基于涡旋识别方法的高速列车尾涡结构的讨论

利用改进型延迟分离涡模拟方法对缩尺比例1:30的高速列车简化模型的绕流流场进行数值计算,主要针对近尾流区的涡旋结构展开具体讨论.通过不同的涡旋识别方法,发现在尾涡结构中,高涡量的强涡旋主要聚集于尾车附近,而涡量较低但处于相对稳定状态的涡旋分布在大部分尾流空间中.对此,主要基于最新提出的涡旋定义及其物理意义认为,由于边界层在尾部发生的流动分离,剪切变形以及高涡量的扩散对强涡旋的形成发挥着重要的作用,而涡旋会被较强的剪切旋转拉伸,使得局部复杂的流动表现出突出的湍流特性;另一方面,尽管涡强度明显下降,但是在强剪切应变迅速衰减的情况下,流向涡核中的涡旋涡量是主要的,此时,在较接近地面的情况下,流体微团以涡核为中心的旋转运动使得涡旋与地面之间的相互作用成为主导的流动机制.虽然涡强度会相对缓慢地衰减,但是从湍流能量产生的角度,该机制对涡旋的自维持发挥重要的作用,从而使尾涡结构能够相对稳定地存在于尾流流动中.     

高速列车近尾流区湍流涡旋的数值计算及动力学讨论

主要针对缩尺比例1:30的高速动车组空气动力学模型的近尾流区域流场进行数值模拟,分析讨论了湍动能和湍流能量的产生,得到以下结论:对应于各展向位置的湍动能沿流向的变化规律与近尾流区涡旋结构携带能量的展向外移现象有关;在尾车鼻端附近,湍流涡旋具有显著的湍动能,并且各方向上的能量分量具有相同的量级,反映出湍流涡旋是高度三维的流动结构;靠近尾车鼻端的近尾流区湍流涡旋具有较强的从平均流动中提取能量的能力,并且结果表明,来自于车体底面和侧面的剪切流动沿垂向分别在一定空间范围内发挥重要的影响作用;大涡特征尺度沿流向增大,其中较小的涡旋对湍流能量的产生有主要贡献,同时,由于受到列车侧面较厚剪切层的影响,对湍流能量有贡献的涡旋所对应的积分尺度范围增大,从而使位于尾车鼻端附近的涡流能够在更大的流向范围内获取用于维持湍流尾流的能量.     

水下发射航行体尾涡不稳定性分析

在水下连续发射过程中前一发航行体尾流会对后一发航行体运动姿态稳定性产生流动干扰现象. 因此, 研究尾流中涡旋结构演变机理对解决多弹体水下连续发射流动干扰难题具有重要的意义. 本文采用改进型分离涡模型与能量方程, VOF多相流模型与重叠网格技术相结合方法, 对航行体水下发射尾流演变过程开展精细化模拟研究, 其中模拟结果和实验吻合度较好, 验证了本文数值方法的有效性. 以航行体尾流区域为重点研究对象, 分析了尾流区瞬态流场分布, 讨论了横流强度和雷诺数对尾涡结构演变以及脉动压力分布特性的影响. 结果表明: 由于尾流区高速流体核心区与低速自由流相互作用导致Kelvin-Helmholtz不稳定现象出现, 可以清晰地发现涡旋结构在剪切力的作用下发生脱落. 在横流条件下, 航行体尾端脱落的涡环与涡腿形成发卡涡, 而多个发卡涡沿轴向间隔排列组成发卡涡包存在于尾流中. 随着横流强度增大, 形成多级发卡涡包结构, 而导致脉动压力二次峰值均出现的主要原因是尾流涡旋流场演变引起的. 随着雷诺数的增大, 尾流中由圆柱形涡和U型涡组成的二次涡结构逐渐明显, 不稳定性加强.      

基于相交不稳定性的尾流自消散机翼实验研究

飞机尾流是复杂的流动现象,相关控制的研究常采用简化模型,抓住主要矛盾进行尾流不稳定性的学术探索. 采用结构化矩形机翼模型,通过添加扰流片来模拟襟翼的一种作动方式,引入一对与主翼涡反向的小涡,以期诱发尾涡的瑞利-路德维希相交不稳定性. 改变模型在水槽中的拖曳速度以及机翼攻角,采用粒子图像速度场仪定量研究单主翼尾涡发展特性以及双涡相互作用特性. 研究表明,未添加扰流片时,尾涡环量在45 个翼展内相对于初始环量衰减了10%;而添加了扰流片的实验中,在较好的实验参数组合情况下,主翼尾涡环量较初始环量降低35%~45%. 结果表明添加适当扰流片产生的反向小涡能诱发与主翼尾涡的相交不稳定性,在尾流涡系中引入自消散机制,加速机翼尾涡的消散过程,达到提早消弱尾涡的目的.      

