菱形口射流与超音主流的相互作用

基于在不同射流角(10°, 27.5°, 45°, 90°)和射流总压(0.1 MPa, 0.46 MPa)下,对音速次膨胀射流通过菱形口喷射到马赫5横穿主流的实验及圆形射流器的对比实验,研究了次膨胀射流与超音横穿主流相互作用流场, 实验包括横截面流场的Pitot和锥静压力, 获得横截面马赫数、 压力分布．结果表明近壁面低马赫数半圆区为尾区,尾区附近边界层减薄．脱体激波高度向自由流扩展,激波形状更弯曲, 低马赫数区较大．高射流压力和射流角增加羽流涡度,激波位置较高．90°菱形和圆形喷射器有更强的羽流涡度,但圆形喷射器的低马赫数区较小．前沿渐细的变壁面的斜面物增加羽流涡度,反之,双变壁面的斜面物减弱羽流涡度．通过表面激波形状、中心平面激波及横截面激波模化三维激波形状,激波总压损失用正激波关系式通过马赫数法向分量估计．激波总压损失随射流角和动压比的减小而减小,最大损失发生在90°圆形和菱形喷射器．     

偏斜来流对圆柱跨声速绕流的影响

采用大涡模拟方法数值研究了偏斜角为60°的偏斜圆柱跨声速绕流.基于非偏斜圆柱跨声速绕流的实验和数值研究工作,来流Mach数取为0.75,Reynolds数取为2×105.通过与相同参数的非偏斜圆柱跨声速绕流对比,分析了偏斜来流对柱体受力和流动特性的影响.由于偏斜来流的流动控制,偏斜圆柱的阻力比非偏斜圆柱的阻力减小高达45%,而振荡力仅受到较小的抑制.偏斜圆柱流场的可压缩性被弱化,激波和小激波消失,而整体流动的模态未改变.偏斜来流使得偏斜圆柱后的剪切层变得更为稳定,进而提升柱体背压.在剪切层的初始发展阶段,剪切层的扰动涡斜脱泻模态和快速动能衰减是偏斜圆柱剪切层更为稳定的两个主要原因.     

具有H2引燃的CH4，煤油超声速混合的三维数值研究

用双流体模型和分区算法,气相多组分N－S方程和液相Euler方程分别用迎风TVD格式和NND格式进行数值求解,相间相互作用项方程用二阶Runge-Kutta法求解,并首次对H2引燃的CH4和煤油横向喷射,混合问题进行了数值研究,结果表明：与喷射加碘空气的PLIF结果相比,本文和实验结果符合得很好,对直通道,后面喷嘴的穿透深度大,煤油较气态 穿透深度大,但难以被卷吸到喷嘴前的回流区内,流场出现无煤油区,H2和CH4均可扩散到回流区内,CH4和煤油引燃机理不同,对后台阶直通道,存在两个回流区,H2可扩散到这两个回流区,但CH4只能被卷吸到其喷嘴前的回流区,后台阶直通道增强混合的效果和引燃点火的可靠性更好。     

具有H_2引燃的CH_4、煤油超声速混合的三维数值研究

用双流体模型和分区算法 ,气相多组分N_S方程和液相Euler方程分别用迎风TVD格式和NND格式进行数值求解 ,相间相互作用项方程用二阶Runge_Kutta法求解·　并首次对H2 引燃的CH4 和煤油横向喷射、混合问题进行了数值研究·　结果表明 :与喷射加碘空气的PLIF结果相比 ,本文和实验结果符合得很好·　对直通道 ,后面喷嘴的穿透深度大 ,煤油较气态燃料的穿透深度大 ,但难以被卷吸到喷嘴前的回流区内 ,流场出现无煤油区·　H2 和CH4 均可扩散到回流区内 ,CH4 和煤油引燃机理不同·　对后台阶直通道 ,存在两个回流区 ,H2 可扩散到这两个回流区 ,但CH4 只能被卷吸到其喷嘴前的回流区·　后台阶直通道增强混合的效果和引燃点火的可靠性更好     

