Liyong Zou

Parametric study of cylindrical converging shock waves generated based on shock dynamics theory

In our previous work, the technique of generating cylindrical converging shock waves based on shock dynamics theory was proposed. In the present work, a further study is carried out to assess the influence of several parameters including the converging angle θ0, the incident planar shock Mach number M0, and the shock tube height h on the wall profile and the converging shock wave. Combining the high-speed schlieren photography and the numerical simulation with the shock dynamics theory, the characteristics of wall profiles, cylindrical converging shock waves, and thermodynamic properties for different controllable parameters are analyzed. It is found that these parameters have great effects on shapes of the wall profile and experimental investigation favors large values of M0 and h and moderate θ0. The experimental sequences of schlieren images indicate that the shocks moving in the converging part are of circular shapes, which further verifies the method in our previous work. In addition, the changes of ...

On interaction of shock wave with elliptic gas cylinder

The interaction of a planar shock with one elliptic heavy-gas (SF6) cylinder surrounded by air is investigated experimentally. By changing the aspect ratio of the elliptic cylinder, the influence of the initial shape on the evolution of the interface is visualized by a series of dynamic photos utilized by a high-speed camera. It is found that the longer the axis perpendicular to the shock front, the faster and the severer the deformation of the gas cylinder. This can be explained mainly by the different amount of vorticity produced by the misalignment between the density gradient and the pressure gradient. When the vertical axis is much longer than the horizontal axis, the vorticity production is mainly concentrated at the upper and lower corners, which rolls up in time, and results in a structure of big vortex-pair. When the horizontal axis is much longer than the vertical axis, the baroclinic vorticity production distributes at almost every position along the interface, which leads to a faster rolling up of vortices, and even second vortex may develop at later times.Graphical Abstract

Aspect ratio effect on shock-accelerated elliptic gas cylinders

The evolution of an elliptic heavy-gas (SF6) cylinder accelerated by a planar weak shock wave is investigated experimentally using particle imagevelocimetry(PIV) diagnostics, and the emphasis is on the aspect ratio effect on shock-elliptic cylinder interaction. Experiments are conducted at five different aspect ratios (the ratio of length in streamwise and spanwise directions) varied from 0.25 to 4.0. PIV raw images and quantitative flow field data are obtained at t = 0.6 ms after the shock impact. As the aspect ratio increases, the interface morphology develops faster owing to more vorticity produced along the interface and smaller vortex spacing between the two vortex cores. For each case in this study, the maximal fluctuating velocity locates at the middle point of the two counter-vortices. The histograms of fluctuating velocity reveal that a distinct double-peak structure appears in the largest aspect ratio case in comparison with a single-peak structure in the smallest aspect ratio case. The vortex velocities predicted by the theoretical model [G. Rudinger and L. M. Somers, “Behaviour of small regions of different gases carried in accelerated gas flows,” J. Fluid Mech. 7, 161–176 (1960)] agree well with the experimental ones. With the increase of aspect ratio, the maximal value of vorticity increases as well as the circulation, and more low-magnitude quantities are generated, which indicates the formation of multi-scale flow structure in the late mixing process. It is found that the experimental circulation of the vortex motion is reasonably estimated by the ideal point vortex-pair model.

Interaction of rippled shock wave with flat fast-slow interface

The evolution of a flat air/sulfur-hexafluoride interface subjected to a rippled shock wave is investigated. Experimentally, the rippled shock wave is produced by diffracting a planar shock wave around solid cylinder(s), and the effects of the cylinder number and the spacing between cylinders on the interface evolution are considered. The flat interface is created by a soap film technique. The postshock flow and the evolution of the shocked interface are captured by a schlieren technique combined with a high-speed video camera. Numerical simulations are performed to provide more details of flows. The wave patterns of a planar shock wave diffracting around one cylinder or two cylinders are studied. The shock stability problem is analytically discussed, and the effects of the spacing between cylinders on shock stability are highlighted. The relationship between the amplitudes of the rippled shock wave and the shocked interface is determined in the single cylinder case. Subsequently, the interface morphologies and growth rates under different cases are obtained. The results show that the shock-shock interactions caused by multiple cylinders have significant influence on the interface evolution. Finally, a modified impulsive theory is proposed to predict the perturbation growth when multiple solid cylinders are present.The evolution of a flat air/sulfur-hexafluoride interface subjected to a rippled shock wave is investigated. Experimentally, the rippled shock wave is produced by diffracting a planar shock wave around solid cylinder(s), and the effects of the cylinder number and the spacing between cylinders on the interface evolution are considered. The flat interface is created by a soap film technique. The postshock flow and the evolution of the shocked interface are captured by a schlieren technique combined with a high-speed video camera. Numerical simulations are performed to provide more details of flows. The wave patterns of a planar shock wave diffracting around one cylinder or two cylinders are studied. The shock stability problem is analytically discussed, and the effects of the spacing between cylinders on shock stability are highlighted. The relationship between the amplitudes of the rippled shock wave and the shocked interface is determined in the single cylinder case. Subsequently, the interface morphologi...

A PIV study on the interaction of double Richtmyer-Meshkov-unstable elliptic gas cylinders

激波冲击双气柱既包含了激波与界面的相互作用又包含了界面之间的相互作用, 是探索实际应用中Richtmyer-Meshkov (RM)不稳定性演化的很好的桥梁. 利用高分辨率的粒子图像测速(PIV)技术获得了4种不同初始中心间距的双椭圆气柱在平面激波冲击下的速度场、涡量场和环量, 定量表征了界面相互作用对RM不稳定性演化的影响. 界面相互作用较弱时, 双椭圆气柱的演化由两对涡对结构主导, 内涡弱于外涡, 且随着中心间距的减小内涡的弱化愈加显著; 界面相互作用较强时, 双椭圆气柱演化模式发生转变, 内涡消失, 只由一对涡对结构主导, 外涡随中心间距的减小而增强. 界面相互作用导致双椭圆气柱的演化行为与单椭圆气柱的演化行为不同, 典型的特征是内涡的削弱及涡对结构的旋转运动. 双椭圆气柱演化行为与双圆气柱演化行为也存在差别, 表现为强界面相互作用时外涡随中心间距的减小而增强.

The Richtmyer-Meshkov instability by diffracted incident shock waves and reshock

The developments of Richtmyer-Meshkov instability at gas interface subjected to a diffracted incident shock wave of low Mach number and reshock wave have been experimentally studied using the high-speed schlieren photography and laser sheet technique. The motionless and stable interface is formed in a vertical shock tube by using a downward flow of nitrogen (light gas N2) and an upward flow of heavy gas SF6, which meet at the desired interface location and exit from the lateral slots in the test section walls. The interaction yields of shock waves with rigid cylinder is complex, including the primary incident shock, reflected bow shock, Mach shock, slip lines originated from the triple Mach shock. The understanding of interaction of this with interface, will contribute to the knowledge of the perturbation originated from the complex flow. The planar shock wave diffracted over a cylindrical column produces local perturbation at the gas interface. It has been demonstrated that the thickness of the interface grows slowly under the incident shock wave, while spike and bubble are clearly observed upon reshock. Moreover, the interaction between the reshock wave and the boundary layer generates wall vortices, which enhances the development of Turbulence Mixing Zone (TMZ). The reflected rarefaction wave from wall-end also accelerates the development of TMZ.