Dimple Edge Research Articles

Flow structure analysis of nanofluid impingement on modified target surface under different design parameters

The flow structures of jet impingement dominate heat and mass transfer process, even the whole thermal performance. In this study, we have inspected the flow structures and mechanism of nanofluid jet impingement onto a dimpled target surface with different design parameters. Investigations are performed for the relative depth of dimple ([Formula: see text]), the jet-to-plate spacing ([Formula: see text]), nanoparticle volume concentration ([Formula: see text]), and Reynolds number (Re) ranging to explore the mechanism of flow structure variations. Results indicate that these parameters have a significant effect on the flow structure of nanofluid jet impingement near the dimpled target surface. The flow begins to separate after passing the edge of the dimple along with the curvature of a dimple. [Formula: see text] will affect the form and location of flow separation and reattachment, and [Formula: see text] will affect the intensity of separation flow. The length of the flow separation bubble varies in different [Formula: see text] cases. When [Formula: see text] increases, the impinging energy and the velocity near the dimple edge decreases. The different Re has little effect on the length of the flow separation bubble and the tendency of the pressure coefficient (Cp). These results can provide further mechanism inspiration for the design of the flow structure of nanofluid jet impingement.

Effect of dimple edge ratio on the thermo-aerodynamic performance in turbulent channel flow

Effect of dimple edge ratio on the thermo-aerodynamic performance in turbulent channel flow

Fabrication of oxide-free dimple structure on germanium via electrochemical jet machining enhanced by opposing laser irradiation

The electrochemical machining (ECM) of germanium (Ge) wafer can be significantly improved in terms of efficiency and localization by introducing laser irradiation. However, residual oxide particles usually exist when using neutral electrolyte. In this study, the NaNO3 electrolyte jet is formed via a cathodic nozzle and impacts perpendicularly on the bottom of the Ge wafer, at which position the nanosecond (ns) laser irradiates from opposing side to locally and precisely enhance the electrical conductivity. In this way, the dimple structure without any detectable oxidation products can be efficiently fabricated. Characterization of surface morphology is then amply conducted. The radial lines consisting of regular micro-waving structure are usually noticeable, especially near the dimple edge, which may be attributed to the rapid and radial electrolyte flow along the concave sidewall. In contrast, irregular corrugated pattern can be observed near the dimple center, which may be caused by the electrolyte turbulence after impingement. In addition, parametric experimental study has been carried out, and the effects of the applied voltage, laser power and electrolyte concentration on the dimple profile and surface characteristics have been respectively detailed discussed. Finally, simulation study of the electrolyte flow field is conducted to reveal the distribution regularities of velocity and pressure, to deepen the understanding of formation mechanism of the complex surface morphology.

Fabrication of micro-structured surface with controllable randomness by using FTS-based diamond turning

Random micro-structured surfaces (RSS) have great application potentials as various functional surface elements, such as optical diffusers, with homogeneous surface properties. Conventional fabrication methods for RSS, such as chemical etching, can no longer meet modern requirements of high precision and controllability. In this study, a novel method was proposed for fabricating micro dimple arrays with controllable randomness in dimple size and depth by using fast tool servo (FTS)-based single-point diamond turning. Firstly, for tool path generation, a computer program was developed to periodically arrange small patches of quadratic surfaces over a large surface area. Then the position, height, and size of each quadratic surface were adjusted by applying random values, whose dynamic range was preset, to control the spatial difference among the dimples. Finally, the designed RSS was machined by continuous and segment cutting methods, respectively, to evaluate their effects on dimple edge formation. Results showed that the segmented cutting method improved the edge accuracy, thus was effective for RSS machining. The machined surface matched precisely with the designed surface with a form error of 10 nm level. The machining method proposed in this study enables high-precision patterning of mold inserts for molding/imprinting RSS on polymer materials.

Effects of Micro Dimple’s Topography Parameters on Wear Resistance of Laser Textured AlCrN Coating Under Starved Lubrication Condition

Due to good impact wear performance and abrasion resistance, AlCrN coating has been widely used on stamping die. But because the tribological property of the surface is poor, in practical application, coating often wears or spalls. To furtherly enhance the coating’s wear resistance, coating combined with laser surface textures was applied. In this study, micro dimples with different depths and diameters were fabricated on different samples and their depth-to-diameter ratios also varied. Reciprocating pin-on-disk tests under starved lubrication conditions were performed to compare samples’ wear resistance. The wear morphology was observed under confocal microscopy and scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) to investigate the influence mechanism of topography parameters on wear resistance. The results showed that optimal topography parameters could be found to achieve the best wear resistance. When the depth-to-diameter ratio was small, serious adhesive wear occurred due to insufficient ability of storing wear debris; when the ratio was large, the surface suffered severe adhesion wear due to poor lubrication condition. In addition, for micro dimples with small depth-to-diameter ratio, the coating on the dimple wall could cracked because of horizontal friction force. Furthermore, for dimple with large diameter, the dimple edge could interlock with the upper sample edge, causing serious wear. The contribution provided reference for the topography parameters selection of textured coating under starved lubrication conditions.

