Semi-infinite Surface Research Articles

A new 3D plastoelastohydrodynamic lubrication model for rough surfaces

Plastoelastohydrodynamic lubrication of rough surfaces (R-PEHL) is a cutting-edge area of research in interface fluid-structure coupling analysis. The existing R-PEHL model calculates the elastic-plastic deformation of rough surface by the Love equation in a semi-infinite space smooth surface, which deviates from the actual surface. Therefore, it is an innovative work to study the exact solution of elastic-plastic deformation of rough surface and its influence on the solution results of R-PEHL model. In this paper, a new contact calculation model of plastoelastohydrodynamic lubrication (PEHL) with three-dimensional (3D) rough surface is proposed by integrating numerical method of EHL and finite element method. The new model eliminates an original error introduced by the assumption of semi-infinite space in contact calculation, providing wide applicability and high accuracy. Under the given rough surfaces and working conditions, the study reveals that: (1) the oil film pressure calculated by the new model is lower than that of the smooth surface in semi-infinite space by 200–800 MPa; (2) the Mises stress of the new model is 2.5%–26.6% higher than that of the smooth surface in semi-infinite space; (3) compared with the semi-infinite space assumption, the rough surface plastic deformation of the new model is increased by 71%–173%, and the local plastic deformation singularity may appear under the semi-infinite space assumption; (4) the plastic deformation caused by the first contact cycle on the rough surface of the new model accounts for 66.7%–92.9% of the total plastic deformation, and the plastic deformation of the semi-infinite space accounts for 50%–83.3%. This study resolves the contradiction between the smooth surface assumption and the rough surface in the existing R-PEHL model, establishing a solid logic foundation for the accurate solution of R-PEHL model.

Scaling laws for natural convection boundary layer of a Pr > 1 fluid on a vertical solid surface subject to a sinusoidal temperature in a linearly-stratified ambient fluid

The understanding of the transient behavior of natural convection boundary layer (NCBL) on a heated vertical solid surface under various heating conditions is of fundamental significance and application importance. In this study, scalings for the parameters representing the behavior of unsteady NCBL flow of a linearly-stratified Pr > 1 fluid on a semi-infinite vertical solid surface heated with a time-varying sinusoidal temperature at different development stages are developed with a scaling analysis, in terms of Ra, Pr, s, and fn, which are the Rayleigh number, Prandtl number, stratification number, and frequency of the sinusoidal temperature, respectively. These scalings are validated and quantified with a series of numerical simulations over wide ranges of Ra, Pr, s, and fn. The frequency of the fluctuations experienced by the NCBL behavior at the transitional stage, due to the stratification of the ambient fluid, is also analyzed, and it is shown that the previously obtained scaling for the unsteady NCBL case with the constant heat flux heating condition is basically applicable for the current case, Ra and fn have additional effects as well due to the time-varying nature of the applied temperature.

Crack length directivity effects on guided-wave acoustic emission: Numerical investigation of radiation patterns

This paper investigates the effect of a finite-sized crack surface on acoustic emission (AE) generated at the crack tip in a thin metallic plate using a 3D time-domain finite element model. Directivity effects on the first arrivals are of particular focus, and the AE is interpreted in terms of a guided mode decomposition. Crack lengths from 0–10 plate thicknesses are studied, in addition to a semi-infinite crack surface, for frequency content in the neighborhood of 0.1–0.4 MHz-mm. Far-field radiation patterns for the fundamental symmetric (S0) and shear horizontal (SH0) guided modes are measured as a function of crack length. The results show an increase in radiation behind the crack tip for both modes and across all considered crack lengths. The surface wave traveling along the crack length appears to be one of the main drivers behind this increase, due to mode conversion after reaching the opposite end of the crack. However, a similar increase is also observed for the semi-infinite crack case, in which there is no mode conversion (the opposite end of the crack is never reached). A radiation offset (RO) metric is introduced to capture this behavior. Parametric studies of the RO across center frequency and bandwidth are presented. Findings suggest that this metric may be of use for AE-based crack length estimation.

Wave Diffraction from a Bicone Conjoined with an Open-Ended Conical Cavity

The problem of axially symmetric TM-wave diffraction from a bicone conjoined with an open-ended conical cavity is analysed rigorously. The scatterer is formed by the perfectly conducting semi-infinite and truncated semi-infinite conical surfaces; the spherical termination of an internal area of the truncated cone creates the open-ended cavity. In this paper the certain physical aspects of diffraction which are known to cause mathematical difficulties are considered. It includes an accurate analysis of the wave-mode transformation phenomena at the open end of the cavity, as well as a study of wave radiation from the cavity into the biconical waveguide. The primary outcome of this paper is a precise treatment of the wave diffraction problem mentioned above using new techniques and establishing new properties of resonance modes’ penetration into the biconical waveguide region.

