Computational Fluid Dynamics Modeling of Projectile Drag in Baffled-Tube Ram Accelerator

Abstract

The ram accelerator operates on principles similar to those of a ramjet engine. In the baffled tube ram accelerator (BTRA), projectiles operate below the Chapman-Jouguet detonation velocity in a thermally choked propulsive mode. Geometry variations of the projectile can significantly change its performance. This investigation aimed to determine the drag of projectiles with varying geometries using computational fluid dynamics (CFD). Nose cone angle, tail cone angle, shoulder diameter, and shoulder length were varied to investigate their influence on drag. The geometric parameters considered here were based on previous experiments conducted in a BTRA. The velocity of the projectile was also varied to understand its effect on drag. ANSYS FLUENT, utilizing the Reynolds-Averaged Navier-Stokes (RANS) equations with the k-omega SST turbulence model was used to solve the transient CFD models. Dynamic mesh with a constant velocity of 1000 m/s (Mach 2.82) was used to understand the interaction between the projectile and the baffle. A non-reactive methane-air mixture at 300 psig was set as the working fluid. The results indicated that a smaller nose cone angle produces less drag compared to a larger nose cone angle. Similarly, a smaller tail cone angle also contributed to lower drag. An increase in shoulder diameter led to higher drag when the shoulder length was fixed. The variation in shoulder length yielded interesting results, i.e., while shoulder lengths of 2 and 3 times the baffle length produced similar drag, a shoulder length of 2.5 times the baffle length resulted in 12% lower drag. For reference purposes, the projectile drags in smooth bore and free-flight were also determined.