考虑熵层效应的扫掠激波/湍流边界层干扰特性研究

为了探究熵层对扫掠激波/湍流边界层干扰特性的影响规律,采用仿真方法对尖鳍/钝板物理模型进行研究。结果表明：扫掠激波上游的熵层厚度随着平板前缘钝化半径的增大而增加,同时边界层厚度也随着熵层厚度的增加而增加。熵层的引入并不改变扫掠激波/湍流边界层干扰固有的准锥形相似特性,也不会改变拟锥原点(virtual conical origin, VCO)的位置,仅会改变干扰形成的上游影响线和分离线的角度。扫掠激波/湍流边界层干扰形成的锥形主旋涡和角涡的尺度随着熵层厚度的增加而增大。上游熵层的引入增大了下游扫掠激波/湍流边界层干扰区的总压损失,但扫掠激波/湍流边界层干扰自身造成的相对总压损失并不受上游熵层的影响。     

Numerical analysis of shock wave and supersonic turbulent boundary interaction between adiabatic and cold walls

Direct numerical simulations of shock wave and supersonic turbulent boundary layer interaction in a 24° compression ramp with adiabatic and cold-wall temperatures are conducted. The wall temperature effects on turbulence structures and shock motions are investigated. The results are validated against previous experimental and numerical data. The effects of wall cooling on boundary layer characteristics are analysed. Statistical data show that wall cooling has a significant effect on the logarithmic region of mean velocity profile downstream the interaction region. Moreover, the influence of wall temperature on Reynolds stress anisotropy is mainly limited in the near-wall region and has little change on the outer layer. As the wall temperature decreases, the streamwise coherency of streaks increases. Based on the analysis of instantaneous Lamb vector divergence, the momentum transport between small-scale vortices on cold-wall condition is significantly enhanced. In addition, spectral analysis of wall pressure signals indicates that the location of peak of low-frequency energy shifts toward higher frequencies in cold case. Furthermore, the dynamic mode decomposition results reveal two characteristic modes, namely a low-frequency mode exhibiting the breathing motion of separation bubble and a high-frequency mode associated with the propagation of instability waves above separation bubble. The shape of dynamic modes is not sensitive to wall temperature.     

超声速层流/湍流压缩拐角流动结构的实验研究

在Ma=3.0的超声速风洞中, 分别对上游边界层为超声速层流和湍流, 压缩角度为25°和28°的压缩拐角流动进行了实验研究. 采用纳米粒子示踪平面激光散射(NPLS)技术获得了流场整体和局部区域的精细结构, 边界层、剪切层、分离激波、回流区和再附激波等典型结构清晰可见, 测量了超声速层流压缩拐角壁面的压力系数. 从时间平均的流场结构中测量出分离激波、再附激波的角度和再附后重新发展的边界层的增长情况, 通过分析时间相关的流场NPLS图像, 可以发现流场结构随时间的演化特性. 实验结果表明: 在25°的压缩角度下, 超声速层流压缩拐角流动发生了典型的分离, 边界层迅速增长失稳转捩, 并引起一道诱导激波, 流场中出现了K-H涡、剪切层和微弱压缩波结构, 而超声速湍流压缩拐角流动没有出现分离, 湍流边界层始终表现为附着状态; 在28° 的压缩角度下, 超声速层流压缩拐角流动进一步分离, 回流区范围明显扩大, 诱导激波、分离激波向上游移动, 再附激波向下游移动, 分离区流动结构复杂, 相比之下, 超声速湍流压缩拐角流动的回流区范围明显较小, 边界层增长缓慢, 流场中没有出现诱导激波、K-H涡和压缩波, 流动分离区域的结构也相对简单, 但分离激波的强度则明显更强. 关键词： 压缩拐角 层流 湍流 流动结构     

压缩拐角激波/湍流边界层干扰特性分析

用大涡模拟方法对Mach数3.0下的压缩拐角激波/湍流边界层干扰问题进行数值研究.对拐角上游平板区域边界层转捩及湍流进行模拟,设定平板区域长度,使得转捩过程于平板区域发生并充分完成,从而在拐角处产生激波/湍流边界层相互干扰,研究激波/湍流边界层的作用机理.研究表明:流场能够在非定常扰动激励下迅速转捩,并于平板区发展为完全湍流;湍流边界层与激波相互作用过程中,拐角附近分离区较层流情况明显减小;展向不同区域分离区大小差异较大,局部区域分离现象消失.     

