脉冲激光与正激波相互作用过程和减阻机理的实验研究

纳秒脉冲激光具有峰值功率密度高、易于击穿空气形成等离子体这一突出优势,在降低超声速波阻方面具有重要应用价值.以深刻揭示减阻机理为目的,针对激光与正激波相互作用这一基本物理现象开展实验研究.发展高精度纹影技术以测量复杂激波结构,时间分辨率达到30 ns,空间分辨率达到1 mm;搭建快速PIV实验系统以定量测量流场速度和涡量,时间分辨率达到500 ns.探明了激光等离子体引致的球面激波和高温低密度区域特性,揭示了激光等离子体在正激波冲击下的流动特性与演化规律,并结合数值模拟结果阐明了脉冲激光等离子体降低超声速波阻的根本原因.研究表明:激光等离子体引致激波的初始马赫数随着激光能量而增大,形状由水滴形逐渐发展为球面形,传播速度随着时间降低,在50μs后接近于声速;高温低密度区域初始近似于球形,而后从激光入射方向的下游开始失稳,形成尖刺结构;在正激波冲击下,高温低密度区域演化为上下对称的双涡环结构,尺寸随着激光能量而增大.涡的卷吸和逆流可改变飞行器头部激波结构,是流场重构的重要形式,引起飞行器表面压力的大幅降低,是引起超声速飞行器波阻降低的重要机理.     

脉冲激光与正激波相互作用过程和减阻机理的实验研究

纳秒脉冲激光具有峰值功率密度高、易于击穿空气形成等离子体这一突出优势,在降低超声速波阻方面具有重要应用价值.以深刻揭示减阻机理为目的,针对激光与正激波相互作用这一基本物理现象开展实验研究.发展高精度纹影技术以测量复杂激波结构,时间分辨率达到 30ns,空间分辨率达到 1mm;搭建快速~PIV 实验系统以定量测量流场速度和涡量,时间分辨率达到 500ns.探明了激光等离子体引致的球面激波和高温低密度区域特性,揭示了激光等离子体在正激波冲击下的流动特性与演化规律,并结合数值模拟结果阐明了脉冲激光等离子体降低超声速波阻的根本原因.研究表明:激光等离子体引致激波的初始马赫数随着激光能量而增大,形状由水滴形逐渐发展为球面形,传播速度随着时间降低,在50$\mu$s 后接近于声速;高温低密度区域初始近似于球形,而后从激光入射方向的下游开始失稳,形成尖刺结构;在正激波冲击下,高温低密度区域演化为上下对称的双涡环结构,尺寸随着激光能量而增大.涡的卷吸和逆流可改变飞行器头部激波结构,是流场重构的重要形式,引起飞行器表面压力的大幅降低,是引起超声速飞行器波阻降低的重要机理.      

反射激波作用下三维凹气柱界面演化的数值研究

激波与气柱相互作用是Richtmyer-Meshkov不稳定性研究的经典案例. 单次激波与二维气柱相互作用已得到广泛关注, 但是反射激波再次冲击气柱 (尤其是三维气柱) 的研究较少, 相关演化规律和机理尚不清楚. 反射激波再次冲击演化中的气柱界面会产生新的斜压涡量, 影响涡量的输运和分布, 从而影响界面的演化. 本文采用自主开发的HOWD (high-order WENO and double-flux methods) 程序, 研究了马赫数为1.29的平面激波冲击N激波与气柱相互作用是Richtmyer-Meshkov不稳定性研究的经典案例.单次激波与二维气柱相互作用已得到广泛关注,但是反射激波再次冲击气柱(尤其是三维气柱)的研究较少,相关演化规律和机理尚不清楚.反射激波再次冲击演化中的气柱界面会产生新的斜压涡量,影响涡量的输运和分布,从而影响界面的演化.本文采用自主开发的HOWD (high-order WENO and double-flux methods)程序,研究了马赫数为1.29的平面激波冲击N_2气柱(气柱外为SF_6)的演化过程,并考察了反射激波对二维和三维凹气柱界面演化的影响规律.在数值模拟中,选取了不同的反射距离(定义为气柱和反射边界的距离),得到了二维和三维凹气柱在反射激波冲击前后的完整演化图像,提取了气柱上特征点位置随时间变化的定量数据,重点分析了不同演化阶段气柱几何特征及斜压涡量分布的变化趋势.研究表明,反射距离决定着反射激波作用气柱时的激波形状和气柱形态,从而影响斜压涡量的生成和分布,进而改变气柱的不稳定性演化过程.对于三维气柱,不同高度截面上的斜压涡量分布不同,从而诱导出复杂的三维演化结构.     

