Investigating the Direction and Location Effect on the Dual Bell Nozzle Flow Through Secondary Injection

Dual Bell nozzles are increasingly investigated as an altitude-compensating technology in rocket propulsion, offering higher efficiency across varying ambient pressures compared to conventional bell nozzles. Their geometry includes an inflection point along the nozzle wall, designed to induce controlled flow separation at low altitudes and flow reattachment at higher altitudes. However, unstable or premature separation can cause performance losses and side loads. To mitigate these issues, active flow control techniques have been introduced, with secondary injection proving to be particularly effective in manipulating flow behavior, delaying separation, and enhancing overall nozzle performance. In this research, a computational fluid dynamics (CFD) investigation is carried out to analyze the effect of secondary injection directions and locations on flow separation and stability within a Dual Bell nozzle. Four injection orientations (tangential, horizontal, inclined, and normal) are systematically studied at three nozzle sites: the inflection point, the middle of the base nozzle, and the middle of the extension nozzle. Numerical simulations are performed using ANSYS FLUENT, applying the Navier–Stokes transport equations, continuity, and energy equations in conjunction with the k-ω SST turbulence model to capture shear stress transfer. Due to nozzle symmetry, only half of the geometry is modelled. The analysis focuses on the resulting flow field, including shockwaves, boundary layer interactions, separation points, and reattachment zones. The results demonstrate that both injection direction and placement strongly influence flow separation characteristics. Tangential injection at the inflection point is identified as the most effective configuration, generating streamwise vortices that control shock-boundary layer interactions, delay separation onset, and stabilize the transition between operating modes with minimal pressure loss. NPR effect on the flow behavior is studied for the optimum case. Overall, the investigation confirms that secondary injection is a promising active flow control method for optimizing stability and efficiency in Dual Bell nozzles.