Effect Of Dimples Research Articles

Analysis of numerical results for two-pass trapezoidal channel with different cooling configurations of trailing edge: The effect of dimples

Internal cooling of the gas turbine blades is imperative for the enhanced inlet temperature of gas turbine. In this paper, numerical results of two-pass trapezoidal channel provided with dimples for internal cooling of a gas turbine blade are discussed. Three configurations of dimples are compared and enhancement in heat transfer has been reported. Also pressure drop along the channel for all cases has been predicted. It is found that for the channel with inline arranged dimples having H/d ratio equal to 0.25, maximum heat transfer enhancement is attained. Staggered arrangement of the same dimples reduces pressure drop but also effects enhancement in heat transfer negatively. The thermal performance of the dimples size suggests that H/d ratio of dimples is an important parameter and its optimum value should be found out.

Computational Analysis of Different Shapes of Dimple on Wing

Dimple is a slight indentation in a surface. Dimples create turbulence by creating vortices which delays the boundary layer separation resulting in decrease of drag, increasing aerodynamic efficiency, manoeuvrability and also the angle of stall. The present work focused on the understanding of the effect of dimples on boundary layer separation, lift, drag, critical angle of attack, aerodynamic efficiency of wings. The airfoils without any dimples and with circular dimples as inward and outward on are studied. Types of dimples considered in 3D studies are circular and octagon dimple then computational analysis is done using ANSYS FLUENT CFD software, applying subsonic flow, in three dimensional co-ordinate system. The results are compared with a straight wing without dimples. Then suggestions and conclusions are made.

Effect of Elliptical Dimples on Hydrodynamic Lubrication

This paper develops an analytical model to investigate the effect of elliptical dimples on hydrodynamic lubrication of surface contact. The hydrodynamic pressure generated by elliptical dimples is solved by the multi-grid method. The evaluation criterion of hydrodynamic effect of dimensionless average pressure is calculated and presented with the variation of elliptical dimple depth, area density, slender ratio and inclination angle. The results indicate that geometrical parameters of elliptical dimples have an obvious influence on the hydrodynamic pressure. The hydrodynamic lubrication performance can be ameliorated by optimizing the geometrical parameters of elliptical dimples according to the operating parameters of the mechanical components.

Experimental and numerical investigation of heat transfer, temperature distribution, and louver efficiency on the dimpled louvers fin banks

Numerical models have been developed by commercial package Fluent 6.3 to simulate the air flow through single dimple, simple, and dimpled louvers in low and medium Reynolds numbers. Experiments have also been conducted to measure the temperature and heat transfer in these geometries. Heat transfer augmentation of 8% has been observed by implying dimples on louver at the same mass flow rate. For accurate investigation of the effect of dimple, a single dimpled surface has been modeled numerically and experimentally. The simulation revealed that these heat transfer and temperature augmentations occur due to existence of a circulation region created by dimple. Additionally, the effects of louver’s thermal resistance on temperature distribution over the louver surface have been considered to gain the actual contours. Continuous temperature gradients have been observed over the louver surface with the highest temperature at the base of the louver and the lowest temperature at the middle of the louver. Louver efficiency has been introduced to assess the dimpled louver performance for low and medium Reynolds numbers. It has been observed that dimpled geometry has satisfactory louver efficiency. Good agreement has been observed between experimental and numerical models.

Aerodynamic Analysis of Dimple Effect on Aircraft Wing

Aerodynamic Analysis of Dimple Effect on Aircraft Wing

COMPUTATIONAL INVESTIGATION OF DIMPLE EFFECTS ON HEAT TRANSFER AND FRICTION FACTOR IN A LAMILLOY COOLING STRUCTURE

Good heat transfer performance with a moderate pressure drop penalty contributes to the gas turbine engine lifetime and guaranteeing engine efficiency. In this study, the dimple effects for a Lamilloy® (Allison Advanced Development Corporation, Indiana, IN, USA) cooling structure on the heat transfer and friction factor are numerically investigated. The dimple is positioned directly under the jet impingement nozzle. The Reynolds number ranges from 10,000 to 70,000, the dimple normalized depth is between 0 and 0.3, and the dimple normalized diameter varies from 1 to 2.5. The results for the flow field, target surface heat transfer, pin fin surface heat transfer, friction factor, and solid domain outer-wall temperature are included. For comparison, a Lamilloy cooling structure without the dimple is considered as the baseline. The results show that the dimple significantly increases the local heat transfer due to flow reattachment and recirculation. With an increase in the normalized dimple depth, the heat transfer on the target surface is first augmented due to the increase of flow reattachment and recirculation, and then it is decreased due to the large toroidal vortex. However, an increase in the dimple depth results in reduction of the pin fin surface heat transfer. As the dimple diameter increases, the target surface heat transfer is also first augmented due to the increase in the flow reattachment and recirculation, and then it is decreased as the flow separation increases. The thermal performance indicates that the intensity of the heat transfer enhancement depends on the depth and diameter of the dimple.

