Numerical investigation of the influence of geometric parameters on thrust vectoring performance of a conical nozzle

Low-Mach thrust vectoring control for micro turbojet powered low-altitude manned air vehicles requires an externally attached conical vector nozzle that redirects the exhaust flow. However, the geometric rules governing the compromise between thrust retention and vectoring effectiveness remain insufficiently quantified. In this study, a three-dimensional steady compressible Reynolds-averaged Navier–Stokes model was established for the engine aft section, vector nozzle, and near-field flow region. The diameter ratio λ_D and length-to-diameter ratio λ_L were used to characterize the outlet expansion level and streamwise development length of the vector nozzle, respectively. A total of 128 non-deflected configurations were first calculated, and representative deflected cases were further analyzed at θ=5^∘,10^∘,15^∘,20^∘. The numerical setup was supported by grid independence, engineering consistency, and turbulence model sensitivity checks. The results show that the nozzle thrust first increases and then decreases with increasing λ_D, with the maximum thrust appearing at λ_D=1.0∼1.2 under non-deflected conditions. The thrust also exhibits a non-monotonic variation with λ_L, indicating that outlet expansion and internal flow development must be geometrically matched. Under deflected conditions, the thrust optimal and thrust vectoring efficiency optimal λ_D are different: λ_D≈1.2 favors thrust retention, whereas λ_D≈0.8 improves flow turning and vectoring efficiency. It provides a favorable balance between wall guided momentum redirection and suppression of separated asymmetric flow at λ_L≈1.8. Flow field analysis indicates that the performance variation is governed by outlet expansion matching, asymmetric pressure redistribution, wall attachment, and separation development. Within the investigated operating and geometric range, λ_D=0.8 and λ_L=1.8 are identified as a favorable design candidate rather than a universal optimum, providing an important reference for vector nozzle parameter selection in micro turbojet vector control applications.