Thesis Defence: Computational evaluation of leading-edge droops for performance enhancement of wind turbine rotors

Najiba Akther, supervised by Dr. Joshua Brinkerhoff, will defend their thesis titled “Computational evaluation of leading-edge droops for performance enhancement of wind turbine rotors” in partial fulfillment of the requirements for the degree of Master of Applied Science in Mechanical Engineering.

An abstract for Najiba’s thesis is included below.

Examinations are open to all members of the campus community as well as the general public. Registration is not required for in-person defences.

---

ABSTRACT

This thesis evaluates a solution to leading-edge erosion based on droop attachments bonded to sub-MW turbine blades with a focus on leading-edge transition delay that also yields notable aerodynamic improvements at the intermediate Reynolds numbers that characterize sub-MW turbines. This thesis presents a detailed investigation of the transition process from a series of high-fidelity large eddy simulations (LES) to evaluate the physical basis of the aerodynamic benefits of the droop attachments. The incompressible flow over two sub-MW airfoils is evaluated in drooped and non-drooped conditions. First, a National Renewable Energy Laboratory (NREL) S822 wind turbine airfoil is simulated for a chord Reynolds number of 500,000 and an angle of attack of 11.2 degrees. Second, a Natural Laminar Flow (NLF-416) airfoil is simulated for a chord Reynolds number of 500,000 and an angle of attack of 10.18 degrees. For each, the selection of the best-performing droop shape is conducted using the Boundary Element Momentum theory. Comparisons of drooped and non-drooped airfoils under the same conditions are objectively performed, evaluating the aerodynamic forces and boundary layer parameters. The modified S822 airfoil shows an approximately 15% increment in glide ratio over the base airfoil, which is a result of a delay in the onset and progression of laminar-to-turbulent transition on the suction surface. The instability amplification mechanisms in the suction-surface boundary layers are compared, showing that the boundary-layer transition is dominated by the inviscid Kelvin-