On hairpin vortex generation from near-wall streamwise vortices

The generation of a hairpin vortex from near-wall streamwise vortices is studied via the direct numerical simulation（DNS） of the streak transient growth in the minimal channel flow at Re_τ- 400.The streak profile is obtained by conditionally averaging the DNS data of the fully developed turbulent channel flow at the same Reynolds number.The near-wall streamwise vortices are produced by the transient growth of the streak which is initially subjected to the sinuous perturbation of the spanwise velocity.It is shown that the arch head of the hairpin vortex first grows from the downstream end of the stronger streamwise vortex and then connects with the weaker,opposite-signed streamwise vortex in their overlap region,forming a complete individual hairpin structure.The vorticity transport along the vortex lines indicates that the strength increase and the spatial expansion of the arch head are due to the stretching and the turning of the vorticity vector,respectively.The hairpin packets could be further produced from the generated individual hairpin vortex following the parent-offspring process.

A Numerical Study on Supersonic Combustion Optimization Based on the Streamwise Vortex-Couple Method

In this paper,some typical methods to promote mixing in supersonic combustion are reviewed,and the fluid-dynamic mechanism underpinning the development of the supersonic shear layer in the presence of a streamwise vortex is analyzed through computational fluid dynamics.It is proven that the streamwise vortex-couple method is an excellent approach to enhance mixing.A specific combustor design is proposed accordingly.

Active control of turbulence for drag reduction based on the detection of near-wall streamwise vortices by wall information

The spatial relations between the measurable wall quantities （streamwise shear stress τwx, spanwise shear stress τwz, and pressure fluctuations Pw） and the near-wall streamwise vortices （NWSV） are investigated via direct numerical simulation （DNS） databases of fully developed turbulent channel flow at a low Reynolds number. In the stan- dard turbulent channel flow, the results show that all the wall measurable variables are closely associated with the NWSV. But after applying a stochastic interference, the relation based on τwx breaks down while the correlations based on Pw and τwz are still robust. Hence, two wall flow quantities based on Pw and τwz are proposed to detect the NWSV. As an appli- cation, two new control schemes are developed to suppress the near-wall vortical structures using the actuation of wall blowing/suction and obtain 16 % and 11% drag reduction, respectively.

Numerical simulation of quasi-streamwise hairpin-like vortex generation in turbulent boundary layer

A mechanism for generation of near wall quasi-streamwise hairpin-like vortex （QHV） and secondary quasi-streamwise vortices （SQV） is presented. The conceptual model of resonant triad in the theory of hydrodynamic instability and direct numerical simulation of a turbulent boundary layer were applied to reveal the formation of QHV and SQV. The generation procedures and the characteristics of the vortex structures are obtained, which share some similarities with previous numerical simulations. The research using resonant triad conceptual model and numerical simulation provides a possibility for investigating and controling the vortex structures, which play a dominant role in the evolution of coherent structures in the near-wall region.

Relationship between wall shear stresses and streamwise vortices in turbulent flows over wavy boundaries

The relationship between wall shear stresses and near-wall streamwise vortices is investigated via a direct numerical simulation(DNS) of turbulent flows over a wavy boundary with traveling-wave motion. The results indicate that the wall shear stresses are still closely related to the near-wall streamwise vortices in the presence of a wave. The wave age and wave phase significantly affect the distribution of a two-point correlation coefficient between the wall shear stresses and streamwise vorticity. For the slow wave case of c/Um = 0.14, the correlation is attenuated above the leeward side while the distribution of correlation function is more elongated and also exhibits a larger vertical extent above the crest. With respect to the fast wave case of c/U_m=1.4, the distribution of the correlation function is recovered in a manner similar to that in the flat-wall case. In this case, the maximum correlation coefficient exhibits only slight differences at different wave phases while the vertical distribution of the correlation function depends on the wave phase.

