Entry and exit dynamics of symmetrical projectiles

Amphibious drones are a type of unmanned vehicle that can both fly and swim. Typically, these drones will hover above the water then gently land on the surface. We are investigating the possibility of amphibious drones that can dive straight into and out of the water without slowing down. In this study we investigate symmetrical projectiles in canonical shapes—a sphere and a cone—that are driven into and out of a tank of water at a high Reynolds Number, roughly matching that of common military and commercial drones. There is an extensive body of research on entry body dynamics, however this study adds the exit dynamics as well. A closed motor-pulley system with the projectiles attached to a ⅛ in wire drives them out of the water at about 55 m/s. Images of the projectile, air cavite and splash are captured by a high speed camera at 6000 frames per second. We measure the forces on the object as it crosses the surface of water into air and observe the post-exit flight. The high speed images determine how changing the aspect ratio of the projectile affects the splash as the object exits the water. This shows that there is less water entrainment into the air, making it easier for the object to exit the water and maintain its speed. The amount of water entrained from the exit is examined in the same manner as air for entry body dynamics in cavity formation.