逆压梯度下层流分离泡转捩的大涡模拟

本文采用有限体积法,发展了适合复杂几何边界条件的可压缩流动大涡模拟程序,通过对槽道湍流的计算,验证了程序的可靠性。对低雷诺数、逆压梯度环境中的层流分离泡转捩现象进行了数值模拟,分析了其统计特性及大尺度相干结构的演化规律．初步的结果表明:大涡模拟可以很好地处理层流分离泡转捩问题,既能给出满意的统计平均结果,又可以通过相干结构的演化细致描述分离泡转捩的动力学过程。     

R-T界面不稳定性及湍流混合的大涡模拟

采用大涡模拟(LES)方法研究R-T(Rayleigh Taylor)不稳定性,显示了不稳定发展的小扰动阶段、变形阶段、规则非线性阶段、不规则非线性阶段和湍流混合阶段界面演化与特征,分析了湍流混合层厚度增长的规律,同时还研究了湍流二阶相关量如动量、质量和能量通量在大尺度和亚格子尺度所占分量,验证了LES方法是处理R-T界面不稳定性和湍流混合的一种有效方法.     

基于三角波瓣混合器的超声速流场精细结构和掺混特性

在超声速吸气式混合层风洞中,采用基于纳米粒子的平面激光散射(NPLS)技术对平板混合层和三角波瓣混合器诱导的混合层流场精细结构进行了对比实验研究.上下两层来流的实测马赫数分别为1.98和2.84,对流马赫数为0.2.NPLS图像清晰地展示了Kelvin-Helmholtz涡、流向涡、波系结构以及大尺度涡结构的配对合并过程.通过对比分析时间相关的NPLS流场图像,发现了大尺度拟序结构随时间发展演化的非定常特性.基于流动显示结果,采用分形维数和间歇因子指标对流场结构和混合特性进行了定量分析.实验研究表明,三角波瓣混合器诱导的流向涡结构显著提高了上下两层来流的掺混效率,其流动远场的分形维数突破了平板混合层中完全湍流区的分形维数值,达到了1.88,流场结构表现出明显的破碎性,有利于流动在标量层面的扩散和掺混.流动间歇性分析表明,流向涡与展向涡的相互剪切作用主导着混合层的掺混特性,同时由于流向涡的卷吸作用,三角波瓣混合器诱导的混合层混合区域更大,更多的流质被卷入混合区完成混合.     

湍流边界层内流向涡的数值模拟研究

采用准二维共振三波作为湍流边界层近壁区相干结构初值,用直接数值模拟方法计算了流动从二维结构发展到三维结构并且伴随流向涡生成的整个过程,分析结果显示流向涡对湍流动能和质量传输有着重要作用,是湍流边界层相干结构的重要特征和运动形式.     

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.     

“反尖端”界面不稳定性数值计算分析

利用可压缩多介质黏性流动和湍流大涡模拟代码(MVFT),在超算平台上对"反尖端"界面不稳定性及其诱发的湍流混合问题进行了大规模三维数值模拟分析。数值模拟结果清晰地显示了冲击波加载界面后分解产生的冲击波、稀疏波、压缩波及其在SF6气体中的运动和相互作用,以及波多次加载界面的复杂过程,波和界面的每一次作用都会加速湍流混合区的发展和物质混合。"反尖端"界面受冲击波加载后发生反相而形成典型的大尺度壁面气泡和中心轴尖钉结构,该大尺度结构基本确定了湍流混合区的平均几何特征和包络范围而不依赖计算网格。高分辨率的计算网格下,捕捉到了更精细的小尺度湍涡结构和更强的湍流脉动,显示了湍流混合区所具有的复杂结构和特征。     

密度非均匀流场中冲击加载双模态界面失稳现象的数值模拟

利用可压缩多介质黏性流动和湍流大涡模拟的二维计算程序MVFT-2D,针对初始非均匀流场密度为高斯分布、马赫数Ma=1.27激波作用下的双模态界面失稳现象,进行了数值模拟研究。数值模拟结果表明,处于非均匀流场中的双模态振幅耦合效应较弱,而且低密度区的初始大振幅界面扰动增长最快,高密度区的初始小振幅界面扰动增长最慢。通过进一步分析可知,在一定初始振幅范围内,非均匀流场低密度区的振幅增长率较高,混合区域更宽,湍动能较大,受初始振幅影响较大,导致该区域界面不稳定演化较快。其变化规律与均匀流场呈现相反趋势,说明非均匀流场界面不稳定性的发展规律与均匀流场存在较大差异。     

