Drag crisis

Plot of drag coefficient against Reynolds number for rough or smooth spheres. A sharp drop is observed around Reynolds of 100000 to 1000000 for either.
The drag coefficient of a sphere drops at high Reynolds number (number 5 on the graph). The effect occurs at lower Reynolds numbers when the ball is rough (such as a golf ball with dimples) than when it is smooth (such as a table tennis ball).

In fluid dynamics, the drag crisis (also known as the Eiffel paradox[1]) is a phenomenon in which drag coefficient drops off suddenly as Reynolds number increases. This has been well studied for round bodies like spheres and cylinders. The drag coefficient of a sphere will change rapidly from about 0.5 to 0.2 at a Reynolds number in the range of 300000. This corresponds to the point where the flow pattern changes, leaving a narrower turbulent wake. The behavior is highly dependent on small differences in the condition of the surface of the sphere.

  1. ^ Birkhoff, Garrett (2015). Hydrodynamics: A study in logic, fact, and similitude. Princeton University Press. p. 41. ISBN 9781400877775.