Tribological properties of PTFE/laser surface textured stainless steel under starved oil lubrication

Surface textures with dimples were fabricated on stainless steel surface by a Nd:YAG pulsed laser. Friction and wear behaviors were investigated by rubbing against PTFE under starved oil lubrication and the effect of dimple density from 2.0% to 20.9% was examined. The result shows that friction coefficient of stainless steel is reduced from 0.08 to 0.05 due to texturing, and the longevity of oil film is prolonged by 50%. The dimple density of 7.9–8.8% appears the optimal tribological performance with the friction coefficient of 0.055 and wear rate of 5.2×10−7mm3/Nm. The beneficial effect of dimples is to act as oil reservoirs and the sink for capturing wear debris under starved lubrication.

Effects of dimples on laminar boundary layers

A series of direct numerical simulations of the flow past a flat plate with two and eight rows of dimples in a staggered arrangement is carried out. The Reynolds number based on the boundary layer thickness and freestream velocity near the inflow plane is 1000 and the dimples are spherical with a depth to diameter ratio of 0.1. The incoming flow is laminar and the boundary layer thickness before the dimples is half the dimple depth. At this low Reynolds number the flow is expected to remain laminar over a smooth flat plate. The presence of the dimples triggers instabilities that cause significant momentum transport. It is shown that the shear layer that forms as the flow separates over the first two rows of dimple becomes unstable and sheds coherent vortex sheets. The vortex sheets become unstable and are transformed into packets of horseshoe vortices. When these vortices evolve over a flat plate or over a series of dimples the flow dynamics are very different with important changes in momentum transport across the boundary layer.

Aerodynamics of Ribbed Bicycle Racing Helmets

In competitive cycling, aero-helmets have been used around since 1980 to reduce aerodynamic resistance. Considerable design effort has been made to improve the aerodynamic efficiency of racing bicycle helmets over the years. However, the demand for further improvement has forced helmet manufacturers and designers to introduce new designs progressively. Recently several helmet manufacturers (e.g., LG, Lazer and Giro) have introduced dimples on the outer shell of helmet mimicking the so called ‘Golf-ball’ dimple effects with a view to further reduce aerodynamic drag of the helmet. However, no independently verifiable research so far has been reported in the public domain about the aerodynamic performance of ribbed bicycle helmets compared to smooth surfaced helmets. Hence, the primary objective of this work was to undertake an experimental study on four smooth aero-helmets including two latest model ribbed aero-helmets to understand their aerodynamic performance and the effect of dimples on helmets. The investigation was undertaken in an wind tunnel environment over a range of wind speeds, pitch and yaw angles. The experimental data indicate no measurable advantage between the smooth and ribbed helmets under varied pitch angles and at zero yaw angle.

Improvement in the tribological characteristics of Si-DLC coating by laser surface texturing under oil-lubricated point contacts at various temperatures

This paper presents and discusses the tribological characteristics of the untextured and textured silicon diamond-like carbon (Si-DLC) coatings in the temperature range of room temperature to 200°C. Dimples were produced using a laser surface texturing (LST) technique on a Si-DLC coating deposited onto a bearing steel substrate using a chemical vapor deposition (CVD) system to achieve good tribological behavior. The tribological characteristics of the untextured and textured Si-DLC coatings were investigated by conducting friction tests against an alumina (Al2O3) ball. The friction test results showed that the textured Si-DLC coating led to a lower friction coefficient and wear compared to that of the untextured Si-DLC coating throughout the temperature range tested, which may be attributed to the effects of dimples and microstructural alteration due to high-energy action of the pulsating laser beam.

Study on Tribological Performances for Textured Bearing of ICE

The influence of dimples on the inner surface of big end bearing in internal combustion engine (ICE) on tribological performances of the bearing was investigated based on Navier-Strokes equation and other associated equations. In doing so, the CFD modulus in the software ANSYS12 version is used to analyze the dimple effect on the tribological performances of the bearing using two-way fluid-solid coupling algorithm. Some mechanisms are revealed about the dimple effect on the load-carrying capacity and friction coefficient of oil film, and the deformation and stress for the textured big end bearing.

