Measurement of the sound produced by a tipping trough with fresh and salt water

Abstract

Recent experiments confirm the production of sound by breaking waves at lower frequencies (30 to 500 Hz) with a dipole characteristic. The noise produced has a broadband characteristic associated with the impact and subsequent sounds that have discrete spectral characteristics. Breaking waves are known to produce bubble plumes and bubble clouds; the dynamic evolution of which provides a mechanism for sound production. Since the initial plume and cloud have appreaciable void fractions, compressible resonant oscillations of these structures as a whole or in parts are possible. These bubble plumes would act as compact acoustic monopole sources of sound and due to the pressure release surface would have an effective dipole characteristic. Sufficient energy exists in the initial breaking vorticity and turbulence to excite these regions and to explain measured source levels. These effects have been simulated with a tipping trough experiment that demonstrates the production of low-frequency sound from salt and fresh water tipping trough events. These experimental results are shown to be consistent with the theory of sound radiation from the collective oscillations of bubble plumes.

Effects of Salinity on Bubble Cloud Characteristics

“…Early experiments on the effect of salinity have shown the formation of more bubbles of all sizes in more saline waters [15,18,19]. Observations have also shown narrowing of the bubble size spectrum towards small sizes as the mean bubble radius decreases from fresh to more saline waters [20][21][22]. Summary of recent results [23] confirms that the peak of the bubble size distribution shifts toward smaller bubble sizes with increasing salinity.…”

“…As the volume of the large bubbles dominates α due to a r 3 factor in the definition of the void fraction, a decrease of the number of large bubbles in bubble size distribution can affect α strongly. However, noting that previous experiments have shown at least an order of magnitude increase of the number of small bubbles relative to the number of large bubbles with increasing salinity [20,21], it is possible that the relative contribution of small and large bubbles to α can change.…”

“…Also evident is that there are fewer bubbles at the left-hand side for fresh water (panel (a)). Blenkinsopp and Chaplin [4,5] discussed contrasting results in the literature for bubble cloud characteristics in fresh and saline waters: some investigations report significant differences in plume bubble size distributions, mean void fractions, and/or air entrainment volumes [20,21,29]; other investigations note lack of such differences (see [5] for references). These contrasting reports are based on measurements of bubble cloud characteristics in fresh versus saline waters.…”

“…For example, a laboratory experiment with breaking waves has shown that differences in the contribution of small bubbles to the measured void fraction in freshwater, artificial seawater (≈34 psu), and natural seawater (≈27 psu) is barely detectable [5]. Experiments with a tipping trough showed a difference of only 4% in the void fractions measured in fresh water and brackish water (20 psu) when the number of small bubbles increased by more than an order of magnitude [20,21]. Our results in Figure 7 help to identify salinity value of around 25 psu as the threshold at which the salinity-enhanced production of bubbles smaller than 0.5 mm in diameter prevails; i.e., S ≈ 25 psu marks the salinity threshold at which the relevant contribution of small bubbles (with diameters below 0.5 mm) becomes dominant yielding lower void fraction values.…”

“…Thus bubble clouds have been simulated with different experimental systems. For example, pouring of water in a tank with a tipping trough or bucket [20,21], or collision of two water waves in a tank [29] simulate intermittent breaking events and episodic (discrete) formation of bubble clouds. In contrast, experimental setups using injection of air through porous media such as glass frits or an aquarium diffuser, vertical or inclined plunging water jets, and weirs (in water jet or waterfall configurations) [15,[30][31][32][33][34], simulate continuous (steady) formation of bubble clouds.…”