氢-空气斜爆震波的极曲线分析

本文建立了一种以温度为单变量进行迭代,求解真实气体模型下的斜爆震波波后参数非线性方程组的方法。并通过以加热量和气流偏转角为变量,计算出对应斜爆震波角度和斜爆震波的驻定范围。对于给定的来流条件,基于真实气体模型的斜爆震波的驻定范围比量热完全气体的宽,所以在斜爆震波的计算中必须考虑热物性的影响。随着氢气和空气化学当量比的增加,斜爆震波驻定范围逐步变小。而来流马赫数越大,斜爆震波的驻定范围越大,其CJ爆震波对应的平面偏转角越小。来流初温的升高则使斜爆震波的驻定范围变宽。斜爆震波的驻定需要来流条件与斜面角度等因素的相互匹配。     

凝聚介质中斜激波的反射

结合数值模拟和理论分析,研究凝聚介质中斜激波反射.采用龙格-库塔控制体积间断有限元方法,数值求解"刚性气体"状态方程形式下的欧拉方程组;理论分析凝聚介质中斜激波反射模式;运用激波极曲线理论,给出典型激波强度下正规反射向非正规反射过渡的临界角及波后状态.比较数值模拟结果和激波极曲线理论分析结果,得到典型弱、强斜激波的反射图像.     

爆炸焊接斜碰撞过程的数值模拟

 借助动力学分析软件ANSYS/LS-DYNA 11.0,建立了考虑材料强度的流体弹塑性力学模型,运用光滑粒子流体动力学（SPH）方法,对爆炸焊接斜碰撞过程进行了数值模拟。界面波和射流的模拟结果与爆炸焊接生产实际中的现象较为吻合,说明采用SPH方法对爆炸焊接斜碰撞过程进行模拟研究是有科学价值的。     

35 GPa斜波加载下RDX单晶炸药的动力学行为

利用磁驱动斜波加载10 MA装置和激光干涉测速技术,开展了35 GPa压力下(100)晶向RDX单晶炸药的斜波压缩实验,获得了RDX单晶/LiF单晶窗口界面速度数据。实验结果显示,速度波剖面表现为明显的三波结构,由低到高依次对应弹性波、塑性波和相变波,α-γ相变的起始压力为3.1 GPa。结合修正的多相状态方程和非平衡相变动力学模型,对RDX单晶炸药的斜波压缩过程开展一维流体动力学数值模拟,模拟结果与实验结果基本一致。     

来流边界层效应下斜坡诱导的斜爆轰波

以超声速预混气中的斜爆轰波为研究对象,对其在来流边界层效应下的特性进行了实验研究.在马赫数为3的超声速预混风洞中,通过斜坡诱导产生了斜爆轰波.当来流的当量比较低时,预混气中产生的是化学反应锋面与激波面非耦合的激波诱导燃烧现象.此时边界层分离区中的化学反应放热将使分离区尺度显著增大,流场非定常性显著增强,激波位置剧烈振荡.当来流的当量比较高时,预混气将产生斜爆轰波.此时边界层分离区会影响到斜爆轰波起爆时的形态.在小尺度分离区下,斜爆轰波起爆时呈突跃结构(有横波);在中等尺度分离区下,流场固有的非定常性使斜爆轰波呈间歇突跃结构;在大尺度分离区下,斜爆轰波起爆则呈完全的平滑结构(无横波).     

辛烷-空气混合物爆燃爆震转捩的数值模拟

爆燃向爆震转捩（简称DDT）的问题对爆震波的产生至关重要,是爆震波发动机研究中的核心问题之一。木文针对气态辛烷和空气混合物中产生的燃烧波在一维封闭空间传播的DDT问题,运用化学反应流动基本理论和两步反应模型,在拉格朗日质量坐标系中建立该现象的数学表达,气体动力学方程采用显式（S?）差分格式,化学反应项使用二级精度的Adams方法,开发了程序CULDDT,完成了过程的数值模拟,并与实验数据进行了对比,两者符合较好,为研究辛烷和空气混合物中产生爆震波的过程提供了一种可靠的数值方法。     

基于数值计算的斜流泵级性能改进

采用CFD数值模拟的方法对斜流泵级内部三维紊流流场进行计算,预测了其效率-流量、扬程-流量特性,并与实验结果进行了对比验证。根据得到的流场结果,通过斜流泵级水力尺寸改进-数值模拟分析的循环工作,在最终改进的斜流泵级预测流场中减少了泵内流场的漩涡运动,避免了出口回流的产生,削弱了叶顶间隙泄漏涡,提高了斜流泵级的效率,改进了能量特性,得到了斜流泵级改进前后流场与效率的对比。     

