Vortex dynamics and fluid entrainment of high-speed spore discharge

Abstract

Motivated from the explosive launch of the $Sphagnum$ spores, this study focuses on the vortex interaction of the vortex structures generated from two different sources

a piston jet and a translating upstream bluff body. While there exist numerous studies on the dynamics of a single vortex ring or the interaction of a vortex ring with a stationary body or a wall, little has been known on how the presence of a translating body during the formation process of a vortex ring affects the dynamics of the resulting vortex structure. Using a simplified model, the stroke of the piston and the velocity ratio between the piston velocity and the translating cap velocity were varied. From experimental results from Particle Image Velocimetry and Laser Induced Fluorescence, the dynamics of the generated vortex structures were analyzed in terms of fluid entrainment, fluid transport and circulation. We categorize the vortex structures into three different modes, attached mode, spill mode and detached mode, based on the characteristic structures observed throughout the formation process. The vortex modes are strongly affected by the velocity ratio. The attached mode, observed at the intermediate velocity ratio between the spill mode and the detached mode, shows high fluid entrainment and transport performance from its structure maintaining significantly higher entrainment fraction compared to the other two vortex modes throughout its propagation. Through the analysis of time history of the circulation of the vortex structure generated in the attached mode region, the similarity of its dynamical characteristics with a single vortex ring was observed.