两种湍流模型在高速旋转翼身组合弹箭中的对比研究

弹箭设计、弹道计算和稳定性研究都需要准确预测旋转弹箭的马格努斯力和力矩,国内针对旋转弹箭气动特性的数值模拟工作集中在旋成体上,对带翼外形进行完全时间相关的非定常研究鲜有见到;国外虽然有对带翼外形开展研究,但以验证方法为主,对湍流模型在复杂外形弹箭旋转中的研究未曾见到.采用完全时间相关的非定常N-S方程,对带翼弹箭开展计算,对比了一方程SA(Spalart-Allmaras)湍流模型和两方程k-ωSST(shear-stress-transport)湍流模型对马格努斯效应产生的影响,并分析了旋转导致的边界层和涡非对称畸变,以及周向压力分布和剪切应力分布非对称畸变.结果表明:旋转引起的物面流场参数变化主要体现在弹体中后部,SA和SST湍流模型预测的全弹马格努斯特性与阿诺德工程发展中心(Arnold Engineering Development Center,AEDC)实验及陆军研究实验室(Army Research Laboratory,ARL)的计算结果一致性很好,对动导数而言两湍流模型计算精度相当.两湍流模型计算的弹体左侧流场参数差异比右侧大,分析认为正向旋转使左侧壁面速度方向与来流速度相反,相互阻碍使气流脉动效应更强.壁面附近湍流黏性系数SA结果大于SST结果,y=0截面物面压力SA结果小于SST结果、最大相差6%,摩阻系数SA结果大于SST结果、最大相差35%.SA对旋转产生的分离抑制作用强于SST.     

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.      

两种湍流模型在高速旋转翼身组合弹箭中的对比研究1）

弹箭设计、弹道计算和稳定性研究都需要准确预测旋转弹箭的马格努斯力和力矩,国内针对旋转弹箭气动特性的数值模拟工作集中在旋成体上,对带翼外形进行完全时间相关的非定常研究鲜有见到;国外虽然有对带翼外形开展研究,但以验证方法为主,对湍流模型在复杂外形弹箭旋转中的研究未曾见到.采用完全时间相关的非定常N-S方程,对带翼弹箭开展计算,对比了一方程SA（Spalart-Allmaras）湍流模型和两方程k-!SST（shear-stress-transport）湍流模型对马格努斯效应产生的影响,并分析了旋转导致的边界层和涡非对称畸变,以及周向压力分布和剪切应力分布非对称畸变.结果表明：旋转引起的物面流场参数变化主要体现在弹体中后部,SA和SST湍流模型预测的全弹马格努斯特性与阿诺德工程发展中心（Arnold Engineering Development Center,AEDC）实验及陆军研究实验室（Army Research Laboratory,ARL）的计算结果一致性很好,对动导数而言两湍流模型计算精度相当.两湍流模型计算的弹体左侧流场参数差异比右侧大,分析认为正向旋转使左侧壁面速度方向与来流速度相反,相互阻碍使气流脉动效应更强.壁面附近湍流黏性系数SA结果大于SST结果,y=0截面物面压力SA结果小于SST结果、最大相差6%,摩阻系数SA结果大于SST结果、最大相差35%.SA对旋转产生的分离抑制作用强于SST.     

湍流逆梯度输运的理论与实验研究

湍流中存在的动量、标量的逆梯度输运是当前湍流研究中一个十分重要的方向,它具有深刻的理论意义和重要的实用背景．本文从实验、理论两方面综述了湍流逆梯度输运的研究成果,并指出了一些值得今后进一步研究的方向．     

液-液旋流分离管中强旋湍流的k-ε数值模拟

采用标准k-ε湍流模型,对一种典型液-液旋流分离管中的强旋湍流进行了数值模拟研究.结果表明:该模型对切向速度的数值预报夸大了Rankine涡中的似固核范围,抹煞了似固核外的位涡区;对轴向速度的数值预报未给出中心回流区;对其它流场参数的预报结果也都存在有明显的不合理之处.由此证明这种基于Boussinesq假设的各向同性湍流模型,虽然在管道流、平面射流和无旋流等简单流动问题中经受住了大量计算实践的检验,但在强旋湍流的数值预报方面的确存在有较大缺陷.此问题的解决有赖于对该模型进行必要的修正或转而采用更加高级的各向异性模型.     

