激波和湍流相互作用的数值模拟

本文综述了关于激波和湍流相互作用数值模拟的近期研究进展, 主要包括激波和均匀各向同性湍流、激波和湍流边界层、激波和射流以及激波和尾迹的相互作用. 激波和湍流相互作用特性受到诸多因素的影响,如激波的强度、位置、形状和流动边界以及来流的湍流状态和可压缩性等. 激波和湍流的相互作用会引起流场结构、激波特性和湍流统计特性的显著变化. 最后简要讨论了激波和湍流相互作用数值研究需要关注的一些问题.      

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。     

流场非均匀性对非平面激波诱导的Richtmyer-Meshkov不稳定性影响的数值研究

基于Navier-Stokes方程组，采用可压缩多介质黏性流动和湍流大涡模拟程序MVFT (multi-viscous-flow and turbulence)，模拟了均匀流场与初始密度呈现高斯函数分布的非均匀流场中马赫数为1.25的非平面激波加载初始扰动air/SF₆界面的Richtmyer-Meshkov (RM)不稳定性现象。数值模拟结果表明，初始流场非均匀性将会影响非平面激波诱导的RM不稳定性演化过程。反射激波加载前，非平面激波导致的界面扰动振幅随着流场非均匀性增强而增大；反射激波加载后，非均匀流场与均匀流场条件下的界面扰动振幅差异有所减小。进一步，定量分析流场中环量分布及脉动速度统计量揭示了前述规律的原因。此外，还与平面激波诱导的RM不稳定性进行了简单对比，发现由于非平面激波波阵面区域的涡量与激波冲击界面时产生的涡量的共同作用，使得非平面激波与平面激波诱导的界面失稳过程存在差异。     

基于流动仿真大数据应对旋涡-湍流的研究进展

文章以流体科学进入二十一世纪后，在大规模超算、云存储、数据通信和人工智能为支撑的大数据时代背景下，结合目前在复旦大学航空航天系所构建的热流体湍流直接数值仿真数据库，以及复旦大学团队近期与美国德州大学刘超群教授、上海理工大学蔡小舒教授以及国内水动力学杂志编辑部所合作开展的第三代涡识别技术研究，初步概念性地展示旋涡和湍流，特别是针对有工程实际背景和直接应用价值的壁湍流，在这两个流体力学关键基础议题上的最新认知，和基于大数据深度学习的相关湍流工程模拟实践成果．这些成果包括：(1) 基于第三代涡识别技术的尾迹湍流中的涡运动学和动力学探索；(2) 流-热统一完整的类-1、类-2湍流边界层壁面律构建；(3) 基于第三代涡识别量对Kolmogorov 幂次律的再认知；(4) 基于DNS统计数据和神经网络深度学习构建新型湍流封闭模型及RANS计算实践．通过这些成果展示，论证解决这两个基础流体科学议题的技术路径，进而促进流体及相关学科研究在现代大数据背景下取得实质性进展和突破，并惠及现代流体、气动、水利、动力和化工等工程领域．     

横向紊动射流的数值与实验研究进展

横向紊动射流作为流体运动的一种重要类型,广泛存在于如: 燃气轮机气膜冷却、锅炉燃烧室等的燃烧控制, V/STOL(垂直或短距离起落)飞机、废气排放的控制等工程实际应用中.由于射流的存在,增加了流场的复杂性,流场中同时存在射流剪切层涡、马蹄形涡系、反向旋涡对和尾迹涡等4种涡系结构,这对流体力学理论研究具有重要意义.长期以来,研究人员从理论分析、实验测量和数值模拟方面对横向紊动射流进行了大量的研究工作,目前已经认识了流场中的许多流动特性和流动机理.从数值模拟和实验研究两个方面,比较并分析了国内外横向紊动射流研究的现状和研究结果,评述了不同湍流模型以及不同的实验测量方法对横向紊动射流的预测能力,讨论了存在的问题并对该领域的研究方向进行了展望.      

