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Understanding Vortex Ring Instability During Droplet Impact Onto Thin Liquid Films Using High Speed PIV

Hatim Ennayar (1), Philipp Brockmann (1), Juan Camilo Dueñas Torres (1), Hyoungsoo Kim (2), Jeanette Hussong (1)

1. Technical University of Darmstadt, Darmstadt, Germany
2. Korea Advanced Institute of Science and Technology, Daejeon, South Korea


This study investigates vortex ring instability during droplet impact on thin liquid films using high-speed Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF). Experiments were conducted using two different setups, capturing the droplet impact from both bottom and side views. The study examines how different dimensionless parameters (Reynolds number, Weber number and dimentionless thickness) influence vortex ring behavior in thin liquid films. Vortices were analyzed by identifying the centers using Gamma_2 scalar and calculating the vortex circulation through a two superposed Lamb-Oseen vortex fitting. The effect of film thickness on vortex dynamics reveals that thinner films lead to greater instability due to higher energy at the moment of wall collision, facilitating the formation of additional vortex structures. Experimental phase mapping highlights the critical threshold of Reynolds number and dimensionless thickness for vortex ring instability. Moreover, the number of fingers formed due to vortex ring instabilities increases with higher Reynolds numbers and thinner films, providing a clear relationship between film thickness, impact energy, and the complexity of resulting fluid structures.

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