Physical mechanism for origin of streamwise vortices in mode A of a square-section cylinder

In the present paper, physical mechanism responsible for origin of streamwise vortices in mode A appeared in the threedimensional (3-D) wake transition of a square-section cylinder is investigated. Direct numerical simulations at a Reynolds number of 180 firstly show that such streamwise vorticity is not originated from lateral surfaces. Then through the analysis of local flow field in the immediate neighborhood of rear surface, based on the theory of vortex-induced vortex, a new physical mechanism is identified. At first, the vertical vorticity on rear surface is generated by the intrinsic three-dimensional instability with the same instability wavelength of mode A. Then the streamwise vorticity at a specific sign is induced by such vertical vorticity, convected and concentrated in the shear layers. Finally, streamwise vortices are formed and shed with alternatively shedding spanwise vortices in the near wake. Moreover, the effect of induced spanwise vorticity on original two-dimensional (2-D) spanwise vorticity is also presented in detail.

Numerical studies on evolution of secondary streamwise vortices in transitional boundary layers

Formation and evolution of secondary streamwise vortices in the compressible transitional boundary layers over a flat plate are studied using a direct numerical simu- lation method with high-order accuracy and highly effective non-reflecting characteristic boundary conditions. Generation and development processes of the secondary streamwise vortices in the complicated transitional boundary flow are clearly analyzed based on the of numerical results, and the effects on the formation of the ring-like vortex that is vital to the boundary layer transition are explored. A new mechanism forming the ring-like vortex through the mutual effect of the primary and secondary streamwise vortices is expressed.

斜激波与内凹半圆柱面无粘相互作用

针对三维内转式进气道流动中激波入射内凹壁面主导的复杂波系干扰问题,采用斜激波入射内凹半圆柱面的简化构型,在来流马赫数为6的条件下,通过无粘数值模拟结合理论分析,研究了激波角β_(i)=14°~29°的斜激波在内凹半圆柱面反射形成的三维流场。结果表明,流场对称面均会出现显著高于二维情况的逆压梯度。当β_(i)≤25°时,从侧壁到对称面,斜激波经历了从马赫反射(MR)到规则反射(RR)的转变,形成了MR-RR型流场,转变点处产生的桥激波向对称面延伸,桥激波在对称面反射后产生的压力峰值高于二维斜激波反射;当β_(i)≥25°时,斜激波在侧壁和对称面均发生马赫反射,形成了MR-MR型流场,两种马赫反射分界点处产生的桥激波向侧壁发展,侧壁气流在对称面相撞后产生的压力峰值高于正激波后的压力;当β_(i)=25°时,流场存在MR-RR型和MR-MR型双解现象。通过降维分析理论,揭示了两类流场中转变点和分界点的形成机制,并厘清了桥激波的产生原因和初期演化特征。当β_(i)≥18°时,无粘激波干扰所主导的侧壁气流加剧向对称面汇聚,并在对称面附近产生流向涡对。无粘分析获得的认识,有助于揭示内转式进气道中流动汇聚和流向涡对等现象的形成机理。

双旁侧无隔道超声速进气道参数化设计及流动分析

针对双旁侧无隔道超声速进气道中S弯流道内部流向涡产生的大畸变问题,提出了参数化设计技术,并基于典型飞机前体设计进气道构型,利用Liutex涡识别技术分析S弯流道内部的流动机理,并探索提升进气道出口流场品质的设计技术。研究结果表明:Liutex涡识别理论在进气道流场中能够有效识别流向涡的大小和强度;为抑制流向涡的产生和发展,降低出口畸变,进气道入口处上唇口一侧流道应采用较小的曲率,下唇口一侧则需要增大曲率;S弯流道末端应增大扭曲程度以将流向涡推至进气道出口中间区域。本文研究可有效抑制无隔道超声速进气道出口畸变,为控制S弯流道内部流向涡结构探索新型设计方法。

流向涡对二元进气道流场与性能的影响

针对超声速情况下流向涡进入进气道并影响其正常工作的问题,在来流马赫数2.7条件下开展数值研究。进气道模型使用二维曲面压缩进气道,涡流发生器使用有限展长机翼。通过改变机翼攻角和法向高度,研究了两种强度流向涡从四种位置进入进气道时对流场特性和进气道性能的影响。结果表明,两种强度的流向涡与进气道波系相互作用后均未破碎,属于典型的弱相互作用。进气道内收缩段入口附近的分离区在流向涡影响下出现明显的三维非对称特征。此外,随着流向涡接近边界层,分离区的分离点逐渐向上游移动。在流向涡的影响下,进气道的流量系数和总压恢复系数均出现下降,并且流向涡的强度越强、距离边界层越近,对进气道性能的不良影响越明显。特别地,在最大攻角和最低位置的工况下,进气道内收缩段入口上游形成了大规模的分离区,使得入口截面流场的畸变系数相比均匀来流情况增大了约4倍,流量系数下降了约6.1%,总压恢复系数下降了约17.2%。

