Numerical Study of Transition Between the Two Operating Modes of Dual-Bell Nozzles

Dual bell nozzles may represent a possible alternative to replace conventional bell nozzles in future launcher first stages. Their main feature is the peculiar shape of the divergent section, displaying two bells separated by an inflection point. Although this is a non-optimized shape, it shows two different operating modes that provide at high altitude larger expansion ratios than conventional nozzles, while avoiding sideloads at sea-level. The present study addresses the main critical aspects of the design of the nozzle shape, and the relevant numerical problems that occur in their analysis and design. In particular, tests have been performed to understand the consequence of inadequate grid resolutions in the different operating conditions. Moreover, the transition between their two operating modes must be carefully understood. To this end, the main physical aspects of the flow evolution in time have been analyzed on the basis of the results of a time accurate calculation of the transition between the two main operating conditions.