Dimpled Surface Research Articles

Creep properties of heat-resistant ultrafine-grained Al (HITEMAL©) determined by small punch testing

The creep properties of thermally stable ultrafine-grained Al metal matrix composite strengthened and stabilized by continuous in situ amorphous Al2O3 network (named HITEMAL©) prepared by powder metallurgy (PM) were tested by small punch creep testing (SPCT) in the temperature range from 573 to 673 K. The experimental SPCT results were correlated and compared with the results obtained by conventional tensile creep testing. The mutual correlation between tensile and SPCT creep results based on the transition stress was proposed and used to correctly interpret the measured SPCT data. The creep behaviour of HITEMAL© is characterized by the high values of apparent stress exponents and by the presence of transition stress at which the apparent stress exponents significantly changed. The SPCT discs fractured by the radial cracks at low values of deflection with only limited creep deformation highly localized in the vicinity of the fracture surfaces. The detailed analysis of fractured SPCT discs by scanning electron microscopy showed ductile dimpled fracture surfaces with the presence of unusual high filament-like tearing edges. The comparison of obtained results with other PM Al-based alloys and composites confirmed a significantly higher creep performance of HITEMAL© at used testing conditions.

Effect of generalized dimple shape on load-carrying capacity and leakage characteristics of laser surface textured dry gas seal

A new formula is employed to describe the dimple boundaries by considering edge number, superellipse coefficient, long axis radius and inclined angle, which make it possible to describe the boundaries of more typical dimple patterns by a unifiend equation. A mathematical model for obtaining the pressure distribution of dry gas seal with polygon dimple surface is established. The influence of the key geometrical parameters of polygon dimple on opening force and leakage rate is studied. The optimized geometries of generalized polygon dimple for the maximum opening force are obtained. Results show that an inclined quadrilateral dimple with multi rows in the radial direction presents the largest opening force. The opening forces of the two-sided and quadrilateral dimples with micron-scale are significantly better than that of millimeter apertures, while the opening forces of the triangular, pentagonal and hexagonal dimples with millimeter-scale are larger than that of the micron types. The shape profile parameters, directional parameters and radial dimple number of polygon dimples should be properly designed for excellent LCC and leakage characteristics.

Influence of femtosecond laser textured shapes on the friction characteristics of 7075 aluminum alloy

Different surface dimples (circular, square, and triangular) were fabricated on 7075 aluminum alloy specimens using a femtosecond laser to enhance their wear resistance. The influence of these surface dimples on the friction performance of the aluminum alloy under dry sliding conditions was investigated by comparing the coefficient of friction (COF), wear track morphology, and wear rate. All the textured samples exhibit varying degrees of improvement in the wear resistance compared to the untextured sample, and the circular texture is the most effective. Wear track analysis reveals that abrasive and adhesive wear are the primarily wear mechanisms. Thus, introducing surface dimples significantly enhances the wear resistance of the 7075 aluminum alloy, providing a novel strategy for optimizing its friction performance through design.

The Impact of Using Dimpled Surfaces on Ahmed Body on The Flow Characteristics

Vehicle aerodynamics which directly affects fuel consumption values in vehicles has great importance in terms of vehicle stability, acceleration ability, vehicle noise, and environmental pollution. Ahmed car model is one of the most important generic models, so this popular car model is widely used in the literature on fluid mechanics and vehicle aerodynamics studies. This study investigates the effect of a passive drag reduction technique which is known as dimple surface, on Ahmed car model with a 35° slant angle. The flow control methods were applied numerically also these methods were compared with similar works. Analysis made in this study were conducted with 2.4 million cell numbers and the maximum drag reduction is 3.5 % with optimum dimple surfaces. Moreover, the applicability and manufacturability factors were also evaluated in terms of implementing the dimpling modification to today’s current road vehicles.

Passive flow-field control using dimples for improved aerodynamic flow over a wing

This study explores the efficacy of dimples in influencing the aerodynamic performance of a straight rectangular wing. Computational Fluid Dynamics based numerical simulations were performed to model turbulent flow and quantify the forces exerted on the wing. The k-ω Shear-Stress Transport turbulence model was chosen to solve the underlying equations. To ascertain reliability, the results of numerical simulations were compared with both experimental and simulation results of the previous studies. The impact of various dimple configurations, placed at 15%, 50% and 85% of the chord length, on the aerodynamic performance of the wing was investigated. The evaluation involved analyzing the drag coefficient (CD), lift coefficient (CL), lift-to-drag (L/D) ratio, streamlines and the flow field around wing in both chordwise and spanwise directions. The findings indicated that a wing with a dimpled surface could yield a reduced drag coefficient of up to 6.6% compared to the unmodified wing. This reduction is attributed to the dimples ability to sustain attached airflow and delay flow separation. The results demonstrated negligible deviation in the lift coefficient with the incorporation of dimples. The incorporation of dimples on the wing surface has been demonstrated to enhance the aerodynamic performance of lifting surfaces.

