Transient multiphase flow structure of synchronous water exit for projectiles under different lateral distances

Underwater launch involves the classical high-speed hydrodynamic challenge of cavitation, which becomes more complex when multiple projectiles exit the water synchronously. Strong flow interference during this phase can easily destabilize projectile posture. This study examines how lateral launch spacing affects multiphase flow interference during parallel water exit of rotary models. The analysis employs the Improved Delayed Detached Eddy Simulation method, the Schnerr–Sauer cavitation model, a volume-of-fluid multiphase approach, and overlapping grids. An exit-water experimental system was independently constructed, and the transient flow simulations were validated with excellent experimental agreement. The results show that during water exit, the outer cavity collapses progressively from top to bottom and remains inherently unstable. Numerous small-scale vortical structures near the free surface add complexity to the shoulder-cavity collapse. Under launch conditions of zero successive-launch interval, launch depth of one projectile diameter, initial cavitation number of 0.23, and transverse spacing of 2D–5D, asymmetric cavity morphology diminishes at 4D but only disappears entirely—along with vortex-dynamics and turbulent-energy interference—at 5D. This confirms that flow-field interference persists even after cavity shapes visually appear symmetric.