磁流体压缩管道热电磁流动特性研究

利用磁流体五波模型对低磁雷诺数下压缩管道中磁流体流动进行数值模拟。该模型由带有电磁作用强制项的Navier-Stokes方程组与电势Poisson方程组成,数值格式分别采用严格保证熵条件的熵条件格式和中心差分格式。数值模拟对不同磁作用数下的不同几何外形管道进行数值模拟研究,结果表明在磁流体压缩管道中,由于发生器模式提取...     

Hall效应对三维磁流体发生器的影响

应用三维非理想低磁雷诺数磁流体五方程模型发展了对带有强制项的Navier-Stokes方程组采用熵条件格式, 对椭圆型电势方程采用SOR进行迭代的数值方法,研究了Hall效应对磁流体旁路超燃冲压发动机中磁流体发生器流动及性能的影响.磁流体发生器采用电子束获得有效可靠的电导率. 计算结果表明,Hall效应可引起流场和电场的扭曲, 从而诱导出不稳定二次流的发展与演变,并破坏Joule热的分布. 对这些磁流体现象作出了较详细的分析.最后计算了磁流体发生器的性能参数, 说明Hall效应将导致磁流体发生器的性能下降.      

颗粒物在人体上呼吸道模型中沉降规律的数值研究

本文采用RNG k-ε湍流模型对简化的上呼吸道内的流场进行数值仿真,并结合随机涡相互作用模型对上呼吸道内颗粒的局部运动沉积特性进行了数值模拟研究.结果表明,惯性冲击是微米级颗粒物主要的沉降机理,惯性参量能很好地衡量颗粒的沉降规律.口腔中流动较平稳,颗粒受扰动较小.颗粒由于惯性冲击而运动靠近咽部后部并易于沉积在咽部后壁面...     

湍流冲击射流流动与传热的数值研究进展

湍流冲击射流由于其冲击表面时具有很高的局部传热率和冲击力,被广泛应用于如表面的加热、电子元件的冷却、纸张的干燥和材料的切割等工程应用和工业过程中.由于其流动的复杂性,也常被作为一种理想的测试实例来评价湍流模型的性能.此外,湍升力射流与地面之间的空气动力作用对V/STOL (垂直或短距离起落)飞机的性能具有很大的影响.长期以来,人们从理论分析、实验测量和数值模拟方面对冲击射流进行了广泛而系统的研究,积累了丰富的资料.本文在分析了湍流冲击射流的数值研究现状的基础上,对近年来有关湍流冲击射流流动与传热的数值研究方面的文献有选择地进行了综述,重点评述了不同湍流模型对冲击射流流动与传热的预测能力,讨论了存在的问题并对该领域今后的研究方向进行了展望.      

连铸结晶器内部流场及熔渣卷入的数值模拟

针对连铸结晶器内部流场及熔渣卷入过程 ,提出了两方程k -ε湍流模型和充填法数值计算模型 ,并采用硅油 /水模拟体系代替熔渣 /钢液体系 ,进行了模拟试验和数值计算。对充气造成的浸入式水口水流脉动的影响进行分析。计算结果与试验结果符合良好 ,表明本文所采用的数值模拟方法是适用的     

小波变换在湍流数值研究中的应用

小波变换具有时空双局部性特点,恰好适应了湍流特性。本文主要阐述了小波在湍流数值计算中的两大研究进展:一个是利用连续小波(高斯小波)可使L ap lace算子降阶的特性来求解N-S方程;另一个是正交小波与有限元法相结合的方法——相干涡模拟。指出它们的优缺点及其存在的问题,并对小波在湍流计算中的应用前景作了展望。     

