Let us discuss few important features in an external flow past an airfoil (Fig. 5.7.1) where the flow is dominated by inertial effects. Some of the important features are highlighted below;

• •  The free stream flow divides at the stagnation point.

•  The fluid at the body takes the velocity of the body (no-slip condition).

•  A boundary layer is formed at the upper and lower surface of the airfoil.

•  The flow in the boundary layer is initially laminar and the transition to turbulence takes place at downstream of the stagnation point, depending on the free stream conditions.

•  The turbulent boundary layer grows more rapidly than the laminar layer, thus thickening the boundary layer on the body surface. So, the flow experiences a thicker body compared to original one.

•  In the region of increasing pressure (adverse pressure gradient), the flow separation may occur. The fluid inside the boundary layer forms a viscous wake behind the separated points.

Fig. 5.7.1: Important features in an external flow.