高温非平衡流动中的氧分子振动态精细分析

高超声速流动在头激波压缩后常处于高 温条件下的热化学非平衡状态. 本文采用态-态方法和双温度模型计算分析了一维正激波后和高超声速钝体绕流驻点线上的氧气热化学非平衡流动. 态-态方法将氧气的每个振动能级当成独立的组分,通过耦合 Euler 方程或驻点线上的降维 Navier-Stokes 方程,数值求解得 到了高温流动中的精细热化学非平衡状态. 而双温度模型假设氧气的振动能级服从 Boltzmann 分布,通过求解振动能方程得到振动温度. 一维正激波后热化学松弛过程的计算结果表明,态-态计算预测的温度分布和氧原子浓度分布较好地吻合了文 献中的实验结果,而经典的双温度模型的预测结果误差较大,且不同双温度模型的计算结果比较发散. 态-态方法详细地给出了所有振动能级的变化过程. 无论是正激波还是脱体激波后的流场,都是高振动能级首先得到激发;但是数密度大的低振动能级先达到热平衡,而高能级 分子要经过很长距离后才能达到热平衡. 在驻点附近,复合反应生成的氧气分子处于高振动能级,导致高振动能级分子数密度显著高于平衡分布. 计算还发现,经典双温度模型的离解反应速率明显偏离态-态计算结果,无法准确体现振动离解耦合效应对离解反应 速率的影响,但是 Park 双温度模型将离解失去的振动能取为 0.3$\sim 高超声速流动在头激波压缩后常处于高 温条件下的热化学非平衡状态. 本文采用态-态方法和双温度模型计算分析了一维正激波后和高超声速钝体绕流驻点线上的氧气热化学非平衡流动. 态-态方法将氧气的每个振动能级当成独立的组分,通过耦合 Euler 方程或驻点线上的降维 Navier-Stokes 方程,数值求解得 到了高温流动中的精细热化学非平衡状态. 而双温度模型假设氧气的振动能级服从 Boltzmann 分布,通过求解振动能方程得到振动温度. 一维正激波后热化学松弛过程的计算结果表明,态-态计算预测的温度分布和氧原子浓度分布较好地吻合了文 献中的实验结果,而经典的双温度模型的预测结果误差较大,且不同双温度模型的计算结果比较发散. 态-态方法详细地给出了所有振动能级的变化过程. 无论是正激波还是脱体激波后的流场,都是高振动能级首先得到激发;但是数密度大的低振动能级先达到热平衡,而高能级 分子要经过很长距离后才能达到热平衡. 在驻点附近,复合反应生成的氧气分子处于高振动能级,导致高振动能级分子数密度显著高于平衡分布. 计算还发现,经典双温度模型的离解反应速率明显偏离态-态计算结果,无法准确体现振动离解耦合效应对离解反应 速率的影响,但是 Park 双温度模型将离解失去的振动能取为 0.3$\sim $0.5 倍分子离解能是比较合理的.     

高温非平衡流动中的氧分子振动态精细分析

高超声速流动在头激波压缩后常处于高温条件下的热化学非平衡状态.本文采用态-态方法和双温度模型计算分析了一维正激波后和高超声速钝体绕流驻点线上的氧气热化学非平衡流动.态-态方法将氧气的每个振动能级当成独立的组分,通过耦合Euler方程或驻点线上的降维Navier-Stokes方程,数值求解得到了高温流动中的精细热化学非平衡状态.而双温度模型假设氧气的振动能级服从B oltzmann分布,通过求解振动能方程得到振动温度.一维正激波后热化学松弛过程的计算结果表明,态-态计算预测的温度分布和氧原子浓度分布较好地吻合了文献中的实验结果,而经典的双温度模型的预测结果误差较大,且不同双温度模型的计算结果比较发散.态-态方法详细地给出了所有振动能级的变化过程.无论是正激波还是脱体激波后的流场,都是高振动能级首先得到激发;但是数密度大的低振动能级先达到热平衡,而高能级分子要经过很长距离后才能达到热平衡.在驻点附近,复合反应生成的氧气分子处于高振动能级,导致高振动能级分子数密度显著高于平衡分布.计算还发现,经典双温度模型的离解反应速率明显偏离态-态计算结果,无法准确体现振动离解耦合效应对离解反应速率的影响,但是Park双温度模型将离解失去的振动能取为0.3～0.5倍分子离解能是比较合理的.     

