Page 183 - Wind Energy Handbook

DEFINITION OF DRAG 157

u

du
Velocity variation
dy along a normal to
the surface
Frictional
shear stress τ

Figure A3.1 Creeping Flow Past a Circular Cylinder

Velocity variation
along a normal to
the surface

Figure A3.2 Inviscid Flow Pattern Around a Cylinder

The inviscid flow pattern around a cylinder (Figure A3.2) appears very similar to
that of creeping flow but the nature of the flow is very different indeed. By
definition inviscid flow causes no viscous drag but it also causes no pressure drag,
that is, drag caused by pressure forces aggregated over the whole surface area. The
pressure distribution for the inviscid flow past a cylinder is shown in Figure A3.3,
where the atmospheric pressure p 1 has been subtracted from the pressure around
the surface. The symmetry of the pressure distribution fore and aft shows clearly
that no pressure drag arises. At the nose of the body the flow is brought exactly to
rest and this is called the stagnation point. Another stagnation point occurs at the
rear of the body.