湍流大气中斜程传输光场的相位特性

运用功率谱反演法对斜程传输大气湍流扰动相位屏进行了数值模拟。通过建立激光束初始畸变相位模型,从波前峰谷值和波前功率谱密度函数两个方面,对激光束通过大气湍流后的相位特性进行了研究,重点分析了湍流传输距离和天顶角对激光束波前相位分布的影响。研究结果表明:激光束在湍流大气斜程传输后,其波前相位会发生明显畸变,且空间高频相位较低频相位所占比例明显增加;通过湍流后的波前相位与传输距离及天顶角密切相关,传输距离越长,天顶角越大,相位畸变程度越大,高频相位所占比例越多。     

20 GPa斜波压缩下PBX-14炸药的动力学响应

利用磁驱动加载实验技术和激光干涉测速技术,开展了未反应固体TATB基PBX-14炸药的斜波压缩实验,获得了20 GPa峰值压力下PBX-14炸药的后表面速度波剖面实验数据。基于阻抗匹配修正的迭代Lagrange数据处理方法处理实验数据,获得了0~20 GPa压力范围内PBX-14炸药的压力-相对比容关系、高压声速-粒子速度关系等动力学特性参数。结合等熵状态方程和由实验获得的动力学参数,对PBX-14炸药的斜波压缩实验过程开展了一维流体动力学数值模拟,计算结果与实验结果吻合良好,验证了本实验方法、数据处理方法及选取的物理模型的正确性。     

斜入射角对二维正方形光子晶体带隙的影响

利用平面波法导出了光子晶体本征方程的矩阵形式,以此为基础计算了二维空气孔型正方形光子晶体在正入射时及斜入射时的带结构,并将带结构中混杂在一起的TE波和TM波的模式分离开来以分析斜入射角对带结构的影响.计算结果显示用电场E表示和用磁场H表示计算出来的结果完全一样,与电磁场是相互统一的结论完全吻合.研究还发现,TE波和TM波的第一带隙是完全分开的.随着波矢k的倾斜角度的增大,TE波的第一带隙逐渐减小并在波矢在（－1,0,1）平面内时完全消失.而TM波的带结构则随着波矢k的倾斜角的增大而变得平坦,带隙也是先随之增大,后又逐渐变小直至消失.     

Unstable combustion induced by oblique shock waves at the non-attaching condition of the oblique detonation wave

The instability of oblique shock wave (OSW) induced combustion is examined for a wedge with a flow turning angle greater than the maximum attach angle of the oblique detonation wave (ODW), where archival results rarely exist for this case in previous literatures. Numerical simulations were carried out for wedges of different length scales to account for the ratio of the chemical and fluid dynamic time scales. The results reveal three different regimes of combustion. (1) No ignition or decoupled combustion was observed if a fluid dynamic time is shorter than a chemical time behind an OSW. (2) Oscillatory combustion was observed behind an OSW if a fluid dynamic time is longer than a chemical time behind an OSW and the fluid dynamic time is shorter than the chemical time behind a normal shock wave (NSW) at the same Mach number. (3) Detached bow shock-induced combustion (or detached overdriven detonation wave) was observed if a fluid dynamic time is longer than a chemical time behind a NSW. Since no ignition or decoupled combustion occurs as a very slow reaction and the detached wave occurs as an infinitely fast reaction, the finite rate chemistry is considered to be the key for the oscillating combustion induced by an OSW over a wedge of a finite length with a flow turning angle greater than the maximum attach angle for an ODW. Since this case has not been previously reported, grid independency was tested intensively to account for the interaction between the shock and reaction waves and to determine the critical time scale where the oscillating combustion can be observed.     

Ignition limit and shock-to-detonation transition mode of n-heptane/air mixture in high-speed wedge flows

In this work, oblique detonation of n-heptane/air mixture in high-speed wedge flows is simulated by solving the reactive Euler equations with a two-dimensional (2D) configuration. This is a first attempt to model complicated hydrocarbon fuel oblique detonation waves (ODWs) with a detailed chemistry (44 species and 112 reactions). Effects of freestream equivalence ratios and velocities are considered, and the abrupt and smooth transition from oblique shock to detonation are predicted. Ignition limit, ODW characteristics, and predictability of the transition mode are discussed. Firstly, homogeneous constant-volume ignition calculations are performed for both fuel-lean and stoichiometric mixtures. The results show that the ignition delay generally increases with the wedge angle. However, a negative wedge angle dependence is observed, due to the negative temperature coefficient effects. The wedge angle range for successful ignition of n-heptane/air mixtures decreases when the wedge length is reduced. From two-dimensional simulations of stationary ODWs, the initiation length generally decreases with the freestream equivalence ratio, but the transition length exhibits weakly non-monotonic dependence. Smooth ODW typically occurs for lean conditions (equivalence ratio < 0.4). The interactions between shock/compression waves and chemical reaction inside the induction zone are also studied with the chemical explosive mode analysis. Moreover, the predictability of the shock-to-detonation transition mode is explored through quantifying the relation between ignition delay and chemical excitation time. It is demonstrated that the ignition delay (the elapsed time of the heat release rate, HRR, reaches the maximum) increases, but the excitation time (the time duration from the instant of 5% maximum HRR to that of the maximum) decreases with the freestream equivalence ratio for the three studied oncoming flow velocities. Smaller excitation time corresponds to stronger pressure waves from the ignition location behind the oblique shock. When the ratio of excitation time to ignition delay is high (e.g., > 0.5 for n-C₇H₁₆, > 0.3 for C₂H₂ and > 0.2 for H₂, based on the existing data compilation in this work), smooth transition is more likely to occur.     