舵几何特征对桨-舵系统尾流场演化的影响

发生在桨和舵之间的干扰会影响螺旋桨尾流的演化,导致尾流场中的湍流在下游增强,恶化船舶的振动和噪声性能,深入分析舵几何参数对桨-舵系统尾流场演化的影响能够为推进器尾流场的调节和减振降噪提供新思路.因此,从弦长、剖面和梯形舵入手分析不同的舵几何参数对螺旋桨尾流场演化特性的影响,使用大漩涡模拟方法模拟流场中的湍流结构,对不同舵弦长、剖面下的螺旋桨尾涡结构演化进行了分析,在舵弦长、剖面影响螺旋桨尾流场演化的研究的基础上分析了梯形舵对螺旋桨尾涡结构的影响,进一步分析了梯形舵影响下的螺旋桨尾流场中湍动能的分布.结果表明舵的弦长和剖面均会影响螺旋桨尾流场的演化,这种影响表现为更大的弦长和更厚的剖面会促进螺旋桨梢涡在舵压力面上的偏移,更薄的舵剖面会带来更强烈的螺旋桨毂涡偏移;涡管轮廓和舵表面脉动压力的对比均表明梯形舵会促进螺旋桨尾流场沿逆舵梯度方向偏移,从而导致螺旋桨的尾涡结构在舵两侧及下游呈现不对称分布,桨-舵系统下游的湍流结构与螺旋桨尾涡-舵碰撞过程、螺旋桨尾涡-舵随边涡干扰过程、螺旋桨梢涡-螺旋桨毂涡干扰有关,偏移更大的螺旋桨尾涡结构会在尾流场中更早地引起湍动能增强.     

旋翼尾流与地面干扰时地面涡现象的研究

用Ｎ－Ｓ方程对近地飞行时旋翼尾流与地面干扰时产生的地面涡现象进行了数值计算旋翼对流场的作用由分布在特定区域内的动量源项模拟结果表明，旋翼尾流撞到地面后的卷起和轴向流动的拉伸作用是形成地面涡的原因；地面边界层形成的二次分离涡向地面涡内输入（与尾流所携带的涡量）相反的涡量，而使地面涡保持平衡；地面涡的存在和运动使旋翼附近流场大大改变     

飞机尾涡演变及快速预测的大涡模拟研究

随着我国人民生活水平的提高,航空运输的重要性与日俱增,航班延误问题也日益严重.尾流间隔(保障后机不受前机尾流影响的最小安全间隔)是制约机场效率的关键因素.针对这一工程应用问题,采用大涡模拟方法研究飞机尾涡在大气中的演变特性.研究工作首先发展了飞机尾涡演变的大涡模拟方法,将自适应网格技术应用于飞机尾涡演变的大涡模拟,大幅减少所需的网格量,提高计算效率.提出了升力面尾涡生成方法,在不增加计算量的情况下实现了尾涡卷起过程和远场衰减的组合模拟.在系列算例分析研究基础上,创建了基于大涡模拟计算结果的尾流间隔快速预测系统.该系统可以根据实时大气风场和进出港的前后飞机机型,快速预测并输出所需的尾流间隔.经过与场地测试数据比较表明,在北京市2014年的平均风速条件下,本系统预测的尾流间隔可在现有标准基础上缩减7%～50%,能够有效提高机场容量.     