反射激波冲击重气柱的RM不稳定性数值研究

数值研究了二维气柱在入射激波以及反射激波作用下的Richtmyer-Meshkov(RM)不稳定性发展规律, 采用有限体积法结合网格自 适应技术的VAS2D程序, 精确刻画激波和界面的演化. 入射平面激波的马赫数为1.2, 气柱界面内气体为六氟化硫(SF₆), 环境气体为空气, 激波管的尾端为固壁. 通过改变气柱与尾端之间的距离调节反射激波再次作用已经变形的气柱的时间, 获得不同时刻下已经变形的气柱形态、界面尺寸以及环量演化受到反射激波的影响. 结果表明, 反射激波再次作用气柱时, 气柱所处发展阶段不同, 界面演化规律以及环量随时间的变化也不相同, 反射激波与气柱相互作用过程中的涡量产生和分布与无反射情况差异较大, 揭示了不同情况下界面演化的物理机理.     

柱形汇聚激波冲击球形重气体界面的实验研究

采用高速纹影法实验研究了柱形汇聚激波与球形重气体界面相互作用的 Richtmyer-Meshkov不稳定性问题. 激波管实验段基于激波动力学理论设计, 将马赫数为1.2 的平面激波转化为柱形汇聚激波, 气体界面由肥皂膜分隔六氟化硫(内)和空气(外)得到. 采用高速摄影机在单次实验中拍摄激波运动的全过程, 对柱形激波的形成进行了实验验证, 并进一步观测了汇聚激波与球形气体界面相互作用过程中的波系发展和气体界面变形以及反射激波同已变形界面二次作用的流场演化. 结果表明: 当柱形汇聚激波穿过气泡界面以后, 气泡左侧界面极点沿激波传播方向保持匀速运动, 气泡右侧界面发展成为射流结构, 气泡主体发展成为涡环结构; 在反射激波的二次作用下, 流场中无序运动显著增强并很快进入湍流混合阶段.     

反射激波作用重气柱的Richtmyer-Meshkov不稳定性的实验研究

采用高速摄影结合激光片光源技术,研究了反射激波冲击空气环境中重气体(SF₆)气柱的Richtmyer-Meshkov不稳定性。通过在横式激波管试验段采用可移动反射端壁获得不同反射距离,实现了反射激波在不同时刻二次冲击处于演化中后期的气柱界面,得到了不同的界面演化规律。反射距离较小时,斜压机制对气柱界面形态演化的影响显著,界面衍生出二次涡对结构;反射距离较大时,压力扰动机制的影响显著,界面在流向上被明显地压缩,没有形成明显的涡结构。由气柱界面形态的时间演化图像得到了界面位置和整体尺度随时间的变化,对反射激波作用后气柱界面的演化进行了量化分析。     

RM不稳定过程中预混火焰界面演化及混合区增长预测

预混火焰界面的RM (Richtmyer-Meshkov)不稳定导致的界面混合区增长过程在自然界和工程实践中十分常见,但化学反应对其增长的影响机理仍不明确,反应性界面混合区增长速率的预测也未见报道, 因此,开展RM不稳定过程中火焰界面演化和混合区预测的研究十分必要.本文采用带单步化学反应的Navier-Stokes方程和高精度数值格式,研究了正弦形预混火焰界面在平面入射激波及其反射激波作用下的RM不稳定过程.结果表明, 在入射激波作用后的阶段,除RM不稳定本身导致的界面演化为"钉-帽"和"泡"形结构外,化学反应一方面以预混火焰传播的方式促进了界面中"泡"结构的增长,另一方面通过与涡结构的复杂相互作用促进了"钉-帽"结构的增长.化学反应活性越强, 火焰界面的"泡" 结构和"钉-帽"结构的增长越快.在第一次反射激波作用后的阶段,化学反应以相同的火焰传播方式对"泡"和"钉-帽"结构产生影响, 两者效应相抵,因而导致反射激波作用后的阶段中界面混合区增长不受化学反应活性的影响.根据以上分析,分别针对入射激波和第一次反射激波作用后的火焰界面混合区增长速率提出了相应的预测模型,为探索反应性RM不稳定过程的理论预测方法提供了有益参考.      