Abnormal separated flow intensification and heat transfer enhancement in single-row inclined oval-trench dimples on the narrow channel wall

Intensification of turbulent separated air flow and heat transfer enhancement at the hydrodynamic stabilization length on the periodic section of the narrow channel with inclined one-row oval-trench dimples (OTDs) is analyzed. The periodic section of the channel of height equal to 1, length – 8, and width – 9 is considered at Re=104. The inclined OTD 0.5 in width, 7.05 in length, and 0.25 in depth is located in the center of the heated isothermal wall. The rounding radius of the dimple edge is 0.21. The OTD inclination angle varies from 0° to 52.5°. It is found that the optimal inclination angles in terms of thermal and hydraulic performance (THP) are equal to 45° and 30°. It is observed that at large inclination angles the minimum value of relative friction decreases many times (up to-3.5).

Effect of magnetic field on tribological performance of laser dimple textured surface

Experiments were performed to examine the friction and wear of laser dimple textured surfaces of medium carbon steel under three conditions: normal, degaussing, and an additional magnetic field. The coefficients of friction and wear volume of the laser dimple textured surfaces were measured, and the hardness of the dimple edge were measured. The worn surfaces of the specimens were examined, and the elemental contents of the wear debris were analysed. The wear debris within the laser dimples was examined at regular intervals to analyse the trapping processes. The experimental results indicated that the coefficients of friction and wear volume mass were reduced by the laser dimple textured surface. Compared with the normal condition, the abrasive wear of the laser dimple textured surface was reduced under both the degaussing and additional magnetic conditions, and the coefficients of friction and wear were reduced. Under the additional magnetic condition, uniform debris attachment to the inner walls of the laser dimples occurred. The distributions of the magnetic domains within the specimens affected the trapping process of wear debris during the wear experiments, and the wear-debris trapping capacity of the laser dimples was improved under the additional magnetic condition. The following phenomena occurred under the additional magnetic condition: hardness improvement of the dimple edge, oxidation of the wear debris, and coverage of the worn surfaces by wear debris. The results indicate that the tribological performance of the laser dimple textured surface of medium carbon steel can be improved under degaussing and additional magnetic conditions.

Heat transfer enhancement by surface vortex generators. New basic mechanisms and industrial technologies

New energy effective surfaces structured by vortex generators – inclined oval-trench dimples (OTDs) – and the controlling physical mechanisms of heat transfer enhancement are explored. The narrow 7 × 1 channel with a package of one-row OTDs at Re=104.is considered. Turbulent heat transfer is studied at the channel length where the flow is steady. The OTD width is 1, its depth is 0.3, its length is 7 0.3, and its dimple step is 6. The rounding radius of the dimple edge is 0.2. The influence of the OTD inclination angle θ on the mechanisms of separated flow intensification and heat transfer enhancement in the dimple is analyzed when θ is varied from 42° to 55.5°. The optimal angle θ of order 53° is revealed, at which the relative thermal performance is close to 70% for the heated wall with a very loose packing of dimples equal to 0.235. The optimal angle θ, at which the highest thermal and hydraulic performance (THP=0.175) has been achieved, appears to be close to 47°. The maximum local value of the 4.5–5-fold relative heat transfer growth coincides with that of static pressure in the separated flow zone on the section, where relative friction grows, after it has reached its minimum value up to -4.5.

Flow and Heat Transfer Characteristic of Inclined Oval Trench Dimples With Numerical Simulation

In this work, flow and heat transfer in a channel having oval trench dimples were investigated numerically. 3-D channel flow with a cross-section of 300-mm width and 32-mm height were created using Computational Fluid Dynamic (CFD) with ANSYS, Fluent (V.15.0). Five oval trench dimples with 3-mm depth, 10.0-mm width and 45-mm length arranging with a single row and in-line configuration were located on the bottom surface of the channel. Reynolds number based on hydraulic diameter of the channel were fixed at Re=20,000 whereas a dimple inclined angle defined as the angle of dimple centreline to the mainstream was varied at 0, 15, 30 and 45 degrees. SST turbulent model was used to solve governing Equations. The result show that longitudinal vortex flow occurred at ?=15° to 45° which would be enhance heat transfer on the surface. When inclined angle became larger, the areas of Nusselt number and high total pressure coefficient took place at dimple edge in +Z direction. The peak of average spanwise Nusselt numbers took place for the case of ?=45°. Moreover, the area of high spanwise average Nusselt numbers (>100) for the case ?=30° was the largest.

