Calculation and analysis of added mass for an object during its vertical water exit

The vertical water-exit process of object involves the problem of the two-phase flow and fluid-structure coupling which is one of the key problems that need to be solved urgently in dynamic modeling and controller design of the water-exit stage of the object. Nowadays, added-mass is relatively mature and convenient method of decoupling. Because of the time-variant characteristic of the object's added-mass in water-exit process, the traditional calculation method for the added mass cannot be used directly. For this purpose, in the paper, from the dynamic equations, the computational formula of added-mass is deduced, and a new numerical strategy based on the numerical simulation is proposed to calculate the real-time changes of added mass in different stages of water-exit. On this basis, the added-mass of sphere under different velocity and acceleration conditions are computed by numerical model which is established based on FLUENT. The method is confirmed through the comparison between computational value and theoretical value of the sphere's added-mass in water-exit process. Then, the rules of added mass changes of a sphere and a cylinder during vertical water exit are studied. The results demonstrate that both vertical added mass horizontal added mass of the sphere and cylinder are decreasing in the process of vertical water exit, of which, the vertical added mass changes slowly at a submerged depth of 0.9-0.4 and the horizontal added mass changes approximately linearly. The added inertial moment changes slowly around the submerged depth of 0.5. Added static moment increases first then decreases, and reaches its peak value at the submerged length of 0.7-0.5. The new added mass calculation method is helpful to improve the accuracy of the water dynamics model of various submerged weapons. At the same time, the relevant conclusions also have some reference significance for the object of the water-exit process research, controller design and motion prediction.