INTERFACE EVOLUTIONS AND GROWTH PREDICTIONS OF MIXING ZONE ON PREMIXED FLAME INTERFACE DURING RM INSTABILITY 1)

预混火焰界面的RM (Richtmyer-Meshkov)不稳定导致的界面混合区增长过程在自然界和工程实践中十分常见,但化学反应对其增长的影响机理仍不明确,反应性界面混合区增长速率的预测也未见报道, 因此,开展RM不稳定过程中火焰界面演化和混合区预测的研究十分必要.本文采用带单步化学反应的Navier-Stokes方程和高精度数值格式,研究了正弦形预混火焰界面在平面入射激波及其反射激波作用下的RM不稳定过程.结果表明, 在入射激波作用后的阶段,除RM不稳定本身导致的界面演化为"钉-帽"和"泡"形结构外,化学反应一方面以预混火焰传播的方式促进了界面中"泡"结构的增长,另一方面通过与涡结构的复杂相互作用促进了"钉-帽"结构的增长.化学反应活性越强, 火焰界面的"泡" 结构和"钉-帽"结构的增长越快.在第一次反射激波作用后的阶段,化学反应以相同的火焰传播方式对"泡"和"钉-帽"结构产生影响, 两者效应相抵,因而导致反射激波作用后的阶段中界面混合区增长不受化学反应活性的影响.根据以上分析,分别针对入射激波和第一次反射激波作用后的火焰界面混合区增长速率提出了相应的预测模型,为探索反应性RM不稳定过程的理论预测方法提供了有益参考.     

激波诱导环形SF6气柱演化的机理

基于可压缩多组分Navier-Stokes控制方程,结合5阶加权本质无振荡格式以及网格自适应加密技术和level-set方法,数值模拟了平面激波(Ma=1.23)与环形SF₆气柱(内外半径分别为8和17.5 mm)界面的相互作用过程。相比于之前的实验结果,数值模拟结果揭示了入射激波在界面内4次透射过程中的复杂波系结构,观察到透射激波在内部界面传播时形成自由前导折射结构并向自由前导冯诺依曼折射结构转换的波系演变过程;另外,界面内的复杂激波结构诱导内部下游界面上的涡量发生了3次反向;在界面演化后期,内部界面形成的“射流”结构与下游界面相互作用,诱导界面形成一对主涡、一对次级涡以及一个反向“射流”结构。定量分析了环形界面长度、宽度、位移、环量以及混合率的变化情况,结果表明,内部气柱的存在减弱了前期小涡结构合并形成大涡结构过程中对界面高度与长度的影响,同时提高了重质气体与环境气体的混合率。     

平面激波冲击并排液滴的三维数值模拟研究

采用三维守恒清晰界面数值方法, 研究平面激波冲击并排液滴的动力学过程. 研究的焦点在于激波接触液滴后的复杂波系结构生成, 以及并排液滴相互耦合作用诱导的单个液滴非对称界面演化. 首先, 分析并排液滴之间界面通道内的波系结构发展, 发现在冲击初期由于反射激波相交而形成新的反射激波以及马赫杆; 这些流动现象与液滴另外一侧 (非通道侧) 由激波反射所形成的弯曲波阵面截然不同, 而且所导致的液滴横向两侧流场差异是中后期冲击过程液滴两侧界面非对称演化的主要原因. 其次, 研究冲击中期时, 特别是入射激波已运动至液滴下游并远离并排液滴, 界面形态的演化过程和规律, 揭示通道下游出口处由于气流膨胀导致的界面闭合、以及随后气流阻塞导致的界面破碎等新的流动现象. 最后, 研究液滴间距对并排液滴相互作用的影响规律, 发现液滴间距大小与通道内压力峰值具有明显的关联关系. 研究表明, 更小的液滴间距不仅带来更大的压力峰值, 而且使得峰值出现的时间更早.      

