Dimpled Surface Research Articles

Room temperature and high temperature micro-forming analysis of SS304 foil

ABSTRACT The micro-sheet metal forming (MSMF) processes such as microstamping and micro-deep drawing can be used to mass-produce miniature parts. However, to design the tools and the processes, material behavior of workpiece under various deformation conditions is required. This paper analyzes the material behavior of 200-µm thickness Stainless steel-304 foil with room-temperature tensile (RTT) tests and high-temperature tensile (HTT) tests. The effect of strain rate and temperature on flow stress behavior, surface roughness, microstructural evolution and fracture mechanism have been examined. It is observed that the percentage elongation for HTT tests is significantly higher than as compared to RTT tests. However, the percentage elongation and the tensile strength during HTT tests are dependent on the strain rate and decreases with a decrease in the strain rate. Recrystallization followed by grain growth in the material took place during high-temperature deformation, thereby changing the surface roughness and fracture mechanism. All the RTT specimens showed ductile failure with characteristic trans-granular dimpled fracture surfaces. The HTT specimens, in contrast, have intergranular fracture by grain pull out due to crack propagation along the grain boundaries. The present analysis can be used to guide the designing of micro-sheet metal forming processes to obtain the required micro-parts.

Variations in host surfaces morphology and biology of ciliate epibionts explaining distribution pattern of epibionts in the invasive signal crayfish Pacifastacus leniusculus (Dana, 1852)

AbstractThe present paper reports the distribution of Epistylididae (Sessilida) epibiotic ciliates on the signal crayfish Pacifastacus leniusculus in relation to specialized structures of the host's exoskeleton for the first time. This species is known to maintain a relatively clean carapace, but it is sometimes overgrown by other organisms. Epibionts cover different parts of the signal crayfish body, mainly antennae, antennules, maxillipedes III and pereiopods with inner edges of chelae; however, some parts of the signal crayfish are not fouled. e.g. dorsal surfaces of the carapace and dorsal surfaces of chelae. Using scanning electron microscopy (SEM), topographies of different parts of the crayfish's exoskeleton were examined. Our investigations revealed that the epibiosis in the signal crayfish was determined by morphology of body surfaces. No epibionts were found on smooth surfaces with regular single setae (a known design that prevents fouling in aquatic environments). The dimpled surfaces with regular single micro‐papillae may be considered as a newly described example of self‐cleaning surfaces. Bio‐inspired surface patterning presents promising approaches to the design of novel functional surfaces that can prevent biofouling. The new self‐cleaning design may be used in further technological applications.

Microstructure and Tensile Behavior of a Novel Monocrystalline Co‐Based Superalloy at Different Temperatures

Microstructures and tensile properties of a novel monocrystalline Co‐based superalloy are investigated by experiments and finite element simulation. Microstructures of heat‐treated alloys are composed of γ and cubical γ' precipitates without the presence of the secondary phase, and the fraction of γ' phase is about 86%. The experimental results show that tensile properties of the novel monocrystalline Co‐based superalloy have abnormal yield behavior. In low and medium temperature ranges (R.T. 850 °C), the yield stress increases gradually with temperature increase and the tensile fracture surface is characterized by cleavage rupture. At 850 °C, fracture mechanism is composite of cleavage and microporous. While above 850 °C, the yield stress decreases rapidly, and fracture surface is constituted of mixed fracture surface of dimples and micropores. The simulation results show that the tensile stress‐–strain curves are well fitted to the experimental data at different temperatures.

