Drag reduction for flow past a square cylinder through corner chamfering

This study investigates the unsteady incompressible flow around a square cylinder with different chamfer ratios (CRs) using a commercial finite volume code, ANSYS Fluent. CR ranges from 0.0 (sharp square cylinder) to 0.5 (diamond cylinder) with variable increments. Detailed analysis of flow characteristics is conducted at Reynolds number (Re) = 2100. Additionally, simulation is extended to cover Re, i.e., Re = 100, 500, and 10000. The simulation results show that cylinder with CR = 0.1 outperforms all other cases by enabling a drag reduction of about 60 % at Re = 10⁴. Drag has an inverse relationship with the wake closure length. Time-averaged coefficient of pressure, streamlines, and vorticity contours are also discussed to better understand near-wake features and the physics of drag reduction.

     

Fundamental studies on free stream acceleration effect on drag force in bluff bodies

This paper describes fundamental studies on free stream acceleration effect on drag force in bluff bodies. The flow with gradual velocity increase assumed an accelerated flow. The wind tunnel tests were conducted in order to investigate the difference of aerodynamic characteristics between non-accelerated flow and accelerated flow. The experimental models were a circular cylinder and a square cylinder. In an accelerated flow, the condition of free stream was an acceleration of about 3.6m/s². Experimental Reynolds number varied between form 4.0×10⁴ to 1.64×10⁵. The pressure distributions and the aerodynamic force were measured in both case of nonaccelerated flow and accelerated flow. In case of a circular cylinder, the drag of accelerated flow is lower than that of non-accelerated flow in the low Reynolds number regime. Then, it becomes higher than that of non-accelerated flow in the high Reynolds number regime. On the other hand, in case of a square cylinder, the drag of accelerated flow is higher than that of non-accelerated flow in the whole Reynolds number regime. If a separation point can be movable such as the circular cylinder, the additional momentum due to flow acceleration leads to delay separation and to decrease the drag in the low Reynolds number regime. If a separation point is nearly fixed such as a square cylinder, the additional momentum due to acceleration always affects to increase the drag than that of non-accelerated flow. Based on this research, it is expected that the roof shape of high speed train where the separation point is movable has the advantages to reduce the effect of accelerated flow such as strong crosswind/gust.     

Influence of the edge rounding process on the behaviour of blanked parts: numerical predictions with experimental correlation

Blanking of sheet metal is an important forming process in the automotive industry for the manufacture of mechanical components. The final component shape, obtained at the end of bending or deep-drawing processes, often has sharp edges due to the blanking operation. Concerning passenger safety components, like seat belt anchors, rounding of the edges by punching is necessary to avoid cutting the belt material. In addition to removing the sharp edges, the punching results in work hardening of the material in the rounded zones which results in an increase in the local resistance of the material. In this study, a high-strength low-alloy steel (HSLA S500MC) has been tested with the aim of quantifying the blanking and edge rounding operations. The mechanical behaviour of test specimens is investigated by means of tensile tests and the material is characterised in terms of Vickers micro-hardness. Numerical simulations of the edge rounding process are developed using previously identified material behaviour laws. The residual stress fields are characterised and compared to experimental results. This is done so that numerical simulation can be done in the future to prediction the in-service behaviour of the component. Specimens with rounded edges are compared to specimens that were not submitted to the rounding operation. It is shown that Edge Rounding by Punching improves the component resistance, therefore justifying the use of this process in the manufacture of automotive safety components.     

Pressure distribution and flow development in unsteady incompressible laminar boundary layers

It is shown that in an unsteady flow the friction drag is always accompanied by the form drag whose magnitude is comparable to that of the former and that the pressure around the unsteady boundary layer can be far from that of the inviscid irrotational flow. The unsteady boundarylayer equations and boundary conditions for the external potential flow are modified accordingly and the flow around a circular cylinder which is set impulsively to move in a constant velocity is analysed using these modified boundary-layer equations. The solutions are in power series of \(\sqrt \tau \) rather than τ where τ is the dimensionless time elapsed since the onset of motion, and the form drag, like the friction drag, decreases from infinity in inverse proportion to \(\sqrt \tau \) , when τ is small.     

