Top:

Instantaneous flow past a sphere at Re_D = 15,000. Dye in water shows a laminar boundary layer separating ahead of the equator and remaining laminar for almost one radius. It then becomes unstable and quickly turns turbulent.

Bottom:

Instantaneous flow past a sphere at Re_D = 30,000 with a trip wire. A classical experiment of Prandtl and Wieselsberger is repeated here, using air bubbles in water. A wire hoop ahead of the equator trips the boundary layer. It becomes turbulent, so that it separates farther rearward than if it were laminar (compare with top photograph). The overall drag is thereby dramatically reduced, in a way that occurs naturally on a smooth sphere only at a Reynolds numbers ten times as great.

Contributor: Henri Werlé, 1980 (ONERA). These photographs were taken from "An Album of Fluid Motion," by Van Dyke, Parabolic Press. This book is available at a bargain price of about $25. It might make a nice present for someone.