直圆管突扩通道内宾汉流体湍流流场的数值研究

本文依据牛顿流体中建立的标准ｋ＿ε湍流模型这一基本思想，考虑宾汉流体的本构方程，建立了适用于求解宾汉流体湍流流动的控制方程·采用压力修正算法，实现了宾汉流体速度场与压力场的关联·在理论研究基础上，对直圆管突扩通道内宾汉流体湍流流动进行了数值研究，并探讨了直圆管突扩通道内宾汉流体湍流流动机理·     

关于高超声速绕流问题的激波层方法

切尔诺依(ГГ.Черный)的高超声速绕流激波层的级数解法,不适用于(γ-1)/(γ+1)<<2/(γ十1)M2sin2β(γ=c_p/c_v为绝热指数,M为马赫数,β为激波倾角)的情形。本文只假定在激波邻近存在激波层,在此薄层内,气体密度很大,但不排除(γ-1)/(γ+1)<<2/(γ+1)M2sin2β的情形。     

反射型激波风洞中激波与边界层的相互作用

本文研究了反射型激波风洞中由于非完全反射对激波与壁面边界层相互作用的影响,给出了在反射激波坐标系中计算边界层速度分布、温度分布和马赫数分布的计算方法.算例表明,在计及氮气的平衡真实气体效应的情形下,随着入射激波马赫数M_s的增大,边界层的最小马赫数从壁面处移到边界层内;随着喷管喉道面积的增大,边界层的最小马赫数、反射激波的分叉角α和分叉区后的射流速度均随之减小.计算结果与实验值相比是一致的.     

夹芯型雷达吸波结构的多目标优化

通过引入一个无量纲参数,将重量和雷达吸波性能两个优化目标结合于同一个目标函数中,提出了针对含有多孔材料芯层的夹芯型雷达吸波结构（RASS）的一种多目标优化设计方法．优化模型为承受均布载荷的悬臂夹芯板,考虑4种不同形式的芯层．由传输矩阵法和周期矩量法计算出镜面反射率的平均值,作为表征吸波性能的指标;而面板屈服、芯层剪切破坏和面板起皱则作为优化设计中的力学性能约束．优化结果表明,以填充超轻质海绵体的复合材料二维点阵为芯层的夹芯结构,比含有多孔泡沫或六角蜂窝芯层的夹芯结构更适合作为轻质夹芯型雷达吸波结构．Kagome二维点阵则表现出优于正方二维点阵的吸波性能．     

四个对激波与涡旋有高分辨率的计算格式的比较*

近十年来,计算非定常无粘可压气体力学Euler方程组的高分辨率差分格式有显着进展.本文选择四个近年受到重视的格式,用一较复杂的二维不定常问题作进一步的考验.所选算例为平面激波遇矩形障碍物初始阶段的绕射与反射.在挡板头部有两个尖角点,角点附近流场参量变化剧烈,会有中心稀疏波和集中涡的出现,要模拟好它们,就要求格式有较好的适应性.本文选择特殊的激波马赫数M_s=2.068,使静止坐标系下激波后流速恰为声速,并沿中心稀疏波区从角点发出的一条曲线也有这一现象,以考察各格式在方程组某一特征值恰为零时的计算特点,因零特征值可以使某些格式局部受损.计算结果的图形显示可表明四个格式在激波分辨率,格式粘性、膨胀波的计算、模拟非定常集中涡产生过程的能力等方面的性质.     

一种治愈强激波数值不稳定性的混合方法

HLLC（Harten-Lax-Leer-contact）格式是一种高分辨率格式,能够准确捕捉激波、接触间断和稀疏波.但是使用HLLC格式计算多维问题时,在强激波附近会出现激波不稳定现象.FORCE（first-order centred）格式在强激波附近表现出很好的稳定性,并且其数值耗散比HLL（Harten-Lax-Leer）格式小.分析了HLLC格式和FORCE格式在特定流动条件下的稳定性,构造了HLLC-FORCE混合格式并且进一步结合开关函数来消除HLLC格式的激波不稳定现象.数值试验表明新构造的混合格式不仅能够消除HLLC格式的激波不稳定现象,还最大程度地保留HLLC格式高分辨率的优点.     

