Flow Development and Performance Characteristics of an Axisymmetric Dual-bell Nozzle under Varying Nozzle Pressure Ratio Conditions

Abstract

Dual-bell nozzles (DBNs) provide passive altitude compensation by adapting their effective expansion ratio through controlled flow separation and reattachment. In this study, the flow-regime evolution of an axisymmetric DBN is investigated numerically over a nozzle pressure ratio (NPR) range from 10 to 100. The nozzle geometry was generated using an in-house Method of Characteristics (MoC) design procedure combined with a constant-pressure extension contour. Compressible Reynolds-Averaged Navier–Stokes equations were solved using ANSYS Fluent with the SST k–ω turbulence model. The numerical methodology was validated against experimental wall pressure measurements, Schlieren visualizations, and reference CFD results from the literature. Excellent agreement was obtained for wall pressure distributions and shock-wave structures, while discrepancies between analytical and CFD performance parameters remained below 0.25%. The simulations revealed a progressive transition from restricted-separation to fully attached flow as NPR increases. The separation point moves downstream within the extension contour, accompanied by a gradual weakening of the internal shock system. At high NPR values, fully attached flow is established throughout the extension nozzle, resulting in improved pressure adaptation and enhanced nozzle performance. The proposed framework provides a reliable tool for investigating DBN flow-regime transitions and altitude-compensating nozzle performance.