Experimental characterization of the flow field of square prism with a detached splitter plate at high reynolds number

The characteristics of the flow field of a square prism with detached splitter plate in its wake were investigated by measuring the fluid force on the prism and by visualizing the flow field through particle image velocimetry (PIV) with a high Reynolds number (Re = 10,000). The experimental parameters included the ratios of the splitter and prism widths (H/B = 0.5–1.5) as well as the gap ratios (G/B = 0–2) between the prism and the splitter plate at a high Reynolds number (Re = 10,000). The drag reduction rate of the square prism increased with increasing H/B for the same G/B; meanwhile, it increased and then decreased with increasing G/B for the same H/B. When the detached splitter place was installed, vortices rotating in opposite directions were generated on its upper and lower sides. Reverse flow was caused by the vortices in the wake region of the square prism, and the prism drag was decreased by the reverse flow.     

Study on the unsteady wakes past a square cylinder near a wall

Experimental and numerical studies on the unsteady wake field behind a square cylinder near a wall were conducted to find out how the vortex shedding mechanism is correlated with gap flow. The computations were performed by solving unsteady 2-D Incompressible Reynolds Averaged Navier-Stokes equations with a newly developed ε-SST turbulence model for more accurate prediction of large separated flows. Through spectral analysis and the smoke wire flow visualization, it was discovered that velocity profiles in a gap region have strong influences on the formation of vortex shedding behind a square cylinder near a wall. From these results. Strouhal number distributions could be found, where the transition region of the Strouhal number was atG/D=0.5 - 0.7 above the critical gap height. The primary and minor shedding frequencies measured in this region were affected by the interaction between the upper and the lower separated shear layer, and minor shedding frequency was due to the separation bubble on the wall. It was also observed that the position(y/G) and the magnitude of maximum average velocity (u/u∞) in the gap region affect the regular vortex shedding as the gap height increases.     

Effect of innovative circular shroud fences on a centrifugal fan for augmented performance - A numerical analysis

This paper presents the influence of circular front and back shroud fences on the performance of a backward swept centrifugal fan impeller handling air. The analysis is carried out to determine the main characteristics of the fan i.e. the variation of head and theoretical efficiency for various flow coefficients. The circular fence size is optimized parametrically by varying its diameter and the location on the shroud surface. The effect of the shroud fences on the flow field was analyzed for the fences placed on both front and back shroud surfaces of the impeller separately. The numerical simulation is carried out using an appropriate k - ε turbulence model. The relative flow inside the impeller passages is modeled using the standard sliding mesh technique. The numerical results for the base model (i.e. without the fence) are validated against experimental results obtained from the test rig built specially for validation study and are found to have good agreement. It is found from the analysis that the average percentage increase in head coefficient is significant and is about 2 % higher as compared to the base model for the optimized geometrical configuration of diameter ratio and radial fence location as determined in the study. The optimized geometrical configuration also yields a higher theoretical efficiency of about 2.3 % corroborating the improvement in head coefficient with respect to the base model. Hence the efficacy of optimally placed circular fence on the performance of centrifugal fan is established in this numerical study.     

Steady aerodynamic characteristics of a wing flying over a nonplanar ground surface Part II: Channel

The steady aerodynamic characteristics of a wing flying over a channel are investigated using a boundary-element method. The present method is validated by comparing the computed results with the measured data. Compared with a flat ground surface, the channel fence augmented the lift increase and induced drag reduction. When the fence is lower than the wing height, the gap between the wingtip and the fence does not affect the aerodynamic characteristics of the wing much. When the fence is higher than the wing height, the close gap increased the lift. The induced drag is reduced when the wing is placed near the ground or at the same height as the fence. It is believed that present results can be used in the conceptual design of the high-speed ground transporters flying over the channel.     

