Rate Of Mechanical Energy Dissipation Research Articles

Pull-Off Strength and Mechanical Energy Dissipation in Adhesive Contacts: Experiments and Simulations

This study contributes to the understanding of the adhesive properties in normal contacts, providing valuable information on the influence of various factors on adhesive strength and energy dissipation. The adhesive normal contact between a steel spherical indenter and a soft sheet of elastomer is studied experimentally. The dependencies of contact strength and mechanical energy dissipation in the complete indentation–detachment cycle on the indentation depth, the velocity of the indenter, its radius, thickness, and elastic modulus of the elastomer, the specific work of adhesion, as well as the roughness of the indenter surface, were analyzed. Experimental results are compared with simulations using the boundary element method (BEM), and the reasons leading to discrepancies between experiments and simulations are analyzed. It is shown that over a wide range of experimental parameters, the rate of mechanical energy dissipation can be estimated with sufficient accuracy using a simple empirical relation.

Study of the Wear Resistance of a Friction unit with a Non-Standard Support Profile and a Metal Coating

The theoretical calculations and the results of experiment for a friction unit consisting of a wedge-shaped support surface are presented in this research paper. The slider on the tread surface has a metal alloy. The lubricating fluid is represented by a lubricant with non-Newtonian characteristics; it has the laminar flow in this unit. The friction unit functions when the cavity between the slider and the support ring is completely filled. To solve the theoretical problem under given conditions, the equations of continuity, the rate of mechanical energy dissipation, the values of the expression for the displacement of a non-Newtonian fluid were used. In addition, equations were used to characterize the profile of a slider with a melt on the tread surface. The presented problem was solved by the self-similar variable method with asymptotic expansion in the smallest parameter. Upon calculation, it is given the equations for the first and zero pressure approximations and it is determined the velocity field are determined with and without the molten metal. Further, the friction force and load capacity were found using the non-Newtonian properties of the lubricating fluid and the above structural characteristics of non-standard surfaces for friction contact.

Calculation model for lubrication of bearings with unconventional support surface profile and fusible shaft surface coating

This research work uses the true viscous fluid flow equation for the thin layer, the continuity equation, and the equation describing the radius of the fused shaft coated with a metal alloy taking into account the mechanical energy dissipation rate formula to find the asymptotic and the exact self-similar solution for the zero and first-order approximation for the radial sliding bearing with the support profile adapted to friction and lightalloy coating for incomplete filling of the working gap. We obtained the analytical dependencies for the fused surface radius of metal coating and the speed and pressure field. Besides, we determined the key operating parameters of the reviewed friction couple, the carrying capacity, and the friction force. The authors assess the impact of the parameters associated with the fusing of the coating of the friction-adapted support profile and the length of the loaded area on the carrying capacity and the friction force.

Computational Model for Radial Plain Bearing with Non-Circular Bearing Surface Profile and Fusible Coating on Shaft Surface

The paper presents a study based upon: a Newtonian fluid flow equation (“thin layer”), a continuity equation, and an equation of the molten-profile radius for a shaft coated with a fusible metal alloy; considering a mechanical energy dissipation rate formula, the authors produced an asymptotic and accurate automodel solution for the zero approximation (melting ignored) and first approximation (adjusted for melting) of a radial plain bearing featuring a fusible metal coating and a bearing profile adapted to the specific friction parameters. The paper further presents analytical dependencies describing the molten surface radius, velocity and pressure fields for zero and first approximation. Besides, it determines the key operating parameters of the frictional couple, the bearing capacity, and the friction. It also shows how the parameters arising from the melting of the surface affect the bearing capacity and friction where the bearing surface profile is adapted to the specific conditions of friction.

Mathematical Model of Micropolar Lubricant Considering Viscosity-Pressure Dependence

In the study, based on the micropolar fluid flow equation for a “thin layer”, the continuity equation, the equation describing the profile of the molten contour of the guide coated with a low-melting metal alloy, and the equation for the mechanical energy dissipation rate, asymptotic and exact self-similar solution has been found for the zero (without considering the melting) and first (considering the melting) approximation of wedge-shaped support with the slider support profile adapted to the friction conditions and the low-melting metal coating of the guide surface. The research has taken into account the pressure dependence of the lubricant rheological properties and the melt having micropolar properties in the laminar flow regime. Analytical dependencies have been obtained for the molten surface profile of the low-melting metal coating of the guide and the field of velocities and pressure for the zero and first approximations. Also, the basic performance characteristics of the friction pair under consideration have been determined: the bearing capacity and the friction force. The impact of parameters determined by the coating melt, adapted to the support profile friction conditions, and the parameter characterizing the pressure dependence of the lubricant viscosity on the bearing capacity and friction force has been estimated.