高超声速飞行流场中的最大氧离解度分析

针对大气层内高超声速飞行时的化学非平衡现象,建立了沿驻点线流动的空气中氧离解度的计算模型. 模型假设氮气在氧气未充分离解时不发生离解,并且不涉及边界层内的复合反应. 理论计算发现,空气中氧的离解度随飞行高度的增加呈先增后减的非单调变化规律,其原因是由于化学反应平衡移动与非平衡效应相互作用的结果. 这一结论得到了数值模拟结果的验证,同时也解释了文献中当飞行高度较高时真实气体效应减弱的现象. 基于驻点线的近似理论模型,计算得到了轴对称钝头体绕流流场中的最大氧离解度及边界层外缘温度随飞行速度和高度变化的等值线图谱,相关结果可以为工程设计所用.      

考虑井储和表皮效应的分形介质煤层气压力动态分析

考虑煤层的双重介质特征和介质的分形特征,建立了考虑井筒储存和表皮效应影响的分形介质煤储层非平衡吸附、非稳态条件下的气体流动数学模型,并分别求得了无限大地层条件下中心一口井定产量生产时无因次井底压力的Laplace空间解析解和实空间上的数值解.给出了无因次井底压力及其导数随分形参数、无因次井筒储存系数以及表皮系数等变化的双对数曲线图.     

高超声速飞行流场中的最大氧离解度分析简

针对大气层内高超声速飞行时的化学非平衡现象,建立了沿驻点线流动的空气中氧离解度的计算模型. 模型假设氮气在氧气未充分离解时不发生离解,并且不涉及边界层内的复合反应. 理论计算发现,空气中氧的离解度随飞行高度的增加呈先增后减的非单调变化规律,其原因是由于化学反应平衡移动与非平衡效应相互作用的结果. 这一结论得到了数值模拟结果的验证,同时也解释了文献中当飞行高度较高时真实气体效应减弱的现象. 基于驻点线的近似理论模型,计算得到了轴对称钝头体绕流流场中的最大氧离解度及边界层外缘温度随飞行速度和高度变化的等值线图谱,相关结果可以为工程设计所用.     

热化学非平衡辐射流场数值研究

从耦合辐射的轴对称热化学非平衡N-方程出发,采用双温度、11组元反应气体模型,耦合“线-线”精细辐射模型,利用隐式NND有限差分格式和时间预处理技术数值求解了FIREII飞船热化学非平衡辐射流场,得到了有关辐射和辐射光谱计算结果,并与有关文献的实验结果和计算结果进行了比较。     

轴对称再入舱模型气动热特性数值模拟研究

应用基于块结构网格的有限体积求解方法,对热化学非平衡环境下轴对称再入舱模型的气动热特性进行了数值模拟。控制方程为带化学反应的多组元轴对称N-S方程,空间离散采用VanLeer迎风格式,时间推进为隐式LU-SGS格式;采用7组元7化学反应模型及Park双温模型模拟再入流场的热化学非平衡效应。对Hollis MP-1模型的气动热特性进行了数值模拟,分别就网格效应、湍流模型、流场的热力学性质对流场的气动力、热环境的影响进行了深入研究。研究结果表明:SST模型与k-w1998模型能更准确地计算再入流场热流峰值的位置与大小;在再入舱模型的局部区域,采用热力学非平衡模型计算的物面压强与热流结果要明显低于热力学平衡模型的结果。     

非结构网格热化学非平衡辐射流场数值计算

针对三维热化学非平衡辐射流场设计了基于非结构网格的数值计算方法. 根据原子分子光谱理论逐条计算了100$\sim$1\,500\,nm间N, O, N$^{ + }$, O$^{ + }$的谱线以及N计算流体力学; 辐射; 热化学非平衡; 非结构网格; 有限体积法针对三维热化学非平衡辐射流场设计了基于非结构网格的数值计算方法.根据原子分子光谱理论逐条计算了100～1 500nm间N,O,N+,O+的谱线以及N2,O2,NO,N+2等分子的10个谱带,特别分析了NO的β'带,γ'带,δ带和ε带的辐射特性.采用耦合辐射的双温模型计算热化学非平衡流场,辐射源项通过直接求解辐射输运方程RTE(radiative transport equation)获得.在空间和方向上分别离散后,利用有限体积法求解不同方向上的辐射输运方程.计算得出了再入飞行器前驻点的辐射强度分布.采用该数值方法计算了MUSES-C模型在速度为11.6km/s时的绕流流场及前驻点处的辐射热流密度.并通过对比分析了热辐射对流场的影响.     