A study on the tribological characteristics of duplex-treated Ti–6Al–4V alloy under oil-lubricated sliding conditions

The tribological characteristics of the polished, dimpled and over-coated dimpled specimens were investigated. Dimples were produced on a Ti–6Al–4V alloy specimen using a laser surface texturing (LST). A Cr-doped diamond-like carbon (DLC) film was deposited on a dimpled specimen using an unbalanced magnetron sputtering (UBMS). The effects of dimples and over-coated Cr-doped DLC film on the tribological characteristics were investigated by performing the friction tests against a Cr-plated steel pin. The test results showed that the over-coated dimpled specimen exhibited a lower friction coefficient and wear compared to those of the polished and dimpled specimens, which may be attributed to the storage of wear debris and high hardness. A model of the wear reduction mechanism of the specimens was discussed.

Heat transfer enhancement in mini-channel heat sinks with dimples and cylindrical grooves

The convective cooling heat transfer in mini-channels with dimples, cylindrical grooves and low fins is numerically studied by using the field synergy principle. We solve the synergy angle distribution to examine the mechanisms of the heat transfer enhancement in the enhanced surfaces. The parameter PEC as the evaluation coefficient is employed to study the comprehensive performance of the enhanced surfaces. The results show that the dimple surface presents the highest performance of heat transfer enhancement. The geometry size effects of dimple are studied over a Reynolds number range of 2700–6100, and the most favorable dimple geometric structures are optimized by using the performance evaluation plot of enhanced heat transfer techniques.

The Effect of Micro Dimples Formed on Nylon12/Polyamide-Imide Composite Coatings on Friction and Wear Behavior

The Effect of Micro Dimples Formed on Nylon12/Polyamide-Imide Composite Coatings on Friction and Wear Behavior

Thermal performance comparison between radially rotating ribbed parallelogram channels with and without dimples

The experimental study examines the comparative full-field Nusselt number (Nu) distribution on two opposing enhanced channel walls, including pressure drop coefficient (f), and the thermal performance factor (TPF) of the two radially rotating ribbed channels – with and without dimples. Control conditions were imposed on: Reynolds (Re), rotation number (Ro), buoyancy number (Bu) numbers for both rotating channels, radial outward flows were controlled in the ranges of: 5000⩽Re⩽15000, 0⩽Ro⩽0.3 and 0⩽Bu⩽0.23 respectively. The full-field Nu generated data were recorded and interpreted by the steady-state infrared thermo graphic principle. The isolated interdependent Re, Ro, and Bu impaction on the Nu distribution were accounted; also, the rib-wise average Nusselt number (Nu¯α) at rib and mid-rib locations inclusive over the leading and trailing ribbed walls were recorded. The data for both test section walls, dimpled and non-dimpled were parametrically analysed. For the dimpled test section located opposite one and other, the ribbed wall augmented the near-wall turbulent mixing by means of shedding vortices from the effect of dimples, and the relaxation of shear strain by enlarging channel sectional area thereby causing Nu elevation with a minor f reduction from the reference result generated by the parallelogram ribbed channel at static and rotating conditions. The amplified heat transfer enhancement (HTE) accounting for a minor penalty of pressure drop from adding dimples on to the rib floor led to further TPF elevation. By comparison, the rotational influence on heat-transfer and pressure-drop considered, the performance is similar for both test channels. Due to the combined effect of Coriolis force, rotating buoyancy and the geometric features defined by the test section channels, the rib-wise average Nu¯α/Nu¯α,0 over the leading and trailing walls respectively fall in to the ranges of: 0.71–1.42 and 1.26–1.88 at rib location, and 0.74–1.26 and 1.11–1.77 at mid-rib location for the un-dimpled channel. For the dimpled test channel the ranges were: 0.74–2.07 and 1.11–2.26 at rib location; and 0.75–2.43 and 1.18–2.4 at the mid-rib location. The f coefficients were raised to 1.34–29.8 and 1.81–17.1 times the referenced smooth channel level (f∞) for the parallelogram ribbed channel with and without dimples; developing the TPF factor toward the range of: 0.979–1.575 and 2.62–4.4. A congruous physical data set consistently produced an empirical Nu¯α correlation which evaluated the individual variables, and the interdependent Ro and Bu effect on Nu¯α at rib and mid-rib locations for the parallelogram ribbed dimpled channel to further assist in design applications.