Computational model of a micropolar lubricant with a non-standard support profile and a metal coating at incomplete filling of the working gap

The authors propose an asymptotic and exact self-similar solution for zero (without considering the melt) and the first (considering the melt) approximation of a wedge-shaped sliding support with a profile adapted to friction and a fusible metal coating of the guide surface. The solution is based on the equation of a micropolar liquid flow for a “thin layer”, the continuity equation, as well as the equation describing the profile of the molten contour of a guide coated with a fusible metal alloy. The authors have taken into account the formula of the rate of mechanical energy dissipation as well as rheological properties of the lubricant and the melt, which have micropolar properties in the laminar flow mode at incomplete filling of the working gap. Analytical dependences have been obtained for the profile of the molten surface of the guide coated with a low-melting metal alloy, as well as for the velocity and pressure fields at zero and first approximation. In addition, the main operating characteristics of the friction pair under consideration have been determined: the bearing capacity and the friction force. The article contains estimation of the influence of the parameters conditioned by coating melt and adapted to the friction conditions of the support profile, and the parameter characterizing the rheological properties of the lubricant, as well as the length of the loaded area in terms of bearing capacity and friction force.

Mathematical model of wedge-shaped sliding support with low melting metal coated guide for partially filled working gap

The reported study presents an asymptotic series solution to a system of differential equations in powers of a small parameter K depending on melting and a mechanical energy dissipation rate. Further, a self-similar solution has been found, as a consequence, a profile of velocities, pressure, principal operational characteristics, load capability, and friction force have been determined for a partially grease-filled working gap.

Regenerating Hair in Prevascularized Tissue Space Formed by a Controllable Foreign Body Reaction

AbstractIntracutaneous transplantation of trichogenic cells is a currently endorsed strategy to realize hair regeneration in vivo. However, skin is not the most advantageous transplant site due to the robust mechanical property and deficient physiological perfusion. Herein, a subcutaneous space made of prevascularized collagen fibers (PVCF) generated by controlling the duration of in situ foreign body reaction is reported. In contrast to skin, an optimally preprogrammed PVCF presents a larger tissue volume (171 mm3), low Young's modulus (1/2‐fold), high flexibility high mechanical energy dissipation rate (1.3‐fold), highly permeable surface structure, and plentiful vascular network. Remarkably, cells transplanted into PVCF suffer less apoptosis and necrosis, and generate more mature hairs (≈289 per site) than that in intracutaneous transplantation (≈177 per site). The RNA‐sequencing profiling indicates similar trends under molecular level. This work provides a promising strategy to improve graft site microenvironment physiologically and mechanically, with a low‐cost and simple fabrication process.

Radial Friction Bearing with a Fusible Coating in the Turbulent Friction Mode

This paper presents a method for forming an exact self-similar solution to the problem of hydrodynamic calculation for a radial friction bearing that operates using an electrically conductive liquid lubricant conditioned by the bearing bushing surface melt, taking into account the influence of pressure and temperature on the viscosity and electrical conductivity of the liquid lubricant under adiabatic conditions. The asymptotic solution of a system of differential equations taking into account boundary conditions on the surface of a bearing bushing $$r = - {\Phi (}\theta {\text{)}}$$ has been found in the form of a series in powers of small parameter K caused by the melt and by the rate of mechanical energy dissipation.

Fine Structure, Instabilities, and Turbulence in the Lower Atmosphere: High-Resolution In Situ Slant-Path Measurements with the DataHawk UAV and Comparisons with Numerical Modeling

AbstractA new platform for high-resolution in situ measurements in the lower troposphere is described and its capabilities are demonstrated. The platform is the small GPS-controlled DataHawk unmanned aerial system (UAS), and measurements were performed under stratified atmospheric conditions at Dugway Proving Ground, Utah, on 11 October 2012. The measurements included spiraling vertical profiles of temperature and horizontal wind vectors, from which the potential temperature θ, mechanical energy dissipation rate ε, Brunt–Väsälä frequency N, temperature structure parameter CT2, Thorpe and Ozmidov scales LT and LO, and Richardson number Ri were inferred. Profiles of these quantities from ~50 to 400 m reveal apparent gravity wave modulation at larger scales, persistent sheet-and-layer structures at scales of ~30–100 m, and several layers exhibiting significant correlations of large ε, CT2, LT, and small Ri. Smaller-scale flow features suggest local gravity waves and Kelvin–Helmholtz instabilities exhibiting strong correlations, yielding significant vertical displacements and inducing turbulence and mixing at smaller scales. Comparisons of these results with a direct numerical simulation (DNS) of similar multiscale dynamics indicate close agreement between measured and modeled layer character and evolution, small-scale dynamics, and turbulence intensities. In particular, a detailed examination of the potential biases in inferred quantities and/or misinterpretation of the underlying dynamics as a result of the specific DataHawk sampling trajectory is carried out using virtual sampling paths through the DNS and comparing these with the DataHawk measurements.

