激波与转捩边界层干扰非定常特性数值分析

激波与边界层干扰的非定常问题是高速飞行器气动设计中基础研究内容之一.以往研究主要针对层流和湍流干扰,在分离激波低频振荡及其内在机理方面存在着上游机制和下游机制两类截然不同的理论解释.分析激波与转捩边界层干扰下非定常运动现象有助于进一步加深理解边界层状态以及分离泡结构对低频振荡特性的影响规律,为揭示其产生机理指出新的方向.采用直接数值模拟方法对来流马赫数2.9,24?压缩拐角内激波与转捩边界层干扰下激波的非定常运动特性进行了数值分析.通过在拐角上游平板特定的流向位置添加吹吸扰动激发流动转捩,使得进入拐角的边界层处于转捩初期阶段.在验证了计算程序可靠性的基础上,详细分析了转捩干扰下激波运动的间歇性和振荡特征,着重研究了分离泡展向三维结构对激波振荡特性的影响规律,最后还初步探索了转捩干扰下激波低频振荡产生的物理机制.研究结果表明:分离激波的非定常运动仍存在强间歇性和低频振荡特征,其时间尺度约为上游无干扰区内脉动信号特征尺度的10倍量级;分离泡展向三维结构不会对分离激波的低频振荡特征产生实质影响.依据瞬态脉动流场的低通滤波结果,转捩干扰下激波低频振荡的诱因来源于拐角干扰区下游,与流场中分离泡的收缩/膨胀运动存在一定的关联.     

壁面加热湍流相干结构的子波分析实验研究

通过在平板湍流边界层沿流向固壁表面平行放置若干条通电加热的金属细丝,在平板表面形成沿展向周期性分布的温度场,利用该温度场引起的空气热对流,在湍流边界层近壁区域产生一组沿湍流边界层展向周期分布的大尺度流向涡结构,改变了平板湍流边界层中不同尺度结构及其能量分布。采用对壁湍流多尺度结构的子波分析表明,在湍流边界层近壁区域产生规则的流向涡结构将壁湍流各种尺度湍涡结构不规则的脉动有序地组织起来,抑制了壁湍流各种尺度湍涡结构脉动,特别抑制了能量最大尺度湍涡结构的脉动,减小由于湍流脉动引起的在湍流边界层法向和展向的动量和能量损耗,从而减小了湍流的阻力。     

前体涡发生器对轴对称高超声速进气道激波振荡流动的影响实验

激波振荡是高超声速进气道不起动过程中常见的流动现象,会显著降低进气道气流捕获与压缩效率、产生剧烈的非定常气动力载荷而危害飞行器安全.从激波振荡的控制出发,实验研究了前体转捩带位置的涡发生器对轴对称高超声速进气道激波振荡流动的影响.分别在起动和激波振荡两种进气道流态下,选择无、0.5 mm与1 mm高度涡发生器工况进行对比研究.并采用高速纹影与壁面动态测压同步记录非定常流动特征.结果表明,1 mm高度内的涡发生器对起动状态的进气道主流流场结构、壁面压强分布影响不显著.但对于激波振荡流动,涡发生器会明显缩小外压缩面分离区运动范围,缩短振荡周期,提升振荡周期内壁面压强的时均值.涡发生器的影响程度随其高度的增大而增强,其中振荡周期从无涡发生器的4 ms缩短到1 mm高度涡发生器的3.13 ms.此外,0.5 mm高度涡发生器会使得进气道内部测点的压强振荡幅值整体下降,相比无涡发生器工况的下降幅度可达23%.流场结构与壁面压强信号的分析表明,涡流发生器主要通过其产生的流向涡影响激波振荡流动,包含流向涡对下游边界层的扰动以及流向涡与分离区的相互干扰.     

