Removal of free-stream bubbles in a research water tunnel using an inline cyclone separator

This study evaluates the efficacy of an inline cyclone separator as a means of continuously reducing the free-stream bubble concentration in a research water tunnel. Two sets of experiments are conducted at a cyclone inlet velocity (4.5 m/s) that should remove 100 μm bubble based on a simplified analysis. In the first test, an initial dense field of bubbles is created by intentionally cavitating the pump. Then, the pressure is increased and the time evolution of bubble size distribution in the test section, with and without the cyclone are compared. In the second experiment, the pressure is reduced to a level that causes pump cavitation while monitoring the evolution of bubble statistics, also with and without the cyclone. The bubble size distributions are measured using digital inline holography. Large (>75μm) and small bubbles are reconstructed separately using low-pass & high-pass filtering, and the bubbles are detected using a machine learning based algorithm. In the first experiment, the cyclone doubles the decay rate of bubble concentration, and in the second, it suppresses the increase in bubble density by an order of magnitude. Both experiments indicate that most of the bubbles larger than 60 μm are effectively removed, consistent with expectations.