Definition and Counting of Configurational Microstates in Steady-State Two-Phase Flows in Pore Networks

Steady-state two-phase flow in porous media is a process whereby a wetting phase displaces a non-wetting phase within a pore network. It is an off-equilibrium stationary process—in the sense that it is maintained in dynamic equilibrium at the expense of energy supplied to the system. The efficiency of the process depends on its spontaneity, measurable by the rate of global entropy production. The latter has been proposed to comprise two components: the rate of mechanical energy dissipation at constant temperature (a thermal entropy component, Q/T, in the continuum mechanics scale) and the configurational entropy (a Boltzmann–Gibbs entropy component, klnW), due to the existence of a canonical ensemble of flow configurations, physically admissible to the externally imposed macrostate conditions. Here, we propose an analytical model to account the number of microstates, lnW, in two-phase flows in pore networks. Combinatorial analysis is implemented to evaluate the number of identified microstates per physically admissible internal flow arrangement, compatible with the imposed steady-state flow conditions. Then, Stirling’s approximation is applied to downscale the large factorial numbers. Finally, the number of microstates is estimated by contriving an appropriate mixing scheme over the canonical ensemble of the physically admissible flow configurations. Indicative computations are furnished.

Detachment of particles from bubbles in an agitated vessel

The flotation process is widely used to separate valuable minerals from waste in the minerals industry. It is well known that recovery decreases with increasing particle size, and a reason often given is that large particles detach from bubbles in the turbulent shear flow induced by the impeller in mechanical flotation cells. The energy produced by the impeller is transferred to the liquid in the cell and dissipates throughout the tank. In the impeller region, the turbulence is non-isotropic, but in a very short distance, it becomes isotropic, with the typical cascade of eddies from large to micro-scale length scales.The aim of this study is to observe the behavior of particle–laden bubbles in the turbulent shear flow near a rotating impeller in a flotation cell. An agitated vessel was constructed in which bubbles could be introduced beneath the impeller. Bubbles were generated in a liquid-fluidized bed in a special chamber beneath the cell, in which the fluidizing liquid contained a collector that enabled the particles in the bed to adhere to the bubbles. When the bubbles entered the impeller zone, some particles were observed to detach, while others remained attached to the bubble and rose out of the vessel as a flotation product. The fractional detachment of particles was related to the mechanical energy dissipation rate in the region of the impeller. A standard Rushton turbine was used. The results are compared with theoretical predictions of Schulze (1977, 1982).

Understanding the Effects of Surface Chemistry on Q: Mechanical Energy Dissipation in Alkyl-Terminated (C1−C18) Micromechanical Silicon Resonators

The rate of mechanical energy dissipation in 300-nm-thick, megahertz-range micromechanical silicon resonators is sensitive to single monolayer changes in surface chemistry. Resonators terminated with a single monolayer of methyl groups have significantly higher quality factors (Q's), and thus lower mechanical energy dissipation, than comparable resonators terminated with either long-chain alkyl monolayers (C2H2n+1, n = 2-18) or monolayers of hydrogen atoms. This effect cannot be attributed to mechanical energy dissipation within the alkyl monolayer, as a 9-fold increase in alkyl chain length does not lead to an observable increase in dissipation. Similarly, this effect is not correlated with the chemical structure of the silicon-monolayer interface (e.g., the density of Si-H vs Si-C bonds.) Instead, the chemical trends in resonator quality and stability are consistent with a dissipation mechanism involving the coupling of long-range strain fields to localized, electronically active defects in the monolayer coatings.

Bounds on dissipation in stress-driven flow in a rotating frame

We calculate a rigorous dual bound on the long-time-averaged mechanical energy dissipation rate e within a channel of an incompressible viscous fluid of constant kinematic viscosity ν, depth h and rotation rate f, driven by a constant surface stress τ = ρu 2 * i, where u * is the friction velocity. It is well known that e ≤ e Stokes = u 4 * /v, i.e. the dissipation is bounded above by the dissipation associated with the Stokes flow. Using an approach similar to the variational 'background method' (due to Constantin, Doering & Hopf), we generate a rigorous dual bound, subject to the constraints of total power balance and mean horizontal momentum balance, in the inviscid limit ν→0 for fixed values of the friction Rossby number Ro * = u * /(fh) = GE, where G=τh 2 /(ρν 2 ) is the Grashof number, and E=v/fh 2 is the Ekman number. By assuming that the horizontal dimensions are much larger than the vertical dimension of the channel, and restricting our attention to particular, analytically tractable, classes of Lagrange multipliers imposing mean horizontal momentum balance analogous to the ones used in Tang, Caulfield & Young, we show that e ≤ e max = u 4 * /v-2.93u 2 * f, an improved upper bound from the Stokes dissipation, and e ≥ e min = 2.795u 3 */h, a lower bound which is independent of the kinematic viscosity ν

