俯仰激励下三维液体大幅晃动问题研究

主要研究俯仰激励下三维液体大幅晃动问题,将任意Lagrange-Euler法 (arbitrary Lagrange-Euler, ALE)运动学描述引入到Navier-Stokes方程中,推导了俯仰激励下液体大幅晃 动数值模拟计算公式,并利用Galerkin加权余量法推导了有限元数值离散方程和分步有限 元计算格式,采用ALE分步有限元方法对圆筒形贮腔中的液体大幅晃动进行了数值模拟计 算. 得到了波高、晃动力及晃动力矩等晃动特性的时间变化历程,并对结果进行了分析,揭 示了俯仰激励下三维液体大幅晃动问题的非线性现象.     

振动翼型和襟翼的绕流特性研究

用数值模拟手段详细地研究了振动翼型和襟翼的绕流问题,数值模拟的出发方程为Euler和N-S方程,格式为Bcam-Warming格式的改进型。数值实验主要针对流场的二大特性进行的,即振动对激波的影响和振动对分离的抑制作用,结果表明:(1)随翼型或襟翼的振动激波强度和位置也相应地变化但这一变化滞后于攻角的变化;(2)振幅加大激波强度的变化和激波运动范围也加大;(3)振动频率越高对激波的影响反而较低频时要小;(4)流动条件的不同可使升力回线的走向发生变化;(5)振动对分离有明显的抑制作用。     

非定常俯抑振荡下的横向喷流数值模拟

采用高精度格式数值求解RANS方程,研究了定常状态下横向喷流流场,压力分布计算结果与实验结果基本吻合,并捕捉到喷流干扰流场中多种流动结构.在非定常计算过程中,飞行器的振动引起了法向力和俯仰力矩系数的相位滞后,推力放大因子随俯仰角周期变化.飞行器振动过程中,喷流流场的动态气动特性与稳态喷流有明显的区别,因此在利用横向喷流对飞行器进行姿态控制时,应该考虑由于飞行器姿态的变化对横向喷流所产生的非定常影响问题.     

俯仰激励下液体大幅晃动问题研究

主要研究俯仰激励下液体大幅晃动问题，将ALE(arbitrary Lagrange-Euler)运动学描述引入到Navier-Stokes方程中，推导了俯仰激励下液体大幅晃动数值模拟计算公式，并利用Galerkin加权余量法推导了有限元数值离散方程，采用ALE有限元方法对方形贮腔中的液体大幅晃动进行了数值模拟计算，对结果进行了比较分析，揭示了俯仰激励下液体大幅晃动问题的非线性现象。     

非结构混合网格高超声速绕流与磁场干扰数值模拟

对均匀磁场干扰下的二维钝头体无粘高超声速流场进行了基于非结构混合网格的数值模拟.受磁流体力学方程组高度非线性的影响及考虑到数值模拟格式的精度,目前在此类流场的数值模拟中大多使用结构网格及有限差分方法,因而在三维复杂外形及复杂流场方面的研究受到限制.本文主要探索使用非结构网格(含混合网格)技术时的数值模拟方法.控制方程为耦合了Maxwell方程及无粘流体力学方程的磁流体力学方程组,数值离散格式采用Jameson有限体积格心格式,5步Runge-Kutta显式时间推进.计算模型为二维钝头体,初始磁场均匀分布.对不同磁感应强度影响下的高超声速流场进行了数值模拟,并与有限的资料进行了对比,得到了较符合的结果.     

直接积分法对强迫振动计算精度的讨论

1.前言 以往对积分格式的研究重心多是集中在动力方程右边为零的自由振动下的精度和稳定上。在实际工程中,大多数的工程振动问题都是由强迫振动引起,所以仅讨论在自由振动下某一积分格式的精度是很不全面的。文献[1]讨论了结构阻尼为零,在简谐荷载作用下的Newmark积分格式的数值误差,首次提出了伪共振的概念。由于大多数荷载都可以用富里     

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...     