火星科学实验室气动特性数值分析

针对火星着陆探测器进入-下降-着陆过程的高超声速进入阶段,求解三维流体动力学Navier-Stokes 方程与化学反应动力学模型,分析火星科学实验室进入火星大气时的化学非平衡效应、探测器周围的流场结构和气动特性在化学非平衡效应影响下的变化. 结果表明,CO₂ 在激波后大量分解,消耗大量能量;在化学非平衡效应影响下,探测器头部激波脱体距离大幅减小,尾迹旋涡运动减弱;化学非平衡效应影响下探测器升力系数变化不大,阻力系数高于完全气体,升阻比略低,配平攻角小于完全气体.     

单一水平轴风电机组尾迹的模拟方法与流动机理研究综述

为实现碳达峰、碳中和“3060”目标, 风能将在我国能源体系发挥重要作用. 风力机尾迹是影响风电性能和度电成本的关键因素, 需在风力机布置和控制设计中充分考虑. 本文首先介绍风力机尾迹的数值模拟方法, 包括解析模型、低阶模型、大涡模拟和来流湍流生成方法. 解析模型和低阶模型可快速计算风力机尾迹, 但依赖于模型参数, 且不能或不能准确预测尾迹湍流特性. 结合风力机参数化模型的大涡模拟可准确预测尾迹蜿蜒等湍流特征, 是流动机理研究的有力工具, 可为发展快速预测模型提供数据和理论支撑. 接着, 本文介绍了叶尖涡、中心涡和尾迹蜿蜒并讨论其产生机理. 对于湍流来流, 叶尖涡主要存在于近尾迹. 蜿蜒是远尾迹的主要特征, 影响下游风力机的来流特征. 尾迹蜿蜒的产生有两种机制: 来流大尺度涡和剪切层失稳. 数值和观测结果显示两种机制共同存在. 机舱和中心涡对尾迹蜿蜒有重要影响. 采用叶片和机舱的致动面模型可准确预测尾迹蜿蜒. 研究显示不同风力机尾迹间的湍流特征存在相似性, 为发展尾迹湍流的快速预测模型提供了理论依据. 当前研究多关注平坦地形上的风力机尾迹, 复杂地形和海洋环境下的大气湍流和风力机尾迹的机理复杂, 现有工程模型无法准确预测, 有待深入研究.      

可压缩多介质粘性流体和湍流的大涡模拟

在可压缩多介质粘性流体动力学高精度计算方法MVPPM(multi-viscous-fluid piecewise parabolicmethod)基础上,引入Smagorinsky和Vreman亚格子湍流模型,采用大涡数值模拟方法求解可压缩粘性流体NS(Navier-Stokes)方程,给出适用于可压缩多介质流体界面不稳定性发展演化至湍流阶段的计算方法和二维计算程序MVFT(multi-viscosity-fluid and turbulence)。在2种亚格子湍流模型下计算了LANL(Los Ala-mos National Laboratory)激波管单气柱RM不稳定性实验,分析了气柱的形状、流场速度以及涡的特征,通过与LANL实验和计算结果的比较可知,Vreman模型略优于Smagorinsky模型,MVFT方法和计算程序可用于对界面不稳定性发展演化至湍流阶段的数值模拟。     

搅拌槽流场特性的大涡模拟研究

采用大涡模型结合气液两相流时均方程,对六直叶圆盘涡轮搅拌槽内的流场特性进行了数值模拟。自由水面的捕捉用VOF(Volume of Fluid)法,用Simplec方法求解控制方程。通过模拟得到了搅拌槽内复杂的双涡旋流场结构、转轮叶端尾涡的发展变化情况以及轴向流速的分布规律。通过对比大涡模拟与RNGκ-ε的计算结果,得知大涡模型能模拟出流场内瞬时旋涡的发展变化过程,反映出挡板的存在破坏了圆形搅拌槽的流通模式,提高了叶片附近的混合效率;桨叶区域湍流呈现明显的各向异性,时均流速存在明显的波动性。从而证明了用大涡模拟探讨搅拌槽内湍流现象及流场结构的可靠性。     