应用PIV技术测试涡旋波流场

涡旋波流动作为一种特殊的流动现象,可以使流体在相对较宽的槽道中产生较强的波动和对流混合,从而在小Re数条件下起到强化传质的效果。本文利用PIV流场显示技术,对振荡流在非对称槽道中所形成的涡旋波的产生机理和发展规律进行了实验研究和定量分析,测得了涡旋波流场的速度矢量图,阐明了涡旋波流场周期性变化的特点。分析了Re数和St数对涡旋波流动的影响,并得出了旋涡涡心位置以及涡心处涡量的动态变化规律。     

LARGE EDDY SIMULATION ON THE EVOLUTION AND THE FAST-TIME PREDICTION OF AIRCRAFT WAKE VORTICES

随着我国人民生活水平的提高,航空运输的重要性与日俱增,航班延误问题也日益严重.尾流间隔（保障后机不受前机尾流影响的最小安全间隔）是制约机场效率的关键因素.针对这一工程应用问题,采用大涡模拟方法研究飞机尾涡在大气中的演变特性.研究工作首先发展了飞机尾涡演变的大涡模拟方法,将自适应网格技术应用于飞机尾涡演变的大涡模拟,大幅减少所需的网格量,提高计算效率.提出了升力面尾涡生成方法,在不增加计算量的情况下实现了尾涡卷起过程和远场衰减的组合模拟.在系列算例分析研究基础上,创建了基于大涡模拟计算结果的尾流间隔快速预测系统.该系统可以根据实时大气风场和进出港的前后飞机机型,快速预测并输出所需的尾流间隔.经过与场地测试数据比较表明,在北京市2014年的平均风速条件下,本系统预测的尾流间隔可在现有标准基础上缩减7%~50%,能够有效提高机场容量.     

基于速度梯度张量特征值的陷窝内旋涡分析

作为一种新型的涡流发生器,陷窝具有流动阻力小、综合传热性能高的特点,是现代高性能涡轮叶片内部冷却新技术.旋涡的定量分析是陷窝强化传热优化设计的重要依据.针对在不同陷窝模型下的旋涡结构、分离方式和背景压力变化引起的旋涡强度无法定量分析的问题,本文提出采用涡核速度和涡核速度梯度张量特征值来定量分析旋涡的方法.通过采用涡核处局部坐标系表示的速度矢量和速度梯度张量,得到了涡核的轴向速度、径向速度、旋转角速度、轴向加速度和径向加速度,并在此基础上简化出了用最大轴向速度、最大轴向加速度和最大旋转角速度综合表示的旋涡强度的定量分析方法.用该方法分析了不同深宽比陷窝诱导的旋涡结构,随着深宽比的增大,最大轴向速度、最大轴向加速度和最大旋转角速度均呈现明显的增大趋势,旋涡强度增大.研究表明此方法具有数据处理简单、通用性强、不受分离方式限制、不受背景压力影响的特点,且提取到的数据具有明确的物理意义,适用于各类旋涡定量分析.     

Multisensor Hot Wire Vorticity Probe Measurements of the Formation Field of Two Corotating Vortices

Experimental evidence is reported, regarding the formation of a pair of co-rotating tip vortices by a split wing configuration, consisting of two half wings at equal and opposite angles of attack. Simultaneous measurements of the three-dimensional vector fields of velocity and vorticity were conducted on a cross plane at a downstream distance corresponding to 0.3 cord lengths (near wake), using an in-house constructed 12-sensor hot wire anemometry vorticity probe. The probe consists of three closely separated orthogonal 4-wire velocity sensor arrays, measuring simultaneously the three-dimensional velocity vector at three closely spaced locations on a cross plane of the flow filed. This configuration makes possible the estimation of spatial velocity derivatives by means of a forward difference scheme of first order accuracy. Velocity measurements obtained with an X-wire are also presented for comparison. In this near wake location, the flow field is dictated by the pressure distribution established by the flow around the wings, mobilizing large masses of air and leading to the roll up of fluid sheets. Fluid streams penetrating between the wings collide, creating on the cross plane flow a stagnation point and an “impermeable” line joining the two vortex centres. Along this line fluid is directed towards the two vortices, expanding their cores and increasing their separation distance. This feeding process generates a dipole of opposite sign streamwise mean vorticity within each vortex. The rotational flow within the vortices obligates an adverse streamwise pressure gradient leading to a significant streamwise velocity deficit characterizing the vortices. The turbulent flow field is the result of temporal changes in the intensity of the vortex formation and changes in the position of the cores (wandering).     