Effects of circular trailing edge with the dimple on flow separation of NACA S1210 hydrofoil

Flow separation over a hydrofoil affects its hydrodynamic performance and ultimately reduces the power production of a turbine. Therefore, the different flow control methods have been proposed by the researchers to overcome these challenges. In this study, the combined effects of circular trailing edge with dimple have been investigated numerically on NACA S1210 hydrofoil to control the flow separation of the hydrofoil. The results showed that the coupled effects of the dimple and circular trailing edge modification appeared effective compared to baseline hydrofoil at a wide range of angles of attack. Maximum lift coefficient increments are 9 and 12% at an angle of attack of 10° and 12°, respectively, for circular trailing edge hydrofoil. The maximum increase of glide ratio for the coupled effect (circular trailing edge with outward dimple) is approximately 114% at an angle of attack of 12°. The outcome will be advantageous to design a proper blade profile for horizontal axis current turbines.

Comparisons of tribological properties between laser and drilled dimple textured surfaces of medium carbon steel

PurposeThe study aims to compare tribological properties between laser dimple textured surface and drilled dimple textured surface, and to analyze the influence of dimple hardened edges and ability of trapping wear debris on wear properties of dimple textured surfaces.Design/methodology/approachCircular textured dimples were produced on AISI 1,045 specimen surfaces using laser surface texturing (LST) and drilled surface texturing (DST) methods. Tribological behaviors of LST, DST and non-textured specimens were studied using ball-on-disc tribo-tester. Metallographic structures, dimples and worn surface morphologies were observed using a three-dimensional digital microscope. Hardnesses of substrate and dimple edges were measured.FindingsThere was no obvious difference in wear and friction coefficients between LST and DST specimens. Hardnesses of laser dimple edges were much higher than that of drilled dimple edges and specimen substrate. The hardened materials of laser dimple edge included recast zone and heat affect zone. Laser dimple was cone-shaped and drilled dimple was cylinder-shaped. Drilled dimple had a better ability of trapping wear debris than laser dimple. Non-uniform wear phenomenon occurred on worn surfaces of LST dimple specimens.Originality/valueThe ability of textured dimples to trap wear debris is affected by single dimple volume. Hardened edges of dimples cause non-uniform wear on worn surfaces of LST specimens.

The Wear Behavior of Textured Steel Sliding against Polymers.

Artificially fabricated surface textures can significantly improve the friction and wear resistance of a tribological contact. Recently, this surface texturing technique has been applied to polymer materials to improve their tribological performance. However, the wear behavior of textured tribo-pairs made of steel and polymer materials has been less thoroughly investigated and is not well understood; thus, it needs further research. The aim of this study is to investigate the wear properties of tribological contacts made of textured stainless steel against polymer surfaces. Three polymer materials were selected in this study, namely, ultrahigh molecular weight polyethylene (UHMWPE), polyoxymethylene (POM) and (polyetheretherketone) PEEK. Wear tests were operated through a ring-on-plane mode. The results revealed that the texture features and material properties affected the wear rates and friction coefficients of the textured tribo-pairs. In general, PEEK/textured steel achieved the lowest wear rate among the three types of tribo-pairs investigated. Energy dispersive x-ray spectroscopy (EDX) analysis revealed that the elements of C and O on the contacting counterfaces varied with texture features and indicated different wear behavior. Experimental and simulated results showed differences in the stress distribution around the dimple edge, which may influence wear performance. Wear debris with different surface morphologies were found for tribo-pairs with varying texture features. This study has increased the understanding of the wear behavior of tribo-pairs between textured stainless steel and polymer materials.

