| |
The effect
of the transition to turbulent flow over the surface of bodies is illustrated
opposite. The top diagram shows that the streamline shape now trails
a turbulent wake rather than the well defined center streamline of the inviscid
flow. This wake is dissipating energy in viscous processes. For this shape
skin-friction drag associated with the boundary layer friction losses is about
90% of the total drag on the body the remaining 10% being wake losses.
The
sphere shown in the middle diagram also has a turbulent wake. The flow
over the sphere surface is laminar until it separates from the surface at
about the point of major diameter. For this situation the drag distribution
between surface friction and wake loses has 10% of the drag in the friction
and 90% in the wake.
The bottom picture shows the same sphere but with
a turbulent boundary layer. The flow now stays attached to the sphere over
a longer distance and the wake volume is reduced. Although the turbulent skin
friction is larger than for the laminar case, the wake dissipation is reduced
lowering the overall drag. |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