Trailing-edge noise reduction potential of a locally applied shallow dimpled surface

There is a strong need for airframe noise reduction methods that preserve the aerodynamic performance of lift-generating airframe components. Inspired by the drag reduction studies of van Nesselrooij et al. (2016) and Spalart et al. (2019), the current study explored the trailing-edge noise reduction potential of shallow dimples applied near the trailing edge of a NACA0012 airfoil (approximately the latter 18% of the chord). The effect of shallow dimples on the aerodynamic coefficients is studied by Reynolds Averaged Navier–Stokes (RANS). To investigate the noise reduction credentials of dimples, high-fidelity Overset-Large Eddy Simulations are carried out for a chord based Reynolds number Rec of 4.2 ×105 at zero angle of attack. Near-field analysis revealed that this local application of dimples eliminated the tonal vortex shedding noise, which is otherwise present in the reference configuration without dimples. Furthermore, the application of shallow dimples resulted in the breakdown of the span-wise coherence in the low-to-mid frequency range and a reduction in the convective velocity of the stream-wise convecting eddies. These effects have a favourable impact on the trailing edge noise reduction with a modest 1.8% increase in the overall drag (less than 1 drag count). The far-field sound characteristics revealed that the dimple treatment resulted in the reduction of overall sound pressure levels in the downstream radiation direction (≈ 5.5 dB) and an increase in the upstream radiation direction (≈ 3.5 dB). In a nutshell, this manuscript argues for the inclusion of shallow dimples to the list of potential candidates for passive noise reduction methods, such as serrations, porous materials, and finlets.

Performance Characteristics of Cemented Tailings Containing Crumb Rubber as a Filling Material

Increasing amounts of tailings and waste rubber tires have caused threats to the environment. In order to reuse these solid wastes, this study proposed a cemented tailings material containing crumb rubber recovered from waste tires. Unconfined compression tests and thermal conductivity measurements were conducted to evaluate the feasibility and performance of the proposed cemented tailings. Results revealed that the stress-strain behavior of the cemented tailings changed by the inclusion of crumb rubber. The unconfined compressive strength of cemented tailings decreased with increasing crumb rubber content. The inclusion of crumb rubber increased the failure strain and nonlinearity index of the cemented tailings. It was found that the addition of 4% crumb rubber provided the greatest improvement in ductility. The increase of the rubber content further increased the thermal insulation capacity of the cemented tailings. The low stiffness and the dimpled surface of the crumb rubber were responsible for the reduced compressive strength and thermal conductivity of the cemented tailings. In conclusion, the crumb rubber had both positive and negative effects on the cemented tailings. In practical application, the rubber admixture amount should be flexibly adjusted for different use case to make the cemented tailings material show the most suitable performance characteristic.

Carbon Fiber Surface Functional Landscapes: Nanoscale Topography and Property Distribution.

The ultimate properties of carbon fibers and their composites are largely dictated by the surface topography of the fibers and the interface characteristics, which are primarily influenced by the surface distribution of chemical functionalities and their interactions with the matrix resin. Nevertheless, nanoscale insights on the carbon fiber surface in relationship with its chemical modification are still rarely addressed. Here, we demonstrate a critical insight on the nanoscale surface topography characterization of modified novel carbon fibers using high-resolution atomic force microscopy at multiple length scales. We compare the nanoscale surface characteristics relevant to their role in controlling interfacial interactions for carbon fibers manufactured at two different tensions and two distinct chemically functionalized coatings. We used surface dimple (also known as nanopores) profiling, microroughness analysis, power spectral density analysis, and adhesion and electrostatic potential mapping to reveal the fine details of surface characteristics at different length scales. This analysis demonstrates that the carbon fibers processed at lower tension possess a higher fractal dimension with a more corrugated surface and higher surface roughness, which leads to increased surface adhesion and energy dissipation across nano- and microscales. Furthermore, electrochemical surface modification with amine- and fluoro-functional groups significantly masks the microroughness inherent to these fibers. This results in increased fractal dimension and decreased energy dissipation and adhesion due to the high chemical reactivity in the areas of asperities and surface defects combined with a significant increase in the surface potential, as revealed by Kelvin probe mapping. These local surface properties of carbon fibers are crucial for designing next-generation fiber composites with predictable interfacial strength and the overall mechanical performance by considering the fiber surface topography for proper control of interphase formation.