Origin of the dimpled critical-current-versus-magnetic-field-angle relation in YBa2Cu3O7 films studied using sub-MeV ion irradiation

In high-temperature superconductor (HTSC) films, the magnetic-field-angle (θ) dependence of critical current density (Jc) is often observed to have a characteristic local maximum at intermediate θ between H||ab and H||c. This local maximum appears as a ‘shoulder’ when Jc(θ) has a predominant peak at H||c, and when such a peak is absent, it appears as a ‘dimple edge’ in a very broad dome-like Jc(θ) that is centered and dimpled at H||c. Despite such common observation, there is still no consensus on the physical origin of this anomalous Jc(θ). In this work, to determine this physical origin. we measured the temperature and microstructure dependence of Jc(B, θ) in YBa2Cu3O7 films irradiated with 3 MeV Au, 500 keV Si, and 200 keV B ions (where B is magnetic induction). The film microstructure was controlled by varying the mass of irradiation ions (Mi) known to alter the size and spacing of collision cascades. Results revealed two observations: (i) the dimple structure diminishes with decreasing temperature, and (ii) the maximum pinning force density in a θ profile, which is recorded at the dimple edge, systematically becomes more B-dependent with decreasing Mi. Both experimental findings suggest that the sharp-shaped anomaly in Jc(θ) in HTSC films originates from flux-line-lattice shear and electron mass anisotropy.

SOBEL OPERATOR FOR EDGES DETECTION IN SURFACE TEXTURE ANALYSIS

The classification of surface textures has been changed for the convenience of design, manufacture and metrology. Surface texture can be divided into two types which are engineered and structured surface. One examples of structured surface is dimple edge type of surface. This kind of surface usually has a deterministic structure, with high aspect ratio geometric features. Modern dimple golf ball is the example of structured surface application witches its functional requirement, was to reduce air drag through better hydrodynamic flow around the ball. From the analysis of dimpled surface, process variation can be enhanced and the uniformity of the surface features can be improved. Hence, the work presented in this paper is intended to improve the quality of segmentation method for the dimple edges type of surface and Sobel Operator algorithm is proposed to be used. In Sobel Operator algorithm, two kernels is applied which one kernel identifies the horizontal edges and the other kernel identifies the vertical edges. By passing these two kernels, gradient along x direction and gradient along the y direction can be computed at the different location and finally determined the edges of the surface. Sobel operator application shows that the location of the edges becomes clearer and topography segmentation based on features edges is possible.

Importance and role of grain size in free surface cracking prediction of heavy forgings

The importance and role of grain size in predicting surface cracking of heavy forgings were investigated. 18Mn18Cr0.5N steel specimens with four different grain sizes were tensioned between 900 and 1100°C at a strain rate of 0.1s−1. The nucleation sites and crack morphology were analyzed through electron backscatter diffraction analysis, and the fracture morphology was examined using scanning electron microscopy. The nucleation sites were independent of the grain size, and cracks primarily formed at grain boundaries and triple junctions between grains with high Taylor factors. Grains with lower Taylor factors inhibited crack propagation. Strain was found to mainly concentrate near the grain boundaries; thus, a material with a larger grain size cracks more easily because there are fewer grain boundaries. Fine grains can be easily rotated to a lower Taylor factor to further inhibit cracking. The fracture morphology transformed from a brittle to ductile type with a lowering of grain size. At lower temperature, small dimples on the fracture surfaces of specimens with smaller grain sizes were left by single parent grains and the dimple edge was the grain edge. At higher temperature, dimples formed through void coalescence and the dimple edge was the tearing edge. Finally, the relationship between the reduction in area, grain size, and deformation temperature was obtained.

The secondary buckling transition: wrinkling of buckled spherical shells.

We theoretically explain the complete sequence of shapes of deflated spherical shells. Decreasing the volume, the shell remains spherical initially, then undergoes the classical buckling instability, where an axisymmetric dimple appears, and, finally, loses its axisymmetry by wrinkles developing in the vicinity of the dimple edge in a secondary buckling transition. We describe the first axisymmetric buckling transition by numerical integration of the complete set of shape equations and an approximate analytic model due to Pogorelov. In the buckled shape, both approaches exhibit a locally compressive hoop stress in a region where experiments and simulations show the development of polygonal wrinkles, along the dimple edge. In a simplified model based on the stability equations of shallow shells, a critical value for the compressive hoop stress is derived, for which the compressed circumferential fibres will buckle out of their circular shape in order to release the compression. By applying this wrinkling criterion to the solutions of the axisymmetric models, we can calculate the critical volume for the secondary buckling transition. Using the Pogorelov approach, we also obtain an analytical expression for the critical volume at the secondary buckling transition: The critical volume difference scales linearly with the bending stiffness, whereas the critical volume reduction at the classical axisymmetric buckling transition scales with the square root of the bending stiffness. These results are confirmed by another stability analysis in the framework of Donnel, Mushtari and Vlasov (DMV) shell theory, and by numerical simulations available in the literature.

