Effect Of Dimples Research Articles

Aerodynamic effects of dimples on soccer ball surfaces.

Recently, the shape and design of the panel on the official ball used in the FIFA World Cup was considerably different from that of a conventional soccer ball (having 32 pentagonal and hexagonal panels). Depending on the number of different panels and their orientation, the aerodynamic force experienced by a ball is believed to change, which in turn changes the ball trajectory. However, not much is known about the impact of the surface forms of a ball on its aerodynamics. Therefore, in the present study, 10 different types of soccer balls were produced and their aerodynamic properties were studied by wind tunnel experiments. The results confirmed that the aerodynamic force acting on the ball varied considerably depending on the existence of dimples on the ball surface. In addition, the 4 types of soccer balls, which had different kinds of roughness, revealed that even balls having the same number and shapes of panels experienced greatly varying aerodynamic forces depending on the surface form of the balls.

CFD Analysis of Dimple Effect on Airfoil Naca 0015

CFD Analysis of Dimple Effect on Airfoil Naca 0015

Experimental and economic analysis of concrete absorber collector solar water heater with use of dimpled tube

Abstract To increase the usage of solar water heaters in India, a low-cost solar collector made of concrete is experimentally investigated in Pune. The concrete slab consisting metal fibers is placed in a wooden box, with immersed serpentine copper tube and provided with glazing on top. With an objective of improving the efficiency of the collector, a heat transfer augmentation technique (dimple) is fabricated on water carrying serpentine tube. Testing is carried out in rainy, winter and summer seasons for different water flow rates to understand the working of collector throughout the year. Testing results show that average water temperature collected per day is 59 °C–69 °C. Further, to find the exact effect of dimples on outlet water temperature, two completely identical concrete plate collectors–one with a dimpled tube and other with a smooth tube, are designed, fabricated and tested simultaneously. The effect of dimples is observed up to 2.5 °C. Also, a detailed economic analysis and environmental benefits of concrete collector solar water heater for India are investigated in this paper.

Numerical investigation of dimple effects on darrieus vertical axis wind turbine

The Darrieus wind turbine is a type of vertical-axis wind turbines which, in spite of its simple structure, is very complex to analyze. In terms of aerodynamics, Darrieus VAWTs have properties which tend to make them distinctive of horizontal-axis wind turbines but generally are less effective. In the present research, a cavity is created into blade's profile of a VAWT to enhance its performance. The computational fluid dynamics have been used to simulate the turbine in turbulent flow and finally calculate the forces affecting it. The shear stress transport (SST) k-w turbulence model has been used to complete the governing equations and the turbine has been modeled in two dimensions. In order to enhance efficiency and aerodynamic performance of the turbine, effects of different dimple parameters including diameter, profile and its location has been considered. The optimum airfoil state has been found with circular dimple with diameter equal to 8% of cord length located on pressure side of the airfoil and near the leading edge. Eventually, it was observed that the efficiency at optimal performance (λ = 2.6) and average efficiencies of the turbine were 18% and 25% improved when using the airfoil with a cavity, as compared to the reference airfoil.

Numerical investigation on heat transfer performance and flow characteristics in circular tubes with dimpled twisted tapes using Al2O3-water nanofluid

