Validation of a super-resolution measurement using frequency filtering technique and optical displacement analysis of bubble surface in breathing mode oscillation

A sound is generated when a bubble detaches from a nozzle installed in the water. The sound emission is known as a monopole sound source. This frequency is expressed by Minnaert's equation, and many studies have been conducted on bubble deformation, detachment, and coalescence. However, many physical processes still involve the unsettled phenomena regarding the mechanical behavior of the sound source. In this study, we report that the breathing mode was directly analyzed by an optical measurement, following the simulation of the breathing mode by CFD in the previous report. The present super-resolution measurement gives a sufficient result to discuss the oscillatory motion of bubble surface where the displacement of the bubble is so small that the normal observation cannot capture. In the present research, the numerical simulation of the measurement method was conducted, and measurement of the vibration disc were performed as the preliminary experiments. Thus, the optimum measurement parameters were extracted, and they were applied to the displacement measurement of the breathing mode of the bubble. The results show a good agreement with the CFD ones.