Flow Analysis and Methods of Design for Dual-Bell Nozzles

Dual bell nozzles, that have been proposed to propel main stages of future launchers, show a good compromise between ease of manufacturing and improvement of overall engine performance. Indeed, the main feature of these nozzles is to permit two operating modes, thanks to a geometrical discontinuity in its divergent extension. However, the full achievement of this capability requires further studies: from one hand to design the most efficient geometry, from the other hand to ensure that the transition between the two operating modes takes place without risks in terms of structural loads. These studies largely rely on numerical analyses, which represent an efficient tool to get better understanding of the flow behavior in the nozzle. The present study addresses some numerical problems that may occur in these studies. In particular, it shows how grid resolution and turbulence modeling may affect the quantitative evaluation of the position of separation point. Moreover, some physical aspects of the flow behavior are considered, related to the possible choices of the geometrical shape of the divergent section.