A numerical study on fluid flow around two side-by-side rectangular cylinders with different arrangements

This study numerically examines the flow around a pair of parallel rectangular cylinders placed perpendicular to the direction of the flow using the immersed boundary method at a fixed Reynolds number of 100. A total of eight spacing ratios between 0.1 and 2 are considered in the two arrangements of the cylinders. The two cylinders are arrayed in inline and a staggered arrangement. The pattern of the wake of the two cylinders depends on their arrangements and spacing. The results, show four flow regimes: (i) A single bluff-body regime, (ii) an asymmetric wake regime, (iii) a transition regime, and (iv) a coupled street regime. All flow regimes appear in the case of the inline arrangement. In the case of the staggered arrangement, only the three flow regimes other than the coupled street flow regime are shown. The flow characteristics depend on the flow regime, including the flow structure, drag force, lift force, and frequency. We analyzed the flow characteristics by comparing the flow regimes, vortex shapes, drag and lift coefficients, and Strouhal numbers, which depended on the arrangement. The results of the drag, lift, and Strouhal numbers depend on the interaction of the jet that forms between the cylinders and the adjacent wakes near the cylinder. Therefore, the flow characteristics are sensitive to the arrangement and the distance between the cylinders.

     

Unsteady computation of flow field and convective heat transfer over tandem cylinders at subcritical Reynolds numbers

Unsteady flow and convective heat transfer over single and two tandem cylinders at constant-heat-flux condition in subcritical range of Reynolds number was numerically investigated. Two-dimensional computations were performed by adopting 3-equation k-kl-ω turbulence model using a commercial software FLUENT®. The aim was to investigate the capabilities of k-kl-ω turbulence model for collective flow and heat transport conditions past cylindrical bodies and then to identify a critical spacing ratio for the maximum heat transport. The center-to-center spacing ratio (L/D) was varied in the range from 1.2 to 4.0. Instantaneous path lines and vorticity contours were generated to interpret the interaction of shear layer and vortices from upstream cylinder with the downstream cylinder. Comparison of pressure coefficients, fluctuating and average lift as well as drag coefficients, Strouhal number and the local and average Nusselt numbers with the available literatures indicated a reasonably good agreement. The combined outcome of flow field and heat transfer study revealed a critical spacing ratio of L/D = 2.2. Based on the present investigation, a correlation has been suggested to calculate overall average Nusselt number of the two cylinders placed in tandem.

     

Control of flow around a square cylinder at incidence by using a splitter plate

Passive control of vortex shedding behind a square cylinder at incidence has been conducted experimentally by using a stationary splitter plate for the Reynolds numbers of 3.0×10⁴. The splitter plate was located at the center of the rear face of the square cylinder in tandem. The width of the cylinder and the plate were both chosen to be 30 mm and the incidence angle of the square cylinder was rotated between 0° to 45°. In this study, the combined effects of the splitter plate and angle of incidence on the pressure distributions and vortex-shedding phenomenon were investigated. Vortex shedding frequency was obtained from velocity measurements and aerodynamic force coefficients acted on the cylinder were calculated from pressure distributions. Characteristics of the vortex formation region and location of the flow attachments, reattachments and separation were observed by using the smoke–wire flow visualization technique. For the case with the plate, there is a sudden jump in the Strouhal number in the vicinity of 13° which corresponds to a minimum value of the drag coefficient. At zero angle of incidence, Strouhal number and a drag coefficient of the square cylinder decreased about 20% by means of the splitter plate. Drag reduction was minimum at about 13° and reached its maximum value at about 20°.     

