Presence Of Surface Roughness Research Articles

Numerical investigation for the effects of surface roughness in counter flow microchannel heat exchanger with different channels geometries

This article presents a numerical investigation focused on how roughness of surface affects the overall performance of a counter-flow microchannel heat exchanger (CFMCHE). The study examines various microchannel shapes, including triangles, squares, circles, and trapezoids, to evaluate their performance within the context of CFMCHE. Water with consistent properties flows through aluminum microchannels used as the working fluid. The analysis employs the roughness-viscosity method to assess how surface roughness influences the CFMCHE’s performance. The study’s findings emphasize that the trapezoidal channel shape exhibits the most favorable performance among the shapes investigated. Additionally, among the operational parameters considered, the hydraulic diameter emerges as the most influential factor in determining the most suitable characteristics for CFMCHE. The presence of surface roughness was found to marginally improve thermal performance while slightly reducing hydraulic efficiency. This research provides valuable insights into the intricate interplay between surface roughness and channel shape. Notably, the impact of roughness is more pronounced in the case of the trapezoidal shape compared to other geometries.

The influence of AuNs on the optical properties of GaAs/AlGaAs tunnel-coupled quantum well

Using a numerical approach, we investigated a GaAs/AlGaAs tunnel-coupled quantum well (TCQW) to examine the interplay between electromagnetic waves and gold nanospheres (AuNs) in the presence of an extra SiO2 layer and surface roughness. Our findings demonstrated that the optical efficiency of the response of AuNs was increased in the presence of the SiO2 layer. The extinction cross-section also increased in the presence of surface roughness. Furthermore, we discovered that the orientation of AuNs on the rough surface of TCQW can give rise to a new mode of resonant frequency in the near infrared range. This new mode is advantageous for the TCQW as it is usually seen in AuNs only in the visible range. Lastly, the energy level and wave function of electrons in the TCQW intersubband could be adjusted by an electric field produced by the presence of AuNs. Therefore, this theoretical study could be applied to improve output efficiency and the tuning of performances of optical devices such as solar cells and tunable wavelength photoemitters.

Ultrasonic fortification of interfiber autohesive contacts in meltblown nonwoven materials

Autohesion is a unique class of adhesion that enables the bonding of two identical surfaces by establishing intimate contact at interfaces. Creating intimacy between two identical surfaces poses a challenging task, often constrained by the presence of surface roughness and chemical heterogeneity. To surmount this challenge, we document a variety of autohesive traits in polypropylene-based meltblown nonwovens, accomplished through a facile, scalable, energy-efficient, and cost-effective ultrasonic bonding process. The mean work of autohesion for a single polypropylene bond, serving as a figure of merit, has been computed by extending the classical Johnson−Kendall−Roberts (JKR) theory by factoring in peel strength along with key fiber and structural parameters of nonwoven materials. Achieving a high figure of merit in ultrasonically bonded nonwovens hinges on the synergistic interplay of key process parameters, including static force, power, and welding speed, with the fiber and structural properties acting in concert. In this regard, peel-off force analysis has also been conducted on a series of twenty-seven ultrasonically bonded meltblown nonwovens prepared using a 33 full factorial design by systematically varying process parameters (static force, power, and welding speed) across three levels and extension rate. X-ray microcomputed tomography (microCT) analysis has been performed on select ultrasonically bonded nonwoven samples to discern their bulk characteristics. A broad spectrum of mean work of autohesion for a single polypropylene bond, ranging from 1.88 to 9.93 J/m², has been ascertained by modulating key process parameters.