细长体背风面横向非对称分离的研究

用数值模拟方法研究了超声速情况下，无限长细长体背风面的涡结构。数值模拟的出发方程和计算格式分别为全Ｎ－Ｓ方程和二阶空间精度的ＴＶＤ格式。数值结果给出了圆锥、半球柱体和椭圆锥在不同攻角下的流场结果。结果表明圆锥在攻角α＝１５°，２０°和２５°时背风面呈现明显的稳定非对称横向分离，而半球柱体和椭圆锥在３２．５°和２５°时背风面均未出现非对称的横向分离结构。     

入口速度比对射流拟序结构的影响

为了深入了解湍流流动机理以及湍流拟序结构发现过程的影响因素，本文采用大涡模拟方法对不同入口射流伴流速度比的平面湍射流流动进行了数值模拟。采用分步投影法求解动量方程，亚格子项采用标准Smagorinsky亚格子模式模拟，压力泊松方程采用修正的循环消去法快速求解，空间方程采用二阶精度的差分格式，在时间方向上采用二阶精度的显式差分格式。模拟结果给出了平面射流中湍流拟序结构的瞬态发展演变过程，分析了入口速度比对射流拟序结构发展演化过程及宏观流场形态的影响。为进一步研究射流拟序结构及其在湍流流动中的作用提供了基础。     

关于无振荡、无自由参数有限元格式的研究

利用双曲守恒律方程的Ｔａｙｌｏｒ弱解表达式，建立了有限元法修正方程，选择合适的展开式系数能得到一系列数值格式．通过稳定性分析研究了格式的稳定性、色散误差与有限元修正方程导数项系数之间的关系，该关系与差分法的ＮＮＤ格式一致．在选定格式下，通过ＣＦＬ数可控制有限元离散解的振荡而使格式不含自由参数．最后，用数值算例验证了这一关系，并在二、三维欧拉方程作了推广应用．     

钝头体激波诱导振荡燃烧现象的数值模拟

采用一种改进的化学非平衡流解耦方法对轴对称Euler反应流方程解耦处理,对流项采用五阶WENO格式离散,化学反应源项的刚性采用简化的隐式方法处理,时间步进采用二阶精度的Runge-Kutta方法,对H₂/Air预混气在来流Ma=4.48和Ma=4.79时的激波诱导振荡燃烧现象进行了数值研究. 对比分析了网格尺度的影响,发现计算结果对法向网格尺度比较敏感,流向网格密度的变化对结果影响不大;Ma=4.48时,采用敏感性分析方法对各反应模型进行了对比分析,J和B-W模型在实验所处的温度和压力范围内能够比较准确的预测诱导时间,所得的振荡频率与实验结果相符,所揭示的振荡机理与McVey和Toong振荡机理吻合,而JM模型预测的诱导时间偏长,其振荡频率低于实验观测值;Ma=4.79时,J模型所得的振荡频率与实验值吻合,而B-W模型释热时间短,且对流场波动比较敏感,流场扰动引起了瞬时局部爆震现象,破坏了振荡的规律性;五阶WENO空间离散格式的应用使计算结果精度较好.      

Numerical analysis of pressure oscillations over axisymmetric spiked blunt bodies at Mach 6.80

The pressure oscillations over a forward facing spike attached to an axisymmetric blunt body are simulated by solving time-dependent compressible Navier–Stokes equations. The governing fluid flow equations are discretized in spatial coordinates employing a finite volume approach which reduces the equations to semidiscretized ordinary differential equations. Temporal integration is performed using the two-stage Runge–Kutta time stepping scheme. A global time step is used to obtain a time-accurate numerical solution. The numerical computation is carried out for a freestream Mach number of 6.80 and for spike length to hemispherical diameter ratios of 0.5, 1.0 and 2.0. The flow features around the spiked blunt body are characterized by a conical shock wave emanating from the spike tip, a region of separated flow in front of the hemispherical cap, and the resulting reattachment shock wave. Comparisons of the numerical results are made with the available experimental results, such as schlieren pictures and the surface pressure distribution along the spiked blunt body. They are found to be in good agreement. Spectral analysis of the computed pressure oscillations are performed employing fast Fourier transforms. The surface pressure oscillations over the spike and phase plots exhibit a behaviour analogous to that of the Van der Pol equation for a self-sustained oscillatory flow. Received 28 February 2001 / Accepted 17 January 2002     

