Aerodynamic Analysis on Modified Horizontal-Axis-Wind-Turbine

The increasing of lift force is an important task during the design process of wind turbines.
This leads to a higher performance of wind turbines. The performance of wind turbines
can be increased via either active or passive flow controllers. The aim of this work is to
apply a passive flow controller, namely riblets, on the blades of National Renewable Energy
Laboratory (NREL) phase II. The thesis consists of two main bodies: smooth and tripped
study cases. The smooth study cases begin with studies on 2D aerofoils to investigate fluid
domain geometry and turbulence models’ effects on aerodynamic coefficients. A fixed blade
is then simulated to determine the 2D aerodynamic forces and the pressure distribution for
various angle of attacks (AOA). In addition, the torque of one-third scale rotor and full-scale
rotor of the NREL phase II are computed separately.
Unlike the previous work on riblets, the riblets size of current work is in order of boundary
layer thickness. First, a validation for large-scale riblets on DU 96-W-180 is performed to
compare the results. The tripped cases begin with applying a riblet (step) with h+ = 67-69
on 2D S809. However, the result is unsatisfactory due to missing 3D effect. Different riblet
configurations are then applied on the suction side of a 3D blade. The maximum increase
in lift and reduction in drag occur at l+ = 460 for alpha= 16. However, for a wider range of
AOAs, l+ = 104-157 show an improved performance. The results show that riblets perform
best at low Re and stall AOAs. Finally, riblets are placed on the blades’ suction side of the
NREL phase II. The riblets with l+ = 114 -140 show an improvement in the performance,
which varies between 1.32% and 0.51%, depending on tip speed ratio.
This work confirms that the large-scale riblets are not only able to reduce drag, but also can
increase lift, and therefore improve the performance of wind turbines. Unlike the small-scale
riblets, the large-scale ones act as a vortex generator and cause a recirculated zone in the
wake of large riblets, which leads to delaying stall to a further chord location of the blade. In
addition, and similar to small-scale riblets, the viscous sub-layer thickness of current tripped
cases is increased, thus causing an upward shift of the log-law region and a reduction of the
turbulence statistics. The drag reduction and lift increase depend on AOAs. However, the
riblets positively affect more the skin-friction force for a larger number of riblets at a wider
range of AOAs. This means the contribution of riblets in increasing the area of the blade is
negligible because of the small riblets size compared to the blade size.