One-way fluid structure interaction analysis of a static savonius hydrokinetic turbine under different velocity and surface roughness with different blade materials

Hydrokinetic turbines are prone to a harsh hydrodynamic environment with intricate vortical flows that elevate the probability of failure. The degradation of the blade surface caused by corrosion can impact the blade's hydrodynamic and the structural performance. This paper reports a one-way three-dimensional fluid-structure interaction simulation to analyse the performance of a static Savonius hydrokinetic turbine at varying rotor positions, with different surface roughness and water velocities, in terms of coefficients of static torque, static torque, stresses, and blade deformation. The simulations revealed that the optimum position for the highest coefficient of static torque was at 15° (Cst = 0.30) in reference to the water flow. Increasing the water velocity from 0.4 ms−1 to 0.84 ms−1 improved the turbine static torque due to an increase in the kinetic energy. However, the presence of surface roughness has deterioration effects on the static torque coefficient due to a delayed separation which causes a drag reduction. The simulation predicted no structural failure at 0.4 ms−1 and 0.84 ms−1, but varying materials exhibited varying maximum principal stress and deformation, highlighting the significance of the early development of materials selection process. The maximum von Mises stress and deformation was obtained when the turbine is resting at 45° for aluminium blade (σ = 1.05 MPa, ε = 5.6e−4 mm). The results of this study indicate suitable materials from a hydrodynamic and material perspectives for the construction of the Savonius hydrokinetic turbine, which can be implemented in the design process to potentially save cost and minimize turbine downtime.

Surface Roughness in RANS Applied to Aircraft Ice Accretion Simulation: A Review

Experimental and numerical fluid dynamics studies highlight a change of flow structure in the presence of surface roughness. The changes involve both wall heat transfer and skin friction, and are mainly restricted to the inner region of the boundary layer. Aircraft in-flight icing is a typical application where rough surfaces play an important role in the airflow structure and the subsequent ice growth. The objective of this work is to investigate how surface roughness is tackled in RANS with wall resolved boundary layers for aeronautics applications, with a focus on ice-induced roughness. The literature review shows that semi-empirical correlations were calibrated on experimental data to model flow changes in the presence of roughness. The correlations for RANS do not explicitly resolve the individual roughness. They principally involve turbulence model modifications to account for changes in the velocity and temperature profiles in the near-wall region. The equivalent sand grain roughness (ESGR) approach emerges as a popular metric to characterize roughness and is employed as a length scale for the RANS model. For in-flight icing, correlations were developed, accounting for both surface geometry and atmospheric conditions. Despite these research efforts, uncertainties are present in some specific conditions, where space and time roughness variations make the simulations difficult to calibrate. Research that addresses this gap could help improve ice accretion predictions.

Study the behaviour of transient elastohydrodynamic lubrication contact at motion start-up in the presence of surface roughness

PurposeThis study aims to present a numerical solution for the analysis of the influence of surface roughness as presented by a sinusoidal ripple of different amplitude and wavelength on the performance of transient elastohydrodynamic lubrication at motion start-up under different operational parameters of entraining speed and load as well as different acceleration rates.Design/methodology/approachA statistical asperity micro-contact model represented by a sinusoidal ripple expressed by two parameters (wavelength and undeformed amplitude) is considered. The ball equation of motion is used to calculate the force on the ball as it starts to move. The time-dependent Reynolds equation is solved together with surface deformation and statistical asperity models using the Newton–Raphson technique with the Gauss–Seidel iteration method.FindingsThe behaviour of the film thickness was found to be strongly influenced by the acceleration rate for different ripple amplitude and wavelength parameters. The effect of increasing the final entraining speed will eventually lead to rapid film thickness build-up and increase the film thickness jump at the moment of motion start-up. The effect of increasing applied load is to reduce the deviation of the minimum film thickness jump at the start-up of motion, making its value approximately equal to the steady-state value over the entire run-time period.Originality/valueInfluence of surface roughness for various wavelength and undeformed amplitude on the performance of transient elastohydrodynamic lubrication at motion start-up is presented at different acceleration rates as well as for different operating parameters of entraining speed and load. Ball equation of motion is used to calculate the force on the ball as it starts to move.

