入射和反射激波诱导轻气泡变形和失稳的计算

采用NS方程对激波诱导球形轻气泡的变形失稳过程进行三维数值模拟,验证本文模型的可靠性,讨论气泡变形、涡环形成及其三维失稳过程.结果发现:球形轻气泡在入射和反射激波先后作用下,可以变形并沿流向形成多个旋转方向不同的涡环,而斜压效应是产生这些涡环的主要原因,涡环不仅在自诱导效应和流场作用下运动,而且自身会受到扰动而发生方位角不稳定.当涡环形成以小尺度流向涡为主的复杂结构时,即有可能诱发流场湍流化.     

激波与SF6球形气泡相互作用的数值研究

本文基于大涡模拟方法, 采用高阶精度格式对平面入射激波以及不同反射距离条件下的反射激波与SF₆重气泡相互作用过程进行了三维数值模拟. 数值结果清晰地显示了SF₆重气泡在激波作用下诱导Richtmyer-Meshkov不稳定性过程, 揭示了入射激波以及反射激波在气泡界面聚焦诱导射流的过程, 详细分析了不同反射距离条件下反射激波与SF₆重气泡作用过程及流场结构.     

激波冲击R22重气柱所导致的射流与混合研究

基于大涡模拟, 结合五阶加权基本无振荡格式与沉浸边界法对激波自左向右与R22重气柱作用过程进行了数值模拟. 数值结果清晰地显示了激波诱导Richtmyer-Meshkov不稳定性所导致的重气柱变形过程, 并与Haas 和 Sturtevant 的实验结果符合. 另外, 结果还揭示了入射激波在气柱内右侧边界发生聚焦并诱导射流的过程, 以及在Kelvin-Helmhotz 次不稳定性作用下两个主涡滑移层形成次级涡的过程, 并分析了气柱变形过程中与周围空气的混合机理. 最后, 通过改变反射距离对反射激波与不同变形阶段的气柱的再次作用过程进行了研究. 结果表明: 当激波反射距离较长时, 反射激波与充分变形后的气柱作用, 使其在流向方向上进一步被压缩; 而当激波反射距离较短时, 反射激波会在气柱内发生马赫反射, 两个三波点附近产生两个高压区, 当其传播至气柱左侧边界时对气柱边界造成冲击加速, 诱导两道向左传播的反向射流. 关键词： Richtmyer-Meshkov不稳定性 R22重气柱 反射激波 射流     

涡环发展及其与气泡相互作用的数值模拟

本文发展了三维离散涡模型用于模拟无粘不可压缩涡环的发展及其与气泡的相互作用,涡环被离散为平均分布在其中心线上的球形涡元,由Biot-Sarvart定律叠加每个涡元的诱导速度得到涡环的运动速度.涡环横截面上的涡量分布由二阶高斯分布来近似,而涡环随时间的演化则采用具有二阶精度的预测-校正算法.利用本文发展的三维离散涡模型,首先对圆涡环的发展进行了模拟,运动速度同理论解对比非常吻合,验证了模型的有效性.随后分别模拟了不同强度、不同核径比、不同长短轴比单个椭圆涡环的运动,成功再现了椭圆涡环的长短轴交替互换等现象,得到了不同工况下椭圆涡环随时间发展和演化的定性规律.最后本文将三维离散涡法与气泡运动方程相耦合,对涡环和气泡的相互作用进行了模拟,得到的气泡运动轨迹结果与实验结果非常一致,说明本文提出的三维涡模型可以推广到三维两相的情形.     

激波汇聚效应对球形气泡演化影响的数值研究

利用三维程序,对比研究了汇聚激波及平面激波冲击下SF6球形气泡演化规律的异同,以期发现激波的汇聚效应对界面演化的影响.三维程序采用多组分可压缩欧拉方程,基于有限体积法,利用MUSCL-Hancock格式进行数值求解,可以达到时间和空间的二阶精度.相比平面激波,汇聚激波由于存在曲率,且激波强度以及壁面效应在汇聚激波运行的过程中逐渐增强,使得激波冲击后的流场演化有较大的不同.计算结果表明:汇聚激波作用下,气泡界面的涡结构更加尖锐;气泡内部的透射激波聚焦程度更强,在界面下游附近形成的最高压力大于平面激波算例,由此产生的射流运动速度更快;由于汇聚激波曲率及激波强度的变化,导致界面上涡量的分布规律以及涡量幅值产生较大变化.通过界面上产生的环量以及界面内外气体混合速度的对比表明,汇聚激波更有助于涡量的产生以及气体的混合.因此激波的汇聚效应对气泡界面演化具有重要影响.     