Transforming the Stability, Encapsulation, and Sustained Release Properties of Calcium Alginate Beads through Gel-Confined Coacervation.

Calcium alginate (Ca2+/alginate) gel beads find use in diverse applications, ranging from drug delivery and tissue engineering to bioprocessing, food formulation, and agriculture. Unless modified, however, these gels have limited stability in alkaline media (including phosphate buffers), and their high solute permeability limits their ability to efficiently encapsulate and slowly release water-soluble small molecules. Here, we show how these limitations can be addressed by mixing the alginate solutions used in the bead preparation with the nontoxic anionic polymer polyphosphate (PP). Upon complexing Ca2+ ions, PP undergoes complex coacervation (i.e., liquid/liquid phase separation into a Ca2+/PP-rich coacervate phase and a dilute supernatant phase). At lower PP concentrations, the Ca2+/PP coacervate appears to simply remain dispersed within the beads. Though its presence makes the beads more stable in alkaline media (phosphate-buffered saline and seawater), it has little impact on the bead stiffness, morphology, and (at least in the absence of substantial payload/coacervate association) encapsulation and release properties. When the PP concentrations exceed a critical value, however, Ca2+/PP coacervation within the gelling Ca2+/alginate beads collapses the resulting beads into more compact, interpenetrating polymer networks. Besides their enhanced stability to alkaline environments, these hybrid beads exhibit irregular morphologies with wrinkled and dimpled surface structures and macroscopic (closed) internal pores, and their collapse into these polymer-rich networks also makes them significantly stiffer than their PP-free counterparts. Crucially, these beads also exhibit a much lower solute permeability, which enables highly efficient encapsulation and multiday release of water-soluble small molecules (with the beads encapsulating >90% of the added model payload and sustaining its release over 3-5 d). Collectively, these findings provide a mild and simple (single-step) pathway to generating ionically cross-linked alginate beads with significantly enhanced stability, encapsulation efficiency, and sustained release.

Fabrication of depth-controlled dimples on polymer microsphere and capturing of nano-sized objects

Polymer microspheres containing submicron-sized surface dimples were prepared through the acid-dissolving removal of silica nanoparticles as templates from the surface of core-shell microspheres, which were prepared by modified suspension polymerization of a silica nanoparticle-dispersed polymerizable monomer mixture (methyl methacrylate and divinylbenzene dispersing). We demonstrated depth control of the dimples on the surface of polymer microspheres and evaluated their capturing properties. The depth of the dimples could be facilely controlled by changing the surface polarity of silica nanoparticles (1 µm) as templates during the synthesis of the core-shell microspheres. The depth of the prepared dimples ranged from 250 to 810 nm, and the gate diameter of the dimple was related to its depth owing to the spherical shape of the template. The inner surfaces of the dimples were evaluated based on the adsorption of cationic and anionic fluorescent dyes. Confocal laser microscopy observations indicated that the inner surfaces of the dimples were densely stained with both fluorescent dyes, whereas the outer surfaces of the microspheres were not stained, suggesting that the inner surfaces of the dimples were more hydrophobic. Silica particles smaller than the gate diameter of the dimple were crammed (or jammed) into the inner space of the dimple, and the captured silica particles did not drop off even after washing with water more than five times. Because the larger silica nanoparticles could not be trapped, the dimples could selectively capture the nanosized objects in their inner space.

An experimental investigation into the drag reduction performance of dimpled plates in a fully turbulent channel flow

Dimpled surfaces have gained increasing attention in recent years for their potential to reduce turbulent skin friction, a capability previously acknowledged for its beneficial implications on heat transfer. As a passive drag reduction method, dimpled surfaces offer significant advantages for marine applications due to their effectiveness and practical applicability. However, despite numerous studies, conflicting opinions and inconsistent drag reduction rates persist in the literature. This paper addresses these ambiguities and offers valuable insights into the effectiveness of dimpled surfaces for drag reduction in fully turbulent flows. We conducted an extensive experimental investigation involving various dimple configurations, including different depth-to-diameter ratios, diameters and orientations, utilising a specialised Fully Turbulent Flow Channel facility and a Particle Image Velocimetry system. Our findings demonstrated that circular dimple geometries, particularly those with low depth ratios, can achieve significant drag reduction of up to 27% as the Reynolds number increases. These results highlight the substantial potential of dimpled surfaces for improving energy efficiency in marine applications, where skin friction accounts for a significant portion of the total drag experienced by large vessels.