Between Harmonic Crystal and Glass: Solids with Dimpled Potential-Energy Surfaces Having Multiple Local Energy Minima

Solids with dimpled potential-energy surfaces are ubiquitous in nature and, typically, exhibit structural (elastic or phonon) instabilities. Dimpled potentials are not harmonic; thus, the conventional quasiharmonic approximation at finite temperatures fails to describe anharmonic vibrations in such solids. At sufficiently high temperatures, their crystal structure is stabilized by entropy; in this phase, a diffraction pattern of a periodic crystal is combined with vibrational properties of a phonon glass. As temperature is lowered, the solid undergoes a symmetry-breaking transition and transforms into a lower-symmetry phase with lower lattice entropy. Here, we identify specific features in the potential-energy surface that lead to such polymorphic behavior; we establish reliable estimates for the relative energies and temperatures associated with the anharmonic vibrations and the solid–solid symmetry-breaking phase transitions. We show that computational phonon methods can be applied to address anharmonic vibrations in a polymorphic solid at fixed temperature. To illustrate the ubiquity of this class of materials, we present a range of examples (elemental metals, a shape-memory alloy, and a layered charge-density-wave system); we show that our theoretical predictions compare well with known experimental data.

Deformation modes of an oil-water interface under a local electric field: From Taylor cones to surface dimples

Fluidic interfaces disintegrate under sufficiently strong electric fields, leading to electrohydrodynamic (EHD) tip streaming. Taylor cones, which emit charged droplets from the tip of a conical cusp, are among the most prominent and well-studied examples of EHD instabilities. In liquid-liquid systems, more complex interface deformation modes than simple Taylor cones can be observed, with the interface being pushed away from the electrode, and additional cone structures emerging from the rim of the dimple. In this article, we investigate the mechanisms behind these deformation modes experimentally and numerically, and demonstrate that the presence of droplets triggers the dimple at the interface. In order to characterize the underlying processes, we replace the pin electrode by a hollow metallic needle with a prescribed electrolyte volume flow. The submerged electrospray introduces droplets of an aqueous KCl solution with varying ion concentrations into silicone oils with varying viscosities. By measuring the corresponding electric current and by optical investigation of the interface deformation, we study the system response to variations of the ionic concentration, viscosity, applied voltage as well as flow rate. In addition to the experiments, we use a finite element solver and compute the charge transport due to the droplets in the oil phase. Further, we compute the electric potential distribution, flow field and interface deformation. After calibration of our model with particle tracking velocimetry data of the flow inside the oil phase, we reproduce the experimentally observed dimple at the liquid-liquid interface. In summary, this work highlights the importance of charged droplets for the complex dynamic modes observed when a liquid-liquid interface is exposed to a local electric field.

Using dimpled-pipe surface to reduce submarine landslide impact forces on pipelines at different span heights

The interaction between submarine landslides and pipelines has attracted widespread attention. For a pipeline designer, generalizing practical analysis models, obtaining accurate dynamic forces (i.e., the drag force and lift force), and proposing effective pipeline protection measures are prerequisites for ensuring their safe operation. In this paper, a computational fluid dynamics approach is employed to investigate if a golfball-type dimpled pipeline surface reduces impact forces induced by submarine landslides. Considering technology suspension, geomorphology suspension, environmental erosion suspension and acquired suspension, numerical models are generalized to allow prescribing different pipeline span heights, and are impacted by fluidized homogeneous submarine landslides. For the given Reynolds numbers and span heights, golfball-type dimpled pipelines experience a reduced peak drag force than standard circular pipelines. In addition, the pipeline dimple suppresses the lift force variation and greatly reduces the force amplitude. As a result, pipeline damage could be lessened or even avoided. Also, a methodology for predicting the drag reduction of dimple-surfaced pipelines under various conditions is provided, and the two necessary conditions for the periodic vibration of the lift force are proposed. Finally, the full life cycle of vortices shed from the pipeline and the hydrodynamics during landslides impact on pipelines are revealed providing the needed insights to prevent future submarine pipeline failures.