Gap-flow patterns behind twin-cylinders at low Reynolds number

The flow structures, drag coefficients (C _d ) and vortex shedding characteristics around a single square cylinder and twin side-by-side square cylinders were experimentally investigated with various Reynolds numbers (Re) and gap ratios (g*) in a vertical water tunnel. The Reynolds number (Re) and gap ratio (g*) were 178 < Re < 892 and 0 ≤ g* ≤ 2.5, respectively. The flow patterns and vortex shedding frequency were determined using the particle tracking flow visualization (PTFV). The flow structures, velocity properties, and drag coefficients were calculated using the particle image velocimetry (PIV). The topological flow patterns of vortex evolution processes were plotted and analyzed based on critical point theory. Furthermore, the flow structures behind twin side-by-side square cylinders were classified into three modes — single vortex-street mode, gap-flow mode and couple vortex-streets mode. The maximum C _d occurred in the single vortex-street mode, and the minimum C _d occurred in the gap-flow mode. The highest Strouhal number (St) occurred in the single vortex-street mode, and the lowest St occurred in the gap-flow mode.     

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.     

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。     

基于保偏镜组的外差干涉测量几何误差分析

角锥棱镜由于本身缺陷会导致失偏效应。在平面镜外差干涉仪中,使用一种保偏反射镜组替代角锥棱镜,以减小外差干涉仪的非线性误差。根据这个平面镜外差干涉仪的基本光路图,基于偏振分光棱镜和角锥棱镜的基本光学特性,分析了平面镜外差干涉仪中3个偏振分光棱镜偏摆角、仰俯角和滚动角,保偏反射镜组中2个偏振分光棱镜之间的间距和角度,以及角锥棱镜的偏摆角和仰俯角等误差对干涉仪的影响。推导出外差干涉仪中各个光学元件的最大安装误差,并规定好其加工精度,确保外差干涉仪性能。     

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°.     

Influence of corner radius on the near wake structure of a transversely oscillating square cylinder

The near wake flow field features of transversely oscillating square section cylinders with different corner radii were studied in an attempt to assess the influence of corner radius. The investigation was performed by using particle image velocimetry (PIV) technique in a water channel with a turbulence intensity of 6.5%. Five models were studied with r/B=0, 0.1, 0.2, 0.3 and 0.5 (r is the corner radius and B is the characteristic dimension of the body), and the body oscillation was limited to lock-in condition (at fe/fo=1.0; fe is the excitation frequency and fo is the vortex shedding frequency from a stationary cylinder at the same Re). The corner radius was found to significantly influence the flow features around the bodies. Except for r/B=0.5, for all the other cases of r/B ratios, cycle-to cycle variation in the mode of vortex shedding was observed in the case of oscillating cylinders inducing highly non-linear wake characteristics. Apart from variation in the shedding mode, changes in shedding cycle timing were also observed for sharp and rounded square cylinders. The hgher the r/B ratio, shedding in the near wake was found to be more uniform (lesser variation in shedding cycle timings). Another admissible shedding mechanism is newly identified to operate in the near wake of oscillating cylinders now being called as the ‘passive shedding’ mechanism. Results indicate that increasing the corner radius suppresses the possible instabilities of the cylinder.     

运用数值模拟研究车身局部造型与气动阻力系数的关系

首先,运用FLUENT对标准车进行数值模拟分析,验证了数值模拟的适用性及参数设置的正确性.其次,运用CAD初步设计了微型车的车身,在UG中建立了微型车的三维模型,对模型进行网格划分,将有限元模型导入FLUENT中,计算出车身的阻力系数.在此基础上,就车身局部形状对车身气动阻力系数的影响进行研究,分别得到了不同侧面倾角、...     

Experimental study on synthetic jet array for aerodynamic drag reduction of a simplified car

This paper describes an experimental parametric study of synthetic jet array actuation to reduce the aerodynamic drag of a threedimensional simplified car. By using two configurations of an Ahmed body with 25° and 35° slant angles, we performed wind tunnel tests under different conditions of synthetic jet array. Several parameters, namely, jet location, jet direction, jet momentum coefficient, jet driving frequency, and number and position of activated jets within the actuator array, were considered. The total aerodynamic drag coefficients were compared, and the rear wake flows were studied by using the data obtained from rear surface pressure distribution and flow visualization tests. Results of the parametric study show that the aerodynamic drag exhibits different behavior depending on the location of the jet for each slanted model. Jet direction, jet momentum coefficient, and jet driving frequency affect only the amount of change in the aerodynamic drag. The distribution of the activated jets also affects jet efficiency.     