转捩与湍流对激波边界层干扰及底部流动结构的影响

为研究转捩与湍流对激波边界层干扰及底部流动结构的影响,文章选取了二维与三维高超声速双斜面进气道模型与大钝头着陆器模型,并使用γ-Re_θ转捩模型开展数值模拟研究.研究表明,对于二维进气道模型,随着前缘钝度的增加,激波边界层干扰位置前移,分离区变大,与层流流动情况相比,有转捩流动发生时,激波边界层干扰位置后移,同时分离流动强度变弱,分离区缩小;对于三维进气道模型,其拐角附近的分离流动呈现明显的三维特征,转捩流动也存在三维流动结构,与静风洞状态相比,噪音风洞状态下,有转捩流动发生,对壁面热流影响较大,对激波系影响很小.对于着陆器模型,底部流动发生转捩,使得底部流动由不稳定非定常的流动结构变为稳定定常的流动结构,这有益于姿态控制设计.      

Turbulent scalar flux transport in head-on quenching of turbulent premixed flames: a direct numerical simulations approach to assess models for Reynolds averaged Navier Stokes simulations

The statistical behaviour and the modelling of turbulent scalar flux transport have been analysed using a direct numerical simulation (DNS) database of head-on quenching of statistically planar turbulent premixed flames by an isothermal wall. A range of different values of Damköhler, Karlovitz numbers and Lewis numbers has been considered for this analysis. The magnitudes of the turbulent transport and mean velocity gradient terms in the turbulent scalar flux transport equation remain small in comparison to the pressure gradient, molecular dissipation and reaction-velocity fluctuation correlation terms in the turbulent scalar flux transport equation when the flame is away from the wall but the magnitudes of all these terms diminish and assume comparable values during flame quenching before vanishing altogether. It has been found that the existing models for the turbulent transport, pressure gradient, molecular dissipation and reaction-velocity fluctuation correlation terms in the turbulent scalar flux transport equation do not adequately address the respective behaviours extracted from DNS data in the near-wall region during flame quenching. Existing models for transport equation-based closures of turbulent scalar flux have been modified in such a manner that these models provide satisfactory prediction both near to and away from the wall.     

Constrained large-eddy simulation of supersonic turbulent boundary layer over a compression ramp

A supersonic turbulent boundary layer over a compression ramp is numerically investigated using the constrained large-eddy simulation (CLES) method. The compression corner is characterised by a deflection angle of 24°. The free-stream Mach number is Ma_∞ = 2.9, and the Reynolds number based on the momentum thickness of inlet boundary layer is Re_θ = 2300. The mean and statistical quantities, such as mean velocity, wall pressure and Reynolds stresses, are thoroughly analysed and compared with those from traditional large-eddy simulation (LES), experimental measurement and direct numerical simulation (DNS). It turns out that CLES can predict the friction coefficient, wall-pressure distribution, size of separation bubble, Reynolds stresses, etc. more accurately than traditional LES, and the results are in reasonable agreement with the experimental and/or DNS data. Also discussed are the effects of specific parameterisations of the Reynolds constraint and interfacial positions separating the constrained and unconstrained regions on the performance of the CLES method.     

低Reynolds数翼型绕流主动控制技术

在低Reynolds数条件下,翼型绕流的上表面边界层由于抗逆压梯度能力变差容易发生流动分离,从而形成长层流分离泡.分离泡通常是非定常的,会诱发边界层的转捩、再附并形成湍流边界层.这个过程会使翼型的气动性能急剧下降,并伴随着强非线性效应.转捩后形成的湍流边界层也会产生高摩擦阻力.针对这种现象,文章以NACA0012翼型为例,通过隐式大涡模拟研究了有效的主动控制方案.为了统一分离控制技术和湍流边界层减阻技术,研究了在平板或槽道湍流中取得较好控制效果的壁面垂向反向控制方案.首先利用隐式大涡模拟研究了低Reynolds数条件下NACA0012翼型绕流的流场特征.其次分析并验证了反向控制方案在分离区控制流场的可行性,发现反向控制在分离区的作用相当于基于流场信息的壁面抽吸控制,且控制具有实时性和高效性,控制抽吸了前缘的低能流体,使得翼型前缘附面层变薄,并增强了其抗逆压梯度的能力,较大程度提高了翼型的气动性能.最后在湍流边界层验证了其减阻控制效果,发现反向控制阻断了流向涡的法向输运,抑制了涡结构的发展,并减弱了猝发过程,使得湍流的高摩阻力得到了有效降低.      

Modelling of turbulent scalar flux in turbulent premixed flames based on DNS databases

A transport equation for scalar flux in turbulent premixed flames was modelled on the basis of DNS databases. Fully developed turbulent premixed flames were obtained for three different density ratios of flames with a single-step irreversible reaction, while the turbulent intensity was comparable to the laminar burning velocity. These DNS databases showed that the countergradient diffusion was dominant in the flame region. Analyses of the Favre-averaged transport equation for turbulent scalar flux proved that the pressure related terms and the velocity–reaction rate correlation term played important roles on the countergradient diffusion, while the mean velocity gradient term, the mean progress variable gradient term and dissipation terms suppressed it. Based on these analyses, modelling of the combustion-related terms was discussed. The mean pressure gradient term and the fluctuating pressure term were modelled by scaling, and these models were in good agreement with DNS databases. The dissipation terms and the velocity–reaction rate correlation term were also modelled, and these models mimicked DNS well.     

