Numerical Study on the Effect of Vortex Generators on the Aerodynamic Drag of a High-Speed Train

A relatively high aerodynamic drag is an important factor that hinders the further acceleration of high-speed trains.Using the shear stress transport(SST)k-ωturbulence model,the effect of various vortex generator types on the aerodynamic characteristics of an ICE2(Inter-city Electricity)train has been investigated.The results indi-cate that the vortex generators with wider triangle,trapezoid,and micro-ramp arranged on the surface of the tail car can significantly change the distribution of surface pressure and affect the vorticity intensity in the wake.This alteration effectively reduces the resistance of the tail car.Meanwhile,the micro-ramp vortex generator with its convergent structure at the rear exhibits enhancedflow-guiding capabilities,resulting in a 15.4%reduction in the drag of the tail car.

Application of Vortex Generators to Remove Heat Trapped in Closed Channels

The utilization of vortex generators to increase heat transfer from cylinders installed inside a duct is investigated.In particular,a channel containing eight cylinders with volumetric heat sources is considered for different values of the Reynolds number.The effective possibility to use vortex generators with different sizes to increase heat transfer and,consequently,reduce the surface temperature of the cylinders is examined.Also,the amount of pressure drop inside the channel due to the presence of vortex generators is considered and compared with the cases without vortex generators.The results show that although the addition of generators increases the pressure drop,it strongly contributes to increase the heat transfer coefficient inside the duct(up to 80–90%).

Large Eddy Simulation of New Vortex Generator Enhancing Heat Exchange of Solar Energy

This paper put forward a new-type vortex generator enhancing heat exchange of solar air-drier and air-heater on the gas side,and investigated the mechanism of heat transfer enhancement and drag reduction by the influence of vortex generators on the coherent structure of turbulent boundary layer.The flow and heat transfer characteristics of rectangle channel with bevel-cut half-elliptical column vortex generators were obtained using large eddy simulation(LES)and the hydromechanics software FLUENT6.3.The instantaneous properties of velocity,temperature and pressure in channel were gained.The coherent structure of turbulent boundary layer flow was showed,and the characteristic of vortex induced by inclined-cut semi-ellipse vortex generator and its influence on turbulent coherent structure were analyzed.And the effect mechanism of turbulent coherent structure on flow field,pressure field and temperature field was discussed.Based on the results,the heat transfer coefficient and drag reduction of the new vortex generator with different pitch angles were compared.Sometimes,the coherent effects of the increased wall heat transfer and the decreased skin friction do not satisfy the Reynolds analogy.The turbulent coherent structure can be controlled through the geometry of the vortex generator,so the heat transfer and drag reduction can also be controlled.Then we can seek suitable form of vortex generator and structure parameters,in order to achieve the enhanced heat transfer and flow of drag reduction in the solar air-heater and solar air-drier.

Simulating the Turbulent Hydrothermal Behavior of Oil/MWCNT Nanofluid in a Solar Channel Heat Exchanger Equipped with Vortex Generators

Re-engineering the channel heat exchangers(CHEs)is the goal of many recent studies,due to their great importance in the scope of energy transport in various industrial and environmental fields.Changing the internal geometry of the CHEs by using extended surfaces,i.e.,VGs(vortex generators),is the most common technique to enhance the efficiency of heat exchangers.This work aims to develop a newdesign of solar collectors to improve the overall energy efficiency.The study presents a new channel design by introducing VGs.The FVM(finite volume method)was adopted as a numerical technique to solve the problem,with the use of Oil/MWCNT(oil/multi-walled carbon nano-tubes)nanofluid to raise the thermal conductivity of the flow field.The study is achieved for a Re number ranging from12×10^(3) to 27×10^(3),while the concentration(φ)of solid particles in the fluid(Oil)is set to 4%.The computational results showed that the hydrothermal characteristics depend strongly on the flow patterns with the presence of VGs within the CHE.Increasing the Oil/MWCNT rates with the presence of VGs generates negative turbulent velocities with high amounts,which promotes the good agitation of nanofluid particles,resulting in enhanced great transfer rates.