Strength-ductility synergic enhancement in friction stir welded AA2024 alloy and copper joints: Unravelling the role of Zn interlayer's thickness

This study aims to justify an appropriate condition for enhancement of strength and ductility trade off in dissimilar friction stir welded joints of AA2024/copper having Zn interlayer with various thicknesses. For this aim, friction stir welding (FSW) was conducted at a rotation rate of 950 rpm, traverse speed of 85 mm/min and with tilt angle of 2°, offset of +2 mm and shoulder plunge depth of 0.05 mm. After FSW, the microstructure and mechanical properties of the joints were examined in detail using optical microscopy, scanning electron microscopy equipped with energy dispersive spectroscopy, X-ray diffraction, and tensile test. The tensile results proved that addition of Zn interlayers results in improvement of strength besides ductility by changing the nature of intermetallic compounds in the joints. Furthermore, the accurate analysis indicated that the optimum thickness of Zn interlayers is 0.2 mm in order to obtain the best mechanical properties. The fracture appeared in the AA2024 part instead of the joint interface and more dimple type surface is appeared after fracture. The detail of different intermetallic compounds formed in the interfacial area of the joints is discussed throughly.

Experimental investigation of pressure fields in a single trench dimple

The paper describes the technique and results of an experimental investigation of the pressure distribution over the surface of an oval trench dimple (OTD). The trench dimple in cross-section is a cylindrical segment on one of the flat channel walls. The experiments were carried out at a constant Reynolds number Re D = 3.2 · 104 and variation of the inclination angle relative to the longitudinal axis of the channel in the range φ = 0 – 90°.

Friction factor effect and heat transfer enhancement in combined dimple geometry arrange in different angle to flow direction

One of the most effective passive techniques for heat transfer enhancement in heat exchanger devices is the surface roughness flow augmentation model. In this study, the surface pressure and heat transfer are measured using experimental and computational methods in a rectangular channel. Three aspect ratios, AR, 0.05, 0.035, and 0.025, dimple channel with pitch to depth ratios, P/δ = 6 are tested in this study. The angled dimple surface comprises of two cases; case 1 0 °Circular - 45°Oval dimple (0 °C – 45°O) and Case2 45° Oval - 0° Circular dimple (45° O – 0 °C). The computational study is conducted for Reynolds number, Re 600–11000. The data from the compound-dimple channel is normalized with the smooth channel data of heat transfer and friction factor parameters. It is revealed that friction factors, f, friction factors ratio, f/fo, average Nusselt number, Nuavg, Nusselt number ratios, Nu/Nuo, are dependent on Re. The performance index on the compound dimple surface using the ratio of Nusselt numbers, Nu/Nuo, to friction factors, f/fo. The arrangement and location of the dimple have momentous consequences for the pressure drop parameters f and f/fo. The surface pressure and temperature show that the flow is uniformly distributed in the channel. The performance index improves and Re has strong influence on it. The compound dimple geometry does indicate a strong model for application in heat exchanger devices. Optimum performance of 49% for case 1 and 23% for case 2 are observed.

Numerical study on heat transfer characteristics of swirling flow on dimpled surfaces with effusion holes at turbine blade leading edge

In this paper, we conducted a numerical study to investigate the effect of the offset of the jet holes on heat transfer of swirling flow in a concave target chamber with various dimple structures and effusion holes at the turbine blade leading edge. The distance of the jet holes off the centerline e/d varies from 0 to 2.0. Four types of dimple structure, including spherical dimples (SDs) and oval-trench dimples (OTDs) in the inline and staggered arrangement, are considered. The heat transfer performance of the different leading-edge, impingement-effusion cooling structures is evaluated and compared at a Reynolds number of 30,000 based on the jet hole diameter. Results show that the offset of the jet holes provides 15% higher overall heat transfer performance and more uniform heat transfer of the target surface within the e/d range of 0-2.0. The introduction of the dimple structures on the target surface slightly decreases the overall averaged Nusselt number but enhance the heat transfer quantity due to the clear increase of heat transfer areas. Under the same e/d, the OTD structure, especially with the staggered arrangement, is superior to SD structure.

