The Suspension of Large Bubbles Near the Sea Surface by Turbulence and Their Role in Absorbing Forward-Scattered Sound
The Suspension of Large Bubbles Near the Sea Surface by Turbulence and Their Role in Absorbing Forward-Scattered Sound
复制标题
海面附近大气泡因湍流而悬浮及其在吸收前向散射声中的作用
DOI：
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发表时间：
2013
影响因子：
4.1
通讯作者：
A. Lavery
中科院分区：
文献类型：
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作者：
G. Deane;J. Preisig;A. Lavery
有轶事证据表明，在中高风速条件下（8-15 m·s- 1），破碎波在近表层携带的气泡云可以通过对前向散射声的共振吸收创造一个良性的水下通信通道，减少混响次数和高强度多普勒偏移到达的发生。目前气泡对表面相互作用声音影响的模型显示了两种影响：由于表面附近声速降低而导致的低频声音折射和高频的共振吸收。这些模型包括近表层最大气泡的数量和大小的不确定性，以及它们对风速的依赖。这种不确定性使得对主力频率（10- 30khz）水声通信频带气泡效应的定量预测变得困难。模型计算表明，与最大气泡相关的共振吸收强烈依赖于频率和风速。频率依赖性可以用气泡逃逸半径的概念来解释；这是一个气泡的半径，在此半径内，上层海洋边界层湍流速度波动与气泡终端速度达到平衡。小于逃逸半径的气泡往往被流体湍流困住，而较大的气泡则通过浮力脱气而消失在表面。对逃逸半径的计算提供了一种估计在给定风速下共振吸收的最低频率的方法。初步估计表明，10 kHz的共振吸收开始于10 m风速约8 ms -1时，在13-20 m·s- 1的风速范围内，明显的表面反弹损失低于此频率。
There is anecdotal evidence that under conditions of moderate to high wind speeds (8-15 m ·s- 1), clouds of bubbles entrained in the near-surface layer by breaking waves can create a benign underwater communications channel through the resonant absorption of forward-scattered sound, reducing reverberation times and the occurrence of high-intensity, Doppler-shifted arrivals. Current models for the effects of bubbles on surface-interacting sound show two effects: refraction of low-frequency sound due to reductions in sound speed near the surface and resonant absorption at higher frequencies. These models include uncertainty in the numbers and sizes of the largest bubbles present in the near-surface layer, and their dependence on wind speed. This uncertainty makes quantitative prediction of bubble effects in the underwater acoustic communications band of workhorse frequencies (10-30 kHz) difficult. The model calculations presented here show that resonant absorption associated with the largest bubbles is strongly frequency and wind-speed dependent. The frequency dependence can be explained by the concept of a bubble escape radius; this being the radius of a bubble for which turbulent fluid velocity fluctuations and bubble terminal velocity in the upper ocean boundary layer balance. Bubbles smaller than the escape radius tend to remain trapped by fluid turbulence while larger bubbles are lost to the surface through buoyant degassing. Calculation of the escape radius provides a means of estimating the lowest frequency at which resonant absorption can be expected for a given wind speed. Initial estimates suggest that resonant absorption at 10 kHz begins at 10-m wind speeds of around 8 ms -1, and significant surface bounce losses at frequencies lower than this are expected in the range of wind speeds 13-20 m·s- 1.