激波与SF_6梯形气柱相互作用的数值模拟

基于大涡模拟,结合五阶加权基本无振荡格式以及沉浸边界法对平面入射激波与两种SF_6梯形重气柱的相互作用过程进行了数值模拟,数值结果清晰地显示了激波诱导Richtmyer-Meshkov不稳定性所导致的两种梯形重气柱的变形过程,详细分析了入射激波在两种梯形重气柱界面发生反射、折射、绕射以及折射激波与透射激波在气柱内部来回反射的过程,并研究了该过程中所产生的复杂波系结构,对两种梯形气柱变形过程中与周围空气的混合过程进行了分析;通过记录气柱界面四个特征尺寸随时间的变化对两种梯形气柱界面的不同演化过程进行了定量分析。     

Schardin问题的数值研究

激波绕过三角楔(Schardin问题)时会产生激波马赫反射与绕射、 三角楔尾涡与涡串等复杂物理现象. 本文利用三阶精度加权基本无振荡(WENO)格式、 结构化矩形网格的自适应加密方法与沉浸边界法对Schardin问题进行了数值模拟. 数值结果清晰地显示了激波与三角楔相互作用, 在楔面发生马赫反射以及在楔角绕射诱导主涡的过程, 并与Schardin等的实验结果及相关数值结果完全符合. 另外, 数值结果还详细反映了先前实验与数值结果没有详细讨论的主涡滑移层上的涡串生成机理, 以及激波与涡串相互作用和产生声波的过程.     

Schardin问题的数值研究

激波绕过三角楔(Schardin问题)时会产生激波马赫反射与绕射、 三角楔尾涡与涡串等复杂物理现象. 本文利用三阶精度加权基本无振荡(WENO)格式、 结构化矩形网格的自适应加密方法与沉浸边界法对Schardin问题进行了数值模拟. 数值结果清晰地显示了激波与三角楔相互作用, 在楔面发生马赫反射以及在楔角绕射诱导主涡的过程, 并与Schardin等的实验结果及相关数值结果完全符合. 另外, 数值结果还详细反映了先前实验与数值结果没有详细讨论的主涡滑移层上的涡串生成机理, 以及激波与涡串相互作用和产生声波的     

激波作用下SF6气泡界面演化和射流发展的数值模拟

 数值研究了平面激波冲击氮气环境中SF₆气泡界面的Richtmyer-Meshkov不稳定性,重点关注其中的激波聚焦及射流的产生和发展过程。在入射激波马赫数为1.23的情况下,给出了压力、密度、数值纹影和涡量等物理量的演化图像,定量分析了流场中压力最大值、密度最大值、射流速度、环量和斜压力矩随时间的变化关系。计算结果表明,平面激波冲击SF₆气泡过程有很强的聚能效应,在气泡内部靠近下游极点处发生激波近似理想聚焦和点爆炸现象,直接导致出现二次波系以及向下游运动的细长射流结构。相比入射激波,二次波系产生斜压力矩和涡量的能力要弱得多。     

平面弱激波加载下球形气泡演化的实验研究

 利用高速摄影机直接观测了激波加载下球形气泡的演化过程。采用白色的烟雾颗粒对气泡内透明的测试气体进行染色,直接观察到了气泡界面。通过对平面弱激波加载下轻气体气泡和重气体气泡进行研究,成功验证了轻气体气泡和重气体气泡演化过程中出现的典型界面结构,获得了弱激波加载轻气体气泡中背风涡环的环状几何结构。运用相关方法分析了轻气体气泡实验初期的流场分布,得到的结果与理论分析结果吻合得很好,对于此类实验的数据处理具有一定的启发意义。同时,实验中使用的烟雾颗粒法为今后在球形气泡实验中引入更为精确的平面激光诱导荧光技术（PLIF）、激光粒子图像测速技术（PIV）等实验测试系统提供了布撒示踪粒子的范例。     