前体涡发生器对轴对称高超声速进气道激波振荡流动的影响实验

激波振荡是高超声速进气道不起动过程中常见的流动现象,会显著降低进气道气流捕获与压缩效率、产生剧烈的非定常气动力载荷而危害飞行器安全. 从激波振荡的控制出发,实验研究了前体转捩带位置的涡发生器对轴对称高超声速进气道激波振荡流动的影响. 分别在起动和激波振荡两种进气道流态下,选择无、0.5 mm与1 mm高度涡发生器工况进行对比研究. 并采用高速纹影与壁面动态测压同步记录非定常流动特征. 结果表明,1 mm高度内的涡发生器对起动状态的进气道主流流场结构、壁面压强分布影响不显著. 但对于激波振荡流动,涡发生器会明显缩小外压缩面分离区运动范围,缩短振荡周期,提升振荡周期内壁面压强的时均值. 涡发生器的影响程度随其高度的增大而增强,其中振荡周期从无涡发生器的4 ms缩短到1 mm高度涡发生器的3.13 ms. 此外,0.5 mm高度涡发生器会使得进气道内部测点的压强振荡幅值整体下降,相比无涡发生器工况的下降幅度可达23%. 流场结构与壁面压强信号的分析表明,涡流发生器主要通过其产生的流向涡影响激波振荡流动,包含流向涡对下游边界层的扰动以及流向涡与分离区的相互干扰.      

高超声速激波湍流边界层干扰直接数值模拟研究

高超声速激波与湍流边界层干扰会导致飞行器表面出现局部热流峰值,严重影响飞行器气动性能和飞行安全. 针对高马赫数激波干扰问题,以往数值研究多采用雷诺平均方法,而在直接数值模拟方面的相关工作较为少见. 开展高超声速激波与湍流边界层干扰的直接数值模拟研究,有助于进一步提升对其复杂流动机理认识和理解,同时也将为现有湍流模型和亚格子应力模型的改进提供理论依据. 采用直接数值模拟方法对来流马赫数6.0,34°压缩拐角内激波与湍流边界层的干扰问题进行了研究. 基于雷诺应力各向异性张量,分析了高超声速湍流边界层在压缩拐角内的演化特性. 通过对湍动能输运方程的逐项分析,系统地研究了可压缩效应对湍动能及其输运的影响机制. 采用动态模态分解方法,探讨了干扰流场的非定常运动历程. 研究结果表明,随着湍流边界层往下游发展,近壁湍流的雷诺应力状态由两组元轴对称状态逐渐演化为两组元状态,外层区域则由轴对称膨胀趋近于各向同性. 干扰流场内存在强内在压缩性效应(声效应),其对湍动能输运的影响主要体现在压力--膨胀项,而对膨胀--耗散项影响较小. 高超声速下压缩拐角内的非定常运动仍存在以分离泡膨胀/收缩为特征的低频振荡特性,其物理机制与分离泡剪切层密切相关.      

高超声速激波湍流边界层干扰直接数值模拟研究

高超声速激波与湍流边界层干扰会导致飞行器表面出现局部热流峰值,严重影响飞行器气动性能和飞行安全.针对高马赫数激波干扰问题,以往数值研究多采用雷诺平均方法,而在直接数值模拟方面的相关工作较为少见.开展高超声速激波与湍流边界层干扰的直接数值模拟研究,有助于进一步提升对其复杂流动机理认识和理解,同时也将为现有湍流模型和亚格子应力模型的改进提供理论依据.采用直接数值模拟方法对来流马赫数6.0,34?压缩拐角内激波与湍流边界层的干扰问题进行了研究.基于雷诺应力各向异性张量,分析了高超声速湍流边界层在压缩拐角内的演化特性.通过对湍动能输运方程的逐项分析,系统地研究了可压缩效应对湍动能及其输运的影响机制.采用动态模态分解方法,探讨了干扰流场的非定常运动历程.研究结果表明,随着湍流边界层往下游发展,近壁湍流的雷诺应力状态由两组元轴对称状态逐渐演化为两组元状态,外层区域则由轴对称膨胀趋近于各向同性.干扰流场内存在强内在压缩性效应(声效应),其对湍动能输运的影响主要体现在压力-膨胀项,而对膨胀-耗散项影响较小.高超声速下压缩拐角内的非定常运动仍存在以分离泡膨胀/收缩为特征的低频振荡特性,其物理机制与分离泡剪切层密切相关.     

