A numerical study on the influence of annular gap variation in correctly expanded
sonic coaxial jets, focusing on its effect on mixing characteristics and jet
symmetry, is presented in this paper. The computational simulations were
conducted using a three-dimensional steady-state compressible Reynolds-Averaged
Navier–Stokes (RANS) framework with the Spalart–Allmaras (SA) turbulence model.
Both symmetric (uniform gap) and asymmetric (nonuniform gap) configurations were
simulated. Eccentricity was introduced by offsetting the secondary nozzle by 2
mm downward from the center of the primary nozzle. In symmetric configurations
with uniform annular gaps, the jet exhibited balanced shear-layer development,
uniform entrainment, and symmetric Mach decay characteristics. However, the
asymmetric annular gap configuration exhibited approximately 25–30% earlier
potential core breakdown, 30–35% greater radial jet spreading, and nearly 6–10%
faster centerline velocity decay compared with the symmetric configuration. The
streamline analysis revealed enhanced entrainment, localized recirculation
regions, asymmetric vortex generation, and accelerated momentum diffusion caused
by unequal shear-layer interaction. These results demonstrate that annular gap
asymmetry can serve as an effective passive flow control strategy for enhancing
jet mixing and directional momentum redistribution. Such configurations may be
useful in practical applications including exhaust gas dilution, fuel–air mixing
enhancement in combustors, thrust vectoring, and jet-noise suppression
systems.