Validation of large eddy simulation method for study of flatness and skewness of decaying compressible magnetohydrodynamic turbulence

In the present work we study potential applicability of large eddy simulation (LES) method for prediction of flatness and skewness of compressible magnetohydrodynamic (MHD) turbulence. The knowledge of these quantities characterizes non-Gaussian properties of turbulence and can be used for verification of hypothesis on Gaussianity for the turbulent flow under consideration. Prediction accuracy of these quantities by means of LES method directly determines efficiency of reconstruction of probability density function (PDF) that depends on used subgrid-scale (SGS) parameterizations. Applicability of LES approach for studying of PDF properties of turbulent compressible magnetic fluid flow is investigated and potential feasibilities of five SGS parameterizations by means of comparison with direct numerical simulation results are explored. The skewness and the flatness of the velocity and the magnetic field components under various hydrodynamic Reynolds numbers, sonic Mach numbers, and magnetic Reynolds numbers are studied. It is shown that various SGS closures demonstrate the best results depending on change of similarity numbers of turbulent MHD flow. The case without any subgrid modeling yields sufficiently good results as well. This indicates that the energy pile-up at the small scales that is characteristic for the model without any subgrid closure, does not significantly influence on determination of PDF. It is shown that, among the subgrid models, the best results for studying of the flatness and the skewness of velocity and magnetic field components are demonstrated by the Smagorinsky model for MHD turbulence and the model based on cross-helicity for MHD case. It is visible from the numerical results that the influence of a choice subgrid parametrization for the flatness and the skewness of velocity is more essential than for the same characteristics of magnetic field.     

Numerical modeling of bubble-driven liquid metal flows with external static magnetic field

Three-dimensional numerical simulations are presented considering the impact of a steady magnetic field on a bubble-driven liquid metal flow inside a cylinder. The injection of moderate gas flow rates through a single orifice at the bottom of the fluid vessel results in the formation of a bubble plume. The magnetic field is applied in either vertical or horizontal direction. The calculations were performed by means of the commercial software package CFX using the Euler–Euler multiphase model and the RANS–SST turbulence model. The non-isotropic nature of MHD turbulence was taken into account by specific modifications of the turbulence model. The numerical models are validated with recent experimental results. (Zhang, C., Eckert, S., Gerbeth, G., 2007. The flow structure of a bubble-driven liquid–metal jet in a horizontal magnetic field, J. Fluid Mech. 575, 57–82.) The comparison between the numerical simulations and the experimental findings shows a good agreement. The calculations are able to reproduce a striking feature of a horizontal magnetic field found in the range of moderate Hartmann numbers revealing that such a steady transverse magnetic field may destabilize the flow and cause distinct oscillations of the liquid velocity.     

On the effect of inflow conditions in simulation of a turbulent round jet

This paper investigates the impact of the inflow conditions on simulations of a round jet discharging from a wall into a large space. The fluid dynamic characteristics of a round jet are studied numerically. A numerical method based on the control volume approach with collocated grid arrangement is employed. The k-e{k-\varepsilon} model is utilized to approximate turbulent stresses by considering six different inlet conditions. The velocity field is presented, and the rate of decay at the jet centerline is determined. The results showed that inflow conditions had a strong influence on the jet characteristics. This paper also investigates both sharp-edged and contoured nozzles. The effects of velocity, turbulence intensity, turbulence kinetic energy, and turbulence dissipation rate on flow field characteristics are examined. Results showed that the present simulations in both types of nozzles are in good agreement with experiments when considering the appropriate inflow conditions.     