斜激波入射内凹半圆柱面湍流边界层数值模拟

针对三维内转式进气道中的激波/边界层干扰问题,提出了斜激波入射内凹半圆柱面的简化模型,在来流马赫数为6的条件下,采用数值模拟研究了气流偏转角θ=6°~14°的斜激波与内凹半圆柱面边界层的相互作用。结果表明,从上游至下游,斜激波与内凹半圆柱面干扰产生了以扫掠、入射及反射为主要特征的流动结构。斜激波扫掠内凹半圆柱面边界层诱发流动分离,分离激波与入射激波的干扰类型均从马赫反射转变为规则反射,不满足经典扫掠流场的准锥形特征。扫掠过程中产生的不断增厚的低能流向对称面汇聚,并进入由斜激波入射产生的分离区内,使得分离区长度增加。随着θ的增大,分离区的流谱呈现不同的形态,当θ≥10°时,低能流在对称面撞击后形成流向涡对,当θ=14°时,流谱中的旋风涡消失。当入射激波反射后,低能流集中在对称面处,其高度沿流向持续增加。随着θ的增大,内凹半圆柱面出口截面的总压恢复系数减小,而低能流区的总压恢复系数略有增大。

Streamwise vortex generation by the rod

The technology development related to aerodynamics is leading to ever increasing loads of wings, airfoils and turbine and compressor blades. The increase in aerodynamic forces is often leading to flow separation and depreciation of the aerodynamic performance of flying objects or propulsion systems. Flow control methods are required to avoid these negative effects. In the recent two decades the flow control by means of air-jet vortex generators has been also intensively investigated. In this method a streamwise vortex is introduced by an oblique jet. The necessity to supply air by a pipe system may be considered a disadvantage. In order to eliminate this feature, it has been proposed to put out a rod instead of a jet. It has been shown that the application of a rod can introduce the same effect as a jet, as long as the streamwise vortex generation is concerned and appropriate dimensions are used. The present paper focuses on the influence of rod vortex generators on a flow pattern downstream. The results presented here concern experimental and numerical investigations and provide guidelines for the design of a new flow control method dedicated mainly to external flows.

Streamwise Vortex Interaction with a Horseshoe Vortex

Flow control in turbomachinery is very difficult because of the complexity of its fully 3-D flow structure. The authors propose to introduce streamwise vortices into the control of internal flows. A simple configuration of vortices was investigated in order to better understand the flow control methods by means of streamwise vortices. The research presented here concerns streamwise vortex interaction with a horseshoe vortex. The effects of such an interaction are significantly dependent on the relative location of the streamwise vortex in respect to the leading edge of the profile. The streamwise vortex is induced by an air jet. The horseshoe vortex is generated by the leading edge of a symmetric profile. Such a configuration gives possibility to investigate the interaction of these two vortices alone. The presented analysis is based on numerical simulations by means of N-S compressible solver with a two-equation turbulence model.

圆管湍流近壁结构的DPIV实验研究

采用粒子数字图像测速 (DPIV)对充分发展的圆管湍流进行了研究 ,测量结果和直接数值模拟结果的比较 ,表明作者开发的DPIV技术可取得满意的精度。在此基础上通过瞬时脉动速度场对圆管湍流的近壁结构进行了研究 ,并在近壁结构中检测出涡诱导的喷发和扫掠、低速条带和流向涡结构等拟序结构 。

应用气动斜坡和燃气发生器的超燃燃烧室

为增强超声速燃烧过程中的燃料掺混,设计了一种被动式燃料掺混增强结构:气动斜坡/燃气发生器组合燃料喷注结构,并在直连式超燃试验台上对这种喷注结构进行了纹影、油流谱等冷试和热试试验.同时数值模拟了超声速流场中气动斜坡/燃气发生器组合结构的流动及燃烧特性.结果表明:气动斜坡/燃气发生器组合结构有助于燃料的掺混,掺混效率由单独气动斜坡喷注器情况下的60%提高到了75%;总压损失主要由壁面摩擦产生,气动斜坡和燃气发生器产生的总压损失相对较少;作为燃烧室点火器使用的燃气发生器起着点火和助燃的双重作用.