借助辅助平面确定麻花钻切削角度

为了方便麻花钻的教学,文章通过建立空间辅助平面确定钻头切削角度,使抽象的麻花钻前、后角直观可感,便于学习者理解和应用。     

Effect of Reynolds numbers on flow past four square cylinders in an in-line square configuration for different gap spacings

In this paper two-dimensional (2-D) numerical investigation of flow past four square cylinders in an in-line square configuration are performed using the lattice Boltzmann method. The gap spacing g = s/d is set at 1, 3 and 6 and Reynolds number ranging from Re = 60 to 175. We observed four distinct wake patterns: (i) a steady wake pattern (Re = 60 and g = 1); (ii) a stable shielding wake pattern (80 ≤ Re ≤ 175 and g = 1); (iii) a wiggling shielding wake pattern (60 ≤ Re ≤ 175 and g = 3) and (iv) a vortex shedding wake pattern (60 ≤ Re ≤ 175 and g = 6). At g = 1, the Reynolds number is observed to have a strong effect on the wake patterns. It is also found that at g = 1, the secondary cylinder interaction frequency significantly contributes for drag and lift coefficients signal. It is found that the primary vortex shedding frequency dominates the flow and the role of secondary cylinder interaction frequency almost vanish at g = 6. It is observed that the jet between the gaps strongly influenced the wake interaction for different gap spacing and Reynolds number combination. To fully understand the wake transformations the details vorticity contour visualization, power spectra of lift coefficient signal and time signal analysis of drag and lift coefficients also presented in this paper.     

Sectional finite element analysis of forming processes for aluminum-alloy sheet metals

The sectional finite element analysis of the forming processes for the aluminum-alloy sheet metal known to be planar anisotropic was performed. The two-dimensional rigid-viscoplastic FEM formulation based on the bending augmented membrane theory as well as the anisotropic yield criteria was introduced. For modeling the anomalous behavior of aluminum-alloy sheet metals, Barlat's strain rate potential and Hill's (Journal of the Mechanics and Physics of Solids 1990;38:405–17) non-quadratic yield theory with an isotropic hardening rule were employed. Furthermore, a new method to determine anisotropic coefficients of Barlat's strain rate potential was proposed. For evaluating bending effects in the forming process of aluminum-alloy sheet metals, the bending equivalent forces were calculated in terms of the changes in the interior angle at a node between two linear finite elements and were augmented to the membrane stretch forces. In order to verify the validity of sectional finite element formulation based on the bending augmented membrane theory, the plane strain stretch/draw forming processes of a square cup test were simulated and simulation results are compared with experimental measurements. Friction coefficient was obtained from drawbead friction test. The properties of selected material were obtained from uniaxial tensile tests. Simulation shows good agreement with measurements. For the application of the sectional finite element formulation introduced in this research, the drawing process of a rear seat back upper bracket of passenger cars is simulated assuming plane strain condition. The thinning distribution of the simulation agreed well with that of the measurement, so that the sectional analysis is acceptable in the design and analysis of aluminum-alloy sheet stamping dies.     

Computational investigation on secondary flows in a linear turbine cascade with tapered dual fence

The focus of the present work is to minimize the secondary flow losses inside a linear turbine cascade by means of a novel design of streamwise dual fence. The leading edge and trailing edge of the fences have been modified so as to reduce the total pressure loss coefficient in the passage. The study has been carried out computationally based on RANS simulations with SST turbulence model. Numerous simulations have been undertaken with single fence and dual fence models and compared with the base case model. The dual fence model with tapered trailing edge exhibits significant loss reduction compared to the base case. A suitable fence height ratio (FHR) has been identified for the dual fence model. The FHR = 2 configuration reduces the secondary flow kinetic energy by 78 % within the blade passage and it reduces the exit angle deviation significantly throughout the span. Detailed flow field analysis has been carried out to understand the physical mechanism behind the loss reduction with dual fence models. It is observed that fence-1 breaks the pressure side leg of the horse shoe vortex at the beginning of their formation itself. The radial penetration of the suction side leg of the horse shoe vortex is restrained by fence-2. These combined effects prevent the formation and mixing of two prominent loss core regions thereby avoiding the accumulation of low energy fluid near the suction side of blade.