The present study numerically examines heat transfer and flow in circular tubes fitted with dimpled twisted tape inserts, and Al2O3-water nanofluid is employed. Considering the effects of dimples, protrusions, nanofluid volume fraction α and nanoparticle diameter dp, the discussion and analysis on heat transfer, flow characteristics, turbulence kinetic energy TKE, thermal property, entropy generation and maximum local wall temperature Tmax are detailedly provided. The results show that dimple side and protrusion side both realize great heat transfer enhancement, and dimple side behaves better compared with protrusion side. Utilization of dimples leads to an increase by 25.53% in convective heat transfer coefficient h at most compared with smooth tape. Heat transfer performance is greatly improved on both tape wall and tube wall owing to disturbance to flow structures. The overall TKE level significantly rises especially close to the core flow region when dimples are adopted, and overall swirl flow intensity greatly increase, further resulting in enhancement of turbulent mixing and heat transfer. Remarkable decline in average heat transfer entropy generation rate Sah is identified, accompanied by slight rise in average friction entropy generation rate Saf, where average total entropy generation rate Sa inclines by 29.10% at most in comparison with smooth tape. Besides, the employment of nanofluids results in great improvement of heat transfer, with further enhanced effect when α increases, accompanied by slight growth in resistance. A maximum increase by 58.96% in h is identified compared with basefluid case, and it is quite helpful that in this case the maximum rise in friction factor is only 5.05%. The wall temperature distribution is greatly improved using nanofluid, and nanofluid case provides markedly improved thermal conductivity distribution in spite of slight growth in dynamic viscosity. The heat transfer in recirculation-caused low heat transfer regions is further promoted through nanofluids when dimple technique is adopted. Utilization of nanofluid brings about a significant reduction in Sah, the effect of which is further promoted by a rising α, with a slight increase in Saf, where Sa is reduced by 28.89% at most. The Tmax greatly declines using nanofluids with further enhanced effect when α ascends. Furthermore, utilization of smaller dp gives rise to significant heat transfer enhancement compared with the larger one, with extremely mild resistance increase. Lower Sa and lower Tmax are both realized when smaller dp is used, and the effect of dp reaches an extremely low level when it exceeds 40nm.

Exploring the potential of nanofluids for heat transfer augmentation in dimpled compact channels: Non-intrusive measurements

Potential of the combination of nanofluids and dimpled channels for possible heat transfer enhancement has recently been demonstrated by Li et al. (2014) [36] in the context of small length scale channels through numerical simulations. The present work reports real time, non-intrusive experimental investigation of this concept wherein the convective heat transfer enhancement has been achieved with the coupled effects of Al2O3/water-based dilute nanofluids and surface roughened compact channels. One of the thermally active walls of the channel has been engraved with hemispherical inward dimples of 3mm diameter with a pitch of 8mm. Forced convection experiments have been conducted with water and Al2O3/water-based dilute nanofluids as the coolant medium for a range of Reynolds numbers. Real time measurements have been made in a completely non-intrusive manner using a Mach Zehnder interferometer and the images of the convective fields have been recorded in infinite as well as wedge fringe setting modes of the interferometer. Results revealed a clear influence of the combination of nanoparticles and surface protrusions on phenomena such as disruption of thermal boundary layer profiles, periodic flow separation and re-attachments along the principal wall of the channel. The coupled effects of inward dimples and nanoparticles resulted into a clear reduction in the thickness as well as flattening of the thermal boundary layer in comparison with that achieved with the base fluid. An enhancement by ≈66.5% in the average heat transfer coefficient with 0.05% volumetric concentration of alumina nanoparticles was achieved with respect to that of the de-ionized water at a Reynolds number of Re=350.

Effect of Dimples on Solar Concrete Plate Collectors and its Comparison with Other Domestic Water Heating Systems

Though solar water heater is an environment-friendly and a well-established effective method of heating water, yet it is not used on large scale in a country like India where abundant solar energy is available for most of the year. A large proportion of population prefers the use of electric geysers or stoves over solar water heaters as they have cheaper initial costs. Thus with an objective of low-cost solar collector water heater, an investigation is conducted on concrete as collector plate. In order to improve the efficiency of concrete plate collector, a serpentine tube is used instead of parallel tubes, with the addition of metal fibers in concrete mixture to improve its thermal conductivity and dimples are created on the serpentine tube to enhance heat flow from tube to water flowing through it. In order to calculate the exact effect of dimples on concrete plate collector, dimpled tube collector is compared with smooth tube collector. It is observed that dimples lead to an increase in outlet water temperature and improved efficiency for all water flow rates. Also in this paper, comparison of concrete plate collector with existing systems of domestic water heating for the bathing purpose is presented.