Separation/recontact investigation of a travelling oscillator over a plate with inclusion of surface roughness

It is well understood that when an object travels over a supporting plate, their initial contact is most likely to be lost, particularly in the presence of surface roughness. While there is a voluminous literature on the analysis of plates excited by travelling objects, the existing studies have neglected the possibility of separation between contacting bodies in the problem formulation. On that account, this paper attempts to develop an approach in order to improve the previously-adopted methods by allowing for the separation along with effectively taking care of the subsequent recontact between the travelling superstructure and the supporting plate. Furthermore, by considering the so-called concept of restitution coefficient at the point of recontact, different impact situations are examined and discussed. Numerical examples are then presented to investigate the effect of influential parameters on the behaviour of the system. It is shown that an accurate assumption of the impact condition is necessary to obtain a reliable prediction of the oscillator response. Nevertheless, the plate response, except for some localized outputs, is not as sensitive to the assumed restitution coefficient. The findings also suggest that separation can even take place at low velocities if roughness is involved in the problem. Moreover, it is indicated that neglecting the possibility of separation/recontact as well as surface roughness leads to seriously unsafe results with respect to both the plate and the oscillator response spectra.

Revisiting the Surface Energy Parameters of Standard Test Liquids with a Corrected Contact Angle Method over Rough Surfaces.

Interfacial free energy is a quantitative basis for explaining and predicting interfacial behavior that is ubiquitous in nature. The contact angle (CA) method can determine the surface free energy (γ) as well as Lifshitz-van der Waals (γLW) and Lewis acid/base (γ+/γ-) components of a solid material from its CAs with a set of known test liquids according to the extended Young-Dupré equation. However, the reliability of the "known" parameters of the test liquids is questioned due to the long-neglected surface roughness effect during calibration of the liquids. This study proposed a simple and practicable two-step approach to correct the energy parameters of several test liquids by incorporating Wenzel's surface roughness relationship into CA measurement. Step 1: water and two apolar liquids (diiodomethane and α-bromonaphthalene) were used as benchmarks to calibrate the surface roughness and energy parameters of two reference solids [apolar poly(tetrafluoroethylene) and monopolar poly(methyl methacrylate)], and step 2: the reference solids were used to calibrate any other test liquids by solving the energy parameters from their CAs in the extended Young-Dupré-Wenzel model. Monte Carlo simulation was used to evaluate error transmission and robustness of the model solutions. The obtained energy parameters (γLW/γ+/γ-) of four test liquids (dimethyl sulfoxide, formamide, ethylene glycol, and glycerol) are 28.01/13.68/4.67, 34.95/3.53/37.62, 26.26/7.51/15.74, and 32.99/9.24/26.02 mJ/m2, respectively, and different from the literature values. The liquids were applied to characterize an example solid surface with true γLW/γ+/γ- values of 28.00/1.00/8.00 mJ/m2 and a roughness index (r) of 1.60. Without correction of the liquid parameters, the calculated surface energy, hydration energy, and hydrophobic attraction energy of the solid sample can deviate by 50, 13, and 27%, respectively. This proves the necessity of correcting parameters of the test liquids before they can be used in CA and interfacial energy studies in the presence of surface roughness.

Surface Roughness Explains the Observed Water Contact Angle and Slip Length on 2D Hexagonal Boron Nitride.

Hexagonal boron nitride (hBN) is a two-dimensional (2D) material that is currently being explored in a number of applications, such as atomically thin coatings, water desalination, and biological sensors. In many of these applications, the hBN surface comes into intimate contact with water. In this work, we investigate the wetting and frictional behavior of realistic 2D hBN surfaces with atomic-scale defects and roughness. We combine density functional theory calculations of the charge distribution inside hBN with free energy calculations using molecular dynamics simulations of the hBN-water interface. We find that the presence of surface roughness, but not that of vacancy defects, leads to remarkable agreement with the experimentally observed water contact angle of 66° on freshly synthesized, uncontaminated hBN. Not only that, the inclusion of surface roughness predicts with exceptional accuracy the experimental water slip length of ∼1 nm on hBN. Our results underscore the importance of considering realistic models of 2D materials with surface roughness while modeling nanomaterial-water interfaces in molecular simulations.