并行建表化学加速算法研究及其在气相斜爆轰模拟中的应用

采用详细化学反应机理对气相斜爆轰问题开展数值计算时,由于组分之间的特征时间尺度相差很大,反应源项的直接积分(direct integration,DI)求解通常存在强烈的刚性及非线性现象,导致计算量很大.为了在不损失计算精度的基础上有效减少化学反应过程的计算时间,针对包含2H2+O2详细机理的二维斜爆轰并行计算,提出两...     

Numerical study of self-sustained oblique detonation in a non-uniform mixture

Self-sustained oblique detonation behind a spherical projectile formed in a non-uniform H₂/O₂/Ar mixture was numerically investigated. A hypersonic combustible mixture flow around a 4.76 mm diameter body was modeled to be flowing at 2500 m/s and 100 kPa. The concentration gradient was prescribed applying the Gaussian distribution to hydrogen concentration. Axisymmetric Euler equations including a detailed kinetics of 9 species and 27 elementary reactions were solved with an explicit 2nd-order time integration scheme combined with point implicit method for chemical reaction. Oblique detonation was always obtained when the mixture on the centerline was stoichiometric, as it is for a uniform mixture, and a broader range of equivalence ratio could sustain oblique detonation far from the sphere. Local detonation angle was revealed to reasonably match Chapman-Jouguet analytical solutions with a minor difference attributed to curvature, less reactive composition, and the concentration gradient. Also, a strongly fuel-rich region encountered decoupling of the shock-flame, in which an abrupt deflection of the shock front appeared. These decoupling phenomena can be attributed to a slower kinetics of a less reactive mixture. All of interesting findings in this study will also benefit understanding of various form of detonation in non-uniform mixture taking advantage of the analogy between them.     

Formation and characteristics of composite reaction – Shock clusters in narrow channels

High-speed schlieren visualizations show that a composite reaction-shock cluster structure is formed in the last flame acceleration stage prior to detonation transition for ethylene/oxygen mixture in a narrow channel. The composite structure is bounded by a normal shock at the leading edge of the structure, and series of parallel oblique shocks interweave with reaction front on the other end in the cluster. Propagating velocity of the reaction front at the inception of the cluster is ～ 45–50% of Chapman-Jouguet detonation velocity of the mixture. Reaction front accelerates rapidly after the formation of the reaction-shock cluster, and run into detonation in tens of microseconds except for very lean mixtures. The angle between the parallel oblique shocks in the cluster and the side wall, defined as ω-angle, is found to be constant for a specific mixture as the reaction wave propagates. Dependence of ω-angle on mixture equivalence ratio and channel size are investigated in the study. Analysis shows that DDT distance is linearly proportional to ω-angle, and an empirical correlation is derived.     

Numerical prediction of oblique detonation wave structures using detailed and reduced reaction mechanisms

Modelling of the structure and the limiting flow turning angles of an oblique detonation wave, established by a two-dimensional wedge, requires the implementation of detailed chemical kinetic models involving a large number of chemical species. In this paper, a method of reducing the computational effort involved in simulating such high-speed reacting flows by implementing a systematically reduced reaction mechanism is presented. For a hydrogen - air mixture, starting with an elementary mechanism having eight species in 12 reactions, three alternate four-step reduced reaction mechanisms are developed by introducing the steady-state approximation for the reaction intermediates HO2, O and OH, respectively. Additional reduction of the computational effort is achieved by introducing simplifications to the thermochemical data evaluations. The influence of the numerical grid used in predicting the induction process behind the shock is also investigated. Comparisons of the induction zone predicted by two-dimensional oblique detonation wave calculations with that of a static reactor model (with initial conditions of the gas mixture specified by those behind the nonreactive oblique shock wave) are also presented. The reasonably good agreement between the three four-step reduced mechanism predictions and the starting mechanism predictions indicates that further reduction to a two-step mechanism is feasible for the physical flow time scales (corresponding to inflow Mach numbers of 8 - 10) considered here, and needs to be pursued in the future.     