基于湍流模型的转捩流动数值计算研究

本文通过求解雷诺平均NS方程,采用基于k-ω模型和添加控层流动能方程的三方程湍流模型计算转捩流动。相对于基于实验的转捩关系式模型,本模型对叶轮机械复杂的流动环境提供了更加真实的物理意义.ERCOFTAC T3系列实验和低压涡轮叶栅的算例表明该模型能够自动捕捉流动的转捩过程,但还需要进一步修正以能更加准确考虑各种因素对转捩的影响。     

三维气固两相混合层湍流拟序结构的直接数值模拟

本文对三维气固两相混合层湍流拟序结构进行了直接数值模拟。气相流场应用拟谱方法对Ｎ－Ｓ方程组进行直接求解,通过模拟时间模式的混合层流动,分析流场失稳后涡的卷起、配对、合并及撕裂过程,研究三维混合层湍流拟序结构的演变特征;计算颗粒场时,采用Ｌａｇｒａｎｇｉａｎ方法,针对颗粒不同的Ｓｔｏｋｅｓ数,模拟了颗粒场的瞬态分布,并分析流场三维大涡结构对颗粒分布的影响。     

超声速混合层MHz级超高频流动可视化实验

研究发展了超高频基于纳米示踪的平面激光散射（nano-tracer planar laser scattering,NPLS）技术。基于多腔并联脉冲激光器技术、棱锥分光与短曝光相机集成技术以及高精度同步控制技术,实现了MHz级流场可视化和精细测量。采用超高频NPLS技术研究了对流Mach数Ma_c=0.17,0.26混合层流场,获得了时间序列的混合层高分辨率NPLS图像。采用阵列型涡流发生器开展流动控制研究,分析涡流发生器对混合层发展的影响特性。通过选取典型涡结构,分析了超声速混合层不同发展阶段的涡运动和发展演化规律。发现混合层中段的不稳定性发展阶段,涡结构以平移和旋转为主,伴随一定的拉伸;混合层后段以变形和破碎为主,有大量小尺度结构产生。并且小尺度结构会受到剪切、大尺度结构以及小激波的影响,发生明显的非定常运动。      

Direct Numerical Simulation of the Rayleigh-Taylor Instability with the Spectral Element Method

A novel method is proposed to simulate Rayleigh-Taylor instabilities using a specially-developed unsteady three-dimensional high-order spectral element method code. The numerical model used consists of Navier-Stokes equations and a transport-diffusive equation. The code is first validated with the results of linear stability perturbation theory. Then several characteristics of the Rayleigh-Taylor instabilities are studied using this three-dimensional unsteady code, including instantaneous turbulent structures and statistical turbulent mixing heights under different initial wave numbers. These results indicate that turbulent structures of Rayleigh-Taylor instabilities are strongly dependent on the initial conditions. The results also suggest that a high-order numerical method should provide the capability of simulating small scale fluctuations of Rayleigh-Taylor instabilities of turbulent flows.     

Whistler instability in an electron-magnetohydrodynamic spheromak

A three-dimensional magnetic vortex, propagating in the whistler mode, has been produced in a laboratory plasma. Its magnetic energy is converted into electron kinetic energy. Non-Maxwellian electron distributions are formed which give rise to kinetic whistler instabilities. The propagating vortex radiates whistler modes along the ambient magnetic field. A new instability mechanism is proposed.     

On stability of channel flow of thermoviscous fluid

The paper presents key results on a large-scale entrainment of thermoviscous liquid layers with different temperatures and their further mixing observed in the plane-parallel flow with an inflectional velocity profile. We show that the instability development in the channel is more intensive at the inflection point vicinity and is not related directly to vorticity generation in the near-wall region. The considered flow being unstable relative to the finite-amplitude harmonic disturbances possesses several resonant frequencies initiating the most intense entrainment. Temperature fields are analyzed based on the time-averaged entrainment layer thickness and temperature isoline displacement. We discuss the spectral properties of flow enstrophy, vorticity, and kinetic energy in terms of asymptotics of cascades observed and coherent structures. Okubo-Weiss criterion is used for mapping of four flow zones wherein an active filamentation of the turbulent veil or long-term existence of vortex structures is possible.     

Order-disorder transitions in a turbulent medium

It is shown that ordered turbulent structures are organized in a turbulent medium when the time of flow mixing gets larger than the time of the energy exchange between turbulent pulsations. Transitions of the weak turbulence in a partially ordered state (moderate turbulence) are considered for the cases of beam-plasma, current-convective and drift instabilities.     