基于Stereo-PIV技术的三维发卡涡结构定量测量研究

发卡涡是湍流相干结构研究中最为关注的内容,实现发卡涡三维结构的定量测量并进行流体动力学分析,对深入研究湍流相干结构、实现湍流精准控制等具有重要意义.本研究通过对合成射流装置进行合理控制,使得层流边界层中产生了规则的人造发卡涡结构,进而用体视图像粒子测速仪(Stereo-PIV)锁相实验技术对发卡涡结构所在的三维空间流场进行了定量测量,并得到了一个完整周期内形成的发卡涡三维结构的空间流场. 结果发现,重构所得的三维发卡涡结构质量较高, 实验技术和方案具有可行性.发卡涡结构所在空间流场情况,符合目前人们对于发卡涡、高低速条带、喷射和扫掠事件的常规认识. 此外,对近壁二次流向涡、展向涡量集中区域的展向涡头和强剪切区域、与低速喷射流体相关的汇聚流动和发散流动等有了更细致的认识.同时, 也探讨了"基于二维脉动流场的相关特征去重构发卡涡三维流场"的可行性.为进一步定量探究发卡涡结构的形成演化、不同涡结构的融合及二次诱导等壁湍流相干结构问题提供思路.      

Impact of vacuum degree on the aerodynamics of a high-speed train capsule running in a tube

Motivated by the growing scientific and engineering interest in evacuated tube railway transportation systems, in this paper we numerically study the influence of the vacuum degree on the flow field around a train capsule running in an evacuated tube with circular section. The vacuum degree is increased by lowering the nominal pressure inside the tube. The numerical simulations are fully verified by wind-tunnel experimental data of supersonic flows around a blunt body and in a scramjet combustion chamber, as well as by several numerical results in other related studies. The flow around the train capsule is characterized by a compression region in front of the train, a chocked flow near the train, and a complex highly unsteady region behind the train, where expansions waves and reflecting oblique shock waves exist. The total aerodynamic drag and the vacuum degree are found to be linearly related, revealing that lowering the nominal pressure can have a significantly beneficial effect on the aerodynamic performance of the train capsule. The aerodynamic heating due to compressibility effects and the increased pressure are more prominent along the centreline of the tube than on the tube wall. As the vacuum degree increases, the temperature and pressure differences between the front and the tail of the train and the intensity of the reflected shock waves become less significant, so that the extension of the expansion region in the train wake shortens.     

Study on transient aerodynamic characteristics of parachute opening process

In the research of parachute, canopy inflation process modeling is one of the most complicated tasks. As canopy often experiences the largest deformations and loadings during a very short time, it is of great difficulty for theoretical analysis and experimental measurements. In this paper, aerodynamic equations and structural dynamics equations were developed for describing parachute opening process, and an iterative coupling solving strategy incorporating the above equations was proposed for a small-scale, flexible and flat-circular parachute. Then, analyses were carried out for canopy geometry, time-dependent pressure difference between the inside and outside of the canopy, transient vortex around the canopy and the flow field in the radial plane as a sequence in opening process. The mechanism of the canopy shape development was explained from perspective of transient flow fields during the inflation process. Experiments of the parachute opening process were conducted in a wind tunnel, in which instantaneous shape of the canopy was measured by high velocity camera and the opening loading was measured by dynamometer balance. The theoretical predictions were found in good agreement with the experimental results, validating the proposed approach. This numerical method can improve the situation of strong dependence of parachute research on wind tunnel tests, and is of significance to the understanding of the mechanics of parachute inflation process. The project supported by the National Natural Science Foundation of China (10377006). The English text was polished by Yunming Chen.     

