Liquid Friction Research Articles

Hydropress assembling automation of interference fits with steel and titanium parts

The main aspects of assembling automation of hydropress connections are considered – increasing the assembling performance, its stability and ensuring the quality of hydropress connections. A control system for the technological complex of hydropress assembling on a base of fuzzy logic methods has been developed, it provides the control of technological assembling modes, including: pressing speed, oil pressure, control of the force and pressing length, without identifying the parameters of assembling process, as well as the guaranteed presence of liquid friction in the non-linearity conditions of assembling process. The structure and control algorithm of automated hydropress assembling that ensure the maintenance of the working oil pressure during assembling are proposed. The algorithm is based on a set of feedbacks: pressure and position sensors, that provides a qualitative transition from boundary to liquid friction. A prototype of the technological complex of hydropress assembling of interference fits has been developed, that allows to assemble joints with a coupling diameter up to 50 mm in an automated mode. The results of experimental studies of automated assembling of steel and titanium joints are presented.

Drag Reduction Using Lubricant‐Impregnated Anisotropic Slippery Surfaces Inspired by Bionic Fish Scale Surfaces Containing Micro‐/Nanostructured Arrays

Inspired by the bionic fish scale surfaces containing micro/nanostructured arrays, herein, the applications of lubricant‐impregnated anisotropic slippery surfaces (LIASSs) using laser ablation of aluminum–magnesium alloys are proposed. Different hydrophobic properties are presented on the LIASSs along the parallel direction and the reverse direction defined as directions A and B of bionic fish scale micro/nanostructures. A self‐assembled solid–liquid interface friction test device is set up to demonstrate the drag reduction property of LIASSs. The drag reduction ratios are found to be 51.09% and 44.88% along directions A and B, respectively. With the increase in the velocity, the drag reduction ratios of LIASSs can also be kept near 50%. Simulation models are established to study the drag reduction mechanism of LIASSs in laminar flows. Liquid–liquid repellency has a lubricating effect that can increase mobility and reduce viscous resistance. In this way, it improves the fluidity of the liquid and reduces drag. The drag reduction ratio in direction A is superior to that in direction B for the same flow velocity. The results of the simulation are consistent with the experiments results. LIASSs represents an effective strategy to drag reduction and reducing energy consumption in liquid directional transport and marine vessels.

Investigation on non-uniform friction behaviors of slab during continuous casting based on an inverse algorithm

The non-uniform friction and lubrication are key factors that affect the slab quality during continuous casting. In order to present the complicated friction behavior of slab in the mold, a friction calculation model was built based on a full-scale heat transfer model. An inverse algorithm was applied to calculate the real heat flux by measuring temperatures from thermocouples buried in various locations of the mold. ANSYS software was used to solve the finite-element equations. The results showed that the friction state of the slab surface progressed from a liquid friction zone to a mixed friction zone, and then to a solid friction zone from the meniscus. In the casting direction, the liquid frictional stress gradually increased, which was between 0 and 1 kPa; the mixed frictional stress rapidly increased from 1 to 15 kPa; the solid frictional stress increased linearly, reaching 28.4 kPa at the exit. The slag solidification temperature, slag crystallization temperature and superheat could affect the states of liquid, solid or mixed friction.

Sliding on ice: Real contact area, melted film thickness, and friction force

It is easy to slide on ice because of water films arising as a consequence of frictional melting. Although the friction force of ice and a slider critically depends on the area and thickness of the liquid film as well as the sliding speed, direct experimental visualization and quantification of temporal evolutions of the contact area, film thickness, and the resulting friction force have been scarce to date. Here we develop an experimental technique to visualize the contact area of ice asperities and a high-speed sliding surface in situ based on the optical principle of total internal reflection. We construct a hydrodynamic model to predict the contact area, liquid film thickness and friction force of a model system of hemispherical ice on a flat solid surface. Upon showing good agreement between theory and experiment, we briefly discuss how the fundamental understanding of the friction behavior of a single ice bump can be extended to understand the friction behavior of flat ice surfaces.