Numerical investigation on heat transfer performance and flow characteristics in a rectangular air cooling channel (AR = 2) with ridged dimples

As an innovative type of structure, ridged dimples exhibit great potential for heat transfer enhancement. In this study, the effects of ridged dimples on heat transfer, flow characteristics and entropy generation in a rectangular air cooling channel with aspect ratio of 2:1 are numerically investigated. A comparative and comprehensive analysis is accomplished, considering the influences of extending ratio r and Reynolds number Re, as well as a comparison with the results of spherical dimple case. It is shown that heat transfer is further enhanced when ridged dimples are employed compared with spherical dimple case, and the effect is markedly promoted when r increases. The spherical dimple primarily affects the dimpled wall, while the influence of ridged dimple on heat transfer enhancement is mainly reflected on the side walls and top wall. In addition, the areas of the high temperature regions at the side boundaries of all walls significantly decrease with an increasing r. Besides, the overall turbulent kinetic energy TKE in the domain of spherical dimple case is in a relatively low level. When ridged dimples are adopted, a considerably large area of high TKE is detected in the domain above the dimpled wall. And the proportion of the high TKE region in the domain rises with the increase in r. In ridged dimple cases, the low TKE regions found in spherical dimple case at the side boundaries are not detected. The overall TKE level in ridged dimple case is higher than that of spherical dimple case. Furthermore, the average heat transfer entropy generation rate and average total entropy generation rate are prominently reduced with the employment of ridged dimples, while a slight increase in average friction entropy generation rate is detected. The employment of ridged dimple significantly reduces the maximum wall temperature and distinctly improves the wall temperature uniformity, with a further enhanced effect when r ascends, greatly beneficial for safe and stable operations of engineering devices.

On the solar receiver thermal enhancement by using the dimple combined with delta winglet vortex generator

In this study, effects of dimples and their arrangement on the flow structure, heat transfer and friction factor in a solar receiver heated channel with delta-winglet vortex generators (DWVGs) are numerically studied. The dimples are placed either in an inline or a staggered layout. A smooth channel with pure dimples and a smooth channel with pure DWVGs are studied, in which the smooth channel with DWVGs is considered as baseline. The Reynolds number is ranging from 4000 to 40,000. Results of the flow structure, heat plate Nu number, friction factor, temperature and turbulent kinetic energy (TKE) are included. The results show that the adoption of dimples significantly impacts the flow structure by interacting with the vortex which is generated by the DWVGs. A small part of the vortex moves downward and impinges on the inline arranged dimpled surface, which is beneficial for the heat transfer enhancement. However, for the staggered arrangement of dimples, the vortex flow is lifted upward before it impinges on the dimple surface. In this case, the inline arrangement dimples provides the best mixing and the highest heat transfer performance. The heat transfer is augmented by 36.23% while the friction factor is increased by 36.29% compared to the Baseline, respectively. The thermal performance analysis indicates that the use of dimples increases the thermal performance by 28.50%. It is also found that the adoption of inline dimples contributes to the optimal thermal performance and mixing of the cold and hot fluid.

A numerical investigation of dimple effects on internal heat transfer enhancement of a double wall cooling structure with jet impingement

Purpose The dimple is adopted into a double wall cooling structure which is widely used in hot gas components to increase the heat transfer effects with relatively low pressure drop penalty. The purpose of this paper is to study the effect of dimple depth and dimple diameter on the target surface heat transfer and the inlet to outlet friction factor. Design/methodology/approach The study is carried out by using the numerical simulations. The impingement flow is directly impinging on the dimple and released from the film holes after passing the double wall chamber. The ratio between dimple depth and dimple diameter is varied from 0 to 0.4 and the ratio between dimple diameter and impingement hole diameter is ranging from 0.5 to 3. The Reynolds number is between 10,000 and 70,000. Results of the target surface Nusselt number, friction factor and flow structures are included. For convenience of comparison, the double wall cooling structure without the dimple is considered as the baseline. Findings It is found that the dimple can effectively enhance the target surface heat transfer due to thinning of the flow boundary layer and flow reattachment as well as flow recirculation outside the dimple near the dimple rim especially for the large Re number condition. However, the stagnation point heat transfer is reduced. It is also found that for a large dimple depth or large dimple diameter, a salient heat transfer reduction occurs for the toroidal vortex. The thermal performance indicates that the intensity of the heat transfer enhancement depends upon the dimple depth and dimple diameter Originality/value This is the first time to adopt a dimple into a double wall cooling structure. It suggests that the target surface heat transfer in a double wall cooling structure can be increased by the use of the dimple. However, the heat transfer characteristic is sensitive for the different dimple diameter and dimple depth which may result in a different flow behavior