Direct numerical simulation of a fully developed compressible wall turbulence over a wavy wall

Compressible turbulent channel flow over a wavy surface is investigated by direct numerical simulations using high-resolution finite difference schemes. The Reynolds number considered in the present paper is 3380 based on the bulk velocity, the channel half-width and the kinetic viscosity at the wall. Four test cases are simulated and analysed at Ma_m = 0.33, 0.8, 1.2, 1.5 based on the bulk velocity and the speed of sound at the wall. We mainly focus on the curvature and the Mach number effects on the compressible turbulent flows. Numerical results show that although the wavy wall has effects on the mean and fluctuation quantities, log law still exists in the distribution of the wave-averaged streamwise velocity if the roughness effects are taken into consideration in the scaling of it. Near-wall streaks are broken by the wavy surface and near-wall quasi-streamwise vortices mostly begin at the upslope of the wave and pass over the crest of it. The wavy wall makes the turbulence more active and the flow easier to be blended. From the viewpoint of turbulent kinetic budgets, curvature effects strengthen both the diffusion terms and the dissipation terms. At the same time, they change the properties of the compressibility-related terms and promote more inner energy transferring into turbulent kinetic energy. As the Mach number increases, the reattachment of the mean flow is delayed, which indicates the mean separation bubble becomes larger. Concerning the near-wall coherent structures, the vortices are more sparsely distributed with the increasing of the Mach number. For the supersonic cases, shock waves appear. Though they have little effects on the mean turbulent quantities, they change the structures of the flow fields and induce local separations at the upper wall of the channel.     

低压高负荷涡轮叶栅的大涡模拟

了解预测转捩与层流分离泡,对于低压高负荷叶片设计特别重要.为此,采用大涡模拟方法,对典型高负荷叶栅T106进行了模拟.计算描述了层流分离泡,给出了壁面压力、壁面摩擦力系数的分布,从而准确预测了T106压力面上的层流分离、转捩、湍流再附现象.计算同时刻画了层流分离泡的非定常性质,给出了瞬时压力,进行了频谱分析,确定主频为2500 Hz,并观察到了瞬时涡等有意义的现象.计算同时表明二维大涡模拟计算结果可以复现出湍流的许多重要现象并且精度可以满足工程需要.     

激波与层流/湍流边界层相互作用实验研究

在超声速风洞中,分别对层流和湍流来流条件下的边界层和斜激波(激波强度足以引起流动分离)相互干扰进行了实验研究,利用纳米粒子示踪平面激光散射(NPLS)技术获得了两种条件下流场的精细结构图像;利用粒子图像测速(PIV)技术获得了两种条件下流场的速度场和涡量场;综合运用NPLS结果和PIV结果对比分析了两种流动的瞬时流动结构和时间相关性,实验结果表明:层流边界层内的分离区呈现出狭长的条状,而湍流边界层内分离区呈现出较规则的椭圆;在入射激波上游距入射点较远的位置,层流边界层外围拟序结构会诱导出一系列压缩波系,进而汇聚成空间位置不稳定的诱导激波,而湍流边界层则是在入射激波上游较近的地方直接形成较强且稳定的诱导激波;在入射激波下游,层流边界层内的膨胀区域较小且急促,膨胀后产生的再附激波很弱,而湍流边界层内的膨胀区域较大,膨胀后产生的激波较强。     

空化单气泡外围压强分布

关键词：     

Separated flow behind a backward-facing step under a stationary temperature disturbance

The flow in the separation region of laminar boundary layer behind a rectangular backward-facing step has been experimentally examined under temperature non-uniformity of the flow. The data were obtained in a subsonic wind tunnel at Reynolds numbers M ? 1. The temperature disturbance was generated using a system of Peltier elements provided on the model surface upstream of the separation line. The effect of heating/cooling of the wall on the mean and fluctuating flow components was evaluated using hot-wire measurements. The experimental data were supplemented with calculations of linear-stability characteristics of model velocity profiles in the separated boundary layer. As a result, the response of the separated flow to a stationary thermal perturbation was revealed.     

