Measurements of the drag force on balls in water

Measurements are presented of the drag force on several different balls falling vertically through water in a fish tank, at speeds up to 4.5 m s(-1). Experiments of this type are usually undertaken in a student laboratory by measuring the terminal velocity of an object falling either through air or a viscous liquid. At speeds slightly higher than the terminal velocity, additional effects can be observed visually in water, including flow separation, turbulent flow and the formation of air cavities. Changes in the drag coefficient were observed due to all three effects, although different changes were observed with different balls, depending on the Reynolds number. Cavity formation was found to depend strongly on the roughness of the ball surface as well as the incident ball speed. Results are presented at Reynolds numbers from about 20 000 up to about 250 000.