Influence of Different Vortex Generators on Heat Transfer in Direct Air-Cooled Condensers

通过数值模拟研究了矩形小翼、三角小翼、梯形小翼、柱面梯形翼、柱面三角翼和柱面矩形翼等6种涡流发生器分别安装于直接空冷凝汽器单排蛇形翅片扁管中的压降和换热性能。Re数在600~1800范围内,涡流发生器采用相同高宽比、尾高、攻角以及安装位置。结果表明：矩形小翼的换热及压降增大最多,Re=1729时,相比平直翅片分别增加了14.27%和18.32%。Re=1 729时,柱面梯形翼的压力损失比矩形小翼少4.7%,换热低1.5%。当以（j/j0）/（f/f0）1/3作为综合换热性能评价标准时,柱面梯形翼的综合换热性能最好。另外,考察了柱面梯形翼倾角的影响（0°、6°、12°、20°、24°和26°）,相同工况下,倾角为12°的柱面梯形翼换热最好（较平直翅片增强4.29%~14.1%）、压降最小（增大7.5%~12.8%）且综合换热性能最好。柱面梯形翼因其迎流面积与斜边长度的匹配以及流线型柱面,具有良好的强化传热效果以及较小的压降。

NUMERICAL STUDY OF FLOW IN CONICAL DIFFUSER WITH VORTEX GENERATOR JETS

To develop vortex generator jet （VGJ） method for flow control, the turbulence flow in a 14° conical diffuser with and without vortex generator jets are simulated by solving Navier-Stokes equations with k-ε turbulence model. The diffuser performance, based on different velocity ratio （ratio of the jet speed to the mainstream velocity）, is investigated and compared with the experimental study. On the basis of the flow characteristics using computation fluid dynamics （CFD） method observed in the conical diffuser and the downstream development of the longitudinal vortices, attempt is made to correlate the pressure recovery coefficient with the behavior of vortices produced by vortex generator jets.

Manipulation of vertical fence wake using passive vortex generators

The effect of streamwise vortices generated from passive vortex generators was investigated to manipulate the separation bubble behind the vertical fence. The experiments were carried out in a circulating water channel and the velocity fields were measured using 2D and stereoscopic PIV method.The distance between the vortex generator and fence and the effect of the Reynolds number were investigated. In addition,the effect of boundary layer thickness was also investigated. The averaged recirculation lengths were compared with that of uncontrolled fence flow. The results showed the oscillatory variation of recirculation region appeared under the existence of vortex generators. The reduction of the separation bubble became larger when the fence was submerged in the thick boundary layer with increasing the distance between the generator and fence. When the boundary layer is thin,vortex generator can only suppress the separation bubble under the specific condition.

Experimental and numerical analysis of a demister with vortex generators

An improved wave-plate demister equipped with vortex generators(VGs)has been proposed for wet flue gas desulfurization systems(WFGD)in this work.Numerical and experimental methods were used to evaluate the effect of VGs on the separation of small droplets.Five types of wave-plate demister with different VGs were analyzed.The vortex generators in question included rectangular plates,semi-elliptical plates,square tubes,round tubes,and triangular tubes,respectively.In order to explain the strengthening mechanism,the distribution of flow field,secondary flow,and droplet trajectory were shown,and the effect of VGs on the flow field in the demister was discussed in depth.The simulation results show that the separation performances of the demisters with VGs were significantly improved over that of the initial demister,and the accompanying pressure drop was small.For the vortex generators studied,the rectangular plate fully demonstrated its superior separation performance,followed by semi-elliptical plate.The strengthening effect of VGs was tested through experiments.Experimental data reveal that the average droplet diameter(D_(50))at the outlet of the demister with a vortex generator can be reduced to 23.13μm,whereas this value for the initial demister can be maintained at 32.07μm.Moreover,compared with the original demister 0.81,the overall separation efficiency of the improved demister was improved to 0.92.

