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

Y. Jian, Feng Jin-fu, Qi Duo, Zhang Xiao-qiang, Li Yong-li, Zhang Jing-kun
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引用次数: 2

Abstract

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.
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物体垂直出水过程中附加质量的计算与分析
物体垂直出水过程涉及到两相流及流固耦合问题,是物体出水阶段动力学建模和控制器设计中急需解决的关键问题之一。加质量解耦是目前比较成熟和方便的解耦方法。由于出水过程中物体的附加质量具有时变特性,不能直接采用传统的附加质量计算方法。为此,本文从动力学方程出发,推导了附加质量的计算公式,并提出了一种基于数值模拟的新的计算策略,以计算出水不同阶段附加质量的实时变化。在此基础上,利用FLUENT软件建立的数值模型计算了不同速度和加速度条件下的球面附加质量。通过对出水过程中球体附加质量的计算值与理论值的比较,对该方法进行了验证。然后,研究了球体和圆柱体在垂直出水过程中附加质量的变化规律。结果表明:在垂直出水过程中,球体和圆柱体的垂直附加质量和水平附加质量均呈减小趋势,其中,在0.9 ~ 0.4水深处,垂直附加质量变化缓慢,水平附加质量变化近似线性;附加惯性矩在淹没深度0.5附近变化缓慢。附加静力矩先增大后减小,在淹没长度为0.7 ~ 0.5时达到最大值。新的附加质量计算方法有助于提高各种水下武器水动力学模型的精度。同时,相关结论对出水过程对象的研究、控制器设计和运动预测也具有一定的参考意义。
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