Assessment of detached eddy simulation of a separated flow in a planar nozzle

The sea-level start-up of rocket engines is characterized by the nozzle experiencing a high degree of overexpansion and a consequent internal flow separation with a strong unsteady shock wave boundary layer interaction (SWBLI). This may produce side-loads dangerous for the engine structure. Despite the important amount of experimental, theoretical and numerical work that can be found in literature, the physical mechanism behind this unsteadiness is still not clear and research is going on. In this work, a 3D planar overexpanded nozzle has been simulated by means of the detached eddy simulation (DES) method. In high Reynolds number flows, simulating only the separated flow with the LES method and the attached boundary layer with the RANS method allows the computational cost to be reduced. The unsteady pressure signals have been analyzed by the wavelet decomposition in order to characterize their spectral content. The results indicate that the DES is able to capture the shock oscillations and that the computed characteristic frequency is close to the ones available from literature for the same test case. The value of the fundamental frequency computed in this work is lower than the one predicted by the model of the longitudinal acoustic frequency. The self sustained oscillation is driven by a pressure imbalance between the pressure level downstream the recompression shock and the ambient. The turbulent separated zone seems to have an important part, even if the mechanism is still not clear.