关于湍流标度律的争鸣

研究湍流结构函数的标度律。实验或数值模拟得到的湍流结构函数的标度指数是奇异的。很多学者认为:这一实验事实否定Kolmogorov1941年(K41)提出的正常标度律,各向同性湍流惯性区的标度律是奇异的。近年来作者发表一系列文章,提出不同的观点:由于有限雷诺数效应,有限雷诺数湍流的标度指数不等于真正的惯性区标度指数,湍流结构函数的标度指数的实验数据并不否定K41正常标度律,各向同性湍流惯性区的标度律可能是正常的。惯性区奇异标度律和正常标度律对应的湍流物理本质是完全不同的,因而研究解决这个争论具有重要的意义。      

Numerical and Experimental Study of Rayleigh–Bénard–Kelvin Convection

We performed experimental and numerical studies of combined effects of thermal buoyancy and magnetization force applied on a cubical enclosure of a paramagnetic fluid heated from below and cooled from top. The temperature difference between the hot and cold wall was kept constant. After considering neutral situation (i.e. a pure natural convection case), magnetic fields of different intensity were imposed. The magnetization force produced significant changes in flow (transition from laminar to turbulent regimes), wall-heat transfer (enhancement) and turbulence (turbulence structures reorganization). The strong magnetic field and its gradients were generated by a superconducting magnet which can generate magnetic field up to 10 T and where gradients of the magnetic induction can reach up to 900 T²/m. A good agreement between experiments and numerical simulations was obtained in predicting the integral wall heat transfer over entire range of considered working parameters. Numerical simulations provided a detailed insights into changes of the local wall-heat transfer and long-term time averaged first and second moments for different strengths of the imposed magnetic induction.     

Effect of Inflow Turbulence on an Airfoil Flow with Laminar Separation Bubble: An LES Study

The present paper is concerned with numerical investigations on the effect of inflow turbulence on the flow around a SD7003 airfoil. At a Reynolds number Re_c =?60,000, an angle of attack α =?4^° and a low or zero turbulence intensity of the oncoming flow, the flow past the airfoil is known to be dominated by early separation, subsequent transition and reattachment leading to a laminar separation bubble with a distinctive pressure plateau. The objective of the study is to investigate the effect of inflow turbulence on the flow behavior. For this purpose, a numerical methodology relying on a wall-resolved large-eddy simulation, a synthetic turbulence inflow generator and a specific source term concept for introducing the turbulence fluctuations within the computational domain is used. The numerical technique applied allows the variation of the free-stream turbulence intensity (TI) in a wide range. In order to analyze the influence of TI on the arising instantaneous and time-averaged flow field past the airfoil, the present study evaluates the range 0% ≤ TI ≤?11.2%, which covers typical values found in atmospheric boundary layers. In accordance with experimental studies it is shown that the laminar separation bubble first shrinks and finally completely vanishes for increasing inflow turbulence. Consequently, the aerodynamic performance in terms of the lift-to-drag ratio increases. Furthermore, the effect of the time and length scales of the isotropic inflow turbulence on the development of the flow field around the airfoil is analyzed and a perceptible influence is found. Within the range of inflow scales studied decreasing scales augment the receptivity of the boundary layer promoting an earlier transition.     

