Tandem cavity collapse in a high-speed droplet impinging on a constrained wall

A focusing shock wave can be generated during the high-speed impact of a droplet on a $180^\circ$ constrained wall, which can be used to realise energy convergence on a small scale. In this study, to realise high energy convergence and peak pressure amplification, a configuration of droplets embedded with cavities is proposed for high-speed impingement on a $180^\circ$ constrained wall. A multicomponent two-phase compressible flow model considering the phase transition is used to simulate the high-speed droplet impingement process. The properties of the embedded cavities can influence the collapse pressure peak. The collapse of an embedded single air cavity or vapour cavity, as well as the cavities in a tandem array, is simulated in this study. The physical evolution mechanisms of the impinging droplet and the embedded cavities are investigated qualitatively and quantitatively by characterising the focusing shock wave generated inside the droplet and its interaction with different cavity configurations. The interaction dynamics between the cavities is analysed and a theoretical prediction model for the intensity of each cavity collapse in the tandem array is established. With the help of this theoretical model, the influencing factors for the collapse intensities of the tandem cavities are identified. The results reveal that the properties of the initial shock wave and the interval between the cavities are two predominant factors for the amplification of the collapse intensity. This study enhances the understanding of the physical process of shock-induced tandem-cavity collapse.