Over the past two decades, wind power has become an increasingly important source of electricity worldwide, and large, three-bladed turbines are a familiar sight. While most of those blades are formed as a simple airfoil, researchers have been studying whether new shapes could yield greater performance.
 


The team added winglets and tubercles to wind turbine blades to test their effect on the efficiency of wind turbines.
 
For instance, winglets—extensions to the tip to guide tip vortices rolling off the end of the wing in a manner that increases lift and reduce induced drag —have been used on aircraft for decades to improve performance. And recent research into tubercles, which are small protuberances such as the kind found on the flippers of whales, has shown some promising results in improving airfoil aerodynamics. The tubercles are thought to channel the airflow over the airfoil into more narrow streams, creating higher velocities and more lift, while at the same time reducing drag due to wingtip vortices.
 
A team at the University of Wisconsin-Milwaukee led by Ryoichi S. Amano has looked into the effect of adding both of these features onto a standard wind turbine blade. Their research was published in the ASME Journal of Energy Resources Technology in January 2023.
 
Using both computational fluid dynamics models and physical experiments in an on-campus wind tunnel, the team studied an 8-inch-wide, three-bladed turbine with a relatively new airfoil shape, one that features a higher lift-to-drag ratio, lower thickness, and more curvature than other commonly employed shapes. After getting baseline readings in the wind tunnel, the team developed a CFD model of an 80-foot-diameter turbine, and then modified the model to add winglets and tubercles.
 
The various configurations were simulated at wind speeds of 7.5 m/s, 12.5 m/s, and 17.5 m/s, or 17 mph, 28 mph, and 39 mph, respectively.
 
Compared to the unaltered airfoil, adding winglets improved power output and efficiency at all wind speeds. The best-performing winglet bent 60 degrees from the plane of the blade; turbines with blades featuring that winglet showed power improvement by between 8 percent and 10 percent.
 


The team tested five different sets of tubercles. Only the fourth one shown had any positive impact. Winglets angled at 30, 60, and 90 degrees were also tested.
 
The performance of the tubercles was not so promising. The team tried out five different configurations of various degrees of coarseness. Four of the five demonstrated much worse power production compared to the baseline blade. One set of small and finely spaced tubercles did improve the aerodynamics of the blades, though it was a fraction of what the winglets provided.
 
 

This model shows the vortices shed by the tip of the wind turbine blade as it turns in the wind. Since vortices produce drag on the airfoil, the goal of the research was to find shapes that kept vortices from shedding off the broad surface of the blade.

To test whether the combination of the two features might lead to better results, the team added the three winglet types to four of the tubercle configurations. For each wind speed, the performance of the winglet-tubercle combination was worse than that of the tubercle or the winglet by itself. The researchers concluded that the issue was that the mechanisms the two features used to improve aerodynamics interfered with each other. In their paper, the team wrote, “When tubercles are incorporated with a winglet on the same blade, the tubercle’s mechanism of enhancing the chordwise flow interferes with the ability of the winglet to move the tip vortex away from the rotor plane toward the wake’s downstream direction and hence the inefficient reduction of induced drag on the blade.”
 
Just as with commercial aircraft, it seems possible that wind turbine blades could sport winglets to improve performance in the coming years. But don’t expect to see bumpy blades anytime soon.
 
Jeffrey Winters is editor in chief of Mechanical Engineering magazine.