A study of convective nanofluid flow over a rough slender cylinder under the influence of magnetic field and species diffusion

AbstractThe present work explores the analysis of magnetohydrodynamics nonlinear mixed conv ective nanofluid flow over a vertical slender cylinder in the presence of surface roughness. The application of the present study can be found in the process of coating wires. In fact, during such a process, thin wires in the slender cylinder need to be cooled, and also heat and mass transfer rates need to be controlled through nanofluid and liquid hydrogen to yield better results. By employing nonsimilar transformations, the partial differential equations governing the flow problem are reduced to dimensionless equations. Furthermore, the Quasilinearization technique and implicit finite difference scheme are used to solve the dimensionless governing equations. The novelty of the analysis is the impacts of surface roughness, diffusion of liquid hydrogen, and the presence of nonlinear mixed convective flow over a slender cylinder. The numerical results reveal that the energy transport strength and surface drag coefficient enhance with the roughness parameter values. The nanoparticle volume fraction profile reduces, while nanoparticle Sherwood number enhances with increasing values of velocity ratio parameter. The presence of nanoparticles in the conventional fluid diminishes the energy transfer value significantly for both smooth and rough surfaces. The velocity of the fluid enhances with increasing values of the mixed convection parameter. Due to surface roughness ( ≠ 0), sinusoidal variations of the drag are observed along the length of the cylinder in comparison to smooth surface case ( = 0). The friction between the fluid and the wall rises for a rough surface because pockets of fluid are formed in the cavities due to surface asperities.

Modeling of combined effects of surface roughness and blowing for Reynolds-averaged Navier–Stokes turbulence models

A new modeling strategy adapted to Reynolds-averaged Navier–Stokes turbulence models is proposed to predict combined effects of roughness and blowing boundary conditions. First, an analysis of experimental data is presented, leading to a specific description of the velocity profile in the logarithmic region of transpired turbulent boundary layers over rough walls. This analysis points out the deficiencies of existing roughness corrections to predict the effect of blowing in the presence of surface roughness. Indeed, these corrections tend to underestimate skin friction coefficients and Stanton numbers with the addition of blowing. The failure of existing models derives from an inaccurate estimation of the velocity shift of the logarithmic law given by roughness corrections. Concretely, roughness corrections underestimate the apparent velocity shift of the logarithmic law with blowing. To recover the expected law of the wall, an additional contribution on the velocity shift, characterizing blowing/roughness interactions, is integrated to standard roughness corrections. To that end, a modification of the equivalent sand grain height, adapted to k−ω based turbulence models, is proposed to take blowing effects into account. Furthermore, an extension of Aupoix's thermal correction [B. Aupoix, Int. J. Heat Fluid Flow 56, 160–171 (2015)] to blowing is presented to predict combined thermal effects of roughness and blowing. The assessment of the proposed corrections is performed using k−ω shear stress transport model on a large set of experimental data and proves the relevance of the strategy for incompressible and compressible turbulent boundary layers.

A New Deterministic Model for Mixed Lubricated Point Contact With High Accuracy

Abstract Mixed lubrication is a major lubrication regime in the presence of surface roughness. A deterministic model is established to solve mixed lubricated point contact in this paper, using a new method to solve the asperity contact region in mixed lubrication. Treatment of pressure boundary conditions between elastohydrodynamic lubrication region and asperity contact region is discussed. The new model is capable of calculating the typical Stribeck curve and analyzing the transition of lubrication regime, from full film lubrication to boundary lubrication. Moreover, the final result of the model is independent of pressure initialization. High performance in accuracy and convergence has been achieved, which is of great importance for further lubrication modeling with consideration of nano-scale roughness, intermolecular, and surface forces.

