The Aerodynamics Of The Wind Turbine – Part 5

THE STALL PHENOMENA

The diagrams showing the components of lift and drag illustrate the result of stall. Lift diminishes and drag increases at angles of attack over 15 degrees. The diagrams however do not illustrate the reasons for this stall phenomena.

A stall is understood as a situation during which an angle of attack becomes

so large that the air flow no can longer flow smoothly, or laminar, across the

profile. Air looses contact with the rear side of the blade, and strong turbulence occurs. This separation of air masses normally commences progressively from the trailing edge, so the profile gradually becomes semi-stalled at a certain angle of attack, but a full stall is first achieved at

a somewhat higher angle. From the diagram showing the lift and drag components, one can estimate that the separation at the trailing edge starts at about 12 degrees, where the curve illustrating lift starts to fall. The profile

is fully stalled, and the air flow is separated all over the rear side of the

blade at about 20 degrees.

These figures can greatly vary from profile to profile and also between different thicknesses of the same profile.

When the stall phenomena is used to restrict power output, as in all Bonus

wind turbines, it is important that blades are trimmed correctly. With the steep lift curve, the angle of attack cannot be altered very much, before maximum output also changes, therefore it is essential that the angle of the blade is set at the correct value.

One cannot alter the different angles on the blade itself, once the form, shape and blade molding has been decided upon and fabricated. So we normally talk about calibrating the tip angle. Not because the blade tip has any special magical properties, but we can place a template at the tip, which allows us to

make measurements using a theodolite.

Adjusting of the tip angle can therefore be understood as an example of how the angle of the total blade is adjusted.

Of importance for power output limitation is also the fact that in practice lift and drag normally behave exactly as would be expected from the theoretical

calculations. However this is not always the case. Separation can often occur

before expected, for instance due to dirt on the leading edges, or it can be delayed if the air flow over the profile for some reason or other, is smoother than usual.

When separation occurs before expected, the maximum obtainable lift is not as high as otherwise expected and therefore maximum output is lower. On the other hand, delayed separation can cause continuous excessive power production output.

Accordingly profile types chosen for our blades have stable stall characteristics with little tendency to unforeseen changes. From time to time, however, it is sometimes necessary to actively alter the stall process. This is normally done by alteration to the leading edge, so that a small well-defined extra turbulence across the profile is induced. This extra turbulence gives a smoother stall process.

Turbulence can be created by an area of rougher blade surface, or a triangular strip, fixed on the leading edge. This stall strip acts as a trigger for the stall so that separation occurs simultaneously all over the rear side.

On a wind turbine blade, different air flows over the different profile shapes,

interact with each other out along the blade and therefore, as a rule, it is only necessary to alter the leading edge on a small section of the blade. This altered section will then produce a stall over the greater part of the blade. For example, the Bonus 450 kW Mk III turbine, is usually equipped with a 0.5 meter stall strib, which controls the stall process all over the 17 meter long blade.

SUMMARY

The main points as described in this article can be shortly stated in the

following:

¥ The air flow around a wind turbine blade is completely dominated by the head wind from the rotational movement of the blade through the air.

¥ The blade aerodynamic profile produces lift because of its streamlined shape. The rear side is more curved than the front side.

¥ The lift effect on the blade aerodynamic profile causes the forces of the air to point in the correct direction.

¥ The blade width, thickness, and twist is a compromise between the need for streamlining and the need for strength.

¥ At constant shaft speed, in step with the grid, the angle of attack increases with increasing wind speed. The blade stalls when the angle of attack exceeds 15 degrees. In a stall condition the air can no longer flow smoothly or laminar over the rear side of the blade, lift therefore falls and drag increases.