Innovative Flow Control for Flatback Airfoils
Authors:   , Cene Antonios 
Published: February 2025

As wind turbines grow larger to reduce the levelized cost of energy, their blades grow more slender and require significantly thicker airfoils. However, such airfoils have increased sensitivity to tripping and are very prone to flow separation. One solution to the challenges caused by the very thick traditional airfoils is the use of flatback (FB) airfoils. One method to further increase the performance of FB airfoils is by means of passive flow control devices.
This thesis investigates the aerodynamic and hydrodynamic performance of flatback airfoils, focusing specifically on the FFA-W3-360-FB20 model, to assess its potential applications in enhancing wind turbine technology. With the growing demand for sustainable energy solutions, optimizing the design of wind turbine blades has become increasingly critical for improving their efficiency and performance. This research is grounded in a comprehensive literature review that underscores the significance of aerodynamic optimization in the design of wind turbine blades, particularly under conditions of high Reynolds numbers.
The study employs a dual methodology that combines computational fluid dynamics (CFD) simu­lations with experimental wind tunnel testing. The experimental methodology involved assessing the aerodynamic performance of flatback airfoils, specifically the FFA-W3-360 and FB20 models, in a con­ trolled wind tunnel environment. Aerodynamic forces were measured using a two-force balance system to determine lift and drag coefficients, while pressure taps along the airfoil provided detailed pressure data for aerodynamic calculations. Additionally, microphones were strategically placed to capture vor­tex shedding acoustic measurements, and pressure transducers monitored real-time pressure fluctuations on airfoil TE. The setup facilitated both fixed and free transition cases, with testing conducted at a Reynolds number of approximately 1.8 million to ensure accurate performance evaluation across the different configurations. This approach allows for a detailed analysis of the flow characteristics and per­formance metrics offlatback airfoils. On the computational side, a 2D Reynolds-Averaged-Navier-Stokes equations was used along with an optimisation study for novel flap configurations.
The results reveal a significant 42.6% increase in the Coefficient of Lift (Cl) and an 87.5% increase in the Coefficient of Drag (Cd) for the flatback airfoil when compared to the traditional thin TE airfoil at a 0° angle of attack. The flatback airfoil's performance enhancements can be attributed to its reduced sensitivity to tripped flow. Moreover, a low drag regime within the linear region (0° to 8°), consistent with findings from existing literature is identified.
Several flow control devices were tested to evaluate their impact on the aerodynamic performance of flatback airfoils, with particular focus on the novel Upperflap and Lowerflap configurations, which have not been previously tested on flatback airfoils. The Upperflap configuration emerges as the most effective device, achieving a maximum Cl/Cd ratio of 27.8 at a 7° angle of attack and a 112.6% increase in Cl at 0° with fixed transition conditions. This device demonstrates a 20 dB/Hz reduction in vortex shedding amplitude on the suction side and a 7 dB/Hz reduction on the pressure side, indicating its effectiveness in enhancing flow stability and reducing drag.
The Lowerflap, which yields a 141.7% increase in Cl at 0° but results in an 118.6% increase in Cd, suggesting its suitability for applications where lift is prioritized over drag. Furthermore, the Gurney flap configuration exhibits a 34% increase in Cl and a 44% increase in Cd at 11°. However, increasing the Gurney flap height from 5% to 8% chord leads to higher lift but also increased drag, illustrating the inherent trade-offs in flap design. Additionally, vortex generators placed on the suction side were found to impair performance, while pressure-side VGs effectively restored performance levels to those of free transition when fixed.
The results from the CFD simulations exhibit good agreement in the trends with the experimental findings for both novel flow control devices, confirming their reliability in predicting aerodynamic per­formance. This alignment is particularly notable in terms of performance trends, where the CFD models effectively replicate the observed increases in lift and changes in drag associated with the upper and lower flap configurations.
Moreover, the research examines combinations of these flow control devices, revealing that the upper flap in conjunction with a Gurney flap (8% height) and vortex generators (40% chord) yields the best overall performance in terms of lift, achieving a remarkable 298% increase in Cl compared to the flat back case and a 499% increase compared to the thin TE airfoil. Near-field hydrodynamic measurements provide critical insights into flow patterns and vortex shedding characteristics associated with these configurations, enhancing the understanding of their performance in practical applications.
In conclusion, the present investigation highlights the advantages and disadvantages of flatback air­foils and examined various flow control devices in achieving superior aerodynamic performance, offering a robust framework for future investigations aimed at optimizing wind turbine blade designs. The in­ sights gained from this work not only advance the field of aerodynamic optimization but also promote the development of more effective and sustainable wind energy technologies, contributing significantly to the quest for efficient renewable energy solutions.