逆向喷流流场模态分析及减阻特性研究

逆向喷流减阻的基本原理是利用逆向高速喷流与飞行器绕流的相互作用，使飞行器周围的流场结构发生变化，致使飞行器的气动特性发生改变，从而改善飞行器的气动性能。利用数值模拟方法对轴对称球头、截锥的逆向喷流流场开展了研究，考虑了高温非平衡化学反应对流场的影响。模拟了球头和截锥在不同总压比时流场不同的模态：长穿透流模态（LPM）和短穿透流模态（SPM），得到了不同模态下钝体表面压力、气动力系数和不同模态之间转换的瞬态效应．简单分析了喷流在减阻方面的应用，给出了几个喷口参数与减阻效率之间的关系，提出了喷流减阻工程应用时应考虑的主要因素。     

Numerical simulation of supersonic flow past reentry capsules

The flow fields over ARD (ESA's atmospheric reentry demonstrator), OREX (orbital reentry experiments) and spherically blunted cone-flare reentry configurations are numerically obtained by solving time-dependent, axisymmetric, compressible Navier–Stokes equations for freestream Mach numbers range of 1.2–6.0. The fluid dynamics are discretized in spatial coordinates employing a finite volume approach which reduces the governing equations to semi discretized ordinary differential equations. Temporal integration is performed using the multistage Runge–Kutta time-stepping scheme. A local time step is used to achieve steady-state solution. The numerical simulation is carried out on a structured grid. The flow-field features around the reentry capsule, such as bow shock wave, sonic line, expansion fan and recirculating flow in the base region are obtained. A good agreement is found between the calculated value of aerodynamic drag coefficient of the spherically blunted cone/fare reentry configuration with the experimental data. The effects of geometrical parameters, such as radius of the spherical cap, half cone angle, with sharp shoulder edge and with smooth shoulder edge on the flow-field have been numerically investigated for various reentry configuration which will be useful for optimization of the reentry capsule. PACS 47.11.Df, 47.40.Ki     

Experimental investigation of the effect of Reynolds number on flow structures in the wake of a circular parachute canopy

In the present study, an experimental study was conducted to characterize the effect of Reynolds number on flow structures in the turbulent wake of a circular parachute canopy by utilizing stereoscopic particle image velocime- try （Stereo-PIV） technique. The parachute model tested in the present study was attached by 28 nylon suspension lines and placed horizontally at the test section center of the wind tunnel. The obtained results showed that with the in- crease of Reynolds number, the intensities of the vortices near the downstream region of the canopy skirt were found to increase accordingly. However, the increase of Reynolds number did not result in a significant change in ensemble- averaged normalized x-component of the velocity, ensembleaveraged normalized vorticity, normalized Reynolds stress, and normalized turbulent kinetic energy distributions in the turbulent wake of the circular parachute canopy. The obtained results are very useful to further our understanding about the unsteady aerodynamics in the wake of flexible circular parachute canopies and to constitute a reference for CFD computation.     

Numerical evaluation of passive control of shock wave/boundary layer interaction on NACA0012 airfoil using jagged wall

Shock formation due to flow compressibility and its interaction with boundary layers has adverse effects on aerodynamic characteristics, such as drag increase and flow separation. The objective of this paper is to appraise the prac-ticability of weakening shock waves and, hence, reducing the wave drag in transonic flight regime using a two-dimensional jagged wall and thereby to gain an appropriate jagged wall shape for future empirical study. Different shapes of the jagged wall, including rectangular, circular, and triangular shapes, were employed. The numerical method was validated by experimental and numerical studies involving transonic flow over the NACA0012 airfoil, and the results presented here closely match previous experimental and numerical results. The impact of parameters, including shape and the length-to-spacing ratio of a jagged wall, was studied on aerodynamic forces and flow field. The results revealed that applying a jagged wall method on the upper surface of an airfoil changes the shock structure significantly and disinte-grates it, which in turn leads to a decrease in wave drag. It was also found that the maximum drag coefficient decrease of around 17%occurs with a triangular shape, while the max-imum increase in aerodynamic efficiency (lift-to-drag ratio) of around 10%happens with a rectangular shape at an angle of attack of 2.26?.     