Sensitivity of surface roughness to laser parameters used for polishing additively manufactured Co-Cr alloy

Poor fatigue life, which is impacted by a rough and irregular surface, is a critical challenge that additive manufacturing needs to overcome to facilitate widespread engineering use. Laser polishing, in which a laser irradiates, melts, and smooths a surface through surface tension, is one solution. This work studies the laser polishing of laser powder bed fusion cobalt‑chromium samples through experimentation and microscopy of the surface before and after polishing. The laser processing parameters that contribute to the smoothing performance of four different surface texture parameters are analyzed. It is found that laser power plays an outsized role in smoothing the surface, followed by hatch spacing, scanning velocity, and beam diameter. The greater role of power compared to beam diameter and velocity is explained using analytical temperature models of a Gaussian heat flux on a semi-infinite surface. The influence of the starting surface is also discussed, and the variability in the final laser polished surfaces is partially attributed to the variability that occurs on the original printed surface due to the sporadic adherence of particles and particle agglomerates. This work provides an overview of important processing aspects for furthering the commercialization of the technology for the finishing of additively manufactured parts.

Computational Analysis of Heat Transfer Intensification of Fractional Viscoelastic Hybrid Nanofluids

In the current article, we have performed computational analysis on convection heat transfer of a hybrid nanofluid in occurrences where porous media and the effect of magnetic force are involved. In order to assess the time-fractional derivatives, Caputo’s notion is utilized while the Darcy–Forchheimer model is applied due to the involvement of the porous medium. Moreover, the boundary conditions are assumed to be nonuniform through the equilibrium between the surface tension and shear stress over a semi-infinite permeable flat surface. Keeping in view the complexity of the fractional derivative model and nonuniform boundary conditions, the problem in question is a complicated one. Accordingly, the coupled momentum and energy equation is linearized and the finite difference scheme is then applied and implemented in MATLAB Code R2020b. Furthermore, we have also offered a comprehensive analysis regarding error and convergence of the proposed numerical method. The newly introduced numerical technique to determine the numerical solutions and some unique and interesting deductions are established. From the computational results, one can conclude that the fluid motion in both hybrid and single nanofluids slows down due to magnetic field, porosity, and inertia coefficient as the magnetic and electric fields are synchronized due to the formation of the Lorentz force and viscous interference. We believe that our proposed numerical technique regarding employment of the fractional model for heat transfer application to the hybrid nanofluid over a semi-infinite nonuniform permeable surface along with variable heat flux is not found in the literature so far. Furthermore, the obtained results will be a valuable addition to fractional calculus from an engineering point of view.

From thin “coffee rings” to thick colloidal crystals, through drop spreading inhibition by the substrate edge

“Coffee rings” structures formed by the evaporation of a nanosphere colloidal drop which rests on a substrate can be grown as thick, good quality colloidal crystals if the drop is settled onto the flat upper side of a pillar in such a way that its spreading is inhibited by the pillar edge. The colloidal crystal thus obtained has a torus shape of a 6 mm in diameter (~ 19 mm in length), with a semi-elliptical cross section (width ~ 0.7 mm, height ~ 1 mm) and has a shape factor (the ratio between its width and its height) around 1, much lower than that formed in a classical manner, onto a semi-infinite surface, which is usually very flat. These microsphere crystals can be detached from the substrate as free-standing, cut and manipulated without special precaution. Growing thick rings in only few hours, in a simple set-up, the method could be useful for the researchers which currently use these colloidal crystals as sacrificial templates for inverse-opals synthesis.

Investigation of Cu–water nano-fluid of natural convection hydro-magnetic heat transport in a Darcian porous regime with diffusion-thermo

A theoretical investigation for internal heat generation, suction/injection, diffusion-thermo and nano-particle volume fraction on a chemically reacting hydro-magnetic flow of Cu–water nano-fluid over a semi-infinite vertical surface is presented in this article. The surface has a uniform velocity which is placed in a saturated porous medium. Under thermal equilibrium state, here we consider that the nano-particles are of uniform shape and size. The small perturbation technique is utilized to solve the boundary layer equations, and the solutions are assumed to be oscillatory type. The significant outputs are presented via figures to explain the effects of different controlling variables on the velocity, temperature, concentration, shear stress and rate of heat transfer profiles. As the conductivity of Cupper particles are higher than those of water-based particles, and significantly it is found that the flow velocities are accelerated for the impact of diffusion-thermo/Grashof/internal heat generation. Increasing porosity parameter decelerate the flow velocity for both nano-fluid and base fluid, whereas enhance the temperature for the radiation parameter. For the impact of free convection and diffusion-thermo, growth values of flow velocity are detected for nano-fluid ( $$\mathrm{Cu}$$ ) than the base fluid (water). Furthermore, application of $$\mathrm{Cu}$$ nano-particles can be used in conductive coatings, medical devices, anti-biotic, and textiles. Comparison with previously published works in the limits shows excellent agreement.

