Spherical Dimples Research Articles

Influence of Spherical and Pyramidical Dimples and Bumps on Airfoil Performance in Subsonic Flow

In this study, surface features such as dimples and bumps are introduced to the surface of a NACA 0012 airfoil to study their effect on boundary layer separation, particularly at high angles of attack. Six modified airfoils were designed with dimples and bumps of spherical and pyramidical shapes. A computational fluid dynamics (CFD) analysis was conducted on these models at subsonic flow using Ansys Fluent. The analysis used the Shear Stress Transport k – ω turbulence model at a varying angle of attack (AOA) from 0 to 15°. The velocity contours and streamlines were generated. Also, the lift coefficient, drag coefficient and the lift-to-drag performance ratio were computed and analyzed. The results showed that all surface modifications led to delayed flow separation and flow recirculation. All surface modification also resulted in a decrease in drag at 15°. All designs, except pyramidical protrusions, increased the lift-to-drag ratio (L/D) performance at 15°. It was found that dimples are better than bumps and spherical features are better than pyramidical ones.

Self-similar decay of the drag wake of a dimpled sphere

Recent experimental results from the drag wake of a dimpled sphere show that the observed wake growth and decay are inconsistent with a widely accepted, 100-year-old scaling law for these quantities. While previous experiments have found a similar discrepancy just behind the wake source, this work shows that the new law holds over a full decade in scaling of distance. The theory behind the existing wake scaling law is revisited, and a faulty assumption which may account for the discrepancy is identified.

“Effect of laser textured dimples on tribological behavior of piston ring and cylinder liner contact at varying load”

Surface texturing is playing a vital role in reduction of friction and wear of sliding components in an engine. The dimple shape, dimple size and area ratio are important texture parameters which needs to be studied for the analysis of friction and wear. The laser surface textured OEM (Original Equipment Manufacturer) piston ring segments with different dimple diameters like 150 µm, 300 µm, 500 µm respectively for spherical dimples and different dimple sizes like 150 µm, 300 µm, 500 µm for square shaped dimples shapes were used for investigating the tribological performance of OEM piston ring and cylinder liner segments at a constant speed of 1500 rpm and load of 80 N and 150 N. A linear reciprocating tribometer was used to investigate the tribological performance of the pair at a temperature of 150 °C with the SAE20W40 engine oil. The results shows that there is reduction in the coefficient of friction by 33.56%, 19.69% and 18.57% for 150 µm, 300 µm and 500 µm dimple diameter piston ring segments respectively. Also the square shaped dimples of size 150 µm, 300 µm and 500 µm were tested at 80 N and 150 N loading conditions and observed that there is sudden reduction in the coefficient of friction for 150 µm and 500 µm textured ring segments due to increase in loading from 80 N to 150 N. The reduction in the coefficient of friction was observed 14.73% and 6.14% respectively for 150 µm and 500 µm respectively. Thus through this work an attempt has been made to investigate the tribological performance of ring and liner pair at simulated conditions with laser textured dimples.

Researches Concerning the Lubrication of Profiled Surfaces in Slip Boundary Conditions

The paper investigates the squeeze film process for non-Newtonian fluids between two circular parallel profiled surfaces. The lower surface is characterized by the existence of a cylindrical or spherical dimple in the center, which is specific for profiled surfaces by texturing. In order to integrate the Reynolds equation, the slip boundary conditions on the upper surface have been assumed. Finally, the pressure distribution and the loading capacity of the non-Newtonian film are obtained.

Evaluation and optimization of axial piston pump textured slipper bearings with spherical dimples based on hybrid genetic algorithm

The spherical dimple texture have been designed on the rough surface of slipper bearing for improving the lubrication performance in axial piston pump. In this work, we have investigated and optimized the structure parameters of textures to obtain minimum friction coefficient as well as maximum loading capacity. Optimization of the geometry parameters of dimple texture by the integration of a hybrid evolutionary optimization method based on the sequential quadratic programming and genetic algorithm. Parametric analysis is applied for the evaluation of the impact level of geometry parameters on lubrication performance. The results shows that hybrid genetic method can be used for the optimization of slipper bearing with spherical dimple textures to generate lower friction coefficient and greater capacity of load carrying. The carrying capacity and friction coefficient of slipper bearing demonstrate a 64.8% and 4.5% improvements after multi-objective optimization. When the texture radius and depth are set to 18 µm and 0.8 µm, there exists the greatest load carrying force and lowest friction coefficient. This work presents a key designing guide for axial piston pump textured slipper bearings.