减阻工况下壁面周期扰动对湍流边界层多尺度的影响

通过在平板壁面施加不同频率振幅的压电陶瓷振子周期性扰动,进行了湍流边界层主动控制减阻的实验研究.在压电陶瓷振子最大减阻工况下(80 V和160 Hz),使用单丝边界层探针对压电振子自由端下游2 mm处进行测量,得到不同法向位置流向速度信号的时间序列.通过对比施加控制前后的多尺度分析,发现压电振子产生的扰动只对近壁区产生影响,使得近壁区大尺度脉动降低,小尺度脉动强度增大,而对边界层的外区则基本没有影响.进一步对大尺度和小尺度的脉动信号进行条件平均,发现压电振子产生的扰动对小尺度脉动的影响在时间相位上并不均匀,小尺度脉动强度在大尺度脉动为正时比在大尺度脉动为负时具有更明显的增加.这表明壁面周期扰动主要通过使大尺度高速扫掠流体破碎为小尺度结构,来影响相应的高壁面摩擦事件,从而达到减阻效果.     

减阻工况下壁面周期扰动对湍流边界层多尺度的影响

通过在平板壁面施加不同频率振幅的压电陶瓷振子周期性扰动,进行了湍流边界层主动控制减阻的实验研究.在压电陶瓷振子最大减阻工况下(80 V和160Hz),使用单丝边界层探针对压电振子自由端下游2mm处进行测量,得到不同法向位置流向速度信号的时间序列.通过对比施加控制前后的多尺度分析,发现压电振子产生的扰动只对近壁区产生影响,使得近壁区大尺度脉动降低,小尺度脉动强度增大,而对边界层的外区则基本没有影响.进一步对大尺度和小尺度的脉动信号进行条件平均,发现压电振子产生的扰动对小尺度脉动的影响在时间相位上并不均匀,小尺度脉动强度在大尺度脉动为正时比在大尺度脉动为负时具有更明显的增加.这表明壁面周期扰动主要通过使大尺度高速扫掠流体破碎为小尺度结构,来影响相应的高壁面摩擦事件,从而达到减阻效果.      

双振子同异步振动主动控制湍流边界层减阻实验研究

本文以镶嵌在平板上沿展向对放的两个压电陶瓷振子为主动控制激励器,自主设计了零质量射流主动控制湍流边界层减阻实验方案.在风洞中开展了双压电振子同步和异步振动主动控制湍流边界层减阻的实验研究,实现了压电振子的周期扰动对湍流边界层多尺度相干结构的干扰和调制,施加控制后减小了壁面摩擦阻力,获得减阻效果.当异步控制100 V, 160 Hz工况时得到最大减阻率为18.54%.小波多尺度分析结果表明,施加控制工况中PZT振子的周期性扰动使得小尺度结构的湍流脉动强度增强,改变了近壁区大尺度和小尺度结构的含能分布,且异步控制工况比同步控制工况的减阻效果好.当双振子振动频率为160 Hz时,流向脉动速度的小波系数PDF曲线呈现出波动特征,尾部变宽显著,近壁湍流脉动更加有序和规则,湍流间歇性减弱.对小尺度脉动进行条件相位平均的结果表明,施加PZT周期扰动后使得大尺度结构破碎成为小尺度结构,小尺度脉动强度增强,实现减阻.随着流向位置离PZT振子越来越远,周期性扰动对相干结构的调制作用逐渐减弱.     

湍流边界层等动量区演化机理的实验研究

等动量区是瞬时流场中流体动量接近的局部区域,其生成和分布与相干结构密切相关. 对等动量区的研究有助于更深入认识湍流边界层相干结构,但目前对其演化过程还缺乏实验支持和机理分析. 设计并使用移动式高时间分辨率粒子图像测速技术(TRPIV)系统对光滑平板湍流边界层进行了跟踪测量,用滤波方式对数据进行降噪,结合对直接数值模拟数据的插值结果,获得脉动速度信号. 使用改进方法去掉非湍流的影响,检测边界层内的等动量区,得到其数量的时间序列,结合流向速度概率密度函数分布的变化,分析得出了等动量区的数量在大的时间尺度下从一个稳态到另一个稳态的阶梯状变化特点. 分解不同尺度的脉动速度,对大尺度和小尺度脉动信号进行条件平均,发现大尺度脉动对等动量区数量变化起主要作用,表现为不同速度流体通过发生不同猝发事件改变流向速度概率密度函数分布. 分析流向大尺度脉动空间分布的变化,发现等动量区内常含有多个大尺度脉动区域,不同区域的扩张、收缩、分裂、合并影响流向速度的集中程度,进而导致等动量区数量的变化.      

