58
2 Basic Components
2.1.6 Profile Shapes
The optimal profile shape strongly depends on the particular application. The
NACA 4412 shape used in Fig. 2.3, is, as already mentioned, rather suitable for
applications in pumps and hydraulic turbines. This profile was used frequently in
older designs. The intended flow deflection is small, the Reynolds number moderately high to high, i.e. in the order of 1 to 5 10
6
and with a high degree of turbulence within the main flow. There first is flow acceleration at the suction side,
which keeps the boundary layer laminar (the laminar boundary layer becomes
more stable in accelerating flow). Deceleration follows after that. With a high
Reynolds number and high turbulence in the main flow, the boundary layer quickly becomes turbulent when subjected to an adverse pressure gradient (the laminar
boundary layer becomes less stable in decelerating flow). This is intended, as a
turbulent boundary layer better resists separation than a laminar one. An aerofoil
shape with a pressure distribution as sketched in Fig. 2.5 is therefore termed a turbulent profile, which means that the boundary layer at the suction side is turbulent
on a large part of it. The profile shape is not optimal for applications in pumps and
hydraulic turbines. The predominantly turbulent boundary layer at the suction side
allows a high adverse pressure gradient. Consequently, the attainable lift coefficient is high. But the drag coefficient is rather high as well, due to the higher friction than with laminar flow. Another disadvantage is the strong pressure minimum
at the suction side, which may cause cavitation (local water evaporation) with
some applications. Modern profile shapes have a less deep suction pressure minimum. The laminar part of the leading edge boundary layer is larger, generating a
lower drag coefficient. Nowadays, these profiles are designed with computational
fluid dynamics techniques.
Figure 2.8 shows an aerofoil and the accompanying pressure distribution for
the middle part of a wind turbine blade [5]. The pressure distribution is completely
different from the NACA 4412. Flow circumstances are here: low deflection, slight
acceleration within the general flow, rather high Reynolds number, i.e. 3–5 10
6
and
low turbulence in the main flow. Strong acceleration occurs at the leading edge part
of the suction side, followed by a zone of weak acceleration. The boundary layer
stays laminar until the end of the weak acceleration zone. This succeeds due to the
low turbulence in the oncoming flow. From the beginning of the deceleration phase,
the boundary layer becomes turbulent here as well. This succeeds due to the rather
high Reynolds number. At first, acceleration occurs at the pressure side due to the
high aerofoil thickness. Then follows a weak deceleration intended to increase the
pressure difference between the pressure and the suction sides again. The obtainable
lift coefficient is moderately high (∼ 1.00). The drag coefficient is low (∼ 0.0075), as
the boundary layer at the suction side is laminar over a rather significant part. The
aerofoil shape is termed a laminar profile. The shape is similar to profiles applied
in wings of aircraft.
Present-day aerofoil profiles use the features of the profile shape of Fig. 2.8:
strong acceleration followed by weak acceleration at the suction side leading edge,
2 Basic Components
2.1.6 Profile Shapes
The optimal profile shape strongly depends on the particular application. The
NACA 4412 shape used in Fig. 2.3, is, as already mentioned, rather suitable for
applications in pumps and hydraulic turbines. This profile was used frequently in
older designs. The intended flow deflection is small, the Reynolds number moderately high to high, i.e. in the order of 1 to 5 10
6
and with a high degree of turbulence within the main flow. There first is flow acceleration at the suction side,
which keeps the boundary layer laminar (the laminar boundary layer becomes
more stable in accelerating flow). Deceleration follows after that. With a high
Reynolds number and high turbulence in the main flow, the boundary layer quickly becomes turbulent when subjected to an adverse pressure gradient (the laminar
boundary layer becomes less stable in decelerating flow). This is intended, as a
turbulent boundary layer better resists separation than a laminar one. An aerofoil
shape with a pressure distribution as sketched in Fig. 2.5 is therefore termed a turbulent profile, which means that the boundary layer at the suction side is turbulent
on a large part of it. The profile shape is not optimal for applications in pumps and
hydraulic turbines. The predominantly turbulent boundary layer at the suction side
allows a high adverse pressure gradient. Consequently, the attainable lift coefficient is high. But the drag coefficient is rather high as well, due to the higher friction than with laminar flow. Another disadvantage is the strong pressure minimum
at the suction side, which may cause cavitation (local water evaporation) with
some applications. Modern profile shapes have a less deep suction pressure minimum. The laminar part of the leading edge boundary layer is larger, generating a
lower drag coefficient. Nowadays, these profiles are designed with computational
fluid dynamics techniques.
Figure 2.8 shows an aerofoil and the accompanying pressure distribution for
the middle part of a wind turbine blade [5]. The pressure distribution is completely
different from the NACA 4412. Flow circumstances are here: low deflection, slight
acceleration within the general flow, rather high Reynolds number, i.e. 3–5 10
6
and
low turbulence in the main flow. Strong acceleration occurs at the leading edge part
of the suction side, followed by a zone of weak acceleration. The boundary layer
stays laminar until the end of the weak acceleration zone. This succeeds due to the
low turbulence in the oncoming flow. From the beginning of the deceleration phase,
the boundary layer becomes turbulent here as well. This succeeds due to the rather
high Reynolds number. At first, acceleration occurs at the pressure side due to the
high aerofoil thickness. Then follows a weak deceleration intended to increase the
pressure difference between the pressure and the suction sides again. The obtainable
lift coefficient is moderately high (∼ 1.00). The drag coefficient is low (∼ 0.0075), as
the boundary layer at the suction side is laminar over a rather significant part. The
aerofoil shape is termed a laminar profile. The shape is similar to profiles applied
in wings of aircraft.
Present-day aerofoil profiles use the features of the profile shape of Fig. 2.8:
strong acceleration followed by weak acceleration at the suction side leading edge,