Heat Exchange Between Air and a Liquid Film Flowing Down Along a Profiled Surface

The laws of heat exchange between air and a liquid film flowing down along a solid surface with spherical dimples were investigated experimentally. Three cases of heat transfer were considered: quiescent air, air – liquid counter flow, or their cross flow. In all cases, a significant growth of the heat exchange intensity, especially at air – liquid cross flow, was observed. This is caused by the substantial turbulization of flow and mixing of liquid layers in the film. As a result, it was established that surface profiling (manufacture of dimples) under the optimal conditions leads to an increase in heat exchange intensity by an unexpended factor of 2.5 – 2.8 as compared with a smooth surface, other conditions being equal. The obtained experimental data were generalized in the form of dimensionless dependences Nu vs. Re. The best heat transfer surface can be recommended for use in different heat exchangers.

Experimental and numerical approach to resolve particle deposition on dimpled heat transfer surfaces locally and temporally

When optimizing the performance of heat exchangers, it can be advantageous to introduce structures, such as dimples or ribs on the surface. One especially advantageous solution in terms of thermo-hydraulic efficiency is to use surfaces with spherical dimples, where heat transfer is increased at an only minor pressure loss increase. However, this surface structuring also influences the deposition of particles in aqueous suspensions. The resulting fouling is a serious problem for the design and operation of heat exchangers and can cause significant economic and ecological damage. Despite the knowledge of the importance of fouling and the resulting disadvantages, this knowledge has not been incorporated systematically into the design process, and the criteria of maximizing heat transfer and reducing hydraulic losses remain in the foreground during the design of heat exchangers.In this study, dimpled surfaces with a depth to diameter ration of 0.18,0.26 and 0.35 were evaluated by measuring and comparing their heat transfer ability and pressure loss with and without the occurrence of particulate fouling from aqueous suspensions. The fouling resistance is used to assess the reduction in heat transfer efficiency due to particulate depositions, both local and time resolved. For the first time, a row of dimples is quantitatively assessed regarding its heat transfer efficiency under fouling conditions, and the thermo-hydraulic efficiency is determined experimentally. The results are matched against numerical investigations. The interactions of turbulent flow structures, heat transfer and particulate fouling in aqueous suspension are determined for dimpled surfaces.

Computational fluid dynamics‐based optimization of dimpled steam cracking reactors for reduced CO2 emissions

AbstractSpherical dimples in cylindrical tubes enhance heat transfer and lead to a more uniform radial temperature profile. To combine these positive properties with a low pressure drop, a single dimple was optimized through a genetic algorithm. Multiple design parameters such as width, height, and curvature of the dimple were investigated. Heart‐shaped dimples outperformed spherical dimples. Three‐dimensional reactive simulations of a Millisecond propane steam cracking reactor showed that both the spherically dimpled and the heart‐shaped dimpled coil positively affect the light olefin selectivity, mainly through an increase in propylene selectivity. The optimized dimples could reduce the high pressure drop penalty by 21%. Run length simulations proved that the optimized dimple shape results in an additional run length extension of 18%. Next to this, the fuel rate consumption can be decreased by 6% compared to a bare coil, which could theoretically result in 4% less CO2 emissions.

Plasmonic Janus Microspheres Created from Pickering Emulsion Drops.

Metal nanostructures have been created in a film format to develop unique plasmonic properties. Here, well-defined metal nanostructures are designed on the surface of microspheres to provide plasmonic microgranules. As conventional techniques are inadequate for nanofabrication on spherical surfaces, photocurable emulsion drops with a regular array of silica particles are employed at the interface to create periodic nanostructures. The silica particles, originating from the dispersed phase, fully cover the interface by forming a non-close-packed hexagonal array after drop generation, and slowly protrude to the continuous phase during aging while their interparticle separation decreases. Therefore, hexagonal arrays of spherical dimples with controlled geometry and separation are created on the surface of microspheres by photocuring the drops and removing the particles. Directional deposition of either aluminum or gold results in a continuous film with a hexagonal array of holes on the outermost surface and isolated curved disks in dimples, which renders the hemisphere of microspheres plasmonically colored. The resonant wavelength is controlled by adjusting the aging time, metal thickness, and size of silica particles, providing various plasmonic colors. This granular format of the plasmonic Janus microspheres will open a new avenue of optical applications including active color pixels, optical barcodes, and microsensors.

