Numerical analysis of the water entry process of a projectile with a circular airbag

Abstract

Using the control volume method to describe annular an airtight airbag, we simulated the water entry process of a projectile and its annular airtight airbag based on LS-DYNA combined with the fluid-solid coupling algorithm. We divided this process into seven stages, i.e. those of the projectile slamming water, its air bag falling into water, decelerating in water-entry, hovering in water, floating upward in water, floating out of water, floating on the water surface. We analyzed the projectile's attitude change, deceleration, water entry depth and airbag in different stages in vertical and oblique water-entry. Then we examined the interaction between water and the airbag, covering the changes of the internal pressure, the resultant force of water to the airbag, the relationship between airbag's internal pressure and speed of the water-entry. The results show that the change of the internal pressure of the airbag is mainly affected by the water-entry depth, the movement speed, and the tension of the connecting rope. Further, we calculated the water-entry depth, the deceleration time and the tension peak of the connecting rope under the airbag's different initial internal pressures, and found that the water-entry depth and the deceleration time decrease with the increase of the airbag's initial pressure, while the tension peak of the corresponding connecting rope exhibited an opposite tendency. Therefore, it is necessary to consider the buffering effect, deceleration effect, airbag safety and other factors in the design of airbag parameters.