Abstract: The improved advection upstream splitting method (AUSM+) scheme and the shear stress transport k-ω turbulence model are used to solve the two-dimensional axisymmetric Navier-Stokes equations for investigating the effects of different nozzle structures on the wake flow field characteristics of base-bleed projectile under transient depressurization during launching from muzzle. The structures of wake flow filed and the characteristic parameters of base bleed projectiles with annular and circular nozzles were obtained by numerical simulation. The wake flow field characteristics of base-bleed projectiles with different nozzle sizes were compared and analyzed. Calculated results show that the wake flow field at circular nozzle develops from a highly under-expanded jet to a subsonic jet during transient depressurization. In the case of annular nozzle，a highly under-expanded annular jet is extruded to be a central jet on the centerline，and then develops to a subsonic annular jet，in which an expansion wave at the corner of boattail is weakened effectively and the streamline is smoother in the end of depressurization process.The average static pressure and average static temperature at the base of the annular nozzle are higher than those at the circular nozzle，and the effect of drag reduction is better. Jet size has little effect on the base flow field at the end of the depressurization process，but the depressurization rate is higher with the increase in exhaust area ratio，and the rate of pressure rise becomes slower.

Key words: basebleedprojectile, transientdepressurization, nozzlestructure, wakeflowfiled