A drop of water falling on a water surface entrains a bubble for certain drop diameter–impact velocity combinations. Volume oscillations of the bubble contribute to the underwater noise of rain. A method was developed for optical detection of these volume oscillations in a laboratory environment. The entrained bubble is in the path of a laser beam when the bubble is created. The beam is then directed to a photodetector and the transient signal resulting from oscillations of the optical cross section of the bubble is recorded. This optically obtained record is compared to an acoustic record of each event and the frequencies are in agreement. The general magnitude of the initial radial oscillation is estimated and compared with relevant theoretical results by Og̃uz and Prosperetti [J. Fluid Mech. 228, 417–442 (1991)].
It is known that for a drop of water falling on a water surface a bubble will be entrained for certain drop diameter-impact velocity combinations [Pumphrey et al., J. Acoust. Soc. Am. 85, 1518–1526 (1989)]. Due to the small size of the entrained bubble, the frequency of oscillation (near 14 kHz), and the very small oscillation amplitude, it is extremely difficult to view the volume pulsations of the newly created bubbles. Such oscillations contribute to the underwater noise of rain. A method was developed for optical detection of these oscillations. The entrained bubble is in the path of a laser beam when the bubble is created. The beam was then directed to a photodetector and the transient signal resulting from oscillations of the optical cross section of the bubble was recorded. This optically obtained record was compared to an acoustic record of the same event (obtained by using a hydrophone). The two records agree remarkably well in both time and frequency. The general magnitude of the initial radial oscillation is estimated and compared with theoretical results by H. N. Oguz and A. Prosperetti [J. Fluid Mech. 228, 417–442 (1991)]. [Work supported by ONR.]
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