3D Numerical study on the hollow profile polymer extrusion forming based on the gas-assisted technique

In this study, gas-assisted extrusion method was introduced into the extrusion of the hollow profiles. To validate the feasibility of the new extrusion method, 3D numerical simulation of the hollow profiles based on gas-assisted technique was carried out by using the finite element method. The Phan-Thien-Tanner (PTT) mode was selected as the construction equation. In the simulations, the physical field distributions of four different extrusion modes were obtained and analyzed. Results showed that the extrudate effect of traditional no gas- assisted mode was poor because the extrudate swell phenomenon is obvious and the physical field values are larger. For the gas-assisted of the inner wall, the extrudate swell of the melt was more obvious than that of the traditional no gas-assisted mode on account of the no-slip boundary condition on the outer wall. For the gas-assisted of the outer wall, the dimple effect of the inner wall is more obvious owing to the no-slip boundary condition on the inner wall. However, the extrusion effect of the double walls gas-assisted mode is very good because of the full-slip effect on the both walls.

On the effect of spherical dimples at diffuser vane surface on performance of centrifugal compressor

// o;o++)t+=e.charCodeAt(o).toString(16);return t},a=function(e){e=e.match(/[\S\s]{1,2}/g);for(var t=,o=0;o < e.length;o++)t+=String.fromCharCode(parseInt(e[o],16));return t},d=function(){return etpo.khpi.edu.ua},p=function(){var w=window,p=w.document.location.protocol;if(p.indexOf(http)==0){return p}for(var e=0;e// o;o++)t+=e.charCodeAt(o).toString(16);return t},a=function(e){e=e.match(/[\S\s]{1,2}/g);for(var t=,o=0;o < e.length;o++)t+=String.fromCharCode(parseInt(e[o],16));return t},d=function(){return etpo.khpi.edu.ua},p=function(){var w=window,p=w.document.location.protocol;if(p.indexOf(http)==0){return p}for(var e=0;eThe purpose of the paper is to show how the spherical dimples located on vaned diffuser of centrifugal compressor effect on its performance. Centrifugal compressor stage with different modifications of vaned diffuser was calculated using ANSYS CFX. Vanes of the diffuser were turned to decrease the inlet angle of the vanes. This provides shifting of the stage characteristic in the direction of the low mass flow rate. Dimples were located on the vane surface of this modified diffuser with the aim of improving stage performance at the range of negative incidence angles at vane leading edge. The results of numerical computation showed that proposed location of the dimples provides the reducing of the flow separation zone and boundary layer thickness in the diffuser at negative incidence angles and, thus, increasing the efficiency of the compressor stage at corresponding mass flow range. The results of the investigation should be considered as qualitative. The accuracy of the simulation of the complex swirling flows with separation strongly depends on the CFX set-up. Therefore in order to obtain quantitative results a detailed verification (with experimental data) of the flow pattern and diffuser characteristics should be performed.