Roughness-Induced Adhesive Hysteresis in Self-Affine Fractal Surfaces

It is known that in the presence of surface roughness, adhesion can lead to distinct paths of loading and unloading for the area–load and penetration–load relationships, thus causing hysteretic loss. Here, we investigate the effects that the surface roughness parameters have on such adhesive hysteresis loss. We focus on the frictionless normal contact between soft elastic bodies and, for this reason, we model adhesion according to Johnson, Kendall, and Roberts (JKR) theory. Hysteretic energy loss is found to increase linearly with the true area of contact, while the detachment force is negligibly influenced by the maximum applied load reached at the end of the loading phase. Moreover, for the micrometric roughness amplitude hrms considered in the present work, adhesion hysteresis is found to be affected by the shorter wavelengths of roughness. Specifically, hysteresis losses decrease with increasing fractal dimension and cut-off frequency of the roughness spectrum. However, we stress that a different behavior could occur in other ranges of roughness amplitude.

Meronlike Spin Textures in In-Plane-Magnetized Thin Films

The interfacing of magnetic materials with nonmagnetic heavy metals with a large spin-orbit coupling, such as $\mathrm{Pt}$, results in an asymmetric exchange interaction at the interface due to the Dzyaloshinskii-Moriya interaction (DMI), which in turn leads to the formation of skyrmions and topological spin structures in perpendicularly magnetized multilayers. Here, we show that out-of-plane spin domains with lateral dimensions from 200 nm to 2 $\ensuremath{\mu}\mathrm{m}$ are stabilized in in-plane magnetized $\mathrm{Ta}$/$\mathrm{Co}$/$\mathrm{Pt}$ trilayers. We show that these spin textures are largely insensitive to the direction of the in-plane magnetization switched by either magnetic fields or electric fields applied across a ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ gate dielectric. The results of micromagnetic simulations indicate that the DMI is required for the stabilization of such out-of-plane domains and that the presence of surface roughness helps to stabilize larger structures, in agreement with experimental results. We identify these spin structures as meronlike topological textures, characterized by a perpendicular spin texture in an uniformly in-plane magnetized system.

Deterministic normal contact of rough surfaces with adhesion using a surface integral method.

The fundamental problem of adhesion in the presence of surface roughness and its effect on the prediction of friction has been a hot topic for decades in numerous areas of science and engineering, attracting even more attention in recent years in areas such as geotechnics and tectonics, nanotechnology, high-value manufacturing and biomechanics. In this paper a new model for deterministic calculation of the contact mechanics for rough surfaces in the presence of adhesion is presented. The contact solver is an in-house boundary element method that incorporates fast Fourier transform for numerical efficiency. The adhesive contact model considers full Lennard-Jones potentials and surface integration at the asperity level and is validated against models in the literature. Finally, the effect of surface roughness on the adhesion between surfaces was studied, and it was shown that the root mean square gradient of surface roughness can change the adhesive pressures irrespective of the root mean square surface roughness. We have tested two adhesion parameters based on Johnson's modified criteria and Ciavarella's model. We showed that Civarella's model introduces the most reasonable criteria suggesting that the RMS roughness and large wavelength of surfaces roughness are the important parameters of adhesion between rough surfaces.

Static and dynamic behaviour of aqueous drops on thin lubricating fluid-coated slippery (LCS) surfaces

Inspired by Nepenthes pitcher plants, lubricating fluid infused slippery (LIS) surfaces have recently gained tremendous attention from various research groups. We, alternatively, present lubricating fluid-coated slippery (LCS) surfaces based on smooth solid surfaces. The biggest advantage of LCS surfaces over LIS surfaces is that solid surfaces need not be rough or porous. We discuss that the surface energy of the underlying solid surfaces plays the most important role in preparing the LCS surfaces. Lubricating fluid-coated hydrophilic surfaces result in sinking aqueous drops due to the instability of the thin lubricating film. Additionally, stable and floating aqueous drops are observed on similar hydrophobic surfaces. We also observed that the presence of surface roughness enhances the stability and performance of LCS surfaces. Later we discuss various characteristics of LCS surfaces and its effects on the static and dynamic behaviour of aqueous drops on LCS surfaces.