Power‐Law Fluid Flow Over a Sphere: Average Shear Rate and Drag Coefficient

AbstractBased on the consideration of the rate of mechanical energy dissipation, an expression for the average shear rate for a sphere falling in a power‐law fluid in the creeping flow regime has been deduced. The average shear rate in a power‐law fluid (n<1) appears to be higher than that in an equivalent Newtonian fluid. This in turn has been combined with the numerical predictions of drag coefficient (up to Reynolds number of 100) of a sphere to develop a generalized drag correlation for power‐law liquids encompassing both n > 1 and n < 1 which appears to apply up to much higher values of the Reynolds number. The available experimental data have been used to demonstrate the reliability and accuracy of the new correlation for shearthinning liquids. Also, in the limit of n = 1, this expression reproduces the standard drag curve with a very high accuracy.

Bounds on dissipation in stress-driven flow

We calculate the optimal upper and lower bounds, subject to the assumption of streamwise invariance, on the long-time-averaged mechanical energy dissipation rate , a bound that is independent of the flow viscosity. This scaling (though not the best possible numerical coefficient) can also be obtained directly by applying the same imposed constraints and restricting attention to a particular, analytically tractable, class of mean flows.

Mechanical dissipation versus thermo-mechanical exergy destruction for plane shockwaves in an inviscid perfect gas

In this paper, the physical exergy destruction caused by a shockwave, as an overall measure of the thermo-mechanical dissipation, is compared to the evaluation of mechanical dissipation alone. Travelling and stationary compression shockwaves are considered assuming an inviscid perfect gas with constant specific heats. Although quantities such as the change in exergy are not frame-indifferent, it is shown that mechanical dissipation and exergy destruction are both frame-indifferent. The results of this work reveal that, in many cases, the exergy destruction rate is a small fraction of the mechanical energy dissipation rate typically used to determine losses in gas dynamics. This is particularly true when the initial temperature of the fluid is high relative to the environment temperature. In contrast, the results show that, for very low initial temperatures that are below the environment temperature, the thermo-mechanical dissipation rate can be much greater than mechanical dissipation rate.

The mechanism of suppression or enhancement of three-dimensional surface waves in film flow down a vertical plane

The energy budget in a liquid film flow down a vertical plane subject to three-dimensional disturbances is calculated. The equation relating the average time rate of change of disturbance kinetic energy to the rates of work done by the surface tension, the shear stress, the Reynolds stress, and the rate of mechanical energy dissipation is obtained. Each term in the equation is evaluated in various regions of parameter space to elucidate the physical mechanism of stabilizing an inherently unstable vertical film flow by use of plate oscillation. The Squire theorem, which states that two-dimensional disturbances are more dangerous than three-dimensional ones, is violated in certain parameter ranges. In these parameter ranges the Reynolds stress enhanced by plate oscillation may cause a falling liquid film, which is stable with respect to two-dimensional short waves, to become unstable with respect to three-dimensional disturbances.

True-to-mechanism model of steady-state two-phase flow in porous media, using decomposition into prototype flows

A true-to-mechanism model is proposed, which considers steady-state two-phase flow in porous media (SS2 ϕPM) as a composition of two prototype flows, namely ganglion dynamics (GD) and connected-oil pathway flow (CPF). Coupling of the prototype flows is effected with the simple rule that the macroscopic pressure gradient is the same in both. For a given set of values of the flow system parameters, a domain of admissible flow combinations is obtained. The solution is determined by assuming that each point in this domain has equal probability of being `visited'. This leads to unique values for the flow arrangement variables (FAV), the rate of mechanical energy dissipation, and the relative permeabilities. The new model accounts for the non-linearity of the flow as well as for the effects of all the system parameters (notably those affecting interfaces), and its predictions are in very good agreement with existing data.

The mechanism of surface wave suppression in film flow down a vertical plane

The equation relating the average rate of change of disturbance kinetic energy to the rates of work done by the surface tension, the shear stress, the Reynolds stress, and the rate of mechanical energy dissipation through viscosity in a falling liquid film flow down an oscillating vertical plate is obtained. Each term in the equation is evaluated in various regions of parameter space to elucidate the physical mechanism of stabilizing an inherently unstable vertical film flow by use of plate oscillations [Lin, Chen, and Woods, Phys. Fluids 8, 3247 (1996); Lin and Chen, Phys. Fluids 9, 3926 (1997)].