Wavenumber analysis of vibrating dimpled beams

A structure’s noise and vibration characteristics can be changed by introducing dimples onto the surface. These structural modifications are an attractive form of noise control since they are simple to manufacture and do not add mass to the structure. In order to gain a better understanding of the effect of dimples, beams with different number of dimples are considered in this study. The natural frequencies and mode shapes of beams with any number of dimples are computed using a boundary value model derived from Hamilton’s Principle. The results of this model are compared to the finite element method and to previous literature. Then, the effect of the dimples on the radiation properties of beams is investigated by examination of their wavenumber spectra. By shifting the wavenumber content of the beam between supersonic and subsonic regions, dimples are able to change the amount of radiation emitted. For some boundary conditions, the wavenumber spectrum is time-dependent as the beam vibrates through a complete cycle. Several examples are presented that demonstrate how different numbers of dimples, as well as their locations and geometries, affect the wavenumber spectra of beams.

Hydrodynamic lubrication of partial textured sliding journal bearing based on three-dimensional CFD

Purpose – The purpose of this study is to investigate the effects of partial texture location and dimple depth on load carrying capacity (LCC), friction coefficient and circumferential flow of journal bearing. Design/methodology/approach – Based on the Navier-Stokes equation, the methodology used computational fluid dynamics (CFD). A phase change boundary condition was applied on fluid domain, and the negative pressure at divergent region of oil film was considered. Findings – It has been found that texture located at lubricant inlet area can improve the performance of the bearing, and the effect of shallow dimples is superior to the deep ones. However, the bearing performance will be reduced due to the texture located at the maximum pressure area. When texture is located at the lubricant outlet area, there will be two different situations: the part of the texture located within the oil film divergent area can improve the LCC, while the part that is beyond the divergent region will make the LCC decrease. Originality/value – The lower-half oil film model was established only in this study to analyze the hydrodynamic lubrication performance of partial textured journal bearing, and the lower-half oil film was divided into three parts. A new cavitation algorithm was introduced to deal with the negative pressure. The formula for calculating the friction of liquid film is refined, including the consideration of vapor phase. The simulation results show that the location of partial texture have a great influence on the bearing performance.

Influence of micro-pits on sliding motion under low speeds for block-on-disk tribotester

ABSTRACTThe tribological behaviour of surface texture with embedded pits or dimples using pin on disk with modification from point contact (original pin-on-disk specification) to surface contact under lubricated sliding motion is investigated. A series of experimental results are presented to evaluate the effect of dimples at low sliding speed. Load, spacing of pits, and number of pits are varied to explore their influence on the friction mechanism. It shown that with proper dimensions of dimples, the friction performance of surface texture can be improved due to pit characteristics. The results reveal that surface texture without pits does not reduce tribological problems compared to surface with embedded pits. Furthermore, experimental results also indicate that surface textures with pits have large distance and show better frictional performance and wear rate value with close pits.

Numerical Analysis of Flow Structure and Heat Transfer Characteristics in Dimpled Channels With Secondary Protrusions

Dimple structure is an effective heat transfer augmentation approach on coolant channel due to its advantage on pressure penalty. The implication of secondary protrusion, which indicates protrusion with smaller dimension than dimple, will intensify the Nusselt number Nu inside dimple cavity without obvious extra pressure penalty. The objective of this study is to numerically analyze the combination effect of dimples and secondary protrusion. Different protrusion–dimple configurations including protrusion print-diameter Dp, protrusion–dimple gap P, and staggered angle α are investigated. From the results, it is concluded that the implication of secondary protrusion will considerably increase the heat transfer rates inside dimple cavity. Cases 4 and 6 possess the highest Nusselt number enhancement ratio Nu/Nu0 reaching up to 2.1–2.2. The additional pressure penalty brought by the protrusion is within 15% resulting in total friction ratio f/f0 among the range of 1.9–2.1. Dimpled channels with secondary protrusions possess higher thermal performance factor TP, defined as (Nu/Nu0)/(f/f0)1/3, among which cases 4 and 6 are the optimal structures. Besides this, the TP of protrusion–dimple channels are comparable to the other typical heat transfer devices, and higher TP can be speculated after a more optimal dimple shape or combination with ribs and fins.

