The dual bell altitude compensating nozzle concept represents one of the most promising solution to improve the performance of large liquid rocket first stage engines. This adaptation capability is obtained by means of an inflection point in the nozzle profile. At sea level the flow is separated at this inflection and the engine works with the first bell. At high altitude, the flow reattaches at the wall of the second bell and the engine works with a larger area ratio, so increasing the performance. Several investigators focused on the dual bell flow transition between its two operating modes and the consequent generation of side loads. Currently, one of the most important aspects that still needs a deep investigation is the stability of the flow separation when the dual bell nozzle is operating at sea level. During this condition, the nozzle is highly overexpanded and an internal flow separation takes place, characterized by a shock wave boundary layer interaction (SWBLI) which causes the shedding of vortical structures and strong unsteadiness that produces dangerous side loads. The main aim of this project is to numerically investigate the flow physics in a dual bell nozzle overexpanded flow, to analyze the spectral properties of the shock motion and identify the major sources of unsteadiness in the field. Given the high-Reynolds number of the flow the computational cost is too high for direct numerical simulations (DNS). To overcome this limitation, high-fidelity simulations will be carried out using hybrid RANS/LES methodologies . The continuous wavelet transform will be applied to identify the frequencies contributing to the energy of the fluctuations. A better knowledge of the overexpansion phenomenon will help in predicting and controlling the level of side loads. In such a way it will be possible to design safe dual bell nozzles and to reduce the costs of the access to space.