激波冲击SF6重气泡引发射流的数值模拟

为深入研究重气泡内激波聚焦和射流生成的机理,采用高精度计算格式和高网格分辨率对马赫数为1.23的平面入射激波与SF₆重气泡的作用过程进行数值模拟,计算结果与文献中实验吻合较好。结果显示:入射激波在重气泡内首先在流向上汇聚形成上、下对称的高压区,随后,这对高压区在SF₆重气泡中心对称轴处再次碰撞,完成激波聚焦过程,并在气泡下游界面附近形成远大于初始压力和密度的局部高压高密度区,体现出SF₆重气泡极强的聚能效应;激波聚焦还引起气泡下游界面附近的涡量变化,涡对的旋转能够加速射流形成与发展。因此,SF₆重气泡下游界面附近的高压区和涡量分布对形成射流结构均有促进作用。     

Particle image velocimetry study of the shock-induced single mode Richtmyer–Meshkov instability

Full field particle image velocimetry (PIV) measurements are obtained for the first time in Richtmyer–Meshkov instability shock tube experiments. The experiments are carried out in a vertical shock tube in which the light gas (air) and the heavy gas (SF₆) flow from opposite ends of the shock tube driven section and exit through narrow slots at the interface location. A sinusoidal perturbation is given to the interface by oscillating the shock tube in the horizontal direction. Richtmyer–Meshkov instability is then produced by the interaction with a weak shock wave (M _s  = 1.21). PIV measurements are obtained by seeding the flow with 0.30 μm polystyrene Latex spheres which are illuminated using a double-pulsed Nd:YAG laser. PIV measurements indicate the vorticity to be distributed in a sheet-like distribution on the interface immediately after shock interaction and that this distribution quickly rolls up into compact vortices. The integration of the vorticity distribution over one half wave length shows the circulation to increase with time in qualitative agreement with the numerical study of Peng et al. (Phys. Fluids, 15, 3730–3744, 2003).     

Jet formation in shock-heavy gas bubble interaction

The influences of the acoustic impedance and shock strength on the jet formation in shock-heavy gas bubble interaction are numerically studied in this work. The process of a shock interacting with a krypton or a SF 6 bubble is studied by the numerical method VAS2D. As a validation, the experiments of a SF 6 bubble accelerated by a planar shock were performed. The results indicate that, due to the mismatch of acoustic impedance, the way of jet formation in heavy gas bubble with different species is diversified under the same initial condition. With respect to the same bubble, the manner of jet formation is also distinctly different under different shock strengths. The disparities of the acoustic impedance result in different effects of shock focusing in the bubble, and different behaviors of shock wave inside and outside the bubble. The analyses of the wave pattern and the pressure variation indicate that the jet formation is closely associated with the pressure perturbation. Moreover, the analysis of the vorticity deposition, and comparisons of circulation and baroclinic torque show that the baroclinic vorticity also contributes to the jet formation. It is concluded that the pressure perturbation and baroclinic vorticity deposition are the two dominant factors for the jet formation in shock-heavy gas bubble interaction.     

Experimental investigation of a twice-shocked spherical gas inhomogeneity with particle image velocimetry

Results are presented from an experimental investigation into the interaction of a planar shock wave with a vortex ring. A free-falling spherical soap bubble is traversed by the incident shock wave and develops into a vortex ring as a result of baroclinically deposited vorticity (?r×?p 1 0{\nabla\rho\times\nabla p \neq 0}). The vortex ring translates with a velocity relative to the particle velocity behind the shock wave due to circulation. After the shock wave reflects from the tube end wall, it traverses the vortex ring (this process is called “reshock”) and deposits additional vorticity. Planar Mie scattering is used to visualize the atomized soap film at high frame rates (up to 10,000 fps). Particle image velocimetry (PIV) was performed for an argon bubble in nitrogen accelerated by a M = 1.35 shock wave. Circulation was determined from the PIV velocity field and found to agree well with Kelvin’s vortex ring model.     