串列三圆柱绕流的时均压力分布与气动力

运用刚性模型测压风洞试验方法对单圆柱、不同间距串列双圆柱和串列三圆柱绕流的时均压力分布与气动力进行了研究。首先,进行单圆柱模型和不同间距串列双圆柱模型的绕流试验,试验的雷诺数为3.4×104;其次,通过与单圆柱进行对比,讨论了气动干扰对串列三圆柱时均压力分布与时均阻力的影响规律;最后,通过与串列双圆柱进行对比,讨论了圆柱的数量对干扰规律的影响。试验结果发现,串列三圆柱的绕流存在两个完全不同的流态,其切换的临界间距(L/D)cr在3.5～4.0之间,两个流态下的时均压力分布与时均阻力存在明显的差异。本研究可对实际工程中串列圆柱结构的风荷载取值提供参考。     

A new method for the characterisation of rounded cutting edges

The influence of the cutting edge micro geometry on cutting process and on tool performance is subject of several research projects. Recently published papers focus on optimising the cutting edge rounding. The results are partly inconsistent. Unfortunately, no international standard yet exists to properly describe the cutting edge micro geometry. This is seen as the root cause for detected discrepancies. To develop a common understanding for the influence of rounded cutting edges, it is indispensable to use the same basis to characterise the edge profile. This paper gives a review on existing characterisation methods, analyses the difficulties in their application and discusses different modelling ideas to describe the cutting edge profile. Based hereon, a new algorithm and geometrical parameterisation of the cutting edge is proposed, which reduces uncertainties and difficulties in the application of currently available methods. The proposed method considers measurement uncertainties and is robust against form errors and creates thus the basis required for the study of the influence of rounded cutting edges.     

Numerical study of fluid force reduction on a circular cylinder using tripping rods

A numerical investigation on the effects of small tripping rods on the fluid force reduction on a big structure has been carried out using finite volume method where a configuration of a circular cylinder with two small tripping rods symmetrically placed very near to its front surface is studied. The diameter ratio of the rods and the cylinder is set at 0.08, 0.10 and 0.12, and the gap between the rods and the cylinder is fixed at 0.08 of the cylinder diameter. The angular position of the rods varies from 20° to 60°. The effects of the tripping rods on force reduction, vortex shedding frequency and flow separation have been examined for various arrangements of the rods with Reynolds number focused on 200 for laminar flow and 5.5×10⁴ for a turbulent flow. The results reveal that there exits an optimum position where the time averaged force coefficients acting on the cylinder all reach their minimum values and at the same time Strouhal number meets its maximum. At the optimum position the drag coefficient is reduced by 18% for Re=200 and 59% for Re=5.5×10⁴. Further investigation with tripping rods placed near the separation points is also carried out for Re=5.5×10⁴ and a considerable drag reduction is found.     

驱动电压波形修圆对微流体脉冲惯性力和驱动效果的影响

介绍了微流体脉冲驱动-控制技术,分析了微流体脉冲驱动-控制过程,指出在这一过程中影响微流体流动的主要因素是微流道固壁加速度和流体内部的黏性力。采用"椭圆修圆法"对方波驱动电压进行修圆,针对修圆点的位置决定微流体的驱动方向,获得了不同修圆位置和修圆系数的驱动电压修圆波形。通过实验探索了波形修圆对微流道固壁运动加速度、微流体脉冲惯性力和流体驱动效果的影响规律并进行了机理分析。所得流体体积流量可在0～15.4pl/min连续变化,远小于现有的微流体驱动技术。本文的研究成果可为微流体脉冲驱动-控制技术在微流体系统中的进一步应用提供参考。     

高雷诺数下有限长圆柱绕流阻力特性研究

针对不同长径比的有限长圆柱模型,采用大涡模拟及雷诺平均的方法,对高雷诺数下有限长圆柱绕流阻力特性进行数值模拟和分析,得到了圆柱阻力系数随长径比和雷诺数的变化规律,讨论端面效应对绕流阻力系数的影响。结果表明：在亚临界区内,相同雷诺数下阻力系数随长径比的增大而增大,呈线性变化规律,L/D的对阻力系数的影响明显大于Re对阻力系数的影响;在阻力危机区内,相同雷诺数下阻力系数随长径比的增大而增大,呈二次函数的变化规律,但各工况达到阻力系数“转折点”对应的雷诺数各不相同,基本呈现随雷诺数的增大向前推移的趋势,Re对阻力系数的影响明显大于L/D对阻力系数的影响;在阻力回升区,阻力系数回升的“转折点”随着雷诺数的逐渐增大而向后推移。在各分区内,端面效应对阻力系数的影响随雷诺数增大而更加明显,在高度方向上的最大影响区域约占圆柱总高度的16%。研究结果对有限长圆柱绕流特性的研究及应用具有重要意义和价值。     