Influence of boundary layer on unsteady aerodynamic characteristics of blunt cones in a supersonic flow

A study is made of the influence of the boundary layer on the unsteady aerodynamic characteristics of blunt cones oscillating in a supersonic gas stream about zero angle of attack. A solution to the problem is constructed in the framework of the linear theory of bodies of finite thickness. Such an approach has been used [1–3] in the case of the equations of motion of an ideal gas to calculate the unsteady aerodynamic characteristics of sharp and blunt bodies of various configurations. The influence on these characteristics of the viscosity effects due to the presence on the surface of the body of a laminar boundary layer has been taken into account [4–6] for bodies of the simplest shapes (wedge, cone). The present paper considers the unsteady aerodynamic characteristics of cones and investigates the influence of rounding of the tips and laminar and turbulent flow regimes in the boundary layer.     

Modified Multidimensional Dissipation Scheme on Unstructured Meshes for High-speed Compressible Flow Analysis

A Roe's flux-difference splitting scheme, combining with the entropy fix method according to Van Leer et al., and the H-correction entropy fix method by Pandolfi and D'Ambrosio, is proposed. The presented scheme eliminates unphysical flow behaviors such as a spurious bump of the carbuncle phenomenon that occurs on the bow shock from flow over a blunt body, and the expansion shock generated from flow over a forward facing step. The proposed scheme is further extended to obtain high-order spatial and temporal solution accuracy. The scheme is, in addition, combined with an adaptive meshing technique that generates unstructured triangular meshes to resemble the flow phenomena for reducing computational effort. The entire procedure is evaluated by solving several benchmarks as well as complex steady-state and transient high-speed compressible flow problems.     

钝头体高超声速绕流底部失稳特征数值模拟

利用数值模拟方法对高超声速钝锥及Apollo返回舱底部尾迹流场进行了研究, 分析尾迹流动的失稳过程. 对钝锥模型, 在M_∞=6, Re=1.71× 10⁶(Re以球头半径为参考长度)条件下观察到了底部流动的不稳定性. 不添加任何扰动, 数值模拟首先得到的流动是稳定解, 在底部发展出一个主分离区和一个二次分离区, 流动是轴对称状态. 继续进行计算, 发现二次分离线率先变形, 底部流场发展出非定常周期流动. 对Apollo返回舱模型, 在相同条件下 (Re以前面圆弧半径为参考长度), 数值模拟首先得到的流动同样是稳定解, 出现以二次分离线率先变形为起始的结构失稳, 演化出周期性过程, 但持续时间较短, 很快出现了非周期非对称状态. 研究表明, 高超声速钝锥及Apollo返回舱底部流场均存在不稳定性问题, Apollo返回舱的底部流场更加不稳定.     

Calculation of stationary inviscid flow over three-dimensional bodies

The results of the calculation of inviscid supersonic flow of a perfect gas over a blunt three-dimensional configuration are considered. An explicit finite-difference scheme of second order of accuracy [1] was used for the numerical integration of the hyperbolic system of equations, which was written in divergence form. The region of integration is situated between the body and the outer shock wave. The internal discontinuity surfaces were not separated out and the calculation was made through them. The points on the surface of the body were calculated using relations on characteristics written in a form that makes it possible to calculate flows with strong entropy layers. The results of calculation of flow over a three-dimensional configuration at an angle of attack are given.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza., No. 4, pp. 172–175, July–August, 1980.     