多孔喷嘴射流泵流动模拟与涡结构分析

基于有限体积法和Realizable k-ε湍流模型,对多孔喷嘴射流泵内部流场进行数值模拟和分析,并与试验结果进行对比,验证了数值模拟的可靠性.引入涡动力学理论对喉管内部工作流体和被吸流体的混合机理进行分析,研究了喉管内部流向涡和展向涡的分布及其峰值沿流向的变化.结果表明:多孔喷嘴结构能够加快工作流体和被吸流体的混合,提高射流泵的效率;射流泵喉管内的涡结构对工作流体和被吸流体的混合有重要影响,相对流向涡而言,展向涡的强度较大,在喉管内的衰减较为平缓;在射流泵中,对两股流体起主要掺混作用的是流向涡,其强度和衰减的速率决定了混合效率;在涡量一定的情况下,流向涡越强,衰减越快;展向涡结构越小,越能加速喉管中的混合,从而提高泵的效率;喉管内涡的分布表明,位于喷嘴中心的出口可能导致较大的损失,各孔喷嘴出口应优先沿圆周方向布置.

风切变对风力机尾流湍流特征影响的研究

采用LES方法,对自行开发设计的100 W小型水平轴风力机在有、无风切变两种入流情况下的尾流特征进行了模拟计算,计算中提取了流场稳定后的x、y、z三个方向的瞬间速度、压力等参数的600个数据样本,计算得到了尾流内轴向速度和湍流强度的方均根值,分析了有、无风切变两种入流情况下风力机尾流内的时间平均轴向速度、湍流强度及涡结构等流动特征。结果表明:无风切变入流情况下,尾流内的叶尖涡、中心涡及轴向速度都呈现对称性;而风切变入流情况下,尾流内叶尖涡螺旋轨迹上面部分的螺距明显大于下面部分的螺距,呈现出了非对称性,而中心涡和轴向速度也都呈现出一定的非对称性,且风切变入流情况下尾流内的涡传播距离更远;在y/d<6的近尾流区域内,无论入流是否存在风切变,风力机尾流膨胀现象都不很明显;在相同入流湍流强度条件下,有、无风切变入流情况下尾流内湍流强度的变化规律相似,入流风切变对尾流内湍流强度的影响较小。

扩压叶栅端壁角区流动结构和紊流特性

三维激光多普勒测速技术测量了扩压叶栅叶片和端壁相交的角区中的流动结构和紊流特性。实验结果表明，角区中端壁附面层和叶面附面层的堆积使得叶面附面层变厚；角区内存在较强的通道涡，随流动向下游发展，通道涡核心远离叶面；在前缘产生的马蹄涡逐步消失；通道涡使得角区的紊流动能增加，并影响雷诺正应力的分布。

基于CFD技术的脊状表面湍流流场特性研究

在湍流状态下,利用计算流体力学(CFD)技术,对一系列布置不同尺寸顺流向脊状结构的表面进行了数值模拟。模拟过程采用Reynolds应力方程湍流模型,并且编制程序设定了充分发展的湍流入口边界。通过与光滑表面比较发现,脊状结构附近产生的流向涡是脊状表面减阻或者增阻的重要原因。流向涡改变了近壁面流场结构,引起了边界层近壁区雷诺应力和壁面剪应力的变化,脊状表面也因此出现了减阻或者增阻的现象。

湍流边界层内准流向发卡涡生成的数值模拟

采用根据共振三波理论模型建立的初始准二维流场和直接数值模拟方法,对湍流边界层近壁区流向涡的生成进行了研究.计算得到了准流向发卡涡和次生准流向涡结构的生成过程及其主要特征,讨论了它们的产生机理.作为相干结构的主要特征,利用共振三波理论模型对流向涡结构生成与演化过程的研究为湍流边界层内相干结构的研究与流动控制提供了新的途径和思路.