高温热化学非平衡气动热试验与仿真技术研究进展

临近空间新型飞行器向全空域、更高马赫数发展,面临的气动热环境会越发恶劣,高温流场气动热预测技术是该类飞行器发展的关键技术之一.高超声速气流通过激波压缩或黏性阻滞减速,分子动能转化为内能,产生了高温.高温引起体分子振动、电子激发,伴随离解、电离反应等一系列复杂气动物理现象,其流场气动热预测面临诸多挑战.文章对高温热化学非平衡气动热预测技术的发展情况进行了分析探讨.首先,阐述了国内外高温气动热地面试验技术的发展历程,重点介绍分析了气动热风洞试验设备的模拟能力及目前试验测试技术的研究水平;然后,调研和讨论了高温气动热数值模拟研究现状,分别从热化学模型、辐射输运和壁面催化/烧蚀等多个角度探讨了热化学非平衡流场气动热数值模拟规律;最后,对气动热预测技术的发展趋势进行了讨论,提出了高温气动热试验与仿真技术后续应重点解决的问题.     

高焓湍流边界层壁面摩阻产生机制分析

高超飞行器在中低空以极高马赫数飞行时,飞行器表面会遇到湍流与高温非平衡效应耦合作用的新问题.这种高焓湍流边界层壁面摩阻产生机制是新型高超声速飞行器所关注的基础科学问题,厘清此产生机制可以为减阻方法的设计提供指导,具有重要的工程实用价值.本文选取高超声速飞行时楔形体头部斜激波后的高焓流动状态,开展了考虑高温非平衡效应的湍...     

带喷流激波针流动特性实验研究

采用动态测力、动态测压和纹影等风洞实验技术,对加装了带喷流激波针的钝头体的绕流特性、稳定和非稳模态的形成条件和机理进行了研究.结果表明:带喷流激波针流场存在稳态和非稳态两种模态,超声速喷流的压比大于临界压比时流动处于稳定模态,反之则为非稳模态;增大激波针长度可减小钝头体阻力,但达到一定长度后,进一步减阻的效果不再显著;增大喷流压比能够有效减弱再附激波强度,有利于缓解单独激波针的肩部热斑问题;非稳模态下波系自激振荡对再附激波在钝头体表面所围的区域影响剧烈,振荡是周期性的,且存在确定的主导频率,主导频率随喷流压力比增大而减小;自激振荡的产生是由于喷流出口周围的反压在喷流压比小于临界压比时无法获得持续的平衡而导致.      

Reduction of Peak Heat Fluxes by Supplying Heat to the Free Stream

A supersonic flow past a blunt body in the presence of an incident oblique shock wave is considered. It is shown that by supplying heat to the free stream it is possible substantially to reduce local heat flux peaks on the body surface. The integral heat flux on the body surface increases by only a small fraction of the heat released into the flow.     

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     

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.     

三维大钝头体简谐振动绕流的数值模拟

用数值模拟方法研究了大钝头体外形（双子星座飞船）在超声速下俯仰振动的绕流特性，数值模拟的出发方程为三维全Ｎ－Ｓ方程，格式为二阶迎风ＴＶＤ格式，研究结果给出了物体在俯仰振动过程中激波、分离结构和物面压力分布等非定常特征。     

Reducing the pressure drag of a D-shaped bluff body using linear feedback control

The pressure drag of blunt bluff bodies is highly relevant in many practical applications, including to the aerodynamic drag of road vehicles. This paper presents theory revealing that a mean drag reduction can be achieved by manipulating wake flow fluctuations. A linear feedback control strategy then exploits this idea, targeting attenuation of the spatially integrated base (back face) pressure fluctuations. Large-eddy simulations of the flow over a D-shaped blunt bluff body are used as a test-bed for this control strategy. The flow response to synthetic jet actuation is characterised using system identification, and controller design is via shaping of the frequency response to achieve fluctuation attenuation. The designed controller successfully attenuates integrated base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The effect on the flow field is to push the roll-up of vortices further downstream and increase the extent of the recirculation bubble. This control approach uses only body-mounted sensing/actuation and input–output model identification, meaning that it could be applied experimentally.     