ИССЛЕДОВАНИЕ ТЕМПЕРАТУР В ВОЛЬФРАМОВОМ ЭЛЕКТРОДЕ ПРИ ГОРЕНИИ ДУГИ ОБРАТНОЙ ПОЛЯРНОСТИ

The paper considers the features of energy release in a tungsten electrode under the reverse polarity TIG welding. The study substantiates the statement that the chemical composition of an electrode does not significantly affect the transfer of anode power to it. The specific effective power of an electrode is substantiated and taken as 6 W/A. The authors analyzed the features of arcing on the flat tip of a 3 mm diameter electrode using high-speed video. The analysis identified that at limiting currents ensuring tip melting, the tip heating is uniform over the cross-section. As a design scheme, the authors selected a continuous flat heat source on the semi-infinite rod surface with surface heat transfer. The authors obtained averaged values for volumetric heat capacity сρ=3.2 J/(cm3∙°С) and heat transfer coefficient а=0.3 cm2/s. The current at which the tip melting temperature is reached was taken as a limiting current. Using the limiting current value and start time of the electrode tip melting, the authors calculated the electrode heat transfer coefficient value b. The calculated melting depth for the over-limiting current welding mode showed good coincidence with an experiment. The authors recalculated the b value for the electrodes of 4-, 5-, and 6-mm diameter and calculated limiting currents for these diameters. The design limiting currents for these diameters also showed good coincidence with experimental results. The study showed that the increase of a coefficient up to 0.4 cm2/s does not cause changes in temperature and limiting currents at simultaneous сρ adjustment according to the constant thermal and physical properties сρа0.5. As a result, the authors obtained temperature dependencies for the electrode over time and length. Time dependence of the electrode tip heating allows calculating limiting currents with the decrease in arcing time.

Magneto-optical Kerr effect and signature of the chiral anomaly in a Weyl semimetal in magnetic field

One striking property of the Landau level spectrum of a Weyl semimetal (WSM) is the existence of a chiral Landau level, in which the electrons propagate unidirectionally along the magnetic field. This linearly dispersive level influences the optical properties of WSMs. For example, it was recently shown that a complete optical valley polarization is achievable in a time-reversal symmetric Weyl semimetal placed in a magnetic field\cite% {Bertrand2019}. This effect originates from inter-Landau level transitions involving the chiral Landau level and requires a tilt of the Weyl cones. In this paper, we show how the magneto-optical Kerr effect (MOKE) is modified in a WSM\ with tilted Weyl cones in comparison with its behavior in a normal metal and how a valley polarization can be detected using MOKE. We study both the Faraday (longitudinal) and Voigt (transverse) configurations for light incident on a semi-infinite WSM surface with no Fermi arcs. We use a minimal model of a WSM with four tilted Weyl nodes related by mirror and time-reversal symmetry. In the Voigt configuration, a large peak of the Kerr angle occurs at the plasmon frequency. We show that the blueshift in frequency of this peak with increasing magnetic field is a signature of the chiral anomaly in the MOKE.

CFD modeling of mass transfer in Gas-Liquid-Solid catalytic reactors

This work investigates Gas-Liquid-Solid mass transfer coupled to heterogeneous catalytic reaction using Computational Fluid Dynamics (CFD). The numerical model is based on the Volume-of-Fluid (VOF) approach, coupled with a convection–diffusion equation for mass transfer resolution. First, the numerical method is validated on a falling liquid film over a semi-infinite planar surface. Then, a micro-structured reactor with α-methylstyrene hydrogenation is studied. A good agreement is found between experimental data of Tourvieille et al. (2013) and simulation results. Afterwards, a vertical spherical beads string is investigated. Convective transport by transversal velocities is identified as an important contributor to the overall Gas-Liquid-Solid mass transfer. While the film model is applicable in pure diffusion regimes, the resistances-in-series model is not relevant and over-estimates the real mass transfer by nearly 30% when mass transfer occurs in liquid film flow without bulk.The present work shows how CFD can be an effective tool for predicting hydrodynamic and catalyst geometry effect on mass transfer in Gas-Liquid-Solid reactors.

