Wind Energy

Standard deviation of Cp
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.2 0.4 0.6 0.8 1
x/c
40 Rotation and Turbulence Effects on a HAWT 225
Tu = 4.5%
Tu = 9%
Tu = 12%
Standard deviation
Zone 1 Zone 2 Zone 3
Leading edge
ID
x / c
D
L
D Cp
Zone 4
Trailing edge
Fig. 40.6. Turbulence level effect on standard deviation and definition of displacement
zone from standard deviation curve
The separation point, D, is classically defined as the point where instantaneous
backfill occurs 50% of the time [6]. It may then be assumed to be the
location where the dispersion, σ, is maximum and D Cp may be considered as
the location where the flow is always separated which corresponds to the end
of the zone 3. The point incipent detachment (ID), at which instantaneous
backflow occurs 1% of the time [6], then corresponds to the beginning of the
zone 3. Thus, the standard deviation bump can be interpreted, in a first approach,
as a representation of the separation point displacement zone and the
maximum standard deviation as the middle of this zone.
40.4 Conclusion
Coupled effects of turbulence and rotation on wind turbine airfoil aerodynamics
have been studied. A stall delay has been observed for the highest turbulence
level. The pressure gradient in the separated area shows an increase
with the turbulence level. The instantaneous pressure distributions have been
analysed. It has been assumed that the length of the σ pressure coefficient
bump may be assimilated with the length of the separation point displacement
zone. The method proposed to detect the separation point has to be
validated with, for example, results issue from PIV or LDV experiments.
References
1. Kaimal J.C., Finnigan J.J., (1994) Atmospheric Boundary Layer Flows – Their
Structure and Measurement. Oxford University Press
2. Schreck S. (2004) Rotational Augmentation and Dynamic Stall in Wind Turbine
Aerodynamics Experiments. World Renewable Energy Congress VIII, Denver.
3. Corten G.P. (2001) Flow Separation on Wind Turbine Blades. Thesis, Utrecht
University, Utrecht