Numerical investigation of flow separation in an annular conical aerospike nozzle for underwater propulsion

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 0~1000 Hz 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.