Zhigang Zhai

Generation of cylindrical converging shock waves based on shock dynamics theory

A simple but effective technique is proposed to generate cylindrical converging shock waves. The shock dynamics is employed to design a curved wall profile of the test section in a shock tube. When a planar shock wave propagates forward along the curved wall, the disturbances produced by the curved wall would continuously propagate along the shock surface and bend the shock wave. As an example, the wall profile for an incident shock Mach number of M0=1.2 and a converging angle of 15° is tested numerically and experimentally. Both numerical and experimental results show a perfect circular shock front, which validates our method.

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 ...

Experimental investigation of reshocked spherical gas interfaces

The evolution of a spherical gas interface under reshock conditions is experimentally studied using the high-speed schlieren photography with high time resolutions. A number of experimental sets of helium or SF6 bubble surrounded by air for seven different end wall distances have been performed. Distinct flow structures are observed due to the additional vorticity and wave configuration caused by the reshock. In the air/helium case, the deformation of the reshocked bubble is dependent on the development of the penetrating air jet along the symmetry axis of the bubble. In general, two separate vortex rings can be observed, i.e., one develops slowly, and the other approaches and eventually impinges on the shock tube end wall. In the air/SF6 case, two SF6 jets moving in opposite directions are generated and the oscillation of the interface is observed for small end wall distances, while small scale vortex morphologies on the gas interface are found for large end wall distances. The physical mechanisms of the...

Experimental study of Richtmyer-Meshkov instability in a cylindrical converging shock tube

The interaction of a cylindrical converging shock wave with an initially perturbed gaseous interface is studied experimentally. The cylindrical converging shock is generated in an ordinary shock tube but with a specially designed test section, in which the incident planar shock wave is directly converted into a cylindrical one. Two kinds of typical initial interfaces involving gas bubble and gas cylinder are employed. A high-speed video camera combined with schlieren or planar Mie scattering photography is utilized to capture the evolution process of flow structures. The distribution of baroclinic vorticity on the interface induced by the cylindrical shock and the reflected shock from the center of convergence results in distinct phenomena. In the gas bubble case, the shock focusing and the jet formation are observed and the turbulent mixing of two fluids is promoted because of the gradually changed shock strength and complex shock structures in the converging part. In the gas cylinder case, a counter-rotating vortex pair is formed after the impact of the converging shock and its rotating direction may be changed when interacting with the reflected shock for a relatively long reflection distance. The variations of the interface displacements and structural dimensions with time are further measured. It is found that these quantities are different from those in the planar counterpart because of the shock curvature, the Mach number effect and the complex shock reflection within the converging shock tube test section. Therefore, the experiments reported here exhibit the great potential of this experimental method in study of the Richtmyer-Meshkov instability induced by converging shock waves.

A cylindrical converging shock tube for shock-interface studies

A shock tube facility for generating a cylindrical converging shock wave is developed in this work. Based on the shock dynamics theory, a specific wall profile is designed for the test section of the shock tube to transfer a planar shock into a cylindrical one. The shock front in the converging part obtained from experiment presents a perfect circular shape, which proves the feasibility and reliability of the method. The time variations of the shock strength obtained from numerical simulation, experiment, and theoretical estimation show the desired converging effect in the shock tube test section. Particular emphasis is then placed on the problem of shock-interface interaction induced by cylindrical converging shock waves. For this purpose, membrane-less gas cylinder is adopted to form the interface between two different fluids while the laser sheet technique to visualize the flow field. The result shows that it is convenient to perform such experiments in this facility.

Measurement of a Richtmyer-Meshkov Instability at an Air- SF6 Interface in a Semiannular Shock Tube

We report the first measurements of the perturbation amplitude in the converging Richtmyer-Meshkov instability in a semiannular shock tube. At early stages, the amplitude growth agrees well with the impulsive model considering the geometrical convergence effect. A quick decrease of the growth rate at late time, even to be negative, before the reshock is observed for the first time. The reduction of the growth rate is ascribed to the Rayleigh-Taylor stabilization caused by the interface deceleration motion only presented in the converging circumstance. By reasonably evaluating the Rayleigh-Taylor stabilization, a modified model based on the Bell equation is proposed, which well predicts the perturbation growth in a converging geometry from early to late stages before the reshock. It is also found that the flow compressibility is significant in the converging Richtmyer-Meshkov instability.

A semi-annular shock tube for studying cylindrically converging Richtmyer-Meshkov instability

A novel semi-annular shock tube is realized by combining the advantageous features of two types of existing facilities for generating cylindrically converging shock waves. A high-speed schlieren photography is used to acquire the variation of shock positions versus the time and the evolution of a single-mode gaseous interface subjected to the cylindrical shock. The first experimental results indicate that the semi-annular configuration brings great convenience for interface formation and flow visualization, and the new facility has great potential for cylindrically converging Richtmyer-Meshkov instability studies.

Interaction of cylindrically converging diffracted shock with uniform interface

The Richtmyer-Meshkov instability of an unperturbed air/SF6 interface subjected to a diffracted shock is experimentally studied by high-speed schlieren photography under cylindrical circumstances. The cylindrically converging diffracted shock (CCDS) is produced by a cylindrically uniform shock diffracting around a rigid cylinder(s), and the unperturbed interface is created by a soap film technique. The effects of coupling of multiple rigid cylinders and diverse spacings from the cylinder to interface on a flow field are highlighted. Schlieren images indicate that the amplitude of disturbances on the CCDS increases compared with the local shock radius. After the CCDS impact, a bulge is derived from the interface due to the shock-shock interaction inside the interface, and the number of bulges depends upon the number of cylinders. As the number of cylinders increases, the bulge becomes less pronounced, which is ascribed to additional shock-shock interactions inside the volume. As the distance between the cy...

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.

Richtmyer-Meshkov instability of a three-dimensional SF_{6}-air interface with a minimum-surface feature.

The Richmyer-Meshkov instability of a three-dimensional (3D) SF_{6}-air single-mode interface with a minimum-surface feature is investigated experimentally. The interface produced by the soap film technique is subjected to a planar shock and the evolution of the shocked interface is captured by time-resolved schlieren photography. Different from the light-heavy single-mode case, a phase inversion occurs in the shock-interface interaction and a bubblelike structure is observed behind the shocked interface, which may be ascribed to the difference in pressure perturbation at different planes. The superimposition of spikelike forward-moving jets forms a complex structure, indicating a distinctly 3D effect. Quantitatively, it is also found that the instability at the symmetry plane grows much slower than the prediction of two-dimensional linear model, but matches the extended 3D linear and nonlinear models accounting for the curvature effects. Therefore, the opposite curvatures of the 3D interface are beneficial for suppressing the growth of the instability.