Tobias Bohne, Cristina Díaz-Cereceda, Raimund Rolfes
|Titel:||NUMERICAL MODELLING OF BUBBLE CURTAINS|
The pile driving associated to the construction of offshore wind turbines causes high sound pressure levels in the sea. This noise emission can be potentially dangerous for the marine life, especially mammals. In Germany the limit for underwater noise generated by pile driving activities is 160 dB of sound-exposure level at a distance of 750 m from the pile. To prevent emitted sound levels from exceeding this limit, noise mitigation systems must be used during the construction.The bubble curtain is a commonly used system to reduce the acoustic pressure. Air is pumped through a tube along which a number of holes is distributed and rises due to buoyancy in form of bubbles. The bubble curtain can be placed directly next to the pile or at a certain distance.The physical processes causing the sound attenuation effect of the bubble curtain are not clearly understood. This is partly due to the fact that direct measurements of the properties of the bubble curtain, such as the bubble size distribution, are very difficult to obtain because of the stochastic nature of the bubble flow and the uncertain boundary conditions in the sea .In this paper a numerical method for modelling in detail a bubble curtain is presented. The propagation of a plane wave through a mixture of water and air bubbles is modelled with finite elements. Due to the unaffordable computational cost of modelling the complete bubble curtain, a statistically representative section of it is analysed. In this phase of the modelling the thermal and viscous effects at the bubble-water interface have not been considered.