Mechanisms and characterization of a novel hybrid laser-enhanced particle laden electrochemical fabrication process for high quality micro-dimples on germanium wafers

The first-generation semiconductor materials represented by silicon (Si) and germanium (Ge) still hold a dominant position in many fields, including micro-electromechanical systems (MEMS) and integrated circuits (ICs). The processing of these materials continues to attract significant research attention for improvements in quality and efficiency. This study proposes a novel hybrid laser-enhanced particle-laden electrochemical machining (LEPL-ECM) process, whereby pulsed laser irradiation is utilized to selectively and locally enhance the electrical conductivity of Ge at specific locations. A neutral electrolyte jet, containing abrasive diamond particles, is applied to the location on the wafer surface opposite the laser irradiation, facilitating a localized and enhanced electrochemical dissolution. The effect of micro-particle erosion effectively removes the generated oxides and potential passivation layers, ensuring continuous and efficient electrochemical reactions. Consequently, high-quality micro-dimples with an entrance diameter of about 380–800 μm, a depth of around 138–300 μm, and a quasi-mirror surface with the roughness (Sa) of as low as 59.8 nm can be achieved within 90 s. Furthermore, the effects of the applied voltage, laser power and processing time on the resulting dimple characteristics and surface quality are discussed, along with a detailed morphology characterization. Dense micro-pits, radial streamlined micro-grooves and a special transitional region have been observed on dimple center, sidewall and near the edge, respectively, and the formation mechanism have been analyzed. Finally, a simulation of the electrolyte jet induced fluid pressure and velocity as well as the particle trajectory and distributions within and surrounding the dimple structure was conducted, which reveals that the synergistic mechanism associated with the hybrid material removal process involves both electrochemical corrosion and abrasive erosion.

Effect of Al addition on the microstructure and mechanical properties of IN718 alloy fabricated by laser deposition manufacturing

The IN718 alloys with different Al contents were fabricated by laser deposition manufacturing (LDM) technology. The microstructure and tensile properties of LDMed IN718 alloy with different Al contents before and after homogenization-solution-double aging (HSA) treatment were analyzed. The as-deposited samples show dendrite structure which develops with the addition of Al element. The microstructure of the as-deposited samples mainly consists of the γ matrix, Laves phases and a small amount of carbides. The HSA treatment improves the precipitation of γ″, γ′ and δ strengthening phases, and with the Al content increasing, the γ′ phase increases, but the γ″ phase decreases. With the increasing of Al content, the tensile strength of as-deposited samples increases, but the elongation decreases. For HSA-treated samples, the IN718+0.5Al alloy shows the highest tensile strength and elongation. The tensile fracture surfaces of LDMed IN718 alloys show dimpled surfaces, indicating the transgranular ductile failure mode.

Numerical investigation on flow characteristics and thermal performance of phase change material in spherical capsules with smooth or dimpled surfaces

The utilization of phase change spherical capsules with dimpled surfaces in a packed-bed latent thermal energy storage (PLTES) system provides a novel approach to addressing the intermittency and fluctuation in solar thermal utilization processes. In the context of accounting for wall effects, a comparative analysis of the flow characteristics and thermal performance of smooth and dimpled spherical phase-change capsules is conducted using the Large Eddy Simulation (LES) and an enthalpy-porosity method. Results show that the dimpled spherical capsule exhibits a substantial improvement in thermal performance compared to the smooth spherical capsule, with a maximum increase of 28.3 % in time-averaged Nusselt number when the difference in time-averaged drag coefficient is within 2 %. Compared to the smooth spherical capsules, the transient characteristics of dimpled spherical capsules exhibit significant changes, with the fluctuation amplitude of the drag coefficient and the fluctuation frequency of the lift coefficient both increasing by approximately twice. The local heat transfer performance also exhibits significant differences. The increase in surface temperature enhances the melting rate of PCM inside spherical capsules, but with the temperature rise, the enhancement efficiency decreases. The dimpled structure has a promoting effect on the melting process of PCM inside spherical capsules, and with the increase in surface temperature, this promoting effect becomes more pronounced. The time for PCM to achieve complete melting can be reduced by up to 19.07 %. The work contributes a novel idea for the application of non-smooth structures in PLTES systems.