Nature of the surface heart transfer fluctuation in a hypersonic separated turbulent flow

This paper presents the results of an experimental study of the unsteady nature of a hypersonic separated turbulent flow. The nomimal test conditions were a freestream Mach number of 7.8 and a unit Reynolds number of 3.5×10⁷/m. The separated flow was generated using finite span forward facing steps. An array of flush mounted high spatial resolution and fast response platinum film resistance thermometers was used to make multi-channel measurements of the fluctuating surface heat trtansfer within the separated flow. Conditional sampling analysis of the signals shows that the root of separation shock wave consists of a series of compression wave extending over a streamwise length about one half of the incoming boundary layer thickness. The compression waves converge into a single leading shock beyond the boundary layer. The shock structure is unsteady and undergoes large-scale motion in the streamwise direction. The length scale of the motion is about 22 percent of the upstream influence length of the separation shock wave. There exists a wide band of frequency of oscillations of the shock system. Most of the frequencies are in the range of 1–3 kHz. The heat transfer fluctuates intermittently between the undisturbed level and the disturbed level within the range of motion of the separation shock wave. This intermittent phenomenon is considered as the consequence of the large-scale shock system oscillations. Downstream of the range of shock wave motion there is a separated region where the flow experiences continuous compression and no intermittency phenomenon is observed. The project supported by National Natural Science Foundation of China     

数值研究平板方舵激波-湍流边界层干扰

数值研究了平板方舵激波-湍流边界层干扰流场。模拟出了分离激波与弓型激波砬撞后形成的“λ”激波结构;消晰地显示了分离区中的旋涡结构,发现流场中会出现二次分离涡,并从理论上分析了流场对称面涡心形态与非定常的关系,得到了涡心为不稳定螺旋点或出现极限环是非定常流动特征的新结论。     

Convergence acceleration by self‐adjusted time stepsize using Bi‐CGSTAB method for turbulent separated flow computation

Poor convergence behavior is usually encountered when numerical computations on turbulent separated flow are performed. A design of self‐adjusted stepsize concept both in time span and spatial coordinate systems to achieve faster convergence is demonstrated in this study. The determination of the time stepsize based on the concept of minimization of residuals using the Bi‐CGSTAB algorithm is proposed. The numerical results show that the time stepsize adjusted by the proposed method indeed improves the convergence rate for turbulent separated flow computations using advanced turbulence models in low‐Reynolds number forms. Copyright © 2002 John Wiley & Sons, Ltd.     

The Influence of Periodic Excitation on a Turbulent Separation Bubble

Turbulent separation limits the performance in many engineering applications, for example creating pressure losses in diffuser like flows or stall on aircraft wings. In the present study the turbulent boundary layer flow over a flat plate separating due to an adverse pressure gradient is studied as a model problem and the effect of periodic excitation in both time and space is investigated through direct numerical simulations. Linear stability analysis is used to analyse the sensitivity of the flow with respect to time-periodic excitations. The dependence on position, amplitude and frequency of the forcing is investigated. For a certain frequency range at sufficiently high amplitudes, it is possible to eliminate the separated region. Furthermore, three-dimensional effects are studied by applying a steady spanwise forcing as well as a both time-dependent and spanwise varying forcing. A forcing varying in spanwise direction is shown to be the most effective in eliminating the separated region, whereas two-dimensional time-periodic excitation was not as efficient as it was expected.     