Experiments and Stress-Blended Eddy Simulations of turbulent flow over edge-rounded dimples

Experiments and numerical simulations have been conducted to reveal the detailed turbulent flow and heat transfer characteristics over the surfaces with spherical dimples with different edge schemes, including the sharp edge, the fully rounded edge and the upstream rounded edge. The dimples have the same depth-to-diameter ratio of hd/d = 0.2. Steady-state heat transfer experiments were carried out for the turbulent flow over the three dimpled surfaces within the Reynolds number range of 10,000 ≤ ReDh ≤ 60,000, and the globally averaged heat transfer enhancements and friction factor ratios were obtained. Furthermore, transient liquid crystal thermography technique was used to obtain the local heat transfer characteristics on the dimpled surfaces. In addition, numerical simulations were carried out with a newly developed hybrid RANS-LES method of Stress-Blended Eddy Simulation (SBES) and the local heat transfer enhancement, detailed flow structure and turbulence statistics data were obtained and analyzed. The experimental results show that the best thermal performance is achieved by the dimples with upstream rounded edge, indicating an increase of about 11.4% in total heat transfer enhancement and an increase of about 5.2% in pressure loss when compared to the conventional dimples with sharp edge. The numerical results show that the flow over the dimpled surfaces can be divided into five zones: the mainstream zone, the recirculation zone, the Kelvin–Helmholtz vortex zone, the upwash zone and the rear separation zone, and each zone has its unique flow features. It is also found that the lowest pressure is achieved by the dimple with fully rounded edge, which is due to the absence of flow separation behind the rear edge of the dimple. The upstream-edge-rounded dimples show the reduced recirculation flow in the recirculation zone, the enhanced impingement and upwash flow in the upwash zone near the rear edge. These cause the stronger production of turbulent kinetic energy behind the rear edge, which are responsible for the higher thermal performance.

Assessment of thermo-hydraulic performance of inward dimpled tubes with variation in angular orientations

This paper presents a numerical investigation and assessment of thermal and hydraulic performance of dimpled tubes of varying topologies at constant heat flux of 10kW/m2 and Reynolds numbers ranging from 2300 to 15,000. The performance of the tubes consisting of conical, spherical and ellipsoidal dimples with equivalent flow volumes were compared using steady state Reynolds Averaged Navier Stokes simulations. The ellipsoidal dimples, in comparison to other dimple shapes, demonstrated large increment in heat transfer rate. The variation in the orientation of the ellipsoidal dimples was examined to further improve thermal and hydraulic performances of the tube. A 45° inclination angle of ellipsoidal dimple, from its major axis, increased the thermo-hydraulic performance by 58.1% and 20.2% in comparison to smooth tube and 0° ellipsoidal dimpled tube, respectively. Furthermore, Large Eddy Simulations (LES) were carried out to investigate the role geometrical assistance to fluid flow and heat transfer enhancement for the 45° and 90° ellipsoidal dimpled tubes. LES results revealed a flow channel of connected zones of wakes which maximized fluid-surface contact and therefore enhanced the thermal performance of the tube. In addition, correlations for Nusselt number and friction factor for all angular topologies of ellipsoidal dimpled tube have been proposed.

Aerodynamic performance enhancement of horizontal axis wind turbines by dimples on blades: Numerical investigation

In this study, the effect of dimpled surface blade on the performance of V47-660 kW horizontal axis wind turbine is numerically investigated. For this purpose, the suction sides of the wind turbine blades are passively modified using some spherical dimples. The governing continuity and momentum equations are solved using an incompressible Reynolds-Averaged Navier-Stokes solver and k−ω Shear-Stress Transport turbulent model. The effect of radius, location, and quantity of dimples on the aerodynamic performance of the wind turbine including torque and power generation, flow separation, and thrust load are studied to find an appropriate case. Then the effect of blade pitch angle and wind speed is also examined on the best-dimpled blades. The results show that dimples could be effective in increasing the torque and power generation if they are designed appropriately. Obtained results reveal that for the best situation, dimples could improve the generating torque by around 16.08%.

