Fundamentals of Biomechanics

198 FUNDAMENTALS OF BIOMECHANICS
Figure 8.5. Form drag forces (F D ) result from a vacuum pressure formed in the pocket formed behind a submerged
object (a). Decreasing the pressure in this wake is how contouring the rear profile of an object (streamlining)
decreases form drag (b).
resistance to many human movements.
Some human movements may also use
drag as a propulsive force.
To understand the variations in pressure
drag, one must differentiate two different
kinds of fluid flow in the boundary
layer: laminar and turbulent. The air flow
past a tennis ball can be highlighted by
smoke introduced into a wind tunnel, depicted
in Figure 8.6, which shows both predominantly
laminar and turbulent flow.
Laminar flow typically occurs in low-velocity
conditions with streamlined objects
where the fluid particles can flow relatively
undisturbed in parallel layers. Turbulent
flow occurs when fluid molecules bounce
off the object and each other, mixing in
chaotic fashion.
The kind of fluid flow over an object
also affects pressure drag. At low velocities
the boundary layer is laminar and cannot
flow very far around a non-rotating sphere
before peeling away from the surface
(Figure 8.7a), creating a large form drag. At
Figure 8.6. The air flow past a tennis ball shows both
laminar (L) and turbulent (T) fluid motion. The topspin
on the ball deflects the air flow creating another
fluid force called lift. Photo courtesy of NASA Ames
Research Center Fluid Mechanics Laboratory and
Cislunar Aerospace, Inc.