Experimental investigation of surface roughness effect on a free-flight sphere in a Ludwieg tube

Understanding the aerodynamic coefficients of meteoroid fragments, deorbiting space debris, or launch vehicle stages through atmospheric reentry is essential for ground risk assessments. In high enthalpy flow, surface roughness is a crucial factor affecting the aerodynamic coefficient. In this work, the effect of surface roughness on drag coefficient is investigated experimentally within a Ludwieg tube at a Mach 4 test condition. The test model is a sphere with a 5 mm diameter. Three different types of surface roughness are considered using a pre-heating process. Shadowgraph technique was used to visualize the flow features and model behavior. Based on the acquired high-speed images, the drag coefficients were obtained using an image tracking technique. Results show that the drag coefficient decreases with corresponding increases in surface roughness for the given flow condition, implying the importance of surface roughness effect in ground risk assessments.

     

Unsteady Lift and Drag Forces Acting on the Elliptic Cylinder

A parametric study has been accomplished to figure out the effects of elliptic cylinder thickness, angle of attack, and Reynolds number on the unsteady lift and drag forces exerted on the elliptic cylinder. A two-dimensional incompressible Navier-Stokes flow solver is developed based on the SIMPLER method in the body-intrinsic coordinates system to analyze the unsteady viscous flow over elliptic cylinder. Thickness-to-chord ratios of 0.2, 0.4, and 0.6 elliptic cylinders are simulated at different Reynolds numbers of 400 and 600, and angles of attack of 10°, 20°, and 30°. Through this study, it is observed that the elliptic cylinder thickness, angle of attack, and Reynolds number are very important parameters to decide the lift and drag forces. All these parameters also affect significantly the frequencies of the unsteady force oscillations.     

Aerodynamic drag reduction based on static traveling wave structure

To explore the influence of the traveling wave parameters on the aerodynamic characteristics of Ahmed models, the geometric model of the traveling wave wall is used as the study object, and the influence of the traveling wave geometric parameters on aerodynamic drag is studied by numerical simulation. A cosine-type traveling wave digital model is established on the basis of Ahmed’s original model. Results show that the static traveling wave structure can play a good role in reducing drag under some driving conditions. The traveling wave’s drag reduction characteristics are affected by the layout position, depth, wavelength, and other related parameters and more affected by driving speed. Finally, a group of working conditions with good drag reduction effects is selected, and the principle of drag reduction by using static traveling wave structure is explained qualitatively and quantitatively through velocity field, shear stress, and pressure field.

     

Numerical investigation of fluid flow past a square cylinder using upstream,downstream and dual splitter plates

A two-dimensional numerical study is carried out to analyze the drag reduction and vortex shedding suppression behind a square cylinder in presence of splitter plate arranged in upstream, downstream and both upstream and downstream location at low Reynolds number (Re = 160). Computations are performed using a Single relaxation time lattice Boltzmann method (SRT-LBM). Firstly, the code is validated for flow past a single square cylinder. The obtained results are compared to those available in literature and found to be in good agreement. Numerical simulations are performed in the ranges of 1 ≤ L ≤ 4 and 0 ≤ g ≤ 7, where L and g are the length of splitter plate and gap spacing between the splitter plate and main square cylinder, respectively. The effect of these parameters on the vortex shedding frequency, time-trace analysis of drag and lift coefficients, power spectra analysis of lift coefficient, vorticity contours visualization and force exerted on the cylinder are quantified together with the observed flow patterns around the main cylinder and within the gap spacings. The observed results are also compared with a single square cylinder without splitter plate. We found that at some combinations of L and g, the mean drag coefficient and Strouhal number reach either its maximum or minimum value. It is found that the drag is reduced up to 62.2 %, 13.3 % and 70.2 % for upstream, downstream and dual splitter plates, respectively as compared to a single square cylinder (without splitter plate). In addition, in this paper we also discussed the applications of SRT-LBM for suppression of vortex shedding and reduction of the drag coefficients.     