An investigation of adiabatic spiral vortex flow by means of cross-wire probes

Flow transitions occurring with increase in the Taylor number in an annular gap of radius ratio 0.8, having an imposed axial flow of air of Reynolds number 500 have been studies using the output from a cross-wire probe in a complex digital analysis. Cross and phase spectra, together with auto and cross correlograms, are presented for four Taylor numbers from 10 620 to 12.2 × 10⁶, covering the onset of vortex flow, chaotic flow and turbulent vortex flow. As the Taylor number increases, there is little alteration in the spiral vortex flow in the axial and tangential directions, which oscillates in phase in these two directions. The tangential velocity gradient was seen to become increasingly dominant, with increase in the Taylor number     

分离流动物理特性研究进展

分离流问题的研究在当前流体力学中占有十分重要的地位。它不仅具有重要的理论意义并且具有广泛的应用价值。本文着重评述三维分离流中的基本物理特性和它的流动结构。重点讨论了三维定常分离的两种模式及其判别准则;在三维分离流中应用奇点拓扑理论的定性分析;按照流动的物理特性对三维分离的分类;分离面的物理特性。最后简单介绍了目前在研究湍流边界层分离方面的进展。      

Numerical analysis of turbulent flow separation in a rectangular duct with a sharp 180‐degree turn by algebraic Reynolds stress model

Turbulent flow in a rectangular duct with a sharp 180‐degree turn is difficult to predict numerically because the flow behavior is influenced by several types of forces, including centrifugal force, pressure‐driven force, and shear stress generated by anisotropic turbulence. In particular, this type of flow is characterized by a large‐scale separated flow, and it is difficult to predict the reattachment point of a separated flow. Numerical analysis has been performed for a turbulent flow in a rectangular duct with a sharp 180‐degree turn using the algebraic Reynolds stress model. A boundary‐fitted coordinate system is introduced as a method for coordinate transformation to set the boundary conditions next to complicated shapes. The calculated results are compared with the experimental data, as measured by a laser‐Doppler anemometer, in order to examine the validity of the proposed numerical method and turbulent model. In addition, the possibility of improving the wall function method in the separated flow region is examined by replacing the log‐law velocity profile for a smooth wall with that for a rough wall. The analysis results indicated that the proposed algebraic Reynolds stress model can be used to reasonably predict the turbulent flow in a rectangular duct with a sharp 180‐degree turn. In particular, the calculated reattachment point of a separated flow, which is difficult to predict in a turbulent flow, agrees well with the experimental results. In addition, the calculation results suggest that the wall function method using the log‐law velocity profile for a rough wall over a separated flow region has some potential for improving the prediction accuracy. Copyright © 2007 John Wiley & Sons, Ltd.     

离心叶轮正冲角流场计算的边界层方法

本文使用stanitz快速分析法计算主流,用积分法计算带旋转和曲率的二维不可压湍流边界层.通过流量方程将两者紧密联系起来、同时求解,能够连续计算至分离区.可方便快速地用于估算离心叶轮在正冲角工况下的流场.     

Effect of swirl intensity on the flow and combustion of a turbulent non-premixed flat flame

An experiment in a turbulent non-premixed flat flame was carried out in order to investigate the effect of swirl intensity on the flow and combustion characteristics. First, stream lines and velocity distribution in the flow field were obtained using PIV (Particle Image Velocimetry) method in a model burner. In contrast with the axial flow without swirl, highly swirled air induced streamlines going along the burner tile, and its backward flow was generated by recirculation in the center zone of the flow field. In the combustion, the flame shape with swirled air also became flat and stable along the burner tile with increment of the swirl number. Flame structure was examined by measuring OH and CH radicals intensity and by calculating Damkohler number (Da) and turbulence Reynolds number (Re _T ). It appeared that luminescence intensity decreased at higher swirl number due to the recirculated flue gas, and the flat flames were comprised in the wrinkled laminar-flame regime. Backward flow by recirculation of the flue gas widely contacted on the flame front, and decreased the flame temperature and emissions concentration as thermal NO. The homogeneous temperature field due to the widely flat flame was obtained, and the RMS in the high temperature region was rather lower at higher swirl number. Consequently, the stable flat flame with low NO concentration was achieved.