Abstract: A submerged annular conical aerospike nozzle for a design Mach number of 2.0 has been numerically studied under over-expanded condition. Particular attention has been paid to the flow separation characteristics in aerospike nozzle for underwater propulsion application. The detailed Navier-Stokes flow computations were utilized to elucidate the gas-water interactions under the framework of a Volume of fluid model. The calculation results show that，the intermittently necking/bulging or necking/back-attack phenomenon also exists in the submerged over-expanded annular conical aerospike nozzle flowfield. The back pressure for underwater nozzle is not only determined by the pressure of the water environment but also by the pressure in the gas bubble，and the gas/water interface restricts the gaseous jet as a wall， which is extremely different from the air environment condition. The gas/water interface around the plug flowfield is observed to be severely affected by the pulsation of the nozzle expansion back pressure，and shows continual oscillation between necking and bulging. The flow separation characteristics develop and change along with the jet oscillation. Depending on shock structures in the nozzle and flow separation characteristic on the plug surface，the nozzle exhibits different flow separation regimes which can be broadly classified into five types. Further，the mechanism for unsteady separation in submerged over-expanded aerospike nozzle is different from the one under air environment condition. The predicted wall fluctuating pressure shows a broad-frequency phenomenon with the bandwidth of 0~1000Hz by the current Navier-Stokes computation. As a result of the complicated coupling effects among the back pressure pulsation，the flow separation process and the jet oscillation，flow separation behavior in the present annular conical aerospike nozzle for underwater propulsion shows highly irregular oscillation characteristics.

Key words: Numerical investigation；Underwater propulsion；Multiphase flow；Aerospike nozzle；Flow separation

摘要： 将塞式喷管概念扩展到水下固体火箭发动机应用领域，为了研究高背压环境下塞式喷管的水下流动分离特性，建立水下塞式喷管流动分析模型，并采用流体体积法(VOF)两相流模型对设计马赫数2.0的环喉型圆锥塞式喷管水下工作时的过膨胀流场进行了气/水耦合数值模拟，计算考虑了气体的压缩性和粘性。计算结果显示:圆锥塞式喷管在水下的过膨胀流动也存在间歇性的颈缩、胀鼓以及回击等不稳定现象；与空气环境下的工作条件不同，气/水界面表现出类似于壁面的约束作用，塞锥外流场形成的波系结构由塞锥壁面和内喷管出口下游气/水界面共同决定；水下超声速气体射流的不稳定振荡引起喷管出口背压和气/水界面的脉动，塞锥表面的分离流场随射流的振荡而变化，根据流场激波结构以及塞锥表面分离特征的不同，可以区分为5种不同的分离流动形态；塞式喷管在水下和空气环境下的分离流动振荡的驱动机理不同，水下分离流场的振荡主要受气/液两相相互作用诱导的射流振荡过程的影响，分离流场附近壁面压强振荡频率覆盖0~1000Hz范围内的较宽频带，且没有显著的特征频率。

关键词: 数值研究；水下推进；多相流；塞式喷管；流动分离