Theoretical and Experimental Research on Lubrication Performance of Laser Surface Texturing Cylinder Liner

By applying the laser surface texturing technique to the engine cylinder liner surface,a mixed lubrication theoretical model of cylinder liner-piston ring is established.The model is calculated to reveal the influence of laser surface texturing on tribology performance with barrel-shaped ring.The results show that after the laser surface texturing,the lubrication and friction performance of the cylinder is improved,the effect of dimple is superior to the groove texturing,the minimum oil ratio in the middle regions of the strokes is increased by 29%,while the friction peak is decreased about 30%.By comparing the engine bench test,it is found that after the surface texture cylinder liner,the engine cylinder fuel consumption shows a declining trend relative to the original engine,the effect is improved significantly at a low speed,the maximum rate of fuel consumption is reduced 9.8 g/(kW h),a decline of 4.62%,which confirms that the surface texture technology is effective for reducing engine friction loss.Meanwhile,the other engine performance indicators also are improved to some extent,air leakage,smoke,oil consumption and other parameters is declined,the smoke is declined obviously,the largest leakage is decreased by 34.6%,and the oil consumption is reduced by 33.8%.

Experimental study on hydrodynamic effect of orientation micro-pored surfaces

The orientation of the dimple increases the flow distance in the dimple and produces fluid cumulative effect in the dimple length direction, which leads to obvious hydrodynamic effect as a result. In order to investigate the hydrodynamic effect of orientation dimples, a series of experiments was carried out on a ring-on-ring test. Multi-pored faces were tested with different dimple inclination angles and slender ratios. Film thickness and frictional torque were measured under different conditions of load and rotation speed. Experimental results showed that the orientation dimple could produce obvious dynamic effect by change of the flow direction and the increasing dimple orientation leads to increase of the load capability. The hydrodynamic effect strongly depends on dimple orientation parameters such as inclination angle and slender ratio. A larger load capability can be available by increasing dimple orientation and rotation speed. Experimental results agreed well with the theory that orientation micro-pores can significantly improve hydrodynamic performance of surfaces.

Preliminary Study of the Effect of Micro-Scale Dimple Size on Friction and Wear under Oil-Lubricated Sliding Contact

One of the conclusively presumed effects of micro-scale dimples under oil-lubricated conditions is that can serve as oil reservoirs and play a role in promoting the retention of a lubricating film. Ultrasonic nanocrystal surface modification (UNSM) technology is an emerging effective method for producing micro-scale dimples on a workpiece surface and improving the tribological performance of lubricated friction units. The main object of this research is to understand the effect of micro-scale dimple size on tribological characteristics under oil-lubricated sliding contacts. The current study investigated the effect of micro-scale dimple size on friction and wear reduction through micro-scale dimples. The tribological characteristics of silicon nitride ceramic (Si3N4) ball and S45C carbon steel disk combination in a defined ball-on-disk configuration were determined. It was recognized that friction property has a connection with the size of micro-scale dimple. Overall micro-scale dimpled disk specimens showed better tribological properties in terms of reduced friction coefficient and wear volume loss comparing to the polished specimen.

Effects of Dimples for Drag and Lift on a Sphere with Rotation

The present study clarifies the effects of dimples for drag and lift on a sphere with rotation. The sphere with dimples has 328 arc type dimples on its surface uniformly. The present study measures velocity distribution in wake and pressure distribution on a sphere surface, and visualizes flow around a sphere by the spark tracing method for the case in which Reynolds number Re=0.4×105-1.3×105. For the case of a smooth sphere, separation point on deceleration side shifts downstream remarkably as spin rate ratio (circumferential velocity of a sphere / free stream velocity) increases. The pressure distribution on deceleration side decreases compared to that on acceleration side. The wake becomes small and shifts opposite direction of rotation. Therefore, the drag reduces, and the lift becomes negative. For the case of a sphere with dimples, this phenomenon occurs for the case in which critical Reynolds number Re=0.5×105. The separation point on acceleration side shifts downstream, and the separation point on deceleration side shifts upstream as spin rate ratio increases for the case in which super critical Reynolds number Re=1.3×105. The pressure distribution on acceleration side decreases compared to that on deceleration side. In addition, the pressure distribution on outside dimple decreases compared to that on inside dimple. The wake becomes large and shifts direction of rotation. Therefore, the drag and positive lift increase.

Drag Reduction Effect of Dimples Arranged in Two-Dimensional Quasicrystal Structure

Rotating fluids experiments were carried out by CAP2000+ cone viscometer, to examine the drag reduction properties of dimples arranged in quasicrystal structure. The dimples were fabricated on the surface of duralunmin (LY12) plates. Compared with the periodic arrays, the dimples arranged in quasicrystal structure, especially the 12-fold quasicrystal structure, could significantly reduce the wall shear stress. And the relative drag reduction efficiency changes periodically with the depth of dimple. Flow-visualization experiment verified that the coherent effect of dimples arranged in quasicrystal structure and the fluids could efficiently inhibit the extending intensity of radial secondary flow, which strengthens the drag reduction effect.