基于波传播算法的火焰不稳定性

基于波传播算法构造了多组分反应流的数值格式,利用CH4空气基元反应动力学模型,并采用分离算法,对CH4空气混合物中,入射激波与火焰的相互作用,以及反射激波与火焰的二次作用过程进行了数值模拟.根据计算结果,讨论了激波诱导火焰失稳的发展过程及其特点.结果表明,Helmholtz不稳定、RichtmyerMeshkov不稳定以及反应放热速率对火焰失稳过程有重要影响.计算结果与实验结果进行了比较,对数值方法的有效性进行了验证.     

Experimental investigation of a strongly shocked gas bubble

A free-falling, spherical, soap-film bubble filled with argon is subjected to a planar M=2.88 shock in atmospheric nitrogen; vorticity is deposited on the surface of the bubble during shock interaction, and the Richtmyer-Meshkov instability ensues. The geometrical development of the shocked bubble is diagnosed with laser sheet imaging and a planar slice showing two cross sections of both the major vortex ring and a secondary vortex ring is revealed experimentally for the first time. Quantitative measurements of the experimental data include the vortex velocity defect, and subsequent circulation calculations, along with a new set of relevant length scales. The shock wave strength, leading to a post-shock compressible regime, allows the study of the instability development in a regime between low Mach number shock tube experiments and high Mach number laser driven experiments that has not been investigated previously.     

反应性RM不稳定混合特性研究: 反应活性与介质不均匀性的影响

预混火焰界面的RM（Richtmyer-Meshkov）不稳定现象在自然界和工程实践中十分常见,但目前关于反应性RM不稳定的研究主要集中于均匀介质的情况,而实际中的预混气体往往是非均匀的,因此开展非均匀介质中火焰界面演化和混合特性的研究十分必要。采用带单步化学反应的Navier-Stokes方程和高精度数值格式,研究了预混火焰界面在入射激波及反射激波作用下的RM不稳定过程,考察了化学反应活性以及介质非均匀性对RM不稳定过程中火焰界面混合特性的变化规律的影响。结果表明,在入射激波作用后的阶段,在均匀介质中的火焰界面形态呈现典型的"钉-帽-泡"结构,化学反应活性越强,界面的"泡"结构和"钉-帽"结构增长越快;而在非均匀介质中,火焰界面形态则呈现"钉-钉"结构,界面在流向速度差的诱导下被更大程度地拉伸。在第一次反射激波作用后的阶段,混合区的增长速率不依赖于反应活性和均匀性,仅与流动特性有关。时间尺度的研究表明,大尺度流动是反应性RM不稳定的主导因素,其次是化学反应,最后是小尺度混合,化学反应的强化会抑制大尺度流动,非均匀性会强化大尺度流动。      

磁控条件下激波冲击三角形气柱过程的数值研究

本文基于磁流体动力学方程组,在保证磁场散度为零的条件下,采用CTU+CT（corner transport upwind+constrained transport）算法,对有无磁场控制下激波与重质或轻质三角形气柱相互作用过程进行数值研究.结果表明：无论有无磁场,两气柱在激波冲击下均具有完全不同的波系结构和射流现象.其中,入射激波与重气柱发生常规折射,形成介质射流,而与轻气柱作用则发生非常规折射,形成反相空气射流.无磁场时,气柱在激波冲击下,产生Richtmyer-Meshkov和Kelvin-Helmholtz不稳定性,界面出现次级涡序列,重气柱上下角卷起形成主涡对,轻气柱空气射流穿过下游界面后形成偶极子涡.施加横向磁场后,次级涡序列、主涡对以及偶极子涡均消失.进一步研究表明,在磁场作用下,洛伦兹力将不稳定性诱导产生的涡量向界面两侧的Alfvén波上输运,减少界面涡量沉积,抑制界面卷起失稳.最终,涡量沿界面两侧形成相互远离的涡层,界面不稳定性得到控制.此外,定量分析表明磁场能加快两气柱上游界面的运动,抑制下游界面的运动,且对轻气柱的控制效果更好.     