返回舱高雷诺数再入过程底部流动稳定性

返回舱高雷诺数再入过程中存在肩部高热流、底部阻力无法准确预测以及非定常振动等问题,解决此类问题的关键是分离和转捩等物理现象的准确识别. 本文采用大涡模拟方法细致刻画了返回舱类钝体外形在高雷诺数再入过程中的分离和转捩等物理现象,获得了返回舱底部流动形态以及稳定性特征. 从肩部剪切失稳、底部流动结构失稳、尾迹发展区以及远尾迹区的耦合失稳等多个角度分析了返回舱外形的底部流动失稳机制.研究发现, 返回舱类外形底部流动稳定性主要存在两类失稳模式即肩部剪切失稳模式以及底部流动结构失稳模式,二种模式存在耦合效应, 同时在远尾迹湍流区域存在类卡门涡街的振荡行为.这些认识为理解返回舱外部扰动因素对底部流动的作用机理及返回舱稳定性控制提供了基础理论支撑.      

Active flow control on the 25°Ahmed body using a new unsteady jet

A numerical simulation based on the Large eddy simulation method is carried out on the near wake flow behind a 25° slant angle Ahmed body to analyze and establish a new method to control the near wake flow. An active flow control using a new unsteady jet derived from the traditional synthetic jet is applied to reduce the aerodynamic drag. The control devices are distributed along the separation edges on the rear part of the body. Their effects on the near wake and the rear body by influencing the flow topology and the static pressure distribution are examined respectively. The control frequency of the jet as the key forcing parameter is taken into consideration as well. The different actuation set-ups lead to a max drag reduction of up to 13.6%, which demonstrates a good correlation with the static pressure distribution at the rear end of the body.     

电磁极宽度对圆柱绕流场影响的数值分析

在雷诺数Re=200的情况,利用Maxwell方程直接数值计算表面包覆电极与磁极圆柱体产生的电磁力分布,将其加入到动量方程中,在各种电磁力作用参数和电磁极宽度的组合下,对表面覆盖电磁极圆柱体在弱电解质中的绕流场结构及其升阻力特性进行了数值模拟与分析.结果表明,当电磁极宽度较小时,圆柱体绕流场的分离点越容易接近后驻点,而电磁力对总阻力的影响并不明显,但对压差和摩擦阻力均有明显影响.当电磁极宽度较大时,圆柱体尾部区域越容易产生射流现象,而且总阻力随电磁力作用参数和电磁极宽度增大而减小.在电磁力尚不足以完全抑制周期性涡脱落的情况下,升力幅值随电磁力作用参数增大而减小,但随电磁极宽度则先减小后略有增加,升力脉动频率则均随电磁力作用参数和电磁极宽度增大而增加.研究表明,电磁力可以有效地改善圆柱体绕流场结构,达到减小圆柱体阻力并抑制其脉动升力之目的,因此是圆柱型结构的一种有效流动控制手段.     

Effect of vortex shedding in unsteady aerodynamic forces for a low Reynolds number stationary wing at low angle of attack

This study elucidates the relation between wake vortex shedding and aerodynamic force fluctuations for a low Reynolds number wing from time resolved particle image velocimetry (TR-PIV) experimental measurements. The results reveal a periodic lift and drag variation within the shedding cycle and resolve the frequencies of those fluctuations from a proper orthogonal decomposition (POD) and power spectral density (PSD) analysis. To show the effect of vortex shedding on the body force fluctuations, the evolution of instantaneous aerodynamic forces is compared to the pressure field of the fluid flow and to the vortical structures in the wake of the airfoil. A six step model describing the vortex-force relation is proposed. It shows that changes in lift such as maximum lift and minimum lift are associated with the detachment of a vortex. It also shows that the minimum or local minimum drag value is obtained at the onset formation of a vortex on the airfoil wake. Similarly, the maximum or local maximum drag is obtained at the onset formation of the saddle on the airfoil wake. The model further explains the asymmetry observed in the unsteady drag force evolution. The model can be used to optimize flow control and fluid-structure interaction applications.