Low-frequency underwater sound generation by impacting transient cylindrical water jets

Abstract

The impact of a jet of water onto a still-water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. Results from an experimental study of the noise produced by this process is presented. Preliminary results of this study were reported previously by Kolaini et al. [J. Acoust. Soc. Am. 89, 2452–2455 (1991)]. The densely populated bubble plumes were generated by dropping a fixed volume of water, held in a cylindrical container, onto a still-water surface. High-speed video images reveal the formation of a cylindrical bubble plume with a very high void fraction which grows in size until all the water is injected into the tank. As the leading end of the plume advances, a section of the plume separates near the crater region formed by the jet. After detachment, the separated plume, which is roughly spherical in shape, undergoes volume pulsations, and radiates relatively large-amplitude, low-frequency sound. The nature of the acoustic emissions from bubble plumes depends on the height of the water in the container, the container’s radius, and the velocity of the impacting jet. The natural frequency of oscillation of an individual bubble plume is inversely proportional to the radius of the plume and ranges from a few tens of Hz to over 100 Hz depending upon the void fraction of air contained within the plume. Results obtained with salt water as well as with rough jets are also discussed. The high-speed video observations reveal that immediately following the bubble plume detachment, there is evidence of an axial jet directed downward into the bubble plume and an opposing jet directed upward into the crater formed by the impact. This jet appears to be the physical mechanism that drives the cloud into oscillation. Measurements indicate that the acoustic intensity radiated from bubble plumes correlate with the total potential energy of the water jet.

Coupled hydrodynamic-acoustic modeling of sound generated by impacting cylindrical water jets

“…3 where multiple density lines exist inside the bubble and at the air-liquid surface. This matches the observation obtained from cinematography in the experiment of Kolaini et al, 12 which suggested that the "bubble plume" could be a large air-filled bubble with a roughened surface that contains air-water mixture.…”

“…Water surrounding the neck rushes radially toward the axis of symmetry, pinching off a large bubble plume. Detailed photographic images can be found in Kolaini et al 12 and ͑Szymczak et al͒. 31 The de- tached bubble plume undergoes volume pulsations, constituting a strong acoustic source radiating low-frequency sound.…”

“…It has been recognized by many researchers that the forcing mechanism for bubble oscillations is air entrainment and bubble fission. 3,12 On the theoretical side, Oguz 13 proposed an effective pressure model by considering a distribution of bubbles at the airwater interface. Based on Oguz's model and experimental observation showing that it is the newly entrained bubbles at the leading edge of the breaker that are the primary source of sound from breaking waves, 3,14 Means and Heitmeyer 15 developed a more physically realistic bubble cloud excitation model by treating newly formed bubbles as point sources.…”

Impact of a high-speed train of microdrops on a liquid pool

“…Another example is the impact of a continuous water mass, thus a liquid column or a jet, onto a pool (Kolaini et al 1993;Oguz, Prosperetti & Kolaini 1995;Clanet & Lasheras 1997;Storr & Behnia 1999;Kersten, Ohl & Prosperetti 2003;Szymczak, Means & Rogers 2004;Qu et al 2011). These studies focused on the bubbles resulting from the hydrostatic collapse of the generated cavity.…”

The impact of a translating plunging jet on a pool of the same liquid

“…Kolaini, Roy & Crum (1991) and Kolaini et al (1993) have experimentally considered the problem of a stationary short-duration water jet impacting on a quiescent pool of water. In these experiments, a vertically oriented tube of water was held in place above the pool, which was subsequently released by the sudden rupture of a rubber membrane, allowing a cylinder of water to impact with the pool.…”