高速欠膨胀射流结构及推力特征研究

Researches on the characteristics of structure and thrust of jets underwater with under-expansion

为了研究水下发动机不同工作状态射流结构变化及原因,利用轴对称及VOF模型对水下发动机射流进行仿真,研究发现高马赫数、欠膨胀状态下射流发生"准回击"现象;与具有四个典型过程的过膨胀状态下的射流结构相对比,发现高度欠膨胀下,射流的激波结构阻断了断裂与回击使得回流无法到达壁面完成回击;发动机推力因"准回击"现象骤变,因在激波及膨胀气体共同作用下回击力度受限无法到达壁面而被来流气体带往下游;对长时间射流流场结构研究发现,高度欠膨胀下的射流边界为成一定角度的倾斜直线。

In order to figure out the mechanism about the structure of jets underwater in differ working conditions, a geometrically symmetric model adopted the Volume of Fluid（VOF） multiphase model was utilized to simulate the practical working process. It was drawn that there is a ‘quasi-back-attack＇ in the jets with high Mach number and in under-expansion. Compared with over-expanded jets involving four typical stages, it can be found that the backflow fails to arrive at the wall for the reason that the shock waves, especially in highly expanding flow, stop the fracture of the flow. In addition, the oscillation amplitude of the thrust shows a sharply descend when the ‘quasi-back-attack＇ phenomenon happens, which is attributed to the limited strength of the flow under the influence of shock wave and expanded gas. According to the flow structure simulated in long term, the boundaries of the jets shape are as an inclining straight line.