Heat Transfer Enhancement in the Fields of Mass Forces, Pressure Gradients, Velocity Fluctuations, and Impinging Jets

Results are discussed of the experimental investigations into the effect of mass forces, longitudinal pressure gradient, and other factors on convective heat transfer enhancement in a turbulent flow. Heat transfer intensifiers include protrusions on and dimples in a heat transfer surface. It has been demonstrated that, depending on the intensifier type, the affecting factors have different effects on the convective heat transfer. The investigated combinations of affecting factors and intensifiers yield enhancement, deterioration, or no change in heat transfer. In a flow over concave or convex surfaces of moderate curvature with semispherical dimples, the centrifugal mass forces have the same effect on heat transfer as they do in the flow over smooth curvilinear surfaces. At the same time, in a flow over concave or convex surfaces with semispherical protrusions or transverse ribs, the mass forces do not affect the heat transfer. The investigation of the effect of a streamwise pressure gradient in a separation flow over spherical dimples has demonstrated that relaxation processes in the return flow in a spherical dimple results in a change in the of heat transfer rate opposite in sign compared to the effect pressure gradient in a flow without heat transfer intensifiers. It has been established that with the same relative dimensions of dimples and the same experimental conditions, the array of diffuser-type dimples can enhance heat transfer by a factor of 1.4 as compared with heat transfer in a flow over dimples with separation. The same heat transfer augmentation can be obtained in the array of spherical dimples from the imposition of periodic velocity fluctuations on a turbulent flow. It is shown that calculation of heat transfer for a flow over a surface with heat transfer intensifiers under the effect of disturbing factors sometimes requires correction to the used correlations that consider the effect of these factors.

Experimental analysis on turning of AISI 4340 steel using non-textured, dimple textured and MoS2 coated dimple textured carbide cutting inserts at the rack surface

Forming of dimple texture patterns on the rack surface of titanium coated carbide cutting inserts will produce a positive impact on tribological alterations at the cutting insert-workpiece and cutting insert-chip interfaces. The aim of this research is to identify the effect of MoS2 coated and uncoated dimple textured carbide cutting inserts compared to non-textured cutting inserts during dry machining of the AISI 4340 steel. Straight-line patterns of spherical dimples with various diameter, depth, and density ratio were fabricated on the rack face of the cutting insert using the laser marking method. The responses of cutting insert wear at the cutting insert – chip interface, cutting temperature generated during machining and surface quality of the machined surface are analyzed experimentally. Experimental results show that there is a positive influence on machinability and cutting insert wear decreased by about 10% on optimizing the dimple diameter of 90 µm and the depth of 60 µm. Optimizing the dimple diameter and depth plays a significant role in the dimple texturing forming of the carbide insert.

Experimental investigation of flow and heat transfer in a channel with dimpled plate

This study presents the experimental investigation on the effect of dimpled arrangements on flow and heat transfer characteristics. Three plate surfaces were prepared (smooth, evenly distributed spherical dimples and unevenly distributed spherical dimples) and were placed successively in a channel. The unevenly distributed dimpled plate had the same dimple density with the evenly distributed dimpled plate but had varying transverse pitches to concentrate the dimples at mid-plate in flow direction. Data obtained from the experiment were analysed to determine the performance of each dimpled plate channel. It was observed that the average Nusselt number due to the heat interaction with the air-flow increases with the Reynolds number. The evenly and unevenly dimple plate channels had respectively, 75.7% and 91.8% increase in Nusselt number over the smooth channel. The flow friction factors of the evenly and unevenly dimple plate channels were merely more than that of smooth plate channel by 0.59% and 0.67%, respectively. Thus, the unevenly dimple plate channel had the highest overall thermal-hydraulic performance, followed by the evenly dimple plate channel.