Unsteady boundary layer flow and heat transfer of a viscous fluid over a semi-infinite stationary flat plate: Blasius flow problem

In this paper, a class of similarity solution for the unsteady Navier–Stokes equations which describe the two-dimensional unsteady laminar boundary layer flow of a viscous incompressible fluid over a semi-infinite flat stationary plate surface has been derived. Here, the boundary layer is formed owing to an uniform free stream flow over this plate surface and the unsteadiness in this flow dynamics comes only from the similarity solution which is not yet available in the open literature. For this similarity solution the governing boundary layer equations (momentum and thermal) transform into a set of non-linear ordinary differential equations which conceive two physical parameters, namely, unsteadiness parameter γ and Prandtl number Pr. The impact of these two parameters on the unsteady boundary layer flows as well as on the heat transfer analysis have been discussed in detailed. The present analysis confirms the non-existence of the boundary layer solutions after a certain negative value of γ, while it continues for a large positive value of γ and specifically for the value of γ = 0, we get the steady boundary layer solution for the Blasius flow problem. Here, we have considered the whole range of the Pr values, which includes both the limiting cases Pr → 0 and Pr → ∞, in the discussion of the heat transfer analysis. For these two limiting cases, two approximated separate mathematical formulae for the heat transfer solutions have been derived in terms of the well-known bessel, error and hypergeometric functions, whose results for any given value of γ within the solution domain are well fitted with the corresponding numerical results.

Numerical Study of a Williamson Fluid Past a Semi-Infinite Vertical Plate with Convective Heating and Radiation Effects

In the present paper, we investigated mathematical model of the magnetohydrodynamic flow and heat transfer in an electro-conductive polymer on the externalsurface of a semi-infinite vertical plate under radial magnetic field. Thermal radiation andconvective heating effects are considered at the semi-infinite plate surface with modifiedboundary conditions. The Williamson viscoelastic model is employed which isrepresentative of certain industrial polymers. The non-dimensional, transformedboundary layer equations for momentum and energy are solved with the second orderaccurate implicit Keller box finite difference method under appropriate boundaryconditions. Validation of the numerical solutions is achieved via benchmarking withearlier published results. The influence of Williamson viscoelastic fluid parameter,magnetic body force parameter, convective heating, radiation parameter, stream wisevariable and Prandtl number on thermos-fluid characteristics are studied graphically. Themodel is relevant to the simulation of magnetic polymer materials processing.

Assessment of confinement’s influence on a concrete target’s ability to approximate semi-infinite perforation performance

The ability to accurately evaluate impact penetration resistance of a structural element in an experimental setting often requires the experimental impact specimen configuration to be as close as possible to a structural element in a fielded protective design scheme. This includes the target’s capability to estimate the semi-infinite response of a structural element such as a wall where inertial confinement is provided by undamaged material surrounding the area damaged by impact. Artificially simulating this confinement in an experimental target can take several forms, including use of a circumferential steel ring as a confining medium. Experimental data gathered from impact tests were utilized to create a representative numerical simulation of a confined target design. This simulation evaluated artificial and inertial confinement on concrete perforation by varying target to projectile diameters with and without artificial confinement. Perforation performance was found to be unaffected by confinement type when the target diameter to projectile diameter ratio was 16. Semi-infinite surface approximation was found to occur when the target diameter to projectile diameter ratio was 64 despite the confinement type at higher impact velocities; however, this ratio could be reduced while maintaining semi-infinite performance for lower velocity impacts.

Memory-dependent derivatives (MDD) of magneto-thermal-elastic waves excited by laser pulses for two-temperature theory

ABSTRACT A novel model of the generalized magneto-thermoelasticity with two-temperature theory is investigated. The memory-depended derivative (MDD) during the excitation processes by pulsed laser is established for a time-dependent material. The overlapping between an isotropic homogeneous thermoelastic half-space medium and the non-Gaussian laser pulse in one-dimensional space with time delay is obtained. The semi-infinite bounding surface of the elastic medium is taken as traction free and subject to a thermal shock problem in a time-dependent case. Laplace transformation technique is used to obtain the initial solutions of the main physical field which defined in the governing equations. The temporal complete solutions in Laplace time domain obtained by using the inversion method of the Laplace transform, to obtain the general solution (exact solution) for some physical quantities. Numerical results are performed with discussed for suitable elastic medium and illustrated graphically, taken into consideration the time-delay parameters.

