气泡线性振动时近海面气泡群的声散射

Acoustic scattering from bubble clouds near the sea surface based on linear oscillations

海洋中的不同成因的气泡群是常见的水下声学目标及声呐混响源,因此对水下气泡群进行声学建模意义重大。利用有效媒质理论描述气泡群内部的相速度及声衰减变化,并考虑到海洋中气泡群往往产生于不同界面附近,进一步利用球面波叠加原理描述海面对气泡群散射声波的再辐射,导出了平海面作用下气泡群声散射截面的一般表达式,建立了其声散射模型,研究了单一尺寸及混合尺寸气泡群的声学特性。数值分析表明,气泡群的谐振频率会随其半径或孔隙率增加而降低;由于海面的存在,气泡群声散射截面会随频率进行周期性变化,且随气泡群远离海面,这一变化逐渐加剧。此外,若气泡的黏滞阻尼项在全部阻尼项中占比较高,气泡群声散射强度会在谐振频率附近存在起伏振荡。该模型可为近海面鱼群、气泡羽流及海底泄漏的甲烷气体的声学建模提供一定的理论基础。

Modeling acoustic field scattered by an underwater assembly of gas bubbles or similar resonant monopole scatterers is of considerable theoretical and practical interest. Using effective medium theory to describe the phase speed and attenuation in bubble clouds and using the addition theorem for the spherical wave functions to relate the scattered sound fields of both the bubble cloud and sea surface to a common origin, we obtain a modified scattering cross-section of the cloud near the boundary. Numerical simulations reveal that, with increasing void fraction or increasing cloud radius,the natural frequencies of the cloud can extend to smaller frequencies. Periodic oscillatory perturbations exists in the modified scattering cross-section that is due to the effect of sea surface and such oscillation becomes more considerable with increasing depth. Besides, if the viscous damping dominate the damping term, another oscillation will exists near the resonance frequencies. The approach may serve to model acoustic scattering by fish schools near the sea surface,bubble plumes and methane leaks near the seafloor.