Bubble Plumes and their use for Sound Mitigation

In this work we study bubble plumes in the light of their recent use for sound mitigation of offshore pile driving. We start the thesis by the hydrodynamics of bubble plumes and how to measure the relevant properties. We developed a conductivity based measurement method for void fraction and bubble size measurements. This method is combined with an underwater camera allowing for in-situ calibration of the system. The resulting measurements are presented, and show a different maintained growing angle for different bubble generation manifolds. Further measurements in a 10x31x40 m3 basin are used to calibrate a recently developed model. Based on these results a complete model describing both the integral quantities, such as the void fraction and the width of the plume, as well as the local bubble size distribution is extensively explained. We noted that the slip velocity of the bubbles, required for good agreement between the measurements and the model, needed to be significanlty higher than the slip velocity of a single bubble. We attribute this to the collective effects of the bubbles. Finally we looked at the resulting acoustic performance of bubble curtains. Here we found that splitting the airflow between two manifolds significanlty increases the performance while using the same amount of compressed air. This can be explained by the region of interest of low frequencies, where reflection is the dominant mechanism for sound mitigation. Halving the air flow rate per manifold hardly influences the reflective properties for an individaul bubble curtain. Splitting the airflow rate between two manifolds thus leverages this property. Finally we comment on the applicability of equivalent fluid models in the modelling of the sound mitigation by bubble curtains.