On the presence of spectral shortcut in the energy budget of an asymmetric jet–wake flow in a forward-curved centrifugal turbomachine as deduced from SPIV measurements

Viscous dissipation and its contribution to turbulent kinetic energy (TKE) budget are investigated in the asymmetric jet–wake flow of a forward-curved centrifugal turbomachine. Single-plane three-dimensional turbulent data are obtained using stereoscopic particle image velocimetry (SPIV). Viscous dissipation is indirectly estimated from subgrid-scale (SGS) dissipation (SGS energy flux) by filtering velocity field using a top-hat filter. The filter scale should be within the inertial sub-range and this is ensured by spectral analysis of the measured field. Reduction of turbulent energy flux for smaller filter scales plus underestimation of viscous dissipation as compared with other TKE terms both suggest the presence of spectral shortcut. This bypass energy transfer (from intermediate scales towards dissipative scales) works in parallel with direct SGS energy transfer and affects the classical energy cascade. Analysis of TKE budget in the rotor exit region shows significant radial/circumferential variations in the contributing terms. These variations are mainly due to jet–wake–volute interactions, circumferential asymmetry of volute area and expansion of flow toward the fan outlet.     

Analyse quantitative par moyens optiques des instabilities d'ecoulement en regime supersonique

Previous articles described a method of tomographic visualizations of turbulent mixing zones, stationary structures and instabilities in supersonic flows. This paper shows new improvements of this method which thus becomes quantitative and able to give the nature of such instabilities (unstationary flow) and the parameters of the shock waves oscillations.     

Characteristics and generation of elastic turbulence in a three-dimensional parallel plate channel using direct numerical simulation

Direct numerical simulations(DNSs) of purely elastic turbulence in rectilinear shear flows in a three-dimensional(3D) parallel plate channel were carried out,by which numerical databases were established.Based on the numerical databases,the present paper analyzed the structural and statistical characteristics of the elastic turbulence including flow patterns,the wall effect on the turbulent kinetic energy spectrum,and the local relationship between the flow motion and the microstructures' behavior.Moreover,to address the underlying physical mechanism of elastic turbulence,its generation was presented in terms of the global energy budget.The results showed that the flow structures in elastic turbulence were 3D with spatial scales on the order of the geometrical characteristic length,and vortex tubes were more likely to be embedded in the regions where the polymers were strongly stretched.In addition,the patterns of microstructures' elongation behave like a filament.From the results of the turbulent kinetic energy budget,it was found that the continuous energy releasing from the polymers into the main flow was the main source of the generation and maintenance of the elastic turbulent status.     

利用风廓线雷达谱宽反演晴空湍流耗散率

 从晴天风廓线雷达的回波信号机制出发,研究了利用风廓线雷达谱宽反演晴空湍流耗散率的方法,即从雷达测量的多普勒速度谱宽中提取出湍流因素引起的谱宽,进而由湍流谱宽和湍流动能耗散率的关系计算出湍流动能耗散率。通过对合肥地区风廓线雷达谱宽资料的分析,得到合肥上空300 m高度上湍流动能耗散率的大小分布在10-4~10^-1 m²/s³之间,与理论相符,说明利用雷达谱宽反演湍流耗散率的方法可行。     

Zur spektralen Verteilung der Turbulenzenergie und ihrer Komponenten in turbulenten Grenzschichten

On the Spectral Distribution of Turbulent Energy and Components in Turbulent Boundary Layers KOLMOGOROV 's theory of the inertial subrange of energy spectrum is presented for turbulent boundary layers. As a consequence KOVASZNAY 's formula for the transfer of kinetic energy is confirmed. It is shown that the structure of the turbulent transfer of momentum in boundary layers is analogous to the structure of the transfer of kinetic energy in the spectrum. For the spectral distribution of the turbulent shear stress in the subrange a k^?3 law (k wave number) – apparently in agreement with measurements – is derived from dimensional arguments. It follows that the exchange of energy among the components of turbulent energy in the subrange is some orders of magnitude smaller than the latter.     

Interaction of a Low Frequency Coherent Magneto-Acoustic Wave with a Turbulent Spectrum of Extra-Ordinary Waves

Starting from nonlinear evolution equations of a three-dimensional extra-ordinary wavepacket in a magnetized plasma and introducing a correlation function, equations are derived that describe interaction of a coherent low-frequency magneto-acoustic wave with a turbulent spectrum of high frequency extra-ordinary waves. These equations are used to derive the dispersion relation for a low frequency magnetoacoustic wave in the presence of a spectrum of turbulent high frequency extra-ordinary waves. In the narrow spectrum case, it is found that a small but finite spread in the spectrum of turbulence has a stabilizing influence on the instability which exists for vanishing spectral width. In the wide spectrum case, the effect of a weak turbulent spectrum of extra-ordinary waves is to produce a slight shift in frequency of the coherent low frequency magneto-acoustic wave and to damp or excite the wave.