  The drag force acting on a body moving in a fluid has two components,
friction drag due to fluid viscosity and form drag due to flow separation
behind the body. When present, form drag is usually the most significant
between the two and in many applications, streamlining efficiently reduces or
prevents flow separation. As studied here, when the operating fluid is water, a
promising technique for form drag reduction is to modify the walls of the body
with superhydrophobic surfaces. These surfaces entrap gas bubbles in their
asperities, avoiding the direct contact of the liquid with the wall.
Superhydrophobic surfaces have been vastly studied for reducing friction drag.
We show they are also effective in reducing flow separation in turbulent flow
and therefore in reducing the form drag. Their conceptual effectiveness is
demonstrated by studying numerical simulations of turbulent flow over a bluff
body, represented by a bump inside a channel, which is modified with different
superhydrophobic surfaces. The approach shown here contributes to new and
powerful techniques for drag reduction on bluff bodies.
