Dynamics of a Laser-induced Bubble Near a Convex Free Surface
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This paper experimentally documents the behavior of a laser-induced bubble near a convex paraboloidal free surface with different vertical radius of curvatures, generated in a rotating cylindrical water flask. The shape of the free surface plays a key role on bubble behaviors when bubble collapses. Due to the strong influence of the free surface, the bubble forms a microjet outwards the free surface when it collapses. And the nondimensional bubble collapse time, Tc/Tc’, is smaller than 1, where Tc is the bubble collapse time and Tc’ is the Rayleigh time of bubble collapse with an equivalent maximum radius. As the relative distance between the bubble center and the free surface, h/Rmax, decreases, Tc/Tc’ decreases, which is different with the bubble behavior near a flat rigid boundary, where h is the distance between the bubble center and the flat free surface or the convex free surface center, Rmax is bubble equivalent maximum radius. Compared with the bubble dynamics near a flat free surface, the bubble near a convex free surface presents a milder collapse behavior. Tc/Tc’ increases with the nondimensional radius of curvature of the free surface center, rs/Rmax, decreases, where rs is the vertical radius of curvature of the free surface, which is different from the bubble behavior near a curved rigid boundary. It implies that the convex shape of free surface reduces the Kelvin impulse, and the strength of the microjet when bubble collapses decreases with a decreasing rs/Rmax. Meanwhile the convex free surface provides a focusing mechanism to the splash on the free surface when bubble collapses. The splash gets a higher velocity when rs/Rmax decreases.