when the polynomial degree of the basis and test functions is equal to zero the schemes reduce to classical semi-implicit finite volume methods.While in the 2Dxr model the viscous effects in radial direction are directly obtained from first principles via the Navier-Stokes equations, the 1D model requires an additional closure relation for the wall friction. For both models we perform several tests in order to validate the numerical methods for steady and unsteady flows of compressible and nearly incompressible fluids in elastic and rigid tubes. We also provide numerical convergence results in order to show that the developed schemes achieve high order of accuracy in space.

Significant friction reduction of high-intensity pulsed ion beam irradiated WC-Ni against graphite under water lubrication

Two types of commercial WC-Ni samples were irradiated with the High-intensity pulsed ion beam (HIPIB). Both the surface characteristics and tribo-characteristics of the non-irradiated and irradiated WC-Ni samples, sliding against graphite under water lubrication, were compared. Quite low steady friction coefficients (approximately of 0.02) of the irradiated WC-Ni were observed. The surface topographies and components were investigated. The quite low friction of the irradiated WC-Ni samples was ascribed to the higher fluid retention capability of the latter and the tribofilm formed during sliding.