ILES for mechanism of ramp-type MVG reducing shock induced flow separation

A high order implicit large eddy simulation (ILES) is carried out to study the mechanism of shock induced flow separation reduction under ramp-type MVG control. The mechanism was originally considered as that MVG can generate streamwise vortices which strongly mix boundary layer and the boundary layer becomes more capable to resist strong adverse pressure gradient caused by shock and to keep the boundary layer attached. However, according to our ILES, a chain of ring-like vortices is generated behind the ramp-type MVG and goes further to interact with the shock. When the ring-like vortices pass through the shock, the shock wave is weakened and altered while the vortex structures are quite stable. The instantaneous simulation shows that the spanwise ring-like vortex, not the streamwise vortex, plays a key role to weaken the shock and reduce the shock-induced separation. Detailed investigation on ring-like vortices and shock interaction will be given in this paper.     

Numerical Study of Air Flow Induced by Shock Impact on an Array of Perforated Plates

This study is focused on the propagation behavior and attenuation characteristics of a planar incident shock wave when propagating through an array of perforated plates. Based on a density-based coupled explicit algorithm, combined with a third-order MUSCL scheme and the Roe averaged flux difference splitting method, the Navier–Stokes equations and the realizable k-ε turbulence model equations describing the air flow are numerically solved. The evolution of the dynamic wave and ring vortex systems is effectively captured and analyzed. The influence of incident shock Mach number, perforated-plate porosity, and plate number on the propagation and attenuation of the shock wave was studied by using pressure- and entropy-based attenuation rates. The results indicate that the reflection, diffraction, transmission, and interference behaviors of the leading shock wave and the superimposed effects due to the trailing secondary shock wave are the main reasons that cause the intensity of the leading shock wave to experience a complex process consisting of attenuation, local enhancement, attenuation, enhancement, and attenuation. The reflected shock interactions with transmitted shock induced ring vortices and jets lead to the deformation and local intensification of the shock wave. The formation of nearly steady jets following the array of perforated plates is attributed to the generation of an oscillation chamber for the inside dynamic wave system between two perforated plates. The vorticity diffusion, merging and splitting of vortex cores dissipate the wave energy. Furthermore, the leading transmitted shock wave attenuates more significantly whereas the reflected shock wave from the first plate of the array attenuates less significantly as the shock Mach number increases. The increase in the porosity weakens the suppression effects on the leading shock wave while increases the attenuation rate of the reflected shock wave. The first perforated plate in the array plays a major role in the attenuation of the shock wave.     

Effect of the incident shock wave interacting with transversal jet flow on the mixing and combustion

The interaction between an incident shock wave and a transverse jet flow for mixing and combustion in a supersonic airstream was investigated experimentally and numerically. NO planar laser induced fluorescence (NO-PLIF) and particle imaging velocimetry (PIV) for non-reactive flows and three-dimensional reactive/non-reactive numerical simulations were conducted to examine the effect of the incident shock wave on the three-dimensional flow structure and mixing mechanism between the airstream and the injected gas downstream of the injection slot. Results of NO-PLIF measurement and numerical simulation showed that, in the case without the incident shock wave, injected gas is seldom seen in the recirculation zone just downstream of the injection slot, while the injected gas with higher concentration is almost uniformly distributed in the recirculation zone when the incident shock wave is introduced downstream of the injection slot. Moreover, it was shown by the numerical simulations that the profiles of the local equivalence ratio is in the combustible range due to the enhanced entrainment of the airstream when the incident shock wave is introduced downstream of the injection slot. A large-scale recirculation in the direction parallel to the wall is generated by the three-dimensional flow effects, which enhances the mixing and extends the residence time in the recirculation zone in the case with incident shock wave downstream of the injection slot, the recirculation flow being confirmed successfully by PIV measurements as well. The results of three-dimensional reactive numerical simulations were in good agreement with the experimental flame-holding characteristics at a lower total temperature, which showed that flame-holding can be attained only when the incident shock wave was introduced downstream of the injection slot, confirming that the formation of three-dimensional and large-scale recirculation flow downstream of the injection slot enlarges the recirculation zone and enhances the mixing to produce the conditions for robust flame-holding.