Potential theory for regular and mach reflection of a shock at a wedge

If a plane shock hits a wedge, a self-similar pattern of reflected shocks travels outward as the shock moves forward in time. The nature of the pattern is explored for weak incident shocks (strength b) and small wedge angles 2θ_w through potential theory, a number of different scalings, some study of mixed equations and matching asymptotics for the different scalings. The self-similar equations are of mixed type. A linearization gives a linear mixed flow valid away from a sonic curve. Near the sonic curve a shock solution is constructed in another scaling except near the zone of interaction between the incident shock and the wall where a special scaling is used. The parameter β = c₁θ²_w(γ + 1)b ranges from 0 to ∞. Here γ is the polytropic constant and C₁ is the sound speed behind the incident shock. For β > 2 regular reflection (weak or strong) can occur and the whole pattern is reconstructed to lowest order in shock strength. For β < 1/2 Mach reflection occurs and the flow behind the reflection is subsonic and can be constructed in principle (with an open elliptic problem) and matched. The case β = 0 can be solved. For 1/2 < β < 2 or even larger β the flow behind a Mach reflection may be transonic and further investigation must be made to determine what happens. The basic pattern of reflection is an almost semi-circular shock issuing, for regular reflection, from the reflection point on the wedge and for Mach reflection, matched with a local interaction flow. Assuming their nature, choosing the least entropy generation, the weak regular reflection will occur for β sufficiently large (von Neumann paradox). An accumulation point of vorticity occurs on the wedge above the leading point. © 1994 John Wiley & Sons, Inc.     

Acoustic Radiation from a Circular Cylinder in a Subsonic Stream

The problem of acoustic radiation from a semi-infinite circularcylinder is discussed when a plane wave is propagated out ofthe mouth of the cylinder at which a vortex layer is attached.The effects of convection are included at low Mach numbers anda comparision is made of the radiation in the far field forthe case of propagation in (a) still air, (b) moving air withouta vortex layer and (c) moving air with a vortex layer. It isshown that in general the magnitude of the sound is much greaterdownstream than it is upstream. This directionality becomesmore pronounced in case (b) than in case (a) whilst in the presenceof a vortex layer the field is strengthened in the immediateregion downstream and weakened elsewhere.     

A difference scheme with anti-diffusion terms to improve the computation of contact discontinuities and the study of weak discontinuities in compressible flow

The investigation of Mach reflection formed after the impingement of a weak plane shock wave on a wedge with shock Mach number M_s near 1, is still an open problem[12]. It's difficult for shock tube experiments with interferometer to detect contact discontinuities if it is too weak; also difficult to catch with due accuracy the transition condition between Mach reflection and regular reflection. The interest to this phenomenon is continuing, especially for weak shocks, because there was systematic discrepancy between simplified three shock theory of von Neumann [8] and shock tube results [15] which was named by G. Birkhoff as “von Neumann Paradox on three shock theory” [18].In 1972, K.O.Friedrichs called for more computational efforts on this problem. Recently it is known that for weak impinging shocks it's still difficult to get contact discontinuities and curved Mach stem with satisfactory accuracy. Recent numerical computation sometimes even fails to show reflected shock wave[6]. These explain why von Neumann paradox of the three shock theory in case of weak discontinuities is still a problem of interesting [9,12,14]. In this paper, on one hand, we investigate the numerical methods for Euler's equation for compressible inviscid flow, aiming at improving the computation of contact discontinuities, on the other hand, a methodology is suggested to correctly plot flow data from the massive information in storage. On this basis, all the reflected shock wave , contact discontinuities and the curved Mach stem are determined. We get Mach reflection under the condition when over-simplified shock theory predicts no such configuration[5].     

The reflection of a shock wave from a centre or axis of symmetry at adiabatic exponents from 1.2 to 3

The self-similar problem of the reflection of a shock wave from a centre or axis of symmetry for adiabatic exponents from 1.2 to 3 with a maximum step of 0.1 is solved. The distributions of the main parameters behind the reflected shock wave are obtained.     