Effects of Flow Pulsation and Surface Geometry on Heat Transfer Performance in a Channel With Teardrop-Shaped Dimples Investigated by Large Eddy Simulation

Abstract Large eddy simulation was performed to investigate heat transfer performance of a pulsating flow over teardrop-shaped dimples. A total of six geometries of dimpled surfaces were examined for dimple arrangements of in-line/staggered/original and dimple inclination angle of 0–60 deg. Pulsating flows were generated by sinusoidally varying the volume-averaged velocity. The pulsation frequency and amplitude were changed for the Strouhal number of 0–0.60 and the root-mean-square velocity amplitude normalized by the bulk flow velocity of 0–0.14. The results showed that the surface-averaged Nusselt number and friction factor were larger for the pulsating flow case than for the steady flow case. The surface-averaged Nusselt number ratio and the friction factor increased with the Strouhal number up to the Strouhal number of 0.30. For the Strouhal number larger than 0.30, they decreased with the Strouhal number or stayed almost constant. Consequently, the heat transfer efficiency index increased with the Strouhal number. The increase in the local Nusselt number ratio due to the flow pulsation was observed at the leading-edge region of the dimples. The results of the streamlines near the dimple showed that the swirling separation bubble was located closer to the leading-edge region due to the pulsation, which resulted in the increase of the absolute values of the turbulent heat flux and the local Nusselt number ratio.

Evaluation of methods to counteract multipactor phenomena in X-band waveguides

This study investigates multipactor mitigation techniques in X-band geometries, focusing on surface modifications, surface conditioning, and additive manufacturing. Surface modifications include geometric alterations such as dimpled surfaces. Experimental results demonstrate that 3D-printed copper test pieces can achieve a multipactor onset threshold comparable to traditionally machined test pieces with appropriate sample preparation. Surface conditioning, involving rapid successions of pulses, is explored for its mitigative potential. Among assessed techniques, dimpled configurations demonstrated superior multipactor hold-off compared to non-modified components. Additionally, the conditioning of test pieces significantly increased the single-pulse threshold.

Numerical study of flow and thermal characteristics of pulsed impinging jet on a dimpled surface

This research comprehensively investigates the flow and thermal characteristics of a pulsating impinging jet over a dimpled surface. It analyzes the impact of key parameters (e.g., inlet velocity pulsation functions, pulsation frequency, amplitude, dimple pitch, dimple depth, Reynolds number) on flow patterns and heat transfer. Validated computational fluid dynamics and the Re-normalization group turbulence model are employed to accurately simulate complex turbulent flow behavior. Local and average heat transfer coefficients are calculated and compared to steady impingement cases, revealing the potential benefits of pulsation for heat transfer enhancement. The study also examines how pulsation-induced flow modulation and thermal mixing affect heat transfer mechanisms. Results indicate that combining fluctuating flow with a dimpled surface can improve heat transfer rates. In summary, increasing pulsation amplitude consistently enhances heat transfer, while the effect of frequency varies between impinging and wall jet zones.

EFFECT OF THE NANOFLUID FLOW AND EXTENDED SURFACES ON AN ABRUPT EXPANSION TUBE REGARDING THERMODYNAMIC IRREVERSIBILITY

Numerous scientists have examined circular dimpled surfaces, tubes, and other approaches for enhancing heat transfer. Moreover, the dimples' angle of attack has a substantial effect on the tube's flow and thermal features. This numerical study presents a novel approach to enhancing heat transfer rate in a tube subjected to constant heat flux by incorporating a surface dimpling strategy and evaluates three different tube layouts with elliptical dimpled fins for different working fluids such as DW and Al<sub>2</sub>O<sub>3</sub>/DW nanofluid (φ = 0.5-1.0%) by using ANSYS Fluent v2020R2 under laminar flow conditions. Under identical circumstances, the thermal performances of proposed designs are compared to those of a smooth tube, and the influence of the elliptical dimpled fin angle of attack on these parameters is determined for different Reynolds numbers (1000 ≤ Re ≤ 2000). When comparing the Nusselt number of a tube with/without elliptical dimpled fins, it is found that EDT 1 performed better. Lower Reynolds numbers are shown to result in a greater friction factor. Besides, elliptical dimpled fins promote flow mixing within the tube and the establishment of a thermal boundary layer. At a 135° attack angle (EDT 1), the 1.0% Al<sub>2</sub>O<sub>3</sub>/DW nanofluid is found to be the best-performing nanofluid in the dimpled tube, improving Nu by up to 44.56%. Furthermore, ff presented an increase of 29.18% when comparing ST and EDT 1 flowing 1.0% Al<sub>2</sub>O<sub>3</sub>/DW at Re = 2000, while total S<sub>gen</sub> is diminished by 37.75% in the same conditions.

