FLOW PAST A SPHERE NEAR THE CRITICAL REYNOLDS NUMBER
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.
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.
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.