Symmetry-guaranteed ideal Weyl semimetallic phase in face-centered orthogonal C6

The exploration of topological phases in carbon allotropes offers a fascinating avenue to realize topological devices based on carbon materials. Here, using first-principles calculations, we propose a novel metastable carbon allotrope, which possesses exotically helical carbon chains bridged by quadrangle-rings. This unique structure with sp2-sp3 bonding networks crystallizes in a noncentrosymmetric face-centered orthogonal (fco) lattice with six atoms in a unit cell, thus named fco-C6. The considerable stability of fco-C6 is confirmed by phonon spectra, elastic constants, and ab initio molecular dynamics simulations. More importantly, fco-C6 exhibits extraordinary electronic properties with the minimum number of Weyl points in a time-reversal preserved Weyl system. The symmetry arguments reveal that the Weyl points are guaranteed to lie along the high-symmetry pathes and thus well separated in momentum space, exhibiting the robustness of topologically protected features. We investigate the topological surface states of fco-C6 projected on a semi-infinite (010) surface. There are only nontrivial Fermi arcs across the Fermi surface, which facilitates their measurements in experiments and further applications in carbon allotropes.

Electron-Nanobunch-Width-Dominated Spectral Power Law for Relativistic Harmonic Generation from Ultrathin Foils.

Relativistic high-order harmonic generation from solid-density plasma offers a compact source of coherent ultraviolet and x-ray light. For solid targets much thinner than the laser wavelength, the plasma thickness can be tuned to increase conversion efficiency; a reduction in total charge allows for balancing the laser and plasma driving forces, producing the most effective interaction. Unlike for semi-infinite plasma surfaces, we find that for ultrathin foil targets the dominant factor in the emission spectral shape is the finite width of the electron nanobunches, leading to a power-law exponent of approximately 10/3. Ultrathin foils produce higher-efficiency frequency conversion than solid targets for moderately relativistic (1<a_{0}<40) interactions and also provide unique insight into how the trajectories of individual electrons combine and interfere to generate reflected attosecond pulses.

Large excitonic effect on van der Waals interaction between two-dimensional semiconductors.

An exceptionally large excitonic effect on the van der Waals (vdW) interaction between two-dimensional semiconductors is unraveled using the Lifshitz theory in conjunction with the ab initio GW plus Bethe-Salpeter equation formalism. Upon consideration of the electron-hole interaction, the vdW energy between two atomistic layers separated by 10 000 angstroms can be larger by a ratio of ∼30%, which is an order of magnitude greater than that seen for semi-infinite silicon surfaces. The large influence of the short-range electron-hole interaction on the long-range effect of quantum fluctuations is rooted in the ultra-thin nature of two-dimensional semiconductors which results in not only large exciton binding energy but also amplified roles of low-frequency dielectric responses.

Boundary layer flows of viscoelastic fluids over a non-uniform permeable surface

This study investigates viscoelastic fluid, which possesses both viscous and elastics properties, by employing a fractional derivative model to reveal the stress relaxation phenomenon with distance. The spatial-fractional derivative used in the momentum conservation equation is the Riemann–Liouville type derivative. Further, we use non-uniform boundary conditions subject to the boundary layer equations of the fluid flowing through a semi-infinite permeable flat surface. Owing to the fractional derivative model and non-uniform boundary conditions, this problem is complex. Thus, a finite difference scheme is applied after the coupled continuity equation and momentum equation are decoupled and linearized. The accuracy, convergence, and stability of the numerical method are presented. It is shown that non-uniform mass transfer through a permeable surface considerably affects the velocity boundary layer. By illustrating the physical interactions between the velocity fields in the boundary layer and the permeation mode in the surface, this paper predicts the velocity distributions with varying permeable surface, and also provides the possibility of changing the velocity fields by altering the permeable sheet. The results and numerical technique used in this study will help in the understanding of fractional calculus investigation in engineering.

Laminar boundary layer flow of a combustible gas over a semi-infinite porous surface

Laminar boundary layer flow of a combustible gas under mixed convection over a semi-infinite porous surface is analyzed theoretically and numerically. The governing equations are transformed into a set of partial differential equations which are solved numerically by using finite difference method. To assess the accuracy of numerical results, we also solve these equations by using perturbation method for low frequency range and asymptotic method for high frequency range. Numerical computations have been carried out for several values of parameters involved in the equations, namely the Prandtl number, the Schmidt number, the wall temperature and the mass flux for injected fuel. Results show that the momentum, thermal and concentration boundary layers increase owing to the increase in the mass flux, but increasing Prandtl number reduces the thermal boundary layer and thickens the momentum and concentration boundary layers. Moreover, an expression for the ignition distance involving the pertinent parameters is derived using large activation energy asymptotics. It is found that the ignition distance considerably diminishes upon increasing the above parameters.