Numerical investigations on the flow control over bumped surface circular cylinders

External flow-induced forces on bumped surface circular cylinders in laminar flow regimes are studied at a range of Reynolds numbers up to 200. This study examines three types of bumped surface cylinders, including square, triangular, and dimpled surfaces. The bumped surfaces are set at the top surface of the cylinder. The bumps are arranged between θ = 0° and 90° from the front stagnation point of the cylinder. It is shown that the presence of a bump on the leading edge of the cylinder manipulates the vortex dynamics and leads to significant changes in flow-induced forces. The results show that due to asymmetrical imposed pressure on the cylinder, the generated negative vorticity of the square bumped surface is approximately 28% stronger than those of triangular and dimpled surfaces once θ = 90°. Comparing to a smooth cylinder, significant reductions in drag and lift coefficients are obtained at Re = 150 once a bump is created on the surface of the cylinder apart from the type of geometry. It is found that the maximum time-mean lift coefficient is produced once the bump is located at θ≥ 45°. Furthermore, the minimum drag force is generated for the bumped surface cylinder with θ = 0° and Re = 150.

Influence of Cutting Parameters to Surface Area Roughness in Dimple Machining Using Milling Process

Surface quality is among the predominant criterion in measuring machining process performance, including milling. It is extremely dependent on the process variable, such as cutting parameters and cutting tool conditions. The main intention of this research work is to study the effect of the milling machining parameters, including depth of cut, spindle speed, feed rate as well as machining pattern to the final surface area roughness of the fabricated dimple structure. The concave profile of the dimple is machined at the right angle to a flat Al6061 specimen using a ball end mill attached to a 3-axis CNC milling machine, and the surface area of the concave profile is measured using 3D measuring laser microscope. It is observed that surface area roughness reacts with the spindle speed and feed rate with different tool sizes. Based on the result gained, the work has successfully characterised the influence of studied milling parameters on the dimple surface area roughness, where within the range of the studied parameter, the surface area roughness varies only less than 2.2 μm. The research work will be continued further on the incline milling technique and micro size ball end mill.

Vortex heat transfer enhancement in the separated flow near structured dimpled surfaces

The numerically discovered phenomena of abnormal enhancement of the separated flow in the inclined oval-trench dimple (OTD) and the flow acceleration in the dimpled narrow channel are substantiated experimentally. The analysis of turbulent flow around a deep OTD on the plate and on the channel wall show that within the inclination angle range from 25° to 85°, the pressure drop is seen between the zones of stagnation on the windward slope and of rarefaction in the place where a tornado-like vortex is generated. The velocity field measurements in the narrow channel with two rows of inclined OTDs at the inclination angles of ±45° and ±135° reveal that the shear flow with a maximum velocity in front of the dimple entrance is formed in the flow core. This maximum shear flow velocity exceeds the maximum velocity in the plane-parallel channel.

Improving tribological behavior of laser textured Ti-20Zr-10Nb-4Ta alloy with dimple surface

The tribological behavior of laser textured Ti-20Zr-10Nb-4Ta (TZNT) alloy lubricated with simulated body fluid (SBF) was evaluated. The laser texturing was carried out using a femtosecond laser with a maximum power of 20 W. Dimples with the diameter of 150 μm and area density of 10%, 20%, 30% and 40% were manufactured on TZNT surface. The results of friction and wear test demonstrated that coefficient of friction (COF) and wear rate of all dimple-textured TZNT are lower than that of polished TZNT at the same load. In addition, as area density increases from 10% to 40%, the COF and wear rate decrease significantly. The grooves and delamination on the worn surface indicated that a combination of abrasive and fatigue wear are the wear mechanism of both polished and dimple-textured TZNT. The dimple structure fabricated by laser texturing has a significant anti-wear effect and improves the wear resistance of TZNT.

