Experimental Study On The Interaction Of Planar Shock Wave With Polygonal Gas Cylinders

Richtmyer-Meshkov (RM) instability has been intensively studied in the past decades driven by its wide applications such as in the Inertial Confinement Fusion (ICF) and scramjet engine. RM instability occurs when a shock wave impacts on an initially perturbed interface separating two different fluids. Due to the baroclinic vorticity on the interface produced by the misalignment of pressure and density gradients, the perturbations on the interface will grow with time and eventually result in turbulent mixing. As an important type of RM instability, shock-bubble (gas cylinder) interaction has attracted wide attention. However, it is difficult to generate gas bubble (cylinder) with different shapes except spherical bubble and circular gas cylinder in RM experiment. Furthermore, except rectangle SF6block, other polygonal cylinders have not been experimentally studied in existing literature. In this work, the interaction of a planar shock wave with different polygonal gas cylinders is investigated experimentally and numerically.A novel and simple method of generating polygonal gas interfaces with arbitrary configuration is developed by using thin pins as angular vertex to connect the soap film. The produced polygonal interfaces are applied to shock tube experiment successfully. The experimental and numerical cases are combined to prove that the chamfers and pins existing in the soap film interface have little effect on the large scale evolution of the interfaces. The distortion of the soap film along the vertical direction is analyzed and quantified, which indicates that the distortion can be significantly reduced by decreasing the vertical thickness of the chamber of the shock tube. Six different polygonal cylinders filled with heavy gas and three different polygonal cylinders filled with light gas are generated by using the proposed method and adopted for the experiments of RM instability.The high speed schlieren system captures series of photos and records the whole process of the interaction of the shock with polygonal cylinder during a single shot, which ensures the unicity of the initial condition. The gases concentration in the polygonal interface can be determined by gas dynamics and the numerical procedure, which provides the complete initial condition for the numerical study. For the light gas cylinders, the simulation agrees better with the experiment when the heavier gases combination (such as N2cylinder surrounded by SF6rather than helium cylinder surrounded by air) is adopted to decrease the gas contamination caused by gas penetrating through the soap film and to lower the drag effect caused by comparable mass of the soap film with the gas inside the cylinder.Both experiment and simulation indicate that complex wave patterns appear with the incident shock interacting with polygonal gas interface. For the heavy gas cylinders, the refracted shock wave interacts with the transmitted shock wave, forming new curved shock, Mach stem as well as triple point in the interface. For the light gas cylinders, similar wave patterns appear outside the interface. Different irregular refraction patterns occur at an interface and transitions among them are firstly observed in the experiment for identical incident shock strength. For the irregular refraction of the shock wave in the triangle-2SF6cylinder case, the FPR converts into FNR. For the irregular refraction of the shock wave in the diamond N2cylinder case, the FNR converts into FPR and later to TNR.The effects of initial shape of the interface on the RM instability are illustrated and the emphasis is placed on the shock-shock interaction inside the volume and the baroclinic vorticity on the interface. Numerical results indicate that the rolling up of the vortex at the vertex of the polygonal interface is mainly induced by the baroclinic vorticity on the interface. For the heavy gas interfaces, the intersecting position of the shock waves along the interfaces is different for varies initial interfaces, resulting in the different features of interface instability. These features include a strong jet at the downstream interface for square, a weak jet for triangle-1, two weak jets for rectangle-2and the secondary vortex pair for the other three polygonal interfaces.