Pulsating jet control for manipulating the separation bubble behind the fence

Abstract  

An optimization study of a pulsating jet was performed to manipulate the separation bubble behind the fence. The experiments were carried out in a circulating water channel and the vertical fence was submerged in the turbulent boundary layer. The parameters used for controlling the pulsating jet included the frequency, speed and velocity profile of jet, and the geometries including angle and location of nozzle. In addition, we investigated the effect of upstream boundary layer changed by continuous suction in front of the jet nozzle. Each test case was divided into 20 phases and phase-averaged DPIV method was applied to measure the velocity field. From the results, the main mechanism of reduction of bubble length was the vortex shedding from the recirculating region. The specific jet condition and nozzle geometries which were effective to reduce the separation bubble behind the fence were found.     

Topological evolution near the turbulent/non-turbulent interface in turbulent mixing layer

The topological evolution near the turbulent/non-turbulent interface (TNTI) in turbulent mixing layer is studied by means of statistical analysis of the invariants of velocity gradient tensor (VGT) based on direct numerical simulation data. The dynamics of topological evolution is investigated in terms of the source terms of the evolution equations for the invariants, including the pressure effect term, viscous effect term and interaction term among the invariants. It is found that the local topology of fluid particles at the TNTI evolves from non-focal region to focal region in the plane of the second (Q) and the third (R) invariants of the VGT. The topological evolution is mainly associated with the pressure effect term in the TNTI region. According to the analysis of the evolution of enstrophy and dissipation, the enstrophy increase and the dissipation decrease are revealed in the TNTI region, which are caused by viscous vorticity diffusion near the TNTI. A weak correlation between the strain rate and the rotation rate is found in the TNTI region which is related to the reduction of enstrophy production. The alignments between vorticity and strain near the TNTI are investigated and a strong alignment of the vorticity with the extensive eigenvector direction is identified in the TNTI region.     

Direct numerical simulation of the flow around a wall-mounted square cylinder under various inflow conditions

The flow around a wall-mounted square cylinder of side d is investigated by means of direct numerical simulation (DNS). The effect of inflow conditions is assessed by considering two different cases with matching momentum-thickness Reynolds numbers Re_θ ? 1000 at the obstacle: the first case is a fullyturbulent zero pressure gradient boundary layer, and the second one is a laminar boundary layer with prescribed Blasius inflow profile further upstream. An auxiliary simulation carried out with the pseudo-spectral Fourier–Chebyshev code SIMSON is used to obtain the turbulent time-dependent inflow conditions which are then fed into the main simulation where the actual flow around the cylinder is computed. This main simulation is performed, for both laminar and turbulent-inflows, with the spectral-element method code Nek5000. In both cases the wake is completely turbulent, and we find the same Strouhal number St ? 0.1, although the two wakes exhibit structural differences for x > 3d downstream of the cylinder. Transition to turbulence is observed in the laminar-inflow case, induced by the recirculation bubble produced upstream of the obstacle, and in the turbulent-inflow simulation the streamwise fluctuations modulate the horseshoe vortex. The wake obtained in our laminar-inflow case is in closer agreement with reference particle image velocimetry measurements of the same geometry, revealing that the experimental boundary layer was not fully turbulent in that dataset, and highlighting the usefulness of DNS to assess the quality of experimental inflow conditions.     

Electrical and aerodynamic behavior of arc under shock conditions

Arc-shock interaction in a supersonic nozzle has been investigated for a current range from 200 to 1500 A and for three pressure ratios. The adverse pressure gradient associated with the shock causes flow separation and a broadening of arc cross section. Compared with the shock position in the absence of the arc, the shock center is moved toward upstream and to a region close to the wall. The shock is no longer plane. The center of the shock is not very sensitive to the current, but the shock strength reduces when the current is increased, V-I characteristics under shock conditions are slightly modified     

激波/边界层干扰对等离子体合成射流的响应特性

利用高速纹影系统和数值模拟方法研究了激波/边界层干扰对逆流喷射的等离子体合成射流的响应特性,并揭示了流动控制机理.实验在来流马赫数Ma=3.1的风洞中进行,测试模型采用钝头体和压缩斜坡的组合模型,等离子体合成射流激励器安装在钝头体头部.纹影系统捕捉了放电频率为f=1 kHz和f=3 kHz的激励对附体激波形态和分离激波运动的控制效果.等离子体合成射流使压缩斜坡激波/边界层干扰区域的起始点向下游移动,分离泡尺寸减小,附体激波强度减弱,发生弯曲,再附点移向上游,与此同时分离激波向附体激波逼近.与f=3 kHz激励相比,f=1 kHz激励的射流流量更大,对激波/边界层干扰的影响范围更广、控制效果更好.通过数值模拟,揭示了射流与来流相互作用对下游流场的作用机理:射流与来流相互作用诱导出大尺度旋涡,大尺度旋涡耗散发展增强了近壁面流场的湍流度;压缩斜坡上游近壁面的流场性质发生变化,进而导致了压缩斜坡激波/边界层干扰区域流动的变化.