     

Comparison of flow characteristics around an equilateral triangular cylinder via PIV and Large Eddy Simulation methods

The flow structures around an equilateral triangular cylinder, which is commonly used as a vortex shedder in the vortex flowmeter, were investigated experimentally and numerically. Flow characteristics such as vorticity contours, patterns of sectional streamlines, velocity vectors, velocity fields, Reynolds stress correlations, Strouhal numbers and drag coefficients were examined using the Particle Image Velocimetry (PIV) technique and the Large Eddy Simulation (LES) turbulence model. Experimental studies were performed in an open water channel for Re=2.9×10³, Re=5.8×10³ and Re=1.16×10⁴ based on the equilateral triangle edge. A sharp-tip corner of the cylinder with a triangle cross-section was exposed to the upstream side while the other two sharp-tip corners were placed on the downstream side. Numerical studies were also completed at Reynolds numbers in the range of 2.9×10³≤Re≤1.16×10⁵ to obtain the changes in the Strouhal numbers and drag coefficients. When the results of PIV and LES are considered in the same interval of Reynolds numbers, the maximum and minimum values of each flow pattern were nearly the same. The time-averaged patterns had considerable symmetry with respect to the axis line passing through the sharp-tip corner of the cross-section of the triangular cylinder. The Strouhal number was independent of the Reynolds number and was found to be approximately 0.22. The drag coefficient decreased with increasing Reynolds numbers while increasing the Power Spectral Density (PSD) and the vortex shedding frequency. For the same Reynolds numbers, the experimental and numerical results were in good agreement. Therefore, the LES turbulence model is recommended for applications of flow around this type of bluff body that is generally used in the design of vortex flowmeters to generate vortex shedding.     

车尾凹坑非光滑表面气动减阻分析与优化设计

以凹坑型非光滑车身尾部气动特性为研究对象,探讨了一种将参数化建模、CFD计算和数值寻优方法相结合的非光滑表面气动减阻优化方法。通过分析凹坑型非光滑单元矩形阵列的气动减阻效果,以矩形排布和非光滑单元体尺寸作为优化对象,采用拉丁超立方抽样方法进行试验设计选取样本点。利用CFD仿真得到样本点的响应值,根据响应值建立了Kriging近似模型。在验证了近似模型可信度的基础上,以近似模型为基础进行全局优化。优化结果表明：车辆尾部凹坑单元体矩形排布最大减阻率可达7.9%,较大程度地改善了空气动力学性能。研究结果为汽车非光滑表面减阻和优化提供了理论依据和参考。     

潜艇长圆型舱门密封间隙研究

对潜艇内部平面耐压舱壁上的长圆门在一舱进水条件下的密封间隙进行研究,使用弹簧元模拟卡箍的力学行为,采用有限元方法考察设计状态下,门盖与围栏之间的间隙分布,并分析门盖板厚度以及结构形式对密封间隙的影响规律。研究表明:在计算工况下,卡箍处因总力平衡需要产生基础间隙,该间隙主要受载荷大小和卡箍弹簧刚度的影响;门盖在静水压力载荷作用下因自身变形产生了间隙的另一部分,该部分间隙的分布主要受门盖厚度和结构形式的影响。门盖板的厚度和结构形式影响门盖应力分布和间隙分布,厚度越大,周向边缘壳板的应力水平越小,间隙的最大值也越小。     

Comparative study of a turbulent wall-attaching offset jet and a plane wall jet

The flow field characteristics of a two-dimensional wall-attaching offset jet (WAOJ) are experimentally investigated by comparing with those of a turbulent plane wall jet(PWJ). The mean velocity, the turbulent stresses and triple velocity correlations are measured with a split film probe and anX wire probe. Even with the strong influence of the suction pressure field in the recirculation bubble at the lower corner, it is found that the WAOJ in the wall jet region has a close similarity with the PWJ. Especially, the decay of maximum velocity and the upper jet spread along the maximum velocity line of the WAOJ are virtually the same as those of the PWJ. The mean velocity profile of the WAOJ attains similarity after the jet impingement onto the lower plate. However the profiles of second and third-order moments of fluctuating velocities vary rapidly before the impingement and then relax very slowly to the similarity profiles of the PWJ.