Copyright © A. Filippone (1999-2003). All Rights Reserved.

Rotary Wing Aerodynamics

Wind Turbines


Classification Details

Wind turbines are the most recent technology used for wind energy conversion. Rudimental wind mills have been used in Europe for centuries. The actual systems consists of a mast, a number of freely rotating blades, a power generation system, and various controls (Hansen-Butterfield, 1993).

We will discuss horizontal-axis wind turbines (HAWT), which make up most of the systems in operation today. These machines can be classified according to the position of the mast (upwind or downwind), to the blade articulation (rigid or teetering), aerodynamic control system (stall or pitch regulated), and number of blades.

General Performances

Wind turbines have some peculiar aerodynamic problems. The flow is unsteady most of the time, with combined effects of wind shear, atmospheric turbulence, gusts, yaw mis-alignment, tower shadow.

Three-dimensional effects are also interesting. It has been known for some time that two-dimensional airfoil data are unsuitable for predicting the aerodynamic performances on stall-regulated turbines.

The rotation of the blades delays the static stall, because the centrifugal pumping results in Coriolis acceleration terms that induce favourable pressure gradients. The result is that boundary layer separation is delayed.

Fig. 1 below shows an example of 2D wind tunnel data, as compared with data taked at specified span sections on a rotating blade. Stall does not seem to occur. The normal force coefficient Cn is actually increasing toward large values at large angles of attack.

Normal Force Coefficient
Figure 1: Normal Force Coefficient

Similar effects appear on thrust and torque coefficients (one example shown in Fig. 2). This problem makes it difficult to predict the peak power.

Thrust Coefficient
Figure 2: Thrust Coefficient

Airfoil selection is important in optimizing the rotor performance (which in the general case is quite a complicated problem.) Latest research shows that stall-regulated turbines work best with thin outboard sections with limited CLmax and smooth static stall. Inboard sections must be thick (well above 30%, it turns out).

Aerodynamic Controls

The peak power from a wind turbine can be controlled by letting the blades stall (stall regulated machines), or by pitch adjustment (pitch controlled machines), to allow for the best inflow angles. One obvious difference is that the pitch regulated machines need sophisticated mechanical installations, but the performances do have their own impact.

By letting the blades stall, there is the problem of predicting both the unsteady loads (dynamic stall), and three-dimensional rotating effects.

Related Material

Selected References

  • Spera AD. (editor), Wind Turbine Technology, ASME Press, 1994.

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Copyright © A. Filippone (1999-2003). All Rights Reserved.