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.
The present project is highly multidisciplinary, as it combines computational and theoretical facets of fluid dynamics, control theory of dynamical systems, applied mathematics and advanced scientific computing. If the objectives here proposed are fulfilled, the project will bring significant advances in a crucial aerospace research area and will have a strong impact from the technological point of view. In particular the reduction of Earth-to-orbit launch costs in conjunction with an increase in launcher reliability and operational efficiency are key demands on future space transportation systems. The realization of these vehicles strongly depends on the performance of the engines, which should deliver high performance with low system complexity. Current launchers like Ariane 5 use a parallel configuration. The main stage engine starts at sea level and continues functioning up to almost vacuum conditions. To avoid flow separation phenomena in the nozzle at sea level, the area ratio has to be limited, thus limiting the high altitude performance. For this reason the rocket engines nozzle comes into focus as the sub system with the most promising performance gain. The European Flow Separation Control Device group (FSCD) was initiated to study both flow separation in classical bell nozzles and altitude adapting rocket nozzles such as plug nozzles, dual bell nozzles or nozzles with an extendible exit cone. As a result of this preliminary study, the dual bell nozzle was identified as the most promising concept [1]. A very recent paper [2] evaluated the impact of dual bell nozzles on the payload mass delivered into geostationary transfer orbit (GTO). In this study, the main stage of the standard Ariane 5 ECA configuration was adapted using a redesigned Vulcain 2 rocket engine with dual bell nozzle extension. Ariane 5 ECA launcher is able to to deliver 10.1 t into GTO. With the dual bell nozzle it is estimated to deliver a payload increased by 450 kg, an increment therefore of 4.5% . Assuming a cost of approximately 16000 Euro per kg of payload into GTO, this would lead to an additional value of 7.2 million Euro per Ariane 5 ECA launch. Therefore, the main expected impact of the project is to increase the basic physical knowledge of the shock induced separation in over-expanded advanced nozzle flows. This improved knowledge will help in the development of strategies for the control of flow separation and for the mitigation of side loads. Moreover, this knowledge advancement will help in the development of the dual bell nozzle technology for the next generation launcher.
The outcomes of the project are also expected to have a strong impact from the viewpoint of fundamental science, bringing advances to a critical research area for both computational and theoretical fluid dynamics, as the characterization of shock induced flow separation unsteadiness and the individuation of the main sources of disturbance.
References
[1] Wong, H., Schwane, R., "Numerical Investigation of Transition in Flow Separation in a Dual-bell Nozzle", Fourth Symposium on Aerothermodynamics for Space Vehicles: co-sponsored by European Space Agency. Held October 15-18, 2001, in Capua, Italy. Edited by R. A. Harris. European Space Agency, ESA SP-487, 2002. ISBN: 92-9092-789-5., p.425
[2] Schneider D., Ge¿nin C., Stark R., Fromm C.M., (2014). "Ariane 5 Performance Optimization Using Dual Bell Nozzle Extension". In: Space Propulsion 2014, 19-22 May, 2014, Cologne, Germany.