Optical detection of transient bubble oscillations associated with the underwater noise of rain.

Abstract

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.]

Free decay of shape oscillations of bubbles acoustically trapped in water and sea water

“…(v) Select the desired modulation percentage, 100e, such that the amplitude of shape oscillations is always small relative to the bubble size. Measurements of the amplitude can be made by the method described by Stroud & Marston (1993).…”

“…Free-decay detection system The free decay was recorded by measuring the extinction of light caused by a bubble placed in the path of an expanded laser beam ( figure 2). The technique is known as the pseudo-extinction method (Trinh et al 1988;Lu & Apfel 1990;Stroud & Marston 1993. The expanded laser beam was aligned to pass vertically along the axis of the levitator.…”

Stabilization of a cylindrical capillary bridge far beyond the Rayleigh–Plateau limit using acoustic radiation pressure and active feedback

“…A spatial filter placed at the focus of the lens blocks light reflected and refracted by the bridge from entering the photodiode. The detected optical power is reduced in proportion to the extinction of light by the bridge though it does not give a true measurement of the optical extinction as noted in some of the previous applications of this detection method (Trinh et al 1988;Stroud & Marston 1993).…”

Air entrapment by a falling water mass

“…Several recent papers have been devoted to the related process of air entrainment by impacting drops (Pumphrey & Crum 1988, 1990Pumphrey, Crum & Bjrarnra 1989;Pumphrey & Elmore 1990;Longuet-Higgins 1990;Oguz & Prosperetti 1989, 1990Prosperetti & Oguz 1993;Chahine et al 1991;Stroud & Marston 1993). In the…”