Heat Transfer Enhancement by Vortex Generators for Compact Heat Exchanger of Automobiles

The paper describes the effects of heat transfer enhancement and gas-flow characteristics by wing-type-vortex-generators inside a rectangular gas-flow duct of a plate-fin structure exhaust gas recirculation (EGR) cooler used in a cooled-EGR system. The analyses are conducted using computational fluid dynamics (CFD). The numerical modelling is designed as a gas-flow rectangular duct of an EGR cooler using two fluids with high temperature gas and coolant water whose flow directions are opposite. The gas-flow duct used to separate two fluids is assembled with a stainless steel material. The inlet temperature and velocity of gas flowed inside gas-flow duct are 400°C and 30 m/s, respectively. Coolant water is flowed into two ducts on both a top and a bottom surface of the gas-flow duct, and the inlet temperature and velocity is 80°C and 0.6 m/s, respectively. Wing-type-vortex-generators are designed to achieve good cooling performance and low pressure drop and positioned at the center of the gas-flow duct with angle of inclination from 30 to 150 degrees at every 15 degrees. The temperature distributions and velocity vectors gained from numerical results were compared, and discussed. As a result, it is found that the vortices guided in the proximity of heat transfer surfaces play an important role in the heat transfer enhancement and low pressure drop. The collapse of the vortices is caused by complicated flow induced in the corner constituted by two surfaces inside gas-flow duct.

CFD-Based Simulation and Analysis of Hydrothermal Aspects in Solar Channel Heat Exchangers with Various Designed Vortex Generators

The hydrothermal behavior of air inside a solar channel heat exchanger equipped with various shaped ribs is analyzed numerically.The bottom wall of the exchanger is kept adiabatic,while a constant value of the temperature is set at the upper wall.The duct is equipped with a flat rectangular fin on the upper wall and an upstream V-shaped baffle on the lower wall.Furthermore,five hot wall-attached rib shapes are considered:trapezoidal,square,triangular pointing upstream(type Ⅰ),triangular pointing downstream(type Ⅱ),and equilateral-triangular(type Ⅲ)cross sections.Effects of the flow rates are also inspected for various Reynolds numbers in the turbulent regime(1.2×10^(4)-3.2×10^(4)).The highest performance(η)value is given for the Ⅱ-triangular rib case in all Re values,while the square-shaped ribs show a significant decrease in the η along the achieved Re range.The η value at Remax is 2.567 for the Ⅱ-triangular roughness case.Compared with the other simulated cases,this performance is decreased by about 3.768%in the case of Ⅰ-triangular ribs,15.249% in the case of Ⅲ-triangular ribs,20.802% in the case of trapezoidal ribs,while 27.541% in the case of square ribs,at the same Remax.Also,a comparison ismade with air-heat exchangers that have non-rough walls and contain cross-shaped VGs presented previously,in order to highlight the effectiveness of the rough surface presence in the baffled and finned channels.The obtained results indicated that the triangular-shaped rib(type Ⅱ)has the most significant hydrothermal behavior than the other cases.This indicates the necessity of roughness heat transfer surfaces for finned and baffled channels to improve significantly the performance of the air-heat exchangers they contain.

Study of Eppler 423 Airfoil with Gurney Flap and Vortex Generators

In the past extensive research has been carried out, to study the effect of Gurney flap (GF) on symmetric and cambered airfoil for its usage in low Reynolds number regime. Use of GF at the trailing edge of the airfoil enhances the lift due to increase in the effective camber of the airfoil, which in turn improves the aerodynamic efficiency i.e. Cl/Cd. In the present study, Eppler 423 airfoil is used to first understand the aerodynamics of such a highly cambered airfoil and later GF of various sizes were added on it to understand the change in flow dynamics achieved by adding the GF and their impact on aerodynamic parameters such as Cl, Cd and Cl/Cd. Eppler 423 being a highly cambered airfoil produces high lift coefficient and smoother stall and by adding the GF of various sizes the performance of Eppler 423 improves tremendously and reason for this enhanced performance and effect of size of GF are presented in this paper. Vortex Generators (VG) generate counter rotating vortices that allow the flow to remain attached even at high angles of attack. Also, effect of adding VG at the leading edge of Eppler 423 aerofoil is presented in this paper. At last, results obtained from combination of VG at leading edge and GF at trailing edge on Eppler 423 aerofoil are discussed at length.

