Wind Energy

40
Rotation and Turbulence Effects on a HAWT
Blade Airfoil Aerodynamics
Christophe Sicot, Philippe Devinant Stephane Loyer and Jacques Hureau
40.1 Introduction
Incident flows on wind turbines are often highly turbulent because they operate
in the atmospheric boundary layer (ABL) and sometimes in the wake of
other wind turbines. In the ABL, the turbulence level varies from 5% (offshore)
to 40% and length scale varies typically from 0.001 to 500 m or more [1]. The
blade aerodynamics is greatly influenced by rotation [2]. The flow on wind
turbine blade being often separated due to the angle of attack encountered
which depends on turbulence level, tower shadow or yaw misalignement. The
present study is focused on the coupled influence of turbulence and rotation
on the separation on the blade airfoil. After presenting the experimental
setup, the separation on the blade is studied from average pressure distributions.
The high turbulence levels lead to an important measurement dispersion.
This is the reason why, in a second part, the instantaneous pressure
distributions are analysed.
40.2 Experiment
The wind turbine, tested in the laboratory wind tunnel, was positioned in
a 4 m x 4 m section, 1 m downwind of a square 3 m x 3 m nozzle generating
a free open jet (Fig. 40.1). An isotropic and homogeneous turbulence, with
integral length scale of the order of magnitude of the chord length and three
different streamwise intensities (4.4%, 9% and 12%) was generated using a
square grid mounted in the wind tunnel nozzle (Fig. 40.1). The wind turbine
rotor diameter is 1.34 m, the blades have no twist, a NACA 654-421 airfoil,
and a 71 mm chord length. Surface pressure data have been obtained with one
blade section equipped with 26 pressure taps at three radial positions from
the hub : r
R =0.26, 0.51 and 0.75. Pressure were measured simultaneously on
the 26 taps at a frequency of 200 Hz during 10.25 s.