Certain problems concerning the origin and maintenance of cosmic magnetic fields

How regular magnetic fields in a plasma originated and are sustained is a question of considerable interest, since various objects in space such as spiral arms of galaxies, many stars, and planets have a regular magnetic field [1–3]. Several theoretical studies have been devoted to this theme [4–9]. However, because of the severe mathematical difficulties which arise in solving this problem, its study in the general form is still not possible, and its kinematic aspects are usually considered. In [5] a study is made of the induction of a toroidal magnetic field from the dipole field of a rotating spherical mass under the influence of radial convection. For the process to be undamped it is necessary to maintain a poloidal field, which according to the Cowling theorem may be induced by asymmetric motions from a toroidal field. Parker [6] suggested a specific model for the field increase by motions of the cyclonic type and indicated the possibility of self-regulation of the field maintenance process. Under certain conditions this process has a wave nature, the so-called dynamo-waves. In [7, 8] Braginskii considered the question of maintaining a quasi-stationary field by motions of the medium which differ little from axisymmetric motions. As a result of the large magnetic Reynolds numbers Re_m1 the field decays slowly, and a weak generation mechanism is adequate for maintaining the field.In [9] Birman and Schlüter suggested a fundamentally new mechanism for the formation of magnetic fields in a plasma as a result of the appearance under certain conditions of a vortical component of the mechanical electromotive force. It appears that this mechanism may be the reason for the appearance of cosmic magnetic fields.In the present paper we make a detailed study of the Birman mechanism for the origin of magnetic fields, and we derive the conditions which are imposed on the motion and the characteristics of the completely ionized plasma which are necessary for the appearance of a vortical emf. It is shown that in this case the axisymmetric motions of a highly conducting plasma may generate toroidal magnetic fields of considerable magnitude. Further, a kinematic solution is given of the problem of the determination of the magnetic field and currents which arise in the spherical plasma volume modeling a star. This volume rotates about some axis with a given angular velocity which depends on the spherical coordinates R and . Characteristics corresponding to the mean characteristics of the sun model are taken for specific calculations.     

An extensive numerical benchmark of the various magnetohydrodynamic flows

There is a continuous need for an updated series of numerical benchmarks dealing with various aspects of the magnetohydrodynamics (MHD) phenomena (i.e. interactions of the flow of an electrically conducting fluid and an externally imposed magnetic field). The focus of the present study is numerical magnetohydrodynamics (MHD) where we have performed an extensive series of simulations for generic configurations, including: (i) a laminar conjugate MHD flow in a duct with varied electrical conductivity of the walls, (ii) a back-step flow, (iii) a multiphase cavity flow, (iv) a rising bubble in liquid metal and (v) a turbulent conjugate MHD flow in a duct with varied electrical conductivity of surrounding walls. All considered benchmark situations are for the one-way coupled MHD approach, where the induced magnetic field is negligible. The governing equations describing the one-way coupled MHD phenomena are numerically implemented in the open-source code OpenFOAM. The novel elements of the numerical algorithm include fully-conservative forms of the discretized Lorentz force in the momentum equation and divergence-free current density, the conjugate MHD (coupling of the wall/fluid domains), the multi-phase MHD, and, finally, the MHD turbulence. The multi-phase phenomena are simulated with the Volume of Fluid (VOF) approach, whereas the MHD turbulence is simulated with the dynamic Large-Eddy Simulation (LES) method. For all considered benchmark cases, a very good agreement is obtained with available analytical solutions and other numerical results in the literature. The presented extensive numerical benchmarks are expected to be potentially useful for developers of the numerical codes used to simulate various types of the complex MHD phenomena.     

Numerical and experimental study of the traveling magnetic field effect on the horizontal solidification in a rectangular cavity part 1: Liquid metal flow under the TMF impact

Traveling magnetic field is one of effective tools for controlling phase change processes in metals. A better understanding of electromagnetic impact in such applications can help to improve and simplify technological processes. In this paper, numerical and experimental study of the electromagnetic force generated by traveling magnetic field and its ability to control liquid gallium flow and, consequently, affect the characteristics of solidification and melting processes are evaluated. Three-dimensional numerical model for calculating the magnetic field distribution and electromagnetic force acting on liquid gallium in a laboratory-size rectangular cavity was analyzed. Specific values of the TMF impact were chosen for the cases of interest in order to use such impact in our further work with horizontal gallium solidification process. The traveling magnetic field inductor was designed and built for making appropriate measurements and validating calculations. The analysis was focused on the electromagnetic force and the obtained velocity field. The experimental setup included an ultrasonic Doppler velocimeter for noninvasive measurements of the velocities of liquid metal flow. The comparison of computations with the experiments has shown a good agreement.