Numerical Investigation on Heat Transfer Enhancement Inside a Rect-angular M icrochannel with Vortex Generator using TiO2,Cuo-water Nanofuids

One of the innovative ways to improve heat transfer properties of heat exchangers,is using nanofluids instead of traditional fluids.Due to presence of metal and oxides of metal particles in nanofluids structure,they have better potential in different environments and conditions than conventional fluids and having higher thermal conductivity causes improvements in heat transfer properties.In this research flow of two different nanofluids through a rectangular microchannel containing a different number of longitudinal vortex generators(Ivgs),has been investigated.Simulations conducted under laminar flow boundary condition and for varied Reynolds numbers of 100 to 250.Considered volumetric concentration in this paper is 1,1/6 and 2/3%Results showed,nanofluids and the LVGs notably improve the heat transtfer rates within the microchannel.havg improved with increasing the nanoparticles volume concentrations and Reynolds number,while the op posite trends recognized for pressure drop.havg improved for4 to 12 and 9 to 18%for TO2 and CuO nanofuids,respectively for different volume concentrations in simple microchannel.For lvg-enhanced microchannel the amount of improvements is about 9-14 and 5-10%for CuO and T0,respectively.Also using vortex generators alone improved havg for 15-25%for different number of Ivgs.

Effects of Vortex Generator Jet on Corner Separation/Stall in High-Turning Compressor Cascade

The effects of the vortex generator jet(VGJ)attached at the endwall on the corner separation/stall control are investigated by numerical simulation in a high-turning linear compressor cascade. The results show that the corner separation could be reduced significantly, which results in a wider operation range as well as a more uniform exit flow angle and total pressure profile. At the near-stall operation point, the maximum relative reduction of the total pressure loss is up to 32.5%,, whereas the jet mass ratio is less than 0.4%,. Based on the analysis of the detailed flow structure, three principal effects of the VGJ on the endwall cross flow and corner separation are identified. One is to increase the tangential velocity component opposite to cross flow, thus inhibiting the endwall secondary flow near the jet exit. The second is to suppress the pitchwise extension of the passage vortex as an air fence. The third is to sweep the low energy fluids towards the mainstream on the up-washed side and to transport the mainstream fluids to the endwall to reenergize the boundary layer on the down-washed side.

GENERALIZED ENERGY CONSERVATION AND UNSTABLE PERTURBATION PROPERTY IN BAROTROPIC VORTEX

Based on a barotropic vortex model, generalized energy-conserving equation was derived and twonecessary conditions of basic flow destabilization are gained. These conditions correspond to generalizedbarotropic instability and super speed instability. They are instabilities of vortex and gravity inertial waverespectively. In order to relate to practical situation, a barotropic vortex was analyzed, the basic flow of which issimilar to lower level basic wind field of tropical cyclones and the maximum wind radius of which is 500 km.The results show that generalized barotropic instability depending upon the radial gradient of relative vorticitycan appear in this vortex. It can be concluded that unstable vortex Rossby wave may appear in barotropic vortex.

The Generalized Energy Equation and Instability in the Two-layer Barotropic Vortex

在圆柱的坐标的线性二层的 barotropic 原语方程被用来导出一个概括精力方程，它随后被使用在模型解释螺线波浪的不稳定性。在二层的模型，有不仅不仅概括 barotropic 不稳定性，它掉进 Kelvin-Helmholtz 不稳定性的范围和概括 baroclinic 不稳定性，当上面、更低的基本流动是不同的时。他们也许也是在热带气旋的螺线云带的产生的机制。

气动翼型涡流发生器与风力机翼型同步优化设计

针对目前风力机叶片翼型与涡流发生器相互独立的串行设计方法,导致不能最大限度的设计出气动性能最佳的风力机新叶片加装新型涡流发生器。这里提出带气动翼型的涡流发生器与风力机叶片翼型同步设计方法,利用B样条函数分别表达叶片翼型与涡流发生器气动形状,以最大升阻比及失速前最大升力系数为多目标函数建立优化数学模型,结合CFD技术与粒子群算法对原始叶片翼型DU97-W-300与CLARKY翼型涡流发生器进行同步优化,优化结果表明:相比原始叶片翼段加装CLARKY翼型涡流发生器,优化后的升力系数为2.378,提高了9.3%,升阻比为63.335,提高了7.5%;且对优化前后的流线图及涡量云图进行了对比分析,说明优化后叶片加装涡流发生器翼型可产生更大的诱导涡,能够有效抑制叶片表面流体分离。这里研究对于叶片与涡流发生器的设计及应用具有很好的参考与借鉴意义。