Limited Enhancement of Subatmospheric Boiling on Treated Structured Surfaces With Biphilic Pattern

Abstract Boiling heat transfer suffers deteriorations under subatmospheric conditions, which can be attributed to a shortage of viable nucleation sites at declining pressures. In this work, the possibility of enhancing low-pressure saturated boiling of water using a combination of wettability patterning and structural modifications was experimentally explored. The copper test surface, comprised of an array of circular “dimples” (0.3 mm in depth, 0.5 mm in diameter, and 3.0 mm in pitch), was spray-coated by polytetrafluoroethylene (PTFE) coatings so as to form a matching biphilic pattern with the surface cavities. The resulting dimpled biphilic surface showed appreciable heat transfer enhancement—with a maximum 60% increase of the average heat transfer coefficient of nucleate boiling compared with a flat biphilic surface—down to about 9.5 kPa. Further lowering the pressure to 7.8 kPa, however, was found to lead to diminished performance gains. The visualization study of the bubble departure dynamics revealed signs of additional vapor trapping of the hydrophobic-coated cavities, which can induce uninterrupted bubble regeneration with zero waiting time and explain the qualified enhancement of subatmospheric boiling. Thanks to a potential secondary pinning of contact line inside the hydrophobic cavities, incomplete bubble detachment could prevail at somewhat lower pressures than was otherwise possible without the dimple structure, leaving behind significantly more vapor residues. However, the vapor-trapping capacity was found to decrease with pressure, which provided clues with regard to the reduced efficacy of the surface at even lower pressures.

Differences in the effect of surface texturing on the wear loss of β-type Ti–Nb–Ta–Zr and (α+β)-type Ti–6Al–4V ELI alloys in contact with zirconia in physiological saline solution

The effectiveness of dimple surface texturing via picosecond pulsed laser processing for reducing wear loss was investigated using two types of titanium alloys, β-type Ti–29Nb–13Ta-4.6Zr (TNTZ) and α+β-type Ti–6Al–4V ELI (Ti64). The two alloys showed different modes of wear against zirconia. As the sliding distance increased, the wear loss was observed to increase for Ti64, but not necessarily for TNTZ. The wear debris of Ti64 acted as abrasive particles, but that of TNTZ easily adhered to the surface, and the adhered wear debris turned into a hard wear-protective layer. Therefore, the dependence of wear loss on the sliding distance for these two titanium alloys could be attributed to the difference in the roles of wear debris between each titanium alloy and zirconia. Further, depending on this difference in wear mode, the effect of dimple surface texturing on the wear was found to be different in Ti64 and TNTZ. As the dimples can trap the wear debris, they are effective for reducing wear in Ti64 but are detrimental in TNTZ.

Experimental Study on Flow Behavior and Heat Transfer Enhancement With Distinct Dimpled Gas Turbine Blade Internal Cooling Channel

AbstractThe study presents the investigation on heat transfer distribution along a gas turbine blade internal cooling channel. Six different cases were considered in this study, using the smooth surface channel as a baseline. Three different dimples depth-to-diameter ratios with 0.1, 0.25, and 0.50 were considered. Different combinations of partial spherical and leaf dimples were also studied with the Reynolds numbers of 6,000, 20,000, 30,000, 40,000, and 50,000. In addition to the experimental investigation, the numerical study was conducted using a large eddy simulation (LES) to validate the data. It was found that the highest depth-to-diameter ratio showed the highest heat transfer rate. However, the deepest dimpled channel shows the highest pressure drop, which affects the overall thermal performance of the cooling channel. The results showed that the leaf dimpled surface is the best cooling channel based on the highest Reynolds number's heat transfer enhancement and friction factor. However, at the lowest Reynolds number, partial spherical dimples with a 0.25 depth-to-diameter ratio showed the highest thermal performance.