On the relative role of sea-surface roughness and bubble plumes in shallow-water propagation in the low-kilohertz region

Norton, Guy V.; Novarini, Jorge C.

doi:10.1121/1.1414883

Cited by 18 publications

(12 citation statements)

References 20 publications

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“…However, sound attenuation was probably also influenced by scattering of physical generated sound due to bubbles created by the turbulence near flow obstructions. Plumes of bubbles have been found to absorb and scatter sound (Urick 1983;Norton and Novarini 2001). Scattering is responsible for the deflection of sound energy away from the main propagation direction.…”

Section: Discussionmentioning

confidence: 99%

A flume experiment to examine underwater sound generation by flowing water

et al. 2009

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The hydrogeomorphology and ecology of rivers and streams has been subject of intensive research for many decades. However, hydraulically-generated acoustics have been mostly neglected, even though this physical attribute is a robust signal in fluvial ecosystems. Physical generated underwater sound can be used to quantify hydro-geomorphic processes, to differentiate among aquatic habitat types, and it has implications on the behavior of organisms. In this study, acoustic signals were quantified in a flume by varying hydrogeomorphic drivers and the related turbulence and bubble formation. The acoustic signals were recorded using two hydrophones and analyzed using a signal processing software, over 31 third-octave bands (20 Hz-20 kHz), and then combined in 10 octave bands. The analytical method allowed for a major improvement of the signal-to-noise ratio, therefore greatly reducing the uncertainty in our analyses. Water velocity, relative submergence, and flow obstructions were manipulated in the flume and the resultant acoustic signals recorded. Increasing relative submergence ratio and water velocity were important for reaching a turbulence threshold above which distinct sound levels were generated. Increases in water velocity resulted in increased sound levels over a wide range of frequencies. The increases in sound levels due to relative submergence of obstacles were most pronounced in midrange frequencies (125 Hz-2 kHz). Flow obstructions in running waters created turbulence and air bubble formation, which again produced specific sound signatures.

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Section: Discussionmentioning

confidence: 99%

A flume experiment to examine underwater sound generation by flowing water

et al. 2009

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“…× z (13) In Equation (13), f s is the source frequency, and z is variable depth values, where the signal effect has to be observed. In Figure 6, we have shown the variation of pressure in both the cases --in the presence of, and in the absence of, acoustic source.…”

Section: Analysis Of the Damping Of Acoustic Signal At Near Oceanmentioning

confidence: 99%

Effect of near-surface bubble plumes on the acoustic signal used in UWACNs

Mandal

Misra

Dash

2013

2013 9th International Wireless Communications and Mobile Computing Conference (IWCMC)

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Shallow water environments exhibit significantly prominent spatio-temporal variability; compared to the ones corresponding to deep oceans. An acoustic signal propagating through a shallow water region faces multiple reflections with the ocean surface as well as with the ocean bottom. So, rendering wireless acoustic communication in such type of unpredictable environment is challenging, as the acoustic signal is subjected to different losses such as multipath fading and absorption.In this paper, we analyze the near ocean surface anomalous behavior of acoustic signals, typically observed in Underwater Wireless Acoustic Communication Networks (UWACNs), due to damping in the presence of sub-surface bubble plumes. Bubbles have profound impact on the acoustical properties in the oceanic environment. The injection of bubble plumes near ocean surface renders the subsurface acoustic signal to behave anomalously.

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“…BELLHOP), a practical way is to take into account the presence of a bubble cloud by means of a horizontally-averaged bubble field, taking care to avoid step changes in sound speed (smoothly varying air void fraction). A more advanced approach would be to model also the presence of bubble plumes [18], but this would largely complicate the space and time dependence while it remains to be seen whether such local and discrete phenomena would have a significant structural effect. Therefore, as a compromise between accuracy and efficiency, bubbles will be modelled here as horizontally diffuse clouds and not as discrete plumes.…”

Section: Proceedings Of the 11 Th European Conference On Underwater Amentioning

confidence: 99%

Simulation of an Underwater Acoustic Communication Channel Characterized by Wind-Generated Surface Waves and Bubbles

Dol

Colin

Ainslie

et al. 2013

IEEE J. Oceanic Eng.

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INTRODUCTIONSea surface scattering by wind-generated waves and bubbles is regarded to be the main nonplatform-related cause of the time variability of shallow acoustic communication channels.Simulations for predicting the quality of acoustic communication links in such channels thus require adequate modelling of these dynamic sea-surface effects. It is known that, for frequencies in the range 1-4 kHz, the effect of bubbles on sea surface reflection loss is mainly due to refraction, which can be modelled with a modified sound-speed profile accounting for the bubble void fraction in the surface layer (Hall-Novarini model). The upward refraction induced by the bubble cloud then effectively acts as a catalyst for increasing the rough-surface scattering.In the present work, it is shown that, for frequencies in the range 4-8 kHz, bubble extinction also provides a significant contribution to the surface loss, including both the effects of bubble scattering and absorption. As this is the frequency band adopted in the European Defence Agency (EDA) project RACUN (Robust Acoustic Communication in Underwater Networks [1]), in which the reported research has been conducted, both bubble refraction and extinction effects should be modelled for acoustic channel simulations in RACUN. These model-based channel simulations will be performed by applying a Gaussian-beam ray-tracer (BELLHOP), together with a toolbox for generation of realistic rough sea surfaces based on both fully-developed ocean and short-fetch North Sea wave-height spectra and angular spreading functions (WAFO). SEA SURFACE MODELLING IntroductionThe objective of the present work is to improve channel modelling for underwater acoustic communication by incorporation of the effects of time-varying ambient conditions, especially windgenerated sea surface wave effects. These effects are probably the main cause of time-varying multipath and Doppler spread when both the transmitter and receiver are static. The sea surface dynamics can roughly be divided into two basic mechanisms:1. Periodic vertical motion of the sea surface; 2. Near-surface bubbles created by (breaking) waves.See Figure 1 for a schematic illustration of these effects.In order to keep things practical, we will at this point assume a separation of time scales for the sea surface dynamics and the underwater acoustic propagation. That is, we will treat the problem as "piece-wise frozen". For each "frozen" realization of the sea surface and the bubble distribution, acoustic computations are then performed without accounting for the instantaneous velocity of the sea surface and the bubbles. The main Doppler effects come in as a consequence of the variation of the path lengths between consecutive realizations [2]. This is called the range rate: dL k (t)/dt, with

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On the relative role of sea-surface roughness and bubble plumes in shallow-water propagation in the low-kilohertz region

Cited by 18 publications

References 20 publications

A flume experiment to examine underwater sound generation by flowing water

A flume experiment to examine underwater sound generation by flowing water

Effect of near-surface bubble plumes on the acoustic signal used in UWACNs

Simulation of an Underwater Acoustic Communication Channel Characterized by Wind-Generated Surface Waves and Bubbles

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