Rapid onset of molecular friction in liquids bridging between the atomistic and hydrodynamic pictures

Friction in liquids arises from conservative forces between molecules and atoms. Although the hydrodynamics at the nanoscale is subject of intense research and despite the enormous interest in the non-Markovian dynamics of single molecules and solutes, the onset of friction from the atomistic scale so far could not be demonstrated. Here, we fill this gap based on frequency-resolved friction data from high-precision simulations of three prototypical liquids, including water. Combining with theory, we show that friction in liquids emerges abruptly at a characteristic frequency, beyond which viscous liquids appear as non-dissipative, elastic solids. Concomitantly, the molecules experience Brownian forces that display persistent correlations. A critical test of the generalised Stokes–Einstein relation, mapping the friction of single molecules to the visco-elastic response of the macroscopic sample, disproves the relation for Newtonian fluids, but substantiates it exemplarily for water and a moderately supercooled liquid. The employed approach is suitable to yield insights into vitrification mechanisms and the intriguing mechanical properties of soft materials.

Effect of Hydrogen Bonding between Ions of Like Charge on the Boundary Layer Friction of Hydroxy-Functionalized Ionic Liquids.

Atomic force microscopy has been used to measure the lubricity of a series of ionic liquids (ILs) at mica surfaces in the boundary friction regime. A previously unreported cation bilayer structure is detected at the IL-mica interface due to the formation of H-bonds between the hydroxy-functionalized cations [(c-c) H-bonds], which enhances the ordering of the ions in the boundary layer and improves the lubrication. The strength of the cation bilayer structure is controlled by altering the strength of (c-c) H-bonding via changes in the hydroxyalkyl chain length, the cation charge polarizability, and the coordination strength of the anions. This reveals a new means of controlling IL boundary nanostructure via H-bonding between ions of the same charge, which can impact diverse applications, including surface catalysis, particle stability, electrochemistry, etc.

Optimization and Extended Applicability of Simplified Slug Flow Model for Liquid-Gas Flow in Horizontal and Near Horizontal Pipes

The accurate prediction of pressure loss for two-phase slug flow in pipes with a simple and powerful methodology has been desired. The calculation of pressure loss has generally been performed by complicated mechanistic models, most of which require the iteration of many variables. The objective of this study is to optimize the previously proposed simplified slug flow model for horizontal pipes, extending the applicability to turbulent flow conditions, i.e., high mixture Reynolds number and near horizontal pipes. The velocity field previously measured by particle image velocimetry further supports the suggested slug flow model which neglects the pressure loss in the liquid film region. A suitable prediction of slug characteristics such as slug liquid holdup and translational velocity (or flow coefficient) is required to advance the accuracy of calculated pressure loss. Therefore, the proper correlations of slug liquid holdup, flow coefficient, and friction factor are identified and utilized to calculate the pressure gradient for horizontal and near horizontal pipes. The optimized model presents a fair agreement with 2191 existing experimental data (0.001 ≤ μL ≤ 0.995 Pa∙s, 7 ≤ ReM ≤ 227,007 and −9 ≤ θ ≤ 9), showing −3% and 0.991 as values of the average relative error and the coefficient of determination, respectively.

Controllable Friction of Green Ionic Liquids via Environmental Humidity

Intelligent control of friction is an attractive but challenging topic. In this work, it is investigated if it would be possible to adjust friction in a lubricated contact by controlling environmental humidity. By exploiting the ability to adjust the environmental humidity by various saturated salt solutions, friction behavior of contacts lubricated with Choline l‐Proline ([Cho][Pro]) is modulated in a wide range of relative humidity (RH). The friction increases when the environmental humidity is increased and decreases when water is partially evaporated to a lower RH. It is thus possible to control friction by environmental humidity. The addition of water in ionic liquids (ILs) causes a decrease in viscosity, but as the tests are calculated to be performed in boundary lubrication the viscosity change is not the main factor for the change in friction. The friction sensitivity of RH can be explained by the effect of adhesion on the water uptake from humid air by [Cho][Pro]. Furthermore, the reversible changes of H‐bond types determined by the water content could be another explanation to the altered friction.

A new methodology for a detailed investigation of quantized friction in ionic liquids.

When confined at the nanoscale between smooth surfaces, an ionic liquid forms a structured film responding to shear in a quantized way, i.e., with a friction coefficient indexed by the number of layers in the gap. So far, only a few experiments have been performed to study this phenomenon, because of the delicate nature of the measurements. We propose a new methodology to measure friction with a surface force balance, based on the simultaneous application of normal and lateral motions to the surfaces, allowing for a more precise, comprehensive and rapid determination of the friction response. We report on proof-of-concept experiments with an ionic liquid confined between mica surfaces in dry or wet conditions, showing the phenomenon of quantized friction with an unprecedented resolution. First, we show that the variation of the kinetic friction force with the applied load for a given layer is not linear, but can be quantitatively described by two additive contributions that are respectively proportional to the load and to the contact area. Then, we find that humidity improves the resistance of the layers to be squeezed-out and extends the range of loads in which the liquid behaves as a superlubricant, interpreted by an enhanced dissolution of the potassium ions on the mica leading to a larger surface charge. There, we note a liquid-like friction behavior, and observe in certain conditions a clear variation of the kinetic friction force over two decades of shearing velocities, that does not obey a simple Arrhenius dynamics.

