Bubble collapse analysis and reentrant jet velocity measurements using wOFV*

Experiments were performed to study the collapse of bubbles produced by a spark discharge in a water tank using high speed imaging. A bubble was generated by dischraging a bank of capacitors at a voltage of 10 kV in between two electrodes into liquid water to simulate an underwater explosion. The air pressure above the water was reduced to a partial vacuum in order to increase the time scales associated with the bubble dynamics and to properly scale explosion conditions in the ocean. The coaxial electrodes were placed on the wall of the water tank such that the bubble was hemispherical and so that the reentrant jet that forms as the bubble collapses could be observed. Wavelet-based optical flow velocimetry (wOFV) was applied to high speed video of the bubble collapse, which allowed the velocity profile of the reentrant jet to be measured and for features on the surface of the bubble and within the jet to be tracked in time. Jet centerline velocities were successfully measured in the experiments. These measurements are compared to CFD computations of the bubble dynamics.