Aerodynamic Analysis of Projectiles in Ground Effect at Near-Sonic Mach Numbers

The behavior of projectiles at Mach 0.9, 1.1, and 1.2 when in close proximity to a surface were investigated using a synergy of live-range test firings, wind-tunnel experiments, and computational fluid dynamics simulations; the latter provided the majority of the insight into the problem. Building on work that previously indicated significant ground influence for a projectile at Mach 2.4 due to shock reflections, a similar projectile was investigated for ground clearances of height-to-diameter ratios from 0.25 to 8. It was found that at Mach 0.9 the projectile’s drag, normal force, and pitching moment coefficients changed predictably, and with significant increasing magnitude below h∕d = 2. Drag increased, a strong normal (suction) force acting toward the ground developed with increasing strength of the shock developing on the lower side of the projectile, and a nose-up pitching tendency was established. At the supersonic Mach numbers, the ground influence was present at higher ground clearances, and influential shock reflections occurred; at high clearances, the reflected shock increased the pressure on the lower side, and at low clearances (h∕d < 1), the reflecting expansion waves lowered the pressure to produce a pronounced negative normal force. At the lowest investigated clearances, drag was approximately 30% higher, and the Mach 1.2 cases experienced more substantial fluctuations in the trends. The normal force and pitching moment behaviors would be unlikely to influence the projectile’s path over any realistic distance, but the ground proximity drag increase would significantly alter the projectile’s deceleration and therefore trajectory response.