Design and fabrication of fluid flow characteristic controllable trawl door using a trailing edge flap

A novel trawl door with a trailing edge flap and a trawl door with a hydrophobic surface are proposed to improve the hydrodynamic characteristics of trawl doors. In the former, the flap angle was designed to accommodate rapid changes in water flow velocity, increase the lift force under low-speed conditions, and decrease drag force under high-speed conditions. An evaluation of the hydrodynamic characteristics at different flap angles was conducted through numerical analyses and experimentation. The design of the trawl door with coated hydrophobic surface aims to reduce flow resistance. Both trawl door designs were validated in a circulating water channel ex-periment. At these angles of the trailing edge flap, a 31 % decrement in drag force was observed in the circulating water channel experi-ment, and a 21.3 % decrement in flow resistance was observed when the surface of the trawl door was coated with hydrophobic surface materials.

     

Numerical investigation of cross-flow around a circular cylinder at a low-reynolds number flow under an electromagnetic force

The effect of the electromagnetic force (or Lorentz force) on the flow behavior around a circular cylinder is investigated by computation. Two-dimensional unsteady flow computation forR _e =10² is carried out using a numerical method of finite difference approximation in a curvilinear body-fitted coordinate system by solving the momentum equations including the Lorentz force as a body force. The effect of spatial variations of the Lorentz forcing region and forcing direction along the cylinder circumference is investigated. The numerical results show that the Lorentz force can effectively suppress the flow separation and oscillation of the lift force of circular cylinder cross-flow, leading to reduction of drag.     

两种串列方柱气动性能的试验研究

针对两种布置形式（水平布置和对角布置）的串列双方柱,通过同步测压风洞试验,在雷诺数为Re = 8.0×104、间距比为P/B = 1.75 ~ 5.00（其中P为方柱中心间距、B为方柱边长）条件下,得到了两种布置形式串列双方柱的表面风压,重点研究了对角串列双方柱的气动力、风压分布、Strouhal数等气动性能随方柱间距的变化规律,并与水平串列双方柱进行比较。水平串列双方柱的气动力在P/B = 3.00 ~ 3.50时会发生跳跃现象,下游方柱的平均阻力由负值突变为正值,而对角下游方柱平均阻力系数则均为负值。结果表明：当P/B<3.00时,对角串列双方柱的平均和脉动气动力系数、最大平均负压强度和脉动风压系数均大于水平串列双方柱,而当P/B > 3.00时则情况相反;对角串列双方柱的Strouhal数明显小于相同间距下的水平串列双方柱,且在P/B <3.00时对角串列双方柱的升力功率谱出现了多个峰值。     

Drag force control of flow over wavy cylinders at low reynolds number

Three-dimensional numerical simulations on the laminar flow around a circular cylinder with different diameter along the spanwise leading to a type of sinusoidal waviness, named wavy cylinder are performed at low Reynolds number. A series of wavy cylinders with different combinations of spanwise wavelength and wave amplitude are conducted at a Reynolds number of 100. The optimal range of wavelength and the effect of wave amplitude are obtained. The results show that the 3-D free shear layers from the cylinder are more difficult to roll up to vortex and hence the wake formation lengths of some typical wavy cylinders are larger than that of the circular cylinder and in some cases the free shear layers even do not roll up into vortex behind the cylinder. The mean drag coefficients of the typical wavy cylinders are less than that of a corresponding circular cylinder with the same mean diameter; also the fluctuating lift coefficients are reduced. The reduction of mean drag coefficient and fluctuating lift coefficient of wavy cylinder increases with the value of wavy amplitude. Furthermore, a typical wavy cylinder model at Re=150 is also simulated and found that the control of flow induced vibration by modifing the spanwise wavelength of cylinder has a relationship with the variation of Reynolds number.