Design and validation of a vertical annular shock tube for RM instability study

A vertical annular coaxial diaphragm-less shock tube is designed based on the prin-cipal proposed by Hosseini and Takayama and modified in order to conveniently install the initial interface in the test section and visualize the flow field for the investigation of the Richtmyer-Meshkov (RM)instability.Parametric study is carried out both experimentally and numerically to explore the characteristics of the annular coaxial cylindrical converging shock wave.The varia-tion of pressure behind the shock shows the feasibility and reliability of this shock tube to generate the annular coaxial cylindrical converging shock wave.The pressure variations with time at differ-ent positions in the test section are acquired from the experiment and numerical simulation,and the converging effect of the shock wave is emphasized.After the validation of the converging shock wave,the experiment of RM instability induced by this converging shock wave is con-cerned.For this purpose,a regular octagon air/SF6 interface (the distance from each vertex to the center is 20mm)is generated in the test section by using eight thin wires to restrict the soap films.In this way,the initial interface shape,which is crucial to RM instability study,can be precisely controlled.The influence of the thin wires on the interface evolution is also assessed by numerical simulation and the results indicate that the thin wires have limited effect on the inter-face development at the very early stage.Moreover,because the height of the interface generated is only 5mm,the influence of the gravity can be neglected and the regular octagon soap interface
can be treated as two dimensional.For visualizing the flow field,a continuous laser sheet combined with the high-speed camera is employed and the evolution of the regular octagon air/SF6 interface accelerated by the annular coaxial converging shock wave and its reflected shock wave is captured through the Mie scatting light from the droplets of the soap film.Corresponding numerical simulation is also performed for comparing with the experiment,and a good agreement is found between these two results.During the evolution,the inter-face is first compressed after the incident converging shock wave passage and then the “spike”and “bubble”configurations are generated due to the deposition of the baroclinic vorticity on the interface.Phase reversal occurs on the interface after the reflected shock wave impacts on it,which creates an opposite pressure gradi-ent compared with the initial incident shock.The secondary “spike”configuration is generated at the original“bubble”position while the secondary“bubble”configuration is generated at the original“spike”position and they grow gradually with time.From the observation,it can be found that the interface evolution is quite symmetric which once again verifies the reliability of the shock tube to generate the converging shock wave and the interface formation method.