Heat Transfer Enhancement in a Microtube with an In-Line Package of Spherical and Inclined Oval-Trench Dimples during Transformer-Oil Pumping

The vortex-induced intensification of the laminar (at Re = 308) flow of transformer oil and heat transfer in a microtube with an in-line package of dimples with a depth of 0.26 of the tube diameter is calculated based on the Navier–Stokes equations and the energy equation. The heat-transfer increased by nearly 20 times for eight inclined oval-trench dimples spaced around the circumference and by 13 times for spherical dimples equal in area in comparison with that for a smooth tube; moreover, the relative hydraulic losses grow by approximately 30 and 60%, respectively.

FCC ↔ BCC Phase Transitions in Convex and Concave Hard Particle Systems.

Solid-solid transitions are ubiquitous in nature and are important for technology. Understanding and exploiting transitions are complicated by the fact that multiple transition pathways can exist between small unit cell structures such as face-centered cubic (FCC) and body-centered cubic (BCC). By symmetry, FCC ↔ BCC transitions can occur via a pair of continuous transitions or via a discontinuous, first-order transition. However, how to, or whether it is possible to, select between pathways is unclear. Here, we use particle shape change to induce FCC ↔ BCC transitions in systems where particle valence is malleable. Though some particle shapes can eliminate metastable HCP stacking faults, we find that for both convex and concave particles, transitions are first-order.

Thermal performance investigation of the flat and spherical dimple absorber plate solar air heaters

An experimental investigation has been conducted to determine the thermal performance of a spherical dimple plate solar air heater (SDPSAH) and a flat plate solar air heater (FPSAH) at distinctive environmental conditions at IIT (ISM) Dhanbad (23.79°N), India. The study has been conducted for a mass flow rate of air varying from 0.009 kg/s to 0.028 kg/s. The results indicate that the SDPSAH shows the highest heat transfer rate, which is about 1.51 to 1.64 times higher than the corresponding FPSAH. The maximum rise of the outlet temperature of the SDPSAH is about 4.6 °C greater than the corresponding FPSAH for 0.009 kg/s mass flow rate of air. The instantaneous thermal efficiency of the SDPSAH is about 23.45% to 35.50% greater than the corresponding FPSAH.

On the origin of the drag force on dimpled spheres

It is well established that dimples accelerate the drag crisis on a sphere. The result of the early drag crisis is a reduction of the drag coefficient by more than a factor of two when compared to a smooth sphere at the same Reynolds number. However, when the drag coefficients for smooth and dimpled spheres in the post-critical regime are compared, the latter is higher by a factor of two to three. To understand the origin of this behaviour, we conducted direct numerical simulations of the flow around a dimpled sphere, which is similar to commercially available golf balls, in the post-critical regime. By comparing the results to those for a smooth sphere, it is found that dimples, although effective in accelerating the drag crisis, impose a local drag penalty, which contributes significantly to the overall drag force. This finding challenges the broadly accepted view that dimples only indirectly affect the drag force on a sphere by energizing the near-wall flow and delaying global separation.

Numerical simulation of turbulent nanofluid flow in the narrow channel with a heated wall and a spherical dimple placed on it by using of single- phase and mixture- phase models

The purpose of this study is to numerical simulation of turbulent nanofluid flow in the narrow channel with a heated wall and a spherical dimple placed on it by using of single- phase and mixture- phase models. The nanofluid flow is also simulated with homogeneous and two-phase models for volume fraction of nanoparticles 0, 2, and 4%. The results contain pressure coefficient, friction factor, heat transfer coefficient, and various contours. The results show that by increasing the depth of the cavity, the pressure coefficient becomes more negative (smaller in magnitude), indicating an amplified velocity gradient and the pressure coefficient decreases at all depths, indicating that the vortices have become stronger. Also, at a fixed cavity depth, for every 2% increase in the volume fraction of nanoparticles, the local heat transfer coefficient is enhanced by 2%. By inspection of results, we showed that, the two-phase (mixture) model overestimates the increase in the heat transfer, which is the same for different depths. Finally, the changes in the pressure coefficient of the water-Alumina nanofluid are similar to the changes in the pressure coefficient of the Water-Cu nanofluid. Furthermore, changing the volume fraction of nanoparticles does not affect the vortex strength and pressure distribution.