Re-Entrant Jet Measurement During Large-Scale Gas Bubble Collapse

When a payload exits a submerged launch tube, pressurized gas follows the ejected projectile into the surrounding water and expands between the tube and the upward moving projectile. The gas cavity behind the moving body then deforms and pinches off from the tube and then collapses with the formation of a strong upward reentrant jet. The impact of the high-speed reentrant jet on the projectile presents a threat of damage and imparts an unwanted force to its motion. Previously attempted high speed photography of the reentrant jet has not been successful due to the highly opaque and turbulent bubbly region surrounding the cavity. In order to alleviate this deficiency, we propose in this SBIR to combine high speed video observations of the cavity dynamics with pressure measurements in the water at several locations and impact pressure measurements on the projectile base. The optical method will provide the exterior shape of the evolving cavity, while the acoustic method, combined with a CFD computation of the cavity shape evolution for different input conditions, will enable reconstruction of the reentrant jet shape evolution in space and time using inverse problem optimization techniques that process both the acquired video and the acoustic information. The system will be tested and validated in Phase I using an available small scale uncorking setup in a vacuum tank and then scaled up for the field tests.