Abstracts - KTH Mechanics

Dynamic Lift of Airfoils
R. Grüneberger, T. Bohlen, S. Barth and J. Peinke ∗
For airfoils it is known that the lift coefficient cL depends not only on the angle of
attack α but also on the steadiness of the flow. Varying the angle of attack with an
angular velocity ω = ∂α/∂t the lift force for angles larger than the steady stall angle
clearly depends on ω, too,i.e.cL = f(α, ω). Variations of α and ω can be either due
to different pitch angles of the airfoil or due to fluctuations of amplitude and direction
of the flow as it is the case for wind turbines in atmospheric boundary layers.
We present wind tunnel measurements. In order to quantify the dynamic stall
effect, the lift of an FX79-W-151A airfoil, which is for use on wind turbines, is determined
by the integral pressure distribution at the wind tunnel walls while rotating
the airfoil with defined angular velocities ω. The rotation speed is varied by numeric
control. The pressure measurement is performed by two sets of 40 pressure sensors
at each side wall of the wind tunnel. The temporal resolution of the pressure measurement
is in the range of milliseconds. For stochastic analysis the experiment is
repeated several hundred times. In contrast to static lift values, there is an increase
(overshoot) of lift before flow separation on the suction side occurs, dependent on ω,
see Fig 1.
This knowledge of the maximum lift forces is relevant for the estimation of extreme
mechanical loads on wind turbine blades and therefore of special interest for wind
turbine manufacturers.
∗ ForWind - Center for Wind Energy Research - University of Oldenburg
Figure 1: Increase of the maximum lift coefficient cL with increase of the angular
velocity ω =˙α.
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