双排涡流发生器高度参数对风力机气动性能影响的研究

为了研究双排涡流发生器高度参数对风力机气动性能的影响,以NREL Phase VI风力机叶片为模型,采用CFD方法分别对加装单排涡流发生器、不同高度参数双排涡流发生器共10种模型进行模拟,分析其在不同风速、转速下对风力机叶片气动性能的影响。计算结果表明,所有不同高度参数双排涡流发生器在不同转速、来流风速时,均能提升风力机叶片气动性能,改善风力机流场。其中,第一排涡流发生器与第二排涡流发生器高度差越大时,双排涡流发生器整体流动控制效果最好,即最佳高度参数Case 4(第一排涡流发生器高度3 mm,第二排涡流发生器高度9 mm)前低后高组合。同时,最佳高度参数双排涡流发生器和单排涡流发生器相比能进一步延迟流动分离,取得更好的流动控制效果。

微细粒锡石涡流强化浮选及CFD数值模拟研究

为提高微细粒锡石矿物的浮选回收效果,采用基于湍流涡调控技术的新型涡流浮选装置对微细粒锡石(d50=16.45μm)进行了浮选实验研究,考察了涡流发生器结构及矿浆循环量对微细粒锡石浮选的影响。通过涡流矿化管内部流场CFD数值模拟,分析了涡流矿化改善微细粒浮选效果的原因。浮选实验表明,使用矩形涡流发生器诱导方式且在循环量0.6 m3/h(Re=21 220)时锡石的回收率为92.96%,精矿锡品位为3.32%。在获得精矿锡品位几乎相同的情况下,回收率相较于光管提高了25.73个百分点;CFD模拟分析结果表明内置矩形涡流发生器的矿化管内平均湍流耗散率和平均湍流动能最高,分别是光管的13.01倍和7.03倍,碰撞概率从1.07%提高到2.32%,矩形涡流发生器能显著改善矿化管的湍流环境,增大了微细颗粒-气泡的碰撞概率,从而起到强化微细粒锡石浮选的作用。

风电机组高塔一阶涡激振动特性研究

针对风力发电机组钢制高塔的涡激振动问题,采用固体-流体相结合的仿真方法对停机状态的塔架一阶涡激振动特性进行研究。首先以某2.5 MW、140 m高塔机组为研究对象,建立塔架-叶片耦合结构仿真模型,利用模态和CFD仿真分析塔架一阶涡激振动的共振频率和共振风速范围,表明该风电机组塔架存在发生一阶涡激振动的风险;然后提出一种基于固体-流体迭代快速仿真一阶涡激振动最大位移计算方法,考虑叶片对塔架涡激振动的耦合作用,得到最大振动位移。现场测试表明实测最大一阶涡激振动位移与仿真结果基本相符,验证了该仿真方法的准确性。

风力机涡流发生器斜向布局多目标优化

基于最优拉丁超立方试验设计法确定涡流发生器斜向布局参数和试验方案,耦合计算流体力学方法(CFD)和滑移网格技术计算风力机的推力和扭矩,并耦合反向传播(BP)神经网络与遗传算法,建立风力机涡流发生器斜向布局气动性能模型。通过5组测试算例,建立遗传算法优化BP神经网络的风力机涡流发生器斜向布局气动性能模型,气动性能模型预测值和模拟仿真值的误差与均方根均较小,表明气动模型的准确性较高。耦合多目标遗传算法,构建风力机涡流发生器斜向布局多目标优化方法,开展风力机涡流发生器优化。结果表明:相比原涡流发生器方案,优化后的涡流发生器诱导涡的最大涡量增大10.47%,风力机叶片截面流动分离得到有效抑制,风力机功率提升9.963%,而推力仅增大1.864%,风力机气动性能得到进一步提升。