How does a rapidly moving object enter a quiescent fluid? The experimental
study of this problem parallels the development of high-speed imaging
(Worthington, Edgerton), and recent studies show that there are still new
things to be understood - even in water. I will present our experiments on
the fluid impact of a solid sphere, which produces a cavity and
subsequently an entrained bubble, a splash, and an audible PLUNK! We have
found that this acoustic emission leads to coherent ripples on the bubble
surface, which are fixed in the lab frame, and begin just after the
pinch-off ("deep seal") of the cavity. A potential flow model for a
slightly compressible fluid explains these ripples as the spatial
rectification of the acoustic bubble oscillations by the moving object.
This is joint work with T. Grumstrup and J. B. Keller.
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