Numerical Prediction of Flow Structure and Heat Enhancement with Different Dimple Depth

Aim at finding the relation between flow structure and heat enhancement, this paper change the dimple depth to print diameter from 0.06 to 0.3, with an interval of 0.02, and keep the print diameter constant. Through vortex structure analysis and Nu number integration, find out that: Vortex structure varies with dimple depth. Shallow depth dimple induces horseshoe vortex; middle depth dimple induces symmetric tornado-like vortex; deep depth dimple induces tornado-like vortex. Heat enhancement is closely related to vortex structure. Where spiral separation focus point exists, there the lowest Nu/Nu0 exists. Flow impingement at the windward side of dimple and new viscous layer generation at immediate plane after the dimple enhance heat transfer. The highest Nu/Nu0 exists in the sharp rearward dimple edge. From Fig 6, the deeper the dimple is, the higher the average Nu/Nu0 is, so does the pressure drop. But from 0.06 to 0.2 dimple, heat enhancement increased because of vortex structure changed. From 0.2 to 0.3 heat enhancement increased because of the increased area.

Secondary polygonal instability of buckled spherical shells

When a spherical elastic capsule is deflated, it first buckles axisymmetrically and subsequently loses its axisymmetry in a secondary instability, where the dimple acquires a polygonal shape. We explain this secondary polygonal buckling in terms of wrinkles developing at the inner side of the dimple edge in response to compressive hoop stress. Analyzing the axisymmetric buckled shape, we find a compressive hoop stress with parabolic stress profile at the dimple edge. We further show that there exists a critical value for this hoop stress, where it becomes favorable for the membrane to buckle out of its axisymmetric shape, thus releasing the compression. The instability mechanism is analogous to the formation of wrinkles under compressive stress. A simplified stability analysis allows us to quantify the critical stress for secondary buckling. Applying this secondary buckling criterion to the axisymmetric shapes, we can determine the critical volume for secondary buckling. Our analytical result is in close agreement with existing numerical data.

Vortex mechanism of heat transfer enhancement in a channel with spherical and oval dimples

Vortex mechanism of heat transfer enhancement in a narrow channel with dimples has been investigated numerically using LES and URANS methods. The flow separation results in a formation of vortex structures which significantly enhance heat transfer on dimpled surfaces leading to a small increase in pressure loss. The heat transfer can be significantly increased by rounding the dimple edge and use of oval dimples. To get a deep insight into flow physics LES is performed for single phase flow in a channel with a spherical dimple. The instantaneous vortex formation and separation are investigated in and around the dimple area. Considered are Reynolds numbers (based on dimple print diameter) ReD = 20,000 and ReD = 40,000 the depth to print diameter ratio of Δ = 0.26. Frequency analysis of LES data revealed the presence of dominating frequencies in unsteady flow oscillations. Direct analysis of the flow field revealed the presence of coherent vortex structure inclined to the mean flow. The structure changes its orientation in time causing the long period oscillations with opposite-of-phase motion. Three dimensional proper orthogonal decomposition (POD) analysis is carried out on LES pressure and velocity fields to identify spatio-temporal structures hidden in the random fluctuations. Tornado-like spatial POD structures have been determined inside dimples.

過渡応答法によるディンプル面熱伝達率計測における固体壁内三次元熱伝導補正(熱工学,内燃機関,動力など)

The transient technique using infrared-thermography or liquid-crystal has widely been used for measuring the distribution of local heat transfer coefficients. In this technique, wall surface temperature is measured, and the heat transfer coefficient is calculated so as to accord the measured temperature with the theoretical solution of one-dimensional heat conduction problem. In actual cases of complicated surface geometry, however, three-dimensional heat conduction, caused by the three-dimensionality of the wall surface and the distribution of heat transfer coefficient, occurs in the wall. In this study, the heat transfer coefficient on the hemispherically dimpled surface was measured with an infrared camera, while the three-dimensional heat conduction in the wall was numerically calculated. In the compensation process, modification of the heat transfer coefficient was repeated till the numerical result agreed with the measured surface temperature. The present results showed that the heat transfer coefficient near the dimple edge was overrated, while that within the cavity was underrated. The maximum error induced by the three-dimensional heat conduction was 60% on the leading edge of the dimple, and the error in the other area was about 20% at most. At the dimple edge, the convex geometry increased the surface area where the heat flew into the wall, and consequently temperature rise became larger than the flat part. On the other hand, within the dimple, the concave geometry formed the radially-expanding heat conduction area, and the temperature became lower. The principal factor contributing to the error of the measurement is the three-dimensionality of the surface.