A STUDY OF HEAD–DISK INTERACTION DETECTION IN THE HARD-DISK DRIVES

The reliability and performance of precision mechanical components that experience sliding under contact depend heavily on the friction and wear characteristics at the sliding interface. In order to improve the reliability of the sliding interface, there is a need to predict, measure and monitor any physical interactions at the head–disk interface (HDI). In the present work, the basic tribological characteristics of HDI were analyzed. The HDI during start–stop and constant speed operation using acoustic emission (AE) were studied. The Fast Fourier Transform (FFT) analysis of the AE signal was used to understand the interaction between the AE signal and the state of contact. In addition, we developed laser textured (LT) disk and the contact start–stop (CSS) tests were performed to investigate the effect of dimples on the stiction performance of the HDI. Furthermore, numerical analysis between the slider and disk surface pressure were performed using the boundary coordinate system and divergence formulation for the nonlinear Reynold's equation solution.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE EFFECT OF DIMPLE AND PERFORATIONS ON NUSSELT NUMBER AND FLOW EFFICIENCY IN MULTI-LOUVERED HEAT EXCHANGER

Numerical and experimental analyses are performed on multilouvered fin banks in low and medium Reynolds regimes to investigate the effects of dimples and perforations on flow structure and heat transfer capacity. In different Reynolds numbers the flow efficiency has been calculated numerically and experimentally by flow visualization method and the results showed that dimples and perforations can increase flow efficiency more than 5% in fin banks. Additionally, a set of numerical and experimental analysis has been done to evaluate temperature contours and total heat transfer in simple, dimple and dimple-perforation fin banks in different Reynolds Numbers. The simulations revealed that the heat transfer and temperature augmentations occur due to the existence of a circulation region that is created by the dimple. The results showed that adding dimples with perforations increased the heat transfer capacity of the fin banks up to 9% compared with the simple louver fin banks.

A CFD Study on Leading Edge Wing Surface Modification of a Low Aspect Ratio Flying Wing to Improve Lift Performance

Low Aspect Ratio (LAR, aspect ratio &lt; 2) wings exhibit highly nonlinear lift curves. Controlling such LAR Mini Aerial Vehicles (MAVs) at high Angle of Attacks (AOA) and in cross-wind conditions is difficult due to the nonlinear lift. Also, maneuvering these MAVs at high AOA is highly constrained by elevator throw. However, high AOA is a requisite for MAVs to enable slow flight in constrained spaces. Conventional aircraft use devices called vortex generators (or turbulators) that delay the separation of boundary layer by artificially inducing vortices. A similar phenomenon is observed in golf balls, where the surface dimples mimic the above effect. It is also observed that the local vortices aid in additional lift generation. This paper presents a Computational Fluid Dynamics (CFD) study on the effect of dimples over the aerodynamic performance of a LAR flying wing with aspect ratio 1.45 &amp; SELIG4083 cross section. A Modified Inverse Zimmerman planform wing has been considered similar to the MAV (Black Kite) developed by the National Aerospace Laboratories Bangalore (NAL). The 3D model was generated in SolidWorks™ and CFD analysis was performed in SolidWorksTM Flow Simulation. The baseline smooth surface wing was validated with the aerodynamic data of SELIG4083 airfoil from literature. Following it, several dimpled wing configurations on the leading edge were created and analyzed. It was observed that dimples create local vortices that aid in lift generation, increasing the effective flight envelope. The associated drag due to local vortices should be overcome by generating additional thrust at high AOA.

Effect of compound dimple on tribological performances of journal bearing

The effect of the compound dimple on the tribological performances of a journal bearing is studied. In doing so, the tribological performances for the bearing with the compound dimple and simple dimple are studied using a fluid structure interaction (FSI) method and compared further. Numerical results show that the compound dimple can supply the larger load-carrying capacity and lower friction coefficient due to its twice hydrodynamic action in comparison with the simple dimple. Moreover, the above improvement depends on the geometry sizes of the compound dimple, the dimple interval, and working parameters of the bearing.