Applications of computational fluid dynamics in irrigation engineering

Computational fluid dynamics (CFD) techniques have become an important tool for investigating and predicting flow behavior in many industrial and engineering processes. In the last two decades, CFD has been used for the study, design, and improvement of irrigation equipment. Numerical simulations can be used to predict fluid flow, heat transfer, and chemical reactions within complex systems. The objective of this review is to provide an overview of the uses of CFD in irrigation engineering applications. The paper is organized into two main sections: fundamentals of CFD and irrigation engineering applications. The first section presents the main methods used in numerical simulations, basic equations to predict fluid flow parameters, meshing concerns, and convergence criteria. In the second section, we present applications related to friction and local head losses in pipes, liquid and solid-liquid flow simulation in drippers, chemical scaling, filters, sprinklers, direct-acting pressure-regulating valves, and Venturi injectors. The briefly described applications indicated that CFD modeling can be an accurate, quick, and feasible method for the investigation of flow parameters in irrigation pipes, fittings, emitters, and accessories. The CFD simulations can be useful for designing new products as well as for improving and optimizing existing products. Computational fluid dynamics uses in irrigation engineering must be encouraged, particularly for innovation purposes resulting from the cooperation between academia and irrigation companies.

ОЦЕНКА ВЛИЯНИЯ ПАРАМЕТРОВ МИКРОТЕКСТУРИРОВАНИЯ НА ГИДРОМЕХАНИЧЕСКИЕ ХАРАКТЕРИСТИКИ ПОДШИПНИКОВ КОЛЕНЧАТОГО ВАЛА ДИЗЕЛЯ

The solution to the problem of increasing the motor resource of an internal combustion engine (ICE) is directly related to the reduction of energy losses due to overcoming friction in the elements of systems, mechanisms, and complexly loaded tribo-couplers (TC). Among the mechanical friction losses, a special place isoccupied by the hydromechanical friction losses in the internal combustion engine. The reduction of energy losses to overcome friction is achieved by reducing mechanical losses by limiting the level of loading of the rubbing surfaces, by increasing the proportion of the liquid friction regime for the most critical in terms of reliability resource-determining complex loaded vehicles. For complexly loaded vehicles, the time and magnitude of the acting loads are characteristic, at which the position of the movable element in conjunction is characterized by high eccentricities. Such complexly loaded vehicles include the main and connecting rod bearings of the crankshaft, the “piston guide – cylinder liner” and “piston ring – cylinder liner” couplings, the thrust and thrust bearings of the ICE turbocharger, etc. One of the ways to reduce oil starvation isto texturize the contacting surfaces, which will increase the bearing capacity of a complex bearing due to the creation of many “micro wedges”. In particular, the texturing of the surface of the bearing shells of the crankshaft can be performed in the form of elliptical micro-holes, which allow you to save oil on the friction surface under any operating conditions of the diesel engine. The article provides an overview of the main types of microtexturing of friction surfaces of TC. A calculation model has been created and a calculation analysis program has been developed for the internal combustion engine “crankshaft neck-liner” TC. The calculations ofthe hydromechanical characteristics (HMC) of the vehicle for various types of microtexture were performed using the connecting rod bearing of the diesel engine CHN 13/15 as an example.

АНАЛИЗ ТРАЕКТОРИЙ ДВИЖЕНИЯ ЖЕСТКОГО РОТОРА В КОНИЧЕСКИХ ПОДШИПНИКАХ ЖИДКОСТНОГО ТРЕНИЯ

The calculation procedure of the trajectories of motion of the rigid rotor in the conical liquid friction bearings is offered. The equations set of motion is written for two-bearing rotor in the conical liquid friction bearings.The results are illustrated by the plots of trajectories for the conical bearings with oil lubricant.

