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.

The interaction of a planar shock wave with a spherical gas interface (SF6 or helium) surrounded by air is studied experimentally and numerically. By means of the high-speed schlieren photography with high time resolutions and the numerical method VAS2D, the detailed flow field structures including the evolution of interfaces and the development of wave patterns are obtained. The sequences of schlieren frames of SF6 show that the refracted shock wave converges inside the volume and causes the shock focusing within the bubble interface, resulting in an outward jet. The SF6 jet is different from the krypton jet studied because of the difference in their acoustic impedances. The pressure perturbation plays an important role in the jet formation. Quantitative data of the evolving interface length, height, and vortex spacing as well as the displacements of the interface and the jet are acquired and compared. The generation and distribution of vorticity are also analyzed numerically and are found to be the dominant factors for the interface deformation and the resulting turbulent mixing. Furthermore, the velocities of the interfaces and shock waves obtained from the x-t diagrams in the experimental and numerical photographs are compared with those predicted from one-dimensional gasdynamics. In this way, the air contaminations of both gases are re-evaluated and it is shown that the contamination by air inside the helium bubble is significant while it has just small influence in the SF6 case, which confirms the findings in literatures.

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 M 0 = 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.In some applications of shock tube such as inertial confinement fusion, 1 supernova explosion, 2 and shock wave lithotripsy, 3 converging shocks are needed in order to concentrate energy in a small volume. However, it is difficult to generate a cylindrical converging shock wave in an ordinary shock tube due to the initial shape imperfections and the nonlinear wave interactions. 4-6 Experiments of cylindrical converging shock waves interacting with cylindrical bubble have been conducted in an annual coaxial vertical diaphragmless shock tube. 5 A suggestion of gas lens to generate a cylindrical converging shock wave in a twodimensional wedge geometry was proposed by Dimotakis and Samtaney. 6 This consideration can produce a perfect cylindrical converging shock wave theoretically, although the related experimental results have not yet been found from the open publications. In this letter, a simple but effective technique is proposed in order to avoid difficulties in the formation of cylindrical converging shock wave. Specifically, we apply shock dynamics to design a wall profile with a special shape, which transfers the planar shock wave in a shock tube to a cylindrical one.The shock dynamics is a simple and useful theoretical tool to analyze the process of the propagation of shock waves in various phenomena such as diffraction, reflection, refraction, interaction, and so on. Particularly, the theory of disturbance propagating on shock surface presented by Whitham 7 provides us with such a possibility to analyze complicated phenomena of shock diffraction and interaction. The Chester-Chisnell-Whitham ͑CCW͒ relation is the basis of shock dynamics for the case of a uniform quiescent gas ahead of shock, which refers to Chester, 8 Chisnell, 9 and Whitham, 7,10,11 who obtained the same relation using different methods, independently. According to the shock dynamics, when a planar shock wave propagates forward along a continuously concave wall, the disturbances produced by the curved wall would continuously propagate along the shock front and bend the shock wave such that the contact edge of the wall and the shock wave can keep vertically propagating forward. In order to obtain a perfect cylindrical convergent shock wave, the curvature of the profile line should be calculated by shock dynamics. Only a very brief introduction of the shock dynamics is provided here. The method by applying shock dynam...

Flying insects and swimming fishes have high efficiency and high maneuverability in air and water, respectively. Their wings and fins have evolved for many ages to adapt to propelling in the complex environment. In the paper, an integrative biomimetic robotic fish is proposed and developed, which combines the advantages of insect wings and fish fins to achieve a high agility underwater. In the robotic fish, two caudal fins were equipped at the tail of the robotic fish in parallel as the main propulsion mechanism, the opposite flapping of the two caudal fins generates mutually opposing lateral forces during cruising, which leads to a stable and high-performance swimming. In addition, two pectoral fins that mimic the function of insect wings were equipped at two sides of the robotic fish, which enhances the robotic fish maneuverability in vertical plane. Moreover, a central pattern generator (CPG) model was designed to achieve the versatile maneuvering motions, motion switching, and autonomous swimming with an obstacle avoiding ability. The experiments have demonstrated that the robotic fish can swim more stably and efficiently with versatile maneuver motions by taking advantage of the integrative propulsion mechanism. The developed robotic fish have many potential applications for its agility, stable swimming, and low-cost structure.

The first asymmetric synthesis of tetrasubstituted α‐amino allenoates by a chiral phosphoric acid catalyzed dearomative γ‐addition reaction of 2,3‐disubstituted indoles to β,γ‐alkynyl‐α‐imino esters is reported. This method provides access to a series of highly functionalized tetrasubstituted allenes featuring quaternary stereocenters in high yields, and with excellent regio‐, diastereo‐, and enantioselectivities under mild conditions without by‐product formation. Representative large‐scale reactions and diverse transformations of the products into various scaffolds with potential biological activities render are also disclosed. The mechanism of the reaction was elucidated by control reactions and DFT calculations.