Diffraction of normal shock wave by yawed wedges

The diffraction of normal shock with yawed wedges of small angles have been considered in this paper. Vorticity distribution of the fluid particle over the diffracted shock has been determined for several Mach numbers of the shock wave. The Mach reflection effects have also been investigated when the bend is concave to the flow.     

Diffraction of normal shock wave by yawed wedges

The diffraction of normal shock with yawed wedges of small angles have been considered in this paper. Vorticity distribution of the fluid particle over the diffracted shock has been determined for several Mach numbers of the shock wave. The Mach reflection effects have also been investigated when the bend is concave to the flow.     

Large Eddy Simulations of Swirling Nozzle-Jet Flow Undergoing Vortex Breakdown

We developed a numerical setup to simulate swirling jet flow undergoing vortex breakdown. Our simulation code CONCYL solves the compressible Navier-Stokes equations in cylindrical coordinates using high-order numerical schemes. A nozzle is included in the computational domain to account for more realistic inflow boundary conditions. Preliminary results of a Re = 5000 compressible swirling jet at Mach number M a = 0.6 with an azimuthal velocity as high as the maximum axial velocity (swirl number S = 1.0 ) capture the fundamental characteristics of this flow type: At a certain point in time the jet spreads and develops into a conical vortex breakdown. A stagnation point-flow in the vicinity of the jet axis is clearly visible with the stagnation point located close to the nozzle exit. The stagnation point precesses in time around the jet axis, moving up- and downstream. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Collision of a solar wind shock wave with the Earth’s bow shock. Wave flow pattern

The wave pattern of the flow developed when a solar wind shock wave propagates along the surface of the Earth’s bow shock is studied. The investigation is carried out in the three-dimensional non-plane-polarized formulation within the framework of the ideal magnetohydrodynamic model in which the medium is assumed to be inviscid and non-heat-conducting and to have the infinite conductivity. The global three-dimensional pattern of the interaction which is a function of the latitude and longitude of elements on the surface of the bow shock is constructed as a mosaic of solutions to the problem of breakdown of a discontinuity developed between the states behind the impinging and bow shocks on the moving curve of intersection of their fronts. The investigation is carried out for typical solar wind parameters and interplanetary magnetic field strength in the Earth’s orbit and for several Mach numbers of the interplanetary shock wave, which makes it possible to trace the evolution of the flow developed as a function of the intensity of the shock perturbation of the solar wind. The solution obtained is necessary for interpreting measurements carried out by spacecraft located in the neighborhood of the Lagrange point and the Earth’s magnetosphere.     

Mach configuration in pseudo-stationary compressible flow

This paper is devoted to studying the local structure of Mach reflection, which occurs in the problem of the shock front hitting a ramp. The compressible flow is described by the full unsteady Euler system of gas dynamics. Because of the special geometry, the motion of the fluid can be described by self-similar coordinates, so that the unsteady flow becomes a pseudo-stationary flow in this coordinate system. When the slope of the ramp is less than a critical value, the Mach reflection occurs. The wave configuration in Mach reflection is composed of three shock fronts and a slip line bearing contact discontinuity. The local existence of a flow field with such a configuration under some assumptions is proved in this paper. Our result confirms the reasonableness of the corresponding physical observations and numerical computations in Mach reflection.

In order to prove the result, we formulate the problem to a free boundary value problem of a pseudo-stationary Euler system. In this problem two unknown shock fronts are the free boundary, and the slip line is also an unknown curve inside the flow field. The proof contains some crucial ingredients. The slip line will be transformed to a fixed straight line by a generalized Lagrange transformation. The whole free boundary value problem will be decomposed to a fixed boundary value problem of the Euler system and a problem to updating the location of the shock front. The Euler system in the subsonic region is an elliptic-hyperbolic composite system, which will be decoupled to the elliptic part and the hyperbolic part at the level of principal parts. Then some sophisticated estimates and a suitable iterative scheme are established. The proof leads to the existence and stability of the local structure of Mach reflection.