• Buffeting
• fluid-structure analyses
• Vulcain 2

The transonic base-flow buffeting is an unsteady phenomenon during the ascent which produces high structural loads on the launcher propulsion system, especially on the nozzle. This phenomenon is a critical issue in the optimization of the future launch vehicles: in fact the attempts to improve the propulsive performances of the space vehicles are leading to the use of large area-ratio nozzles, which are more sensitive to the buffeting instabilities than the traditional ones.
The purpose of this thesis is to perform preliminary investigations on the transonic buffeting of the Vulcain 2 nozzle, focusing in particular on the effects of this base-flow instability on the structure of this component.
First of all, the Ariane 5 ECA (i.e. the launch vehicle on which is mounted the analysed nozzle) architecture and the Vulcain 2 engine have been described in Chapter 1. After a brief literature review on buffeting in Chapter 2 (useful also to derive the hypotheses and the data to be used as “references” for the models and the results of this thesis), the main procedures performed to obtain the different models (i.e. the fluid-dynamic and structural ones, carried out through STAR-CCM+® and ABAQUS® software respectively) have been detailed in Chapter 3. This last chapter lists also all the simplified assumptions considered in the realization of the previous models and in the “one-way” coupling process between the fluid-dynamic and the structural solvers. Then the results of the performed fluid-structure analyses have been collected and broadly commented in Chapter 4. Using the data of this chapter, preliminary fatigue and fracture analyses on the structure have been performed in Chapter 5 while in Chapter 6 have been summarized all the obtained conclusions and have been suggested possible improvements and close examinations which could be completed in future works. Finally two appendixes details some aspects only quoted in Chapter 3: in particular Appendix A specifies the turbulence model and the corresponding parameters used in fluid-dynamic analyses while Appendix B suggests a method alternative to the one considered for the performed analyses (through the use of MATLAB® software) to extract the load history for the coupled structural analyses.