Strong mass injection at the surface of a blunt body in a supersonic flow

The case of supersonic flow over a blunt body when another gas is injected through the surface of the body in accordance with a given law is theoretically investigated. If molecular transport processes are neglected, the flow between the shock wave and the surface of the body should be regarded as two-layer, that is, as consisting of the flow in the shock layer between the shock wave and the contact surface and the flow in the layer of injected gas. A numerical solution of the problem is obtained near the front of the body and its accuracy is estimated. Approximate analytic solutions are obtained in the injected-gas layer: a constant-density solution and a solution of the boundary-layer type in the local similarity approximation. Near the flow axis the numerical and analytic solutions are fairly close, but at a distance from the axis the assumptions made reduce the accuracy of the approximate solutions. The flow in question can serve as a gas-dynamic model of a series of problems describing the radiant heating of blunt bodies in a hypersonic flow. In the presence of intense radiative heat transfer, vaporization is so great that the thickness of the vapor layer is comparable with the thickness of the shock layer. Moreover, the thermal shielding of various kinds of obstacles in channels through which a radiating plasma flows can be organized by means of the forced injection of a strong absorber. The formulation of a similar problem was reported in [1], but the results of the solution were not given. A two-layer model of the flow of an ideal gas over a blunt body was used in [2, 3] for the analysis of radiative heat transfer. In [2] the neighborhood of the stagnation point is considered. In [3] preliminary results relating to two-layer flow over blunt cones are presented. The solution is obtained by Maslen's approximate method.Moscow. Translated from Izvestiya Akademii Nauk SSSR. Mekhanika Zhidkosti i Gaza, No. 2, pp. 89–97, March–April, 1972.     

Rotational and vibrational temperature measurements using CARS in a hypervelocity shock layer flow and comparisons with CFD calculations

Broadband single pulse coherent anti-Stokes Raman scattering (CARS) experiments employing a folded-box phase-matching geometry in a pulsed hypervelocity blunt body flow are presented. Rovibrational spectra of molecular nitrogen, produced in the freestream and within the shock layer at moderately high enthalpy (8.4 MJ/kg), are examined. Difficulties peculiar to the application of a single pulse optical technique to a high enthalpy pulsed flow facility are discussed and measurements of flow temperatures are presented. Theoretically calculated values for temperatures based upon algorithms used to determine freestream and shock layer conditions agree well with experimental measurements using the CARS technique. The measurements indicate that thermal non-equilibrium conditions exist within the freestream, and that near thermal equilibrium exists at the point of measurement within the shock layer. The comparison between the experiment and theory in the shock layer is improved by using the measured freestream temperatures as input to the shock layer computations.     

基于滑移速度壁模型的复杂边界湍流大涡模拟

采用滑移速度壁模型实现了浸入边界方法与壁模型相结合的大涡模拟.本文首先分别采用平衡层模型和非平衡壁模型对周期山状流进行数值模拟,以考查在壁模型中考虑切向压力梯度的作用.数值结果表明,流场的压力对本文所采用的壁模型形式并不敏感,但是考虑切向压力梯度可以显著改进壁面摩擦力的计算结果,并且能够准确的预测强压力梯度区以及分离区内的流动平均统计特性.不考虑压力梯度效应的平衡层模型显著低估了壁面摩擦力的分布,同时无法准确预测分离区内的平均速度剖面.非平衡模型的修正项正比于切向压力梯度和壁面法向距离,因此在强压力梯度区或者网格较粗时,计算得到的平均压力和摩擦力分布以及流动的低阶统计量均与参考的实验和计算结果吻合.在此基础上,通过回转体绕流的大涡模拟考查了该方法用于模拟高雷诺数壁湍流的适用性,非平衡壁模型可以准确地捕捉流动的物理结构并较准确地预测其水动力学特性.结果表明,将浸入边界方法与非平衡滑移速度壁模型相结合的大涡模拟,有望成为数值模拟复杂边界高雷诺数壁湍流的工具.