Effect of surface roughness on the motion of a ball through the water column

Abstract

A solid object that enters into the water surface creates air cavity which influences the motion of the object within the water column. Understanding the phenomena which governs the water entering bodies is essential in for the objects that enters from air to liquid column, mainly in the water entering underwater target vehicles and cluster bombs against the opponents’ submarines and torpedoes. The present work is to investigate experimentally the dynamics associated with the motion of metal ball with rough and smooth surfaces into the liquid column that enters from the air to water surface. The deceleration of the solid ball is mainly attributed to the drag encountered by the bodies due to the pinning of three phase contact line (solid-liquid-vapor interface line) in the micro-asperities of the rough solid ball surfaces. At the same time, the pinning effect is negligibly small in the case of solid ball with smooth solid surfaces. Because of this the larger cavity has formed behind the rough solid surfaces that enters into the water surface. Further, the larger air cavity associated with pinning effect at three phase contact line increases the velocity of the rough ball within the water column. This increase in velocity of the rough ball is mainly associated with the reduced drag offered by the water due to the thin layer of air interface between the water and solid surface.