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


TECHNOLOGY AREA(S): Ground/Sea Vehicles, Sensors


ACQUISITION PROGRAM: Strategic Systems Programs (SSP), ACAT I


OBJECTIVE: Develop and validate a new instrumentation sensor suite and associated processing techniques to capture the size, shape, composition, and velocity of a re-entrant jet formed during large-scale bubble collapse near a moving boundary. The sensor system must collect spatially resolved, time-dependent void-fraction, density and velocity data.


DESCRIPTION: When a pressurized volume is suddenly opened, such as when a payload exits a launch tube, there is a sudden “uncorking” of pressurized gas that expands out of the volume. If a moving surface (boundary) is what is initially in place to maintain the pressure in the volume, the gas expands out of the volume and directly behind the moving surface. This “uncorking” event forms a large bubble that expands to a maximum size before collapsing (due to local hydrostatic pressure) and forming a re-entrant jet which then contacts the moving surface. This re-entrant jet presents a threat of damage due to high-speed impact with the moving surface. Re-entrant jet behavior is observed in various other disciplines, including cavitation research and underwater explosions [UNDEX]. In each of these problems, the jet is formed when the bubble is placed near and allowed to collapse into a solid boundary. In spite of the widespread interest in this problem, the dynamics of large-scale bubble collapse are not well understood. This is due in part to the difficulty of performing direct measurements of the bubble collapse phenomena.

To-date, measurements of the uncorking and re-entrant jet phenomena for a moving plate have been limited to qualitative, underwater video of the bubble and limited pressure and load measurements on the moving surface. Visual methods have provided limited information on the jet due to opacity of the bubble interface. The objective is to develop a sensor suite to quantitatively measure the size, shape, and velocity of this re-entrant jet throughout its formation and as it impacts/interfaces with the moving surface. In a notional test environment, the bubble is planned to uncork with an overpressure of approximately 20 pounds per square inch differential (psid) (relative to the local hydrostatic pressure), and then expands out to a diameter of approximately 10 feet. The bubble travels upward due to buoyancy and the influence of the moving surface, which is traveling in the same direction. Bubble collapse occurs after 10-15 feet of upward travel. The bubble then continues to oscillate, resulting in multiple re-entrant jet formations during the time that the moving surface moves to the free surface of the water. At the time when the moving surface broaches the water surface, the jet forms one last time, impacting the moving surface with significant force. Another factor making measurements more difficult to collect is the addition of vented gas from the moving surface, which increases the opacity of the environment that a measurement device would need to be able to resolve. The venting results in a steady stream of small bubbles that surround and join the base bubble that is generated, creating a highly opaque and turbulent region.

Market and historical searches have confirmed that instrumentation suites have not been able to view or collect data on this water jet within this region of vented gas.


PHASE I: Define and develop a new instrumentation sensor suite that can provide accurate measurement of the reentrant jet. Perform analysis, including modeling and simulation and breadboard testing, to ensure concepts can be utilized with major features of the full-scale environment as discussed in the Description section. This environment would include both underwater and surface capture of the phenomena at the described geometries and pressures for this high speed event.


PHASE II: Based on the Phase I effort, develop a large-scale version prototype of the new instrumentation sensor suite for demonstration and validation. The prototype should be delivered at the end of Phase II to be utilized on subscale testing. The large scale version prototype would need to be able to view an approximately 20 ft wide underwater area by 50 feet tall. It would also need to view a 20 ft wide area above water.


PHASE III DUAL USE APPLICATIONS: If Phase II is successful, the small business will provide support in transitioning the technology for Navy use. The small business will develop a plan to determine the effectiveness of the sensing system in an operationally relevant environment. The small business will support the Navy with certifying and qualifying the system for Navy use. As appropriate, the small business will focus on scaling up manufacturing capabilities and commercialization plans. Completed system(s) will be delivered to the Navy for use.


KEYWORDS: Sensor; instrumentation; multi-phase; underwater; launch; bubble collapse; re-entrant jet 

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