On the reductions of airfoil broadband noise through circular dimples

This paper presents systematic and comprehensive experimental investigations into the use of circular dimples to reduce broadband noise on airfoils. The dimples are introduced on the suction surface of symmetric NACA airfoils with different thickness ratios for the control of broadband noise over a wide range of frequencies. The effects of dimple arrangements (inline and staggered) and their locations (1/3rd and 1/5th of the chord from the leading edge) on the noise characteristics are studied in detail to determine the best dimple arrangement, location, and airfoil geometry for low noise. Results indicate that the inline arrangement of dimples positioned at 1/3rdof the chord from the leading edge provides significant noise reductions of about 6–8 dB, particularly from mid to high frequencies. When the dimples are shifted to 1/5thof the chord from the leading edge, the degree of noise reduction decreases, which indicates that the noise reduction performance of airfoils decreases when the dimples are introduced near the leading edge. Further, it reveals that the inline arrangement of dimples on thicker airfoils provides superior noise reductions (6–8 dB) as compared to thinner airfoils (4–6 dB). Flow visualization reveals that the flow field is inconsistent throughout the span of the dimpled surface due to the evolution of small vortices from the dimple corner. It enhances mixing and accelerates flow structures passing over the dimpled surface, which is indicated by the presence of a high degree of spanwise decoherence/phase lag in the hotwire measurements. The presence of higher spanwise decoherence/phase lag due to the presence of the inconsistent flow field leads to significant reductions of far-field noise in dimpled airfoils. Further, the inline arrangement of dimples exhibits much higher levels of noise reduction as compared to staggered ones, due to the presence of a greater degree of spanwise decoherence/phase lag resulting from the substantial disruption of the flow field in the spanwise direction.

A numerical investigation into thermo-fluid characteristics of pulsating jet impingement on a dimpled surface

A two-dimensional axisymmetric numerical model is employed to study the flow and heat transfer attributes of the pulsating air jet impingement on a dimpled surface. The results are compared with the steady jet impingement. The results are examined at a fixed Reynolds number of 5000, over a Strouhal number range of 0.1–0.5, and pulsation amplitude of 15% and 25% for three different nozzle-to-surface separations (z/d = 2, 6, and 10). The pulsation amplitude of 15% has a minor effect on heat transfer from the dimpled surface. However, at 25% pulsation amplitude, significant improvements in the heat transfer rates are obtained in many combinations of jet Strouhal number and jet surface spacing. The value of the optimum Strouhal number provides conditions for the maximum heat transfer rate, which varied with nozzle-to-surface separation distances. Combinations of higher separations and lower Strouhal numbers (and vice versa) produced optimum heat transfer among the cases considered in the present study. The maximum improvement (17.41%) in the average heat transfer over the steady jet was found at z/d = 10 for pulsation at St = 0.1, while at z/d = 6, St = 0.2 provides the highest heat transfer rate. It is urged that the vortices formed in pulse jet close to the natural frequency of vortex formation provide a conducive environment for the vortex growth and their existence, significantly affecting the jet entrainment, mixing, and jet spread, which eventually play the decisive factor in determining the overall heat transfer rates on the dimpled surface.

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.

The Dimple Effect on Flettner Rotors for the Wind Power Drive System of a Sailing Merchant Ship

Wind-powered ship propulsion is investigated in relation to reducing ship exhaust emissions, such as the application of Flettner rotor systems. How to increase rotor thrust's contribution to ship speed is an intriguing research question. This paper investigates the influence of dimples on the surface of a Flettner rotor on its ability to generate thrust for a ship. The CFD code, which includes the Reynolds average Navier-Stokes equation and the k-ω model for turbulent flow, is utilized to analyse three rotor designs: one with a smooth surface and two with dimple surfaces of varying diameters. Lift and drag are computed by taking into account varying spin ratios and the incoming wind angle. The CFD results are validated by comparing them to experimental data gathered in a wind tunnel and determining that their error is less than 5%. The addition of dimpled surfaces to the rotor increases lift by 69.6% and decreases drag by 14.8% due to the smooth surface. For the case study, the two rotors are mounted on the sailing ship 900 DWT and can generate the most thrust to give the ship a speed of 8.5 knots with a spin ratio of 4 and apparent wind angles of 90o or 270o relative to its speed.

Cooling Ability of Smooth and Dimpled Surfaces determined by Experiment and Numerical Simulation

Cooling Ability of Smooth and Dimpled Surfaces determined by Experiment and Numerical Simulation