Effects of build orientation and sample geometry on the mechanical response of miniature CP-Ti Grade 2 strut samples manufactured by laser powder bed fusion

Additive manufacturing can produce a regular three-dimensional mesh containing tiny interconnected wires/struts that form a porous structure. It is reasonable to assume that the properties of these single wires/struts will affect the performance of each unit-cell contained within the porous structure and thereby affect the mechanical properties of the whole porous structure. The effects of strut geometry and build orientation on resulting mechanical properties are examined in this study. Laser powder bed fusion was used to manufacture single strut samples of CP-Ti Grade 2 in the vertical and horizontal orientations with cross-sectional areas ranging from 0.07 mm2 to 2.3 mm2. For samples with cross-sectional areas smaller than 1.5 mm2, the difference in yield strength between the vertical and the horizontal build orientations was 79%, consistent with differences in microstructure scale in these orientations. Build orientation effects were much less pronounced in larger samples where the microstructural scale and features were similar. The presence of surface roughness and process-induced porosity contributed to the significant differences in ductility exhibited in the different orientations. The results indicate that build orientation effects may be significant for as-deposited surfaces and should be considered when designing porous structures consisting of thin struts since these will likely be used with as-deposited surface roughness.

Micromagnetic modeling and imaging of vortex|meron structures in an oxide|metal heterostructure

Using micromagnetic simulations, we have modelled the formation of imprinted merons and anti-merons in cobalt overlayers of different thickness (1-8 nm), stabilised by interfacial exchange with antiferromagnetic vortices in $\alpha$-Fe2O3. Structures similar to those observed experimentally could be obtained with reasonable exchange parameters, also in the presence of surface roughness. We produce simulated meron|antimeron images by magnetic force microscopy (MFM) and nitrogen-vacancy (N-V) centre microscopy, and established signatures of these topological structures in different experimental configurations.

Laser-induced thermal emission of rough carbon surfaces

The decay of thermal emission of rough surface layers of different carbon materials under the pulsed laser excitation is analyzed both theoretically and experimentally. For pulsed laser heating of rough surfaces, computer simulations revealed that laser-induced thermal radiation is mainly emitted by peaks of the surface relief, and the emission decay time depends on the relation between the laser penetration depth and the temperature diffusion length. It is also concluded that the presence of surface roughness can significantly increase the emission decay time. In the experiments, carbon materials with different thermal characteristics were used; however, all of the investigated samples demonstrated close values of the emission decay time. This fact shows that the material's characteristics on the peaks of surface relief are similar for different carbon materials at high temperature.

Effect of axial conduction in integral rough friction stir channels: experimental thermo-hydraulic characteristics analyses

Integral mini-channels fabricated in the metal substrate by friction stir channeling process are different from conventional mini-channels due to irregular shape, and large surface roughness. The effect of axial wall conduction on heat transfer in such mini-channels is significant, which could limit their utilisation as cooling channels in heat transfer based applications. Hence, in this study, we present an experimental analysis of the thermo-hydraulic performance of a friction stir channel to identify the axial wall conduction effect using de-ionized water as working fluid. The thermo-hydraulic performance parameters are analysed in the range of Reynolds number of 500 to 2100. The local wall temperature distribution along the length of the mini-channel is found to be non-uniform due to the significant axial wall conduction effect. The distinct pattern of numerical heat flux distribution is used as evidence to confirm the presence of axial wall conduction effect which is implied by the local wall temperature distribution. The higher average heat transfer and flow characteristics compared to the theoretical predictions for conventional mini-channels are observed as a result of simultaneously developing flow and the presence of surface roughness.