ВПЛИВ ТЕМПЕРАТУРИ НАВКОЛИШНЬОГО СЕРЕДОВИЩА НА ВЛАСТИВОСТІ МОТОРНИХ ОЛИВ

The article presents the results of studies that were conducted on the SMC-2 tribometer according to the ring-sleeve scheme in conditions of non-shocking sliding friction due to the rotational motion of the roller. In order to perform experimental studies of the processes of friction and wear of metal vapors and to study mechanical-physicochemical processes in the interaction of a steel-steel pair, in our experiment we studied motor all-season oils of different chemical composition on the rheological basis: motor oils of semisynthetic origin TEMOL 10 synthetic origin and TEMOL Premium synthetic origin origin of TEMOL Premium 5W-40. The aforementioned oils were tested for 15 min under conditions of temperature rise from minus 180C to plus 750C. The purpose of the work is to minimize the friction processes in the conditions of forced increase of the volume temperature of engine oil in engines of type D-241 and engine of concern Volkswagen VAG ASV 1.9 TDI. The method of investigation is the experimental determination of the rheological features of motor oils and the antifriction properties of friction pairs. According to the results of experimental studies, it is established that the stability of the formation of film thickness with increasing temperature, starting from low temperatures, gives synthetic oils some advantages at low-temperature starting, that is, to the conditions of realization of mixed friction (lubrication starvation). Semi-synthetic oil has some advantages in the formation of the thickness of the lubricating layer under conditions of liquid friction at moderate temperatures, which is explained by the formation of a thickness of greater magnitude, has better viscoelastic properties in these conditions of fluid friction. It is established that oils of semisynthetic and synthetic origin have excellent antifriction properties and this can be explained by their stable to mechanical degradation properties in the conditions of accelerated warming of the internal combustion engine. KEYWORDS: ENGINE OIL, RHEOLOGICAL CHARACTERISTICS, ANTIFRICTION PROPERTIES, FILM THICKNESS.

Dehesion-deformational nature of friction and wear-out

Two friction theories are compared: generally recognized adhesion deformation theory and alternative dehision deformation theory, founded by Prandtl and Deryagin. Concepts and peculiarities of effect of adhesion and cohesion forces of interatomic attraction are clarified. The concept of dehesion forces of interatomic repulsion is instroduced; their features are specified. Dehision deformation theory basic provisions are as follows: Compression of solid bodies results in occurrence of the interatomic repulsion forces, which after reaching the unstable equilibrium point (bifurcation point) can be instantaneously replaced with the cohesion attraction forces. Inversion (replacement) of forces forming new compounds are rare during friction, as bifurcation pressure (processes splitting) in the most pairs of substance is higher than the plasticity limit. Friction forces of solid bodies result from engagement of irregularities of the atomic-molecular roughness at the spots of actual contact. The proportionality of friction force to the normal force (Coulomb’s law) is ensured by increase of the depth of engagement of the atomic roughness, and by increase of the area of spots of actual contact. Displacement results in the cut-out of the contact surface areas. The cut-out particles of wear and tear act as the solid bodies; the particles are pressed in solid conglomerates and scratch the surfaces. Dry friction force is determined by the contact pressure, by the interlayer fragments strength, and by the strength of their links between each other and with the friction surfaces. In case of liquid friction, molecules are moved in a barrier-free irregular field of cohesion forces, thereby determining absence of friction of rest and dependence of friction forces on the flow velocity (Newton’s law), in contrast to gas friction resulting from collision of the freely moving molecules (Maxwell’s law). Liquids, which do not include polarized molecules, are pressed out of the friction area and do not affect the friction force. If lubricant includes SAS, then the friction force can change from maximum solid to minimum liquid depending on viscosity, SAS content, pressure, contact area, and slide-to-roll ratio (Stribeck dependence).

Investigation into methane hydrate reformation in water-dominated bubbly flow

Hydrate reformation may lead to production line blockage in the development of natural gas hydrate reservoirs. However, few studies have focused on the flow characteristics and plugging risks during the hydrate reformation process. Therefore, experiments on hydrate reformation were carried out in a high-pressure flow loop. The hydrate induction time and formation subcooling approached for the first hydrate formation and reformation. The pressure and temperature of the first formation and reformation onset fell in a subcooling band (2.0 ± 0.5 °C). Furthermore, the flow stability of hydrate slurry for the reformation process was relatively poor compared with the first formation. Hydrate particles aggregated more violently during the reformation process when the initial flow rate was 1160 kg∙h−1. Moreover, the hydrate memory effect at the microlevel could be confirmed from two aspects, including an increasing number of methane microbubbles (MMBs) after hydrate dissociation and a shorter time required for the decrease in the number of MMBs during the reformation process. Then, the flow pattern evolutions were summarized for different experimental conditions, and the minimum flow rate of hydrate slurry with the stable flow ability could be predicted using the classical correlation. Finally, a prediction model was developed for predicting the pressure drop in hydrate slurry flow, which considered the hydraulic, particle-aggregation, and hydrate–liquid friction effects. The findings of this work provided an insight into the behavior of methane hydrate reformation in water-dominated bubbly flow, which is an advancing research topic in the field of development of natural gas hydrate reservoirs.

