FLUID DYNAMICS

The following movies show some results obtained by dropping various balls into a fish tank, all filmed at 300 fps.

At low speeds, the flow is laminar. Sometimes, the water separates from the ball and leaves an air cavity behind it.

At higher speeds, the flow can become turbulent. The end result depends on surface roughness and the Reynolds number.

SQUASH ball dropped from a height of 40 cm. See The Physics Teacher,54, 153-155 (2016).

GOLF BALL dropped from (a) 10 cm (laminar) and (b) 70 cm (turbulent). See Eur J Phys 37, 054001 (2016)

SPINNING HAPPY BALL curves to the left due to the Magnus force, then slows down due to the drag force, then rises due to the buoyant force, then reaches terminal velocity on the way up and curves slowly to the right due to the weak (low speed) Magnus force.

Smooth billiard ball Slightly rough billiard ball

SMOOTH BILLIARD BALL dropped from a height of 80 cm (laminar at 3.4 m/s, Re = 194,000)

SLIGHTLY ROUGH BILLIARD BALL dropped from a height of 10 cm (turbulent at 1.3 m/s)

A slightly rough surface makes a big difference to the onset of turbulent flow.

SMOOTH LEATHER BASEBALL (drop height = 10 cm)

SLIGHTLY ROUGH LEATHER BASEBALL (the half marked R was slightly roughened) Drop height = 10 cm

As each ball slows down, turbulent flow stops. Minor turbulence is seen on the rough leather surface.