Numerical simulation of a pulsating flow arising over an axisymmetric spiked blunt body at Mach 2.21 and Mach 6.00

The present paper reports a numerical simulation of the supersonic/hypersonic unsteady flow over a spiked blunt body. Axisymmetric compressible Navier-Stokes equations are solved using a high-resolution unfactored implicit upwind Roe's scheme and a time-accurate pseudo-time method is employed for advancing in time. Unsteady flows arising at Mach 2.21 and Mach 6.00 around a spiked cylinder are simulated and the computational results are compared with measurements. The simulated results are used to increase understanding of the mechanisms of the flow. Received 28 September 1999 / Accepted 26 July 2000     

A local domain‐free discretization method for simulation of incompressible flows over moving bodies

This paper presents a local domain‐free discretization (DFD) method for the simulation of unsteady flows over moving bodies governed by the incompressible Navier–Stokes equations. The discretization strategy of DFD is that the discrete form of partial differential equations at an interior point may involve some points outside the solution domain. All the mesh points are classified as interior points, exterior dependent points and exterior independent points. The functional values at the exterior dependent points are updated at each time step by the approximate form of solution near the boundary. When the body is moving, only the status of points is changed and the mesh can stay fixed. The issue of ‘freshly cleared nodes/cells’ encountered in usual sharp interface methods does not pose any particular difficulty in the presented method. The Galerkin finite‐element approximation is used for spatial discretization, and the discrete equations are integrated in time via a dual‐time‐stepping scheme based on artificial compressibility. In order to validate the present method for moving‐boundary flow problems, two groups of flow phenomena have been simulated: (1) flows over a fixed circular cylinder, a harmonic in‐line oscillating cylinder in fluid at rest and a transversely oscillating cylinder in uniform flow; (2) flows over a pure pitching airfoil, a heaving–pitching airfoil and a deforming airfoil. The predictions show good agreement with the published numerical results or experimental data. Copyright © 2010 John Wiley & Sons, Ltd.     

Magnetic field applied to thermochemical non-equilibrium reentry flows in 2D – five species

In this work, a study involving magnetic field actuation over reentry flows in thermochemical non-equilibrium is performed. The Euler and Navier–Stokes equations are studied. The proposed numerical algorithm is centred and second-order accurate. The hypersonic flow around a blunt body is simulated. Three time integration methods are tested. The reactive simulations involve Earth atmosphere of five species. The work of Gaitonde is the reference to couple the fluid dynamics and Maxwell equations of electromagnetism. The results have indicated that the Maciel scheme, using the Mavriplis dissipation model, yields the best prediction of the stagnation pressure.     

Numerical heat transfer study around a spiked blunt-nose body at Mach 6

An aerospike attached to a blunt body significantly alters its flowfield and influences aerodynamic drag at high speeds. The dynamic pressure in the recirculation area is highly reduced and this leads to the decrease in the aerodynamic drag. Consequently, the geometry of the aerospike has to be simulated in order to obtain a large conical recirculation region in front of the blunt body to get beneficial drag reduction. Axisymmetric compressible Navier–Stokes equations are solved using a finite volume discretization in conjunction with a multistage Runge–Kutta time stepping scheme. The effect of the various types of aerospike configurations on the reduction of aerodynamic drag is evaluated numerically at a length to diameter ratio of 0.5, at Mach 6 and at a zero angle of incidence. The computed density contours are showing satisfactory agreement with the schlieren pictures. The calculated pressure distribution on the blunt body compares well with the measured pressure data on the blunt body. Flowfield features such as formation of shock waves, separation region and reattachment point are examined for the flat-disc spike and on the hemispherical disc spike attached to the blunt body. One of the critical heating areas is at the stagnation point of a blunt body, where the incoming hypersonic flow is brought to rest by a normal shock and adiabatic compression. Therefore, the problem of computing the heat transfer rate near the stagnation point needs a solution of the entire flowfield from the shock to the spike body. The shock distance ahead of the hemisphere and the flat-disc is compared with the analytical solution and a good agreement is found between them. The influence of the shock wave generated from the spike is used to analyze the pressure distribution, the coefficient of skin friction and the wall heat flux facing the spike surface to the flow direction.