Unstructured Adaptive Grid Flow Simulations of Inert and Reactive Gas Mixtures

Unstructured adaptive grid flow simulation is applied to the calculation of high-speed compressible flows of inert and reactive gas mixtures. In the present case, the flowfield is simulated using the 2-D Euler equations, which are discretized in a cell-centered finite volume procedure on unstructured triangular meshes. Interface fluxes are calculated by a Liou flux vector splitting scheme which has been adapted to an unstructured grid context by the authors. Physicochemical properties are functions of the local mixture composition, temperature, and pressure, which are computed using the CHEMKIN-II subroutines. Computational results are presented for the case of premixed hydrogen–air supersonic flow over a 2-D wedge. In such a configuration, combustion may be triggered behind the oblique shock wave and transition to an oblique detonation wave is eventually obtained. It is shown that the solution adaptive procedure implemented is able to correctly define the important wave fronts. A parametric analysis of the influence of the adaptation parameters on the computed solution is performed.     

Detonation simulations in supersonic flow under circumstances of injection and mixing

The unsteady, reactive Navier-Stokes equations with a detailed chemical mechanism of 11 species and 27 steps were employed to simulate the mixing, flame acceleration and deflagration-to-detonation transition (DDT) triggered by transverse jet obstacles. Results show that multiple transverse jet obstacles ejecting into the chamber can be used to activate DDT. But the occurrence of DDT is tremendously difficult in a non-uniform supersonic mixture so that it required several groups of transverse jets with increasing stagnation pressure. The jets introduce flow turbulence and produce oblique and bow shock waves even in an inhomogeneous supersonic mixture. The DDT is enhanced by multiple explosion points that are generated by the intense shock wave focusing of the leading flame front. It is found that the partial detonation front decouples into shock and flame, which is mainly caused by the fuel deficiency, nevertheless the decoupled shock wave is strong enough to reignite the mixture to detonation conditions. The resulting transverse wave leads to further mixing and burning of the downstream non-equilibrium chemical reaction, resulting in a high combustion temperature and intense flow instabilities. Additionally, the longitudinal and transverse gradients of the non-uniform supersonic mixture induce highly dynamic behaviors with sudden propagation speed increase and detonation front instabilities.     

Cell-like structure of unstable oblique detonation wave from high-resolution numerical simulation

A comprehensive numerical study was carried out to investigate the unsteady cell-like structures of oblique detonation waves (ODWs) for a fixed Mach 7 inlet flow over a wedge of 30° turning angle. The effects of grid resolution and activation energy were examined systematically at a dimensionless heat addition of 10. The ODW front remains stable for a low activation energy regardless of grid resolution, but becomes unstable for a high activation energy featuring a cell-like wave front structure. Similar to the situation with an ordinary normal detonation wave (NDW), a continuous increase in the activation energy eventually causes the wave-front oscillation to transit from a regular to an irregular pattern. The wave structure of an unstable ODW, however, differs considerably from that of a NDW. Under the present flow condition, triple points and transverse waves propagate downstream, and the numerical smoke-foil record exhibits traces of triple points that rarely intersect with each other. Several instability-driving mechanisms were conjectured from the highly refined results. Since the reaction front behind a shock wave can be easily destabilized by disturbance inherent in the flowfield, the ODW front becomes unstable and displays cell-like structures due to the local pressure oscillations and/or the reflected shock waves originating from the triple points. The combined effects of various instability sources give rise to a highly unstable and complex flow structure behind an unstable ODW front.     

Formation of transverse waves in oblique detonations

The structure of oblique detonation waves stabilized on a hypersonic wedge in mixtures characterized by a large activation energy is investigated via steady method of characteristics (MoC) calculations and unsteady computational flowfield simulations. The steady MoC solutions show that, after the transition from shock-induced combustion to an overdriven oblique detonation, the shock and reaction complex exhibit a spatial oscillation. The degree of overdrive required to suppress this oscillation was found to be nearly equal to the overdrive required to force a one-dimensional piston-driven detonation to be stable, demonstrating the equivalence of two-dimensional steady oblique detonations and one-dimensional unsteady detonations. Full unsteady computational simulations of the flowfield using an adaptive refinement scheme showed that these spatial oscillations are transient in nature, evolving in time into transverse waves on the leading shock front. The formation of left-running transverse waves (facing upstream) precedes the formation of right-running transverse waves (facing downstream). Both sets of waves are convected downstream away from the wedge in the supersonic flow behind the leading oblique front, such that the mechanism of instability must continuously generate new transverse waves from an initially uniform flow. Together, these waves define a cellular structure that is qualitatively similar to a normal propagating detonation.