Interaction of heat release and vortex breakdown during flame flashback driven by combustion induced vortex breakdown

The interaction of heat release by chemical reaction and the flow dominates flame transition in swirling flows caused by combustion induced vortex breakdown (CIVB). The simultaneous application of 1 kHz high-speed particle imaging velocimetry (PIV) for the analysis of the flow field and OH planar laser-induced fluorescence for the detection of the flame front is particularly useful for the improvement of the understanding of the observed fast CIVB driven flame propagation. For the first time, the combination of both techniques was successfully applied to confined swirling flows. In the study, the flow field characteristics of an aerodynamically stabilized burner system with CIVB are analyzed in great depth. The influence of geometric parameters of the swirl generator was investigated and conclusions concerning the proper burner design of vortex breakdown premix burners are drawn from the experimental results. In particular, the effect of the vortex core with respect to the stability of the swirl stabilized burner is analyzed. The contribution of combustion to vortex breakdown is shown comparing isothermal and reacting flows. The presented data reveals that at the onset of CIVB driven flame transition, the azimuthal vorticity leads to the formation of a closed recirculation bubble at the tip of the internal recirculation zone. Once this bubble propagates upstream, the flame is able to follow and propagate relative to the bulk flow velocity with a velocity far beyond the turbulent flame speed. The interaction of reaction and flow was observed for different volumetric heat releases. The experiments confirm the CIVB theory of the authors, which was initially developed on the basis of a CFD study alone. Both the volume expansion and the baroclinic torque have an effect on whether fast flame propagation occurs. Whereas the volume expansion caused by the heat release stabilizes the flow field and the reaction, the baroclinic torque stimulates flame transition. For upstream propagation the flame tip has to have a position downstream of the stagnation point of the bubble. Else, the required transition inducing force is insufficient and the flame remains stable. In case the flame reaches positions too close or even upstream of the stagnation point, the fast propagation is interrupted or even prohibited. The key finding that the relative position of flame and stagnation bubble governs CIVB is discussed on the basis of high-speed LIF/PIV data as well as chemiluminescence. Since essentially the same behavior has been observed before in tests of a totally different swirler design and flow field, the conclusion can be made that the root cause for CIVB independent of the special geometry has been found.     

Some aspects of streamwise vortex behavior during oblique shock wave/vortex interaction

An experimental study of the flowfield generated by the interaction of a streamwise vortex having a strong wake-type axial Mach number profile and a two-dimensional oblique shock wave was conducted in a Mach 2.49 flow. The experiments were aimed at investigating the dynamics of supersonic vortex distortion and to study downstream behavior of a streamwise vortex during a strong shock wave/vortex encounter. The experiments involved positioning an oblique shock generator in the form of a two-dimensional wedge downstream of a semi-span, vortex generator wing section so that the wing-tip vortex interacted with the otherwise planar oblique shock wave. Planar laser sheet visualizations of the flowfield indicated an expansion of the vortex core in crossing a spherically blunt-nose shock front. The maximum vortex core diameter occurred at a distance of 12.7 mm downstream of the wedge leading edge where the vortex had a core diameter of more than double its undisturbed value. At distances further downstream the vortex core diameter remained nearly constant, while it appeared to become more diffused at distances far from the wedge leading edge. Measurements of vortex trajectory revealed that the vortex convected in the freestream direction immediately downstream of the bulged-forward shock structure, while it traveled parallel to the wedge surface at distances further downstream. The turbulent distorted vortex structure which formed as a result of the interaction, was found to be sensitive to downstream disturbances in a manner consistent with incompressible vortex breakdown. Physical arguments are presented to relate behavior of streamwise vortices during oblique and normal shock wave interactions. Received 7 September 1996 / Accepted 10 February 1998     

三角翼前缘涡的某些破裂形式及特性研究

基于流动显示和PIV技术测量的实验结果，对三角翼前缘涡破裂的一些形式和破裂特性进行了分析和讨论。通过PIV测量所得到的涡量分布证实了在螺旋破裂的情况下，涡核的螺旋方向与前缘涡的旋转方向相反，及双螺旋破裂形式的存在等。进而对螺旋波的形成机理提出了与有关文献不同的看法。