51
2.1 Aerofoils
as well. This adds to the friction drag. The pressure drag is the resultant in the flow
direction of the pressure on the surface. The friction drag is the resultant of the shear
stress on the surface. Pressure drag is rather weak with attached flow, but separation
causes a strong increase. Moreover, the curvature of the streamlines at the suction
side is then strongly reduced. The consequence is that the pressure minimum at the
suction side is weaker than with attached flow. This explains the lift decrease after
separation.
2.1.3 Pressure Distribution
The pressure distribution for optimal operation of a NACA 4412 is sketched in
Fig. 2.5. At the leading edge, the velocity is near to zero and the pressure is near to
the stagnation pressure of the oncoming flow. The stagnation pressure or total pressure is the pressure obtained by bringing the flow to zero velocity in an adiabatic
reversible way. This means that kinetic energy is converted into pressure energy.
For constant density, the stagnation pressure is
2
1
o
2
p
p
v
r
∞
∞ ∞
=
+
. The pressure
is somewhat lower in the leading edge zone. The trailing edge pressure is typically
slightly higher than the pressure of the oncoming flow. The pressure coefficient
shown in Fig. 2.5 is defined by
2
1
p
2
C
( p p ) /
v
r
∞
∞ ∞
=
−
. It is common practice to
plot this coefficient with negative values to the upside in order to have the suction
side at the upside of the figure. At the pressure side of the aerofoil, the pressure is
uniformly decreasing. The boundary layer flow is accelerating everywhere. At the
suction side, flow accelerates at the leading edge, causing a pressure drop. Already
with a low aerofoil load (small angle of attack), the minimum pressure is lower than
the pressure of the oncoming flow (see Fig. 2.1). From the minimum pressure point
to the trailing edge, the boundary layer at the suction side is subjected to an adverse
pressure gradient.
Fig. 2.5 Pressure distribution over an aerofoil with attached flow (NACA 4412)
2.1 Aerofoils
as well. This adds to the friction drag. The pressure drag is the resultant in the flow
direction of the pressure on the surface. The friction drag is the resultant of the shear
stress on the surface. Pressure drag is rather weak with attached flow, but separation
causes a strong increase. Moreover, the curvature of the streamlines at the suction
side is then strongly reduced. The consequence is that the pressure minimum at the
suction side is weaker than with attached flow. This explains the lift decrease after
separation.
2.1.3 Pressure Distribution
The pressure distribution for optimal operation of a NACA 4412 is sketched in
Fig. 2.5. At the leading edge, the velocity is near to zero and the pressure is near to
the stagnation pressure of the oncoming flow. The stagnation pressure or total pressure is the pressure obtained by bringing the flow to zero velocity in an adiabatic
reversible way. This means that kinetic energy is converted into pressure energy.
For constant density, the stagnation pressure is
2
1
o
2
p
p
v
r
∞
∞ ∞
=
+
. The pressure
is somewhat lower in the leading edge zone. The trailing edge pressure is typically
slightly higher than the pressure of the oncoming flow. The pressure coefficient
shown in Fig. 2.5 is defined by
2
1
p
2
C
( p p ) /
v
r
∞
∞ ∞
=
−
. It is common practice to
plot this coefficient with negative values to the upside in order to have the suction
side at the upside of the figure. At the pressure side of the aerofoil, the pressure is
uniformly decreasing. The boundary layer flow is accelerating everywhere. At the
suction side, flow accelerates at the leading edge, causing a pressure drop. Already
with a low aerofoil load (small angle of attack), the minimum pressure is lower than
the pressure of the oncoming flow (see Fig. 2.1). From the minimum pressure point
to the trailing edge, the boundary layer at the suction side is subjected to an adverse
pressure gradient.
Fig. 2.5 Pressure distribution over an aerofoil with attached flow (NACA 4412)