Measuring the size and concentration of micro-scale bubbles has long been recognized as important, since these nuclei directly influence cavitation inception. In this study, a nuclei measurement technique based on Interferometric Particle Imaging (IPI) is developed.

The technique consists of a continuous wave laser, a high-speed camera and a data-processing algorithm. It was first applied in a laboratory setup that consists of a small test tank filled with water in which bubbles of uniform size were produced using a microfluidic device. The IPI technique was validated by measuring the bubbles and comparing the results to those obtained simultaneously using shadowgraphy. The validation showed a systematic underprediction of the size by approximately 3.8%, which is likely due to the measurement uncertainty of the input parameters for the IPI technique.

Overall, however, the IPI technique was judged sufficiently accurate for the current objective, which is to measure the nuclei – produced by electrolysis – from within a ship model in MARIN’s depressurized wave basin. To do this, the laser was fixed inside the ship model and produced a near-vertical beam, slightly in front of the propeller plane. The camera was also inside the model and was observing the laser beam through a transparent window in the hull. In total, ten runs were recorded in which the camera settings, the model speed and the propeller rotation rate were kept constant, while the electrolysis current and the pressure in the towing tank were systematically changed. The preliminary results show that the size distribution of nuclei stays approximately the same with increasing electrolysis current, while the total number of detected nuclei increases. As expected, increasing the basin pressure decreased the size of the bubbles significantly. Finally, some improvements of the current technique are proposed and discussed.

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For more information contact Martijn van Rijsbergen.