Page 186 - Wind Energy Handbook

160 AERODYNAMICS OF HORIZONTAL-AXIS WIND TURBINES

du
dy

Figure A3.6 Boundary Layer Showing the Velocity Profile

A3.3 The Boundary Layer

The reason for the separated flow at the higher Reynolds numbers is the existence
of a thin boundary layer of slow moving fluid, close to the body surface, within
which viscous forces predominates. Outside this layer the flow behaves almost
inviscidly. The drag on the body caused directly by viscosity is quite small but the
effect on the flow pattern is profound.
The drag on an aerofoil can be attributed both to pressure and viscous sources
and the drag coefficient varies significantly with both angle of attack and Reynolds
number.

A3.4 Boundary-layer Separation

Referring to Figure A3.3, the inviscid flow pressure distribution around a cylinder,
fore and aft the pressure is high above and below the pressure is low. The fluid on
the downstream side is slowing down against an adverse pressure gradient and, at
the wall boundary, it slows down exactly to a standstill at the rear stagnation point.
In the real flow the boundary layer, which has already been slowed down by
viscosity, comes to a halt well before the stagnation point is reached and the flow

Boundary layer
outer edge
Increasing pressure

Dividing
streamline Wake: low velocity, low pressure

Point of separation where the normal
velocity gradient becomes zero
Figure A3.7 Separation of a Boundary Layer