Study of longitudinal vibrations of stab knife refiners

The subject of research is the longitudinal vibrations of stab knife refiners. A model of the fibrous layer between the rotor and stator is developed. It is shown that it is advisable to use the Maxwell-Thomson model for liquid friction of the rotor and stator, and the Hooke elastic model for boundary friction. The formula for determining the dynamic stiffness of the fibrous layer is obtained. Dynamic and mathematical models of the refiner in the longitudinal direction are developed. The mathematical model is a system of linear differential equations with periodically varying coefficients (Mathieu-Hill equations). A method for calculating the amplitude of oscillations of the refiner rotor and stator is developed. According to this technique, Andritz 54-60-1c stab refiner was designed. Oscillation parameters of the refiner rotor and stator are determined theoretically and experimentally. The stator oscillation amplitude is 1.6–2.3 times smaller than the rotor oscillation one. In case of boundary friction, the amplitude of oscillations of the stator and rotor increases by 2–3 times in comparison with liquid friction. The oscillation amplitude of the rotor and stator in the longitudinal direction is comparable with the gap between these elements. Therefore, it is recommended while designing refiners, to develop methods and means of vibration protection, and during operation, to prevent boundary friction between headsets. The developed calculation procedure can be used in other industries, for example, mining and metallurgy.

German Bond Yields and Debt Supply

Are Bunds special? This paper estimates the “Bund premium” as the difference in convenience yields between other sovereign safe assets and German government bonds adjusted for sovereign credit risk, liquidity and swap market frictions. A higher premium suggests less substitutability of sovereign bonds. We document a rise in the “Bund premium” in the post-crisis period. We show that there is a negative relationship of the premium with the relative supply of German sovereign bonds, which is more pronounced for higher maturities and when risk aversion proxied by bond market volatility is high. Going forward, we expect German government debt supply to remain scarce, with important implications for the ECB’s monetary policy strategy.

Investigation of the abrasion resistance of stainless-steel composites with Al2O3 reinforcement phase produced by using the capillary forming method

The present paper is relevant with the production and investigation of abrasion resistance of MMCs (Metal Matrix Composites) with metal matrix AISI 304 stainless steel and Al2O3 reinforcement phase. An innovative production method is used for the obtaining of the MMCs by using the conception of the “capillary forming”, where the metal matrix (AISI 304) was forcedly infiltrated in the space between the reinforcement phase (Al2O3) particles by vacuuming the space between reinforcement particles. A weighting method is used to determine the wear resistance by radial load in liquid and dry friction mode. The dynamics of mass variation in the wear process and the wear intensity are determined.

Commercial Mortgage‐Backed Security Pricing with Real Estate Liquidity Risk

AbstractWe propose a structural model with liquidity frictions at the property level for the pricing of commercial mortgages. The model shows that a moderate liquidity shock has a sizable effect on mortgage default risk. The sensitivities of default rates to volatility of property prices, cash payout and interest rates, all increase significantly as liquidity deteriorates. Empirical evidence strongly supports model predictions. The results suggest that failing to account for the effect of real estate illiquidity leads to substantial bias in estimation of default risk, the optimal subordination level and valuation of the structured products.

Research of a fibrous layer at refining in the refiners

A subject of research – a fibrous layer in the refiners at refining. By means of the tension compression chart it is shown that parameters of a fibrous layer are adequately described by Maxwell-Thompson’s model for liquid friction and the Hooke model for boundary friction of a plate. Deborah’s number of a fibrous layer is also investigated. Deborah’s number of a fibrous layer at refining changes in the range from 7 to 2.1·106 and depends on a nature and concentration of the refining material, the angle of knife crossing, the frequency of rotation of rotor and width of the platform of contact. For decrease in power consumption at refining both chips and pulp of high concentration it is recommended to increase the speed of sliding of knife of rotor along stator knife. It is possible to reach by increase in frequency of rotation of rotor and by decrease of an angle of crossing of knife. At refining of pulp of low concentration for decrease in power consumption of refining it is recommended to reduce the frequency of rotation of rotor and to increase the angle of crossing of knife of rotor and stator. At the high density of contact of plate knife in the range from 5 to 106 of Deborah number the deformation component of friction coefficient decreases together with power consumption of refining process.