均匀、振荡及组合流绕平板流动的尾迹研究

本文利用离散涡模型及改进的新生涡产生机制对三种不同来流绕平板的近尾迹进行数值研究。计算讨论了定常流中平板绕流流动的总体特性和近尾迹流场;对于简谐振荡来流,相应于K_c=2.0、4.0 和10.0 分别得到两种不同的尾迹形态。给出了小 K_c 数平板尾迹涡配对、运动的新模式而相应的阻力、惯性力系数计算比以前涡模拟结果更接近于 U 型管实验结果。对于流向组合来流本文模拟了涡锁定及其动力特性并于实验相符,给出了流向扰动对平板绕流流动的影响。     

PIV analysis of flow around a container ship model with a rotating propeller

The flow characteristics of the propeller wake behind a container ship model with a rotating propeller were investigated using a two-frame PIV (Particle Image Velocimetry) technique. Ensemble-averaged mean velocity fields were measured at four different blade phases and ensemble-averaged to investigate the flow structure in the near-wake region. The mean velocity fields in longitudinal planes show that a velocity deficit is formed in the regions near the blade tips and hub. As the flow develops in the downstream direction, the trailing vortices formed behind the propeller hub move upward slightly due to the presence of the hull wake and free surface. Interaction between the bilge vortices and the incoming flow around the hull causes the flow structure to be asymmetric. Contour plots of the vorticity give information on the radial distribution of the loading on the blades. The radial velocity profiles fluctuate to a greater extent under the heavy (J=0.59) and light loading (J=0.88) conditions than under the design loading condition (J=0.72). The turbulence intensity has large values around the tip and trailing vortices. As the wake develops in the downstream direction, the strength of the vorticity diminishes and the turbulence intensity increases due to turbulent diffusion and active mixing between the tip vortices and the adjacent wake flow.     

Numerical study on wake characteristics of high-speed trains

Intensive turbulence exists in the wakes of high speed trains, and the aerodynamic performance of the trailing car could deteriorate rapidly due to complicated features of the vortices in the wake zone. As a result, the safety and amenity of high speed trains would face a great challenge. This paper considers mainly the mechanism of vortex formation and evolution in the train flow field. A real CRH2 model is studied, with a leading car, a middle car and a trailing car included. Different running speeds and cross wind conditions are considered, and the approaches of un- steady Reynold-averaged Navier-Stokes （URANS） and de- tached eddy simulation （DES） are utilized, respectively. Re- suits reveal that DES has better capability of capturing small eddies compared to URANS. However, for large eddies, the effects of two approaches are almost the same. In conditions without cross winds, two large vortex streets stretch from the train nose and interact strongly with each other in the wake zone. With the reinforcement of the ground, a complicated wake vortex system generates and becomes strengthened as the running speed increases. However, the locations of flow separations on the train surface and the separation mechanism keep unchanged. In conditions with cross winds, three large vortices develop along the leeward side of the train, among which the weakest one has no obvious influence on the wake flow while the other two stretch to the tail of the train and combine with the helical vortices in the train wake. Thus, optimization of the aerodynamic performance of the trailing car should be aiming at reducing the intensity of the wake vortex system.     

NUMERICAL STUDY OF PARTICLE DISTRIBUTION IN WAKE OF LIQUID-PARTICLE FLOWS PAST A CIRCULAR CYLINDER USING DISCRETE VORTEX METHOD

Particle-laden water flows past a circular cylinder were numerically investigated. The discrete vortex method (DVM) was employed to evaluate the unsteady water flow fields and a Lagrangian approach was applied for tracking individual solid particles. A dispersion function was defined to represent the dispersion scale of the particle. The wake vortex patterns, the distributions and the time series of dispersion functions of particles with different Stokes numbers were obtained. Numerical results show that the particle distribution in the wake of the circular cylinder is closely related to the particle's Stokes number and the structure of wake vortices: (1) the intermediate sized particles with Stokes numbers, St, of 0.25, 1.0 and 4.0 can not enter the vortex cores and concentrate near the peripheries of the vortex structures, (2) in the circular cylinder wake, the dispersion intensity of particles decreases as St is increased from 0.25 to 4.0.     

斑马鱼直线加速游动试验研究

海洋生物低噪音、高速、髙效游动能力是任何人造水下航行器所无法比拟的. 借助时间解析粒子图像测速技术对斑马鱼直线加速游动过程进行精细流场测量, 对其运动学行为特性和动力学机理进行分析. 同时应用双正交分解对涡量场进行模态分解, 获取流场的时间演化和空间分布特征. 从流动机理的角度探究斑马鱼游动过程的流动结构特征及旋涡动态演化特性. 试验结果表明: 流动可视化展现了整体涡流尾迹的结构分布,方便探究运动特性与旋涡尾迹之间的耦合关系. 斑马鱼从运动开始时体干保持着鲹科式的运动规律, 游动时的动能主要由前几次大幅的摆尾过程提供, 后续的摆尾主要调整方向及姿态. 两次不同方向的摆尾动作会形成一对方向相反的旋涡, 并在时序下旋涡逐渐脱落. 同时尾流的涡量变化在一定程度上反映鱼体的游向的变化. 基于双正交分解分解后的时间演化结果验证本次试验在时间上涡量场具有合理的恒定幅度, 空间分布表明低阶空间模态表征斑马鱼游动的主要涡流动结构, 高阶空间模态表征涡流动的细节结构. 研究鱼类游动时的摆尾推进机制与动力学特性能够为高效率的仿鱼类推进装置设计提供一定科学参考.      

Asymmetric vortex shedding flow past an inclined flat plate at high incidence

This paper reports an experimental investigation of the vortex shedding wake behind a long flat plate inclined at a small angle of attack to a main flow stream. Detailed velocity fields are obtained with particle-image velocimetry (PIV) at successive phases in a vortex shedding cycle at three angles of attack, α=20°, 25° and 30°, at a Reynolds number Re≈5,300. Coherent patterns and dynamics of the vortices in the wake are revealed by the phase-averaged PIV vectors and derived turbulent properties. A vortex street pattern comprising a train of leading edge vortices alternating with a train of trailing edge vortices is found in the wake. The trailing edge vortex is shed directly from the sharp trailing edge while there are evidences that the formation and shedding of the leading edge vortex involve a more complicated mechanism. The leading edge vortex seems to be shed into the wake from an axial location near the trailing edge. After shedding, the vortices are convected downstream in the wake with a convection speed roughly equal to 0.8 the free-stream velocity. On reaching the same axial location, the trailing edge vortex, as compared to the leading edge vortex, is found to possess a higher peak vorticity level at its centre and induce more intense fluid circulation and Reynolds stresses production around it. It is found that the results at the three angles of attack can be collapsed into similar trends by using the projected plate width as the characteristic length of the flow.     

Intrinsic relationship of vorticity between modes A and B in the wake of a bluff body

The intrinsic physical relationship of vorticity between modes A and B in the three-dimensional wake transition is investigated.Direct numerical simulations for the flow past a square-section cylinder are carried out at Reynolds numbers of 180 and 250,associated with modes A and B,respectively.Based on the analysis of spacial distributions of vorticity in the near wake,characteristics of the vertical vorticity in modes A and B are identified.Moreover,the relationship of three vorticity components with specific signs is summarized into two sign laws,as intrinsic physical relationships between two instability modes.By the theory of vortex-induced vortex,such two sign laws confirm that there are two and only two kinds of vortex-shedding patterns in the near wake,just corresponding to modes A and B.In brief,along the free stream direction,mode A can be described by the parallel shedding vertical vortices with the same sign,while mode B is described by the parallel shedding streamwise vortices with the same sign.Finally,it is found out that the|-type vortex is a basic kind of vortex structure in both modes A and B.     

Wind Impact on Single Vortices and Counterrotating Vortex Pairs in Ground Proximity

Wall-resolved large eddy simulations are employed to investigate the behaviour of wake vortices and single vortices in ground proximity at a variety of wind conditions. The six considered strengths of wind, ranging between 0.5 and 4 times the initial wake vortex descent speed, w₀, include practically and theoretically significant wind speeds. A crosswind of 0.5 w₀ may lead to windward stall posing a potential hazard to subsequently landing aircraft, whereas theoretical considerations predict that at 4 w₀ the rebound of the luff vortex is completely suppressed. The same range of wind speeds is also used to investigate the effects of headwind and diagonal wind in order to discriminate between effects of environmental turbulence increasing with wind speed and the direction of the wind shear. The study has been complemented by a number of single vortex computations in order to differentiate between effects related to the mutual interaction of the vortex pair and the individual vortices with the turbulent boundary layer flow. It is shown that vortex ascent, descent, rebound and decay characteristics are controlled by (i) the interaction of the vortices with secondary vorticity detaching from the ground, (ii) the redistribution of vorticity of the boundary layer which is altering the path of the primary vortices by mutual velocity induction, and (iii) the interaction of the vortices with the environmental turbulence.