Characterization of unsteadiness in an overexpanded planar nozzle

The sea-level startup of rocket engines is characterized by the nozzle experiencing a high degree of overexpansion and consequent internal flow separation with a strong unsteady shock-wave/boundary-layer interaction. In this work, a three-dimensional planar overexpanded nozzle, characterized by an internal flow separation, has been simulated by means of the delayed detached-eddy simulation (DDES) method. The unsteady pressure signals have been analyzed by the wavelet decomposition to characterize their spectral content. The results indicate that the DDES approach is able to capture the shock oscillations, and the computed characteristic frequency is close to the ones available from literature for the same test case. 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 of the recompression shock and the ambient. Different nozzle pressure ratios (NPRs) have been simulated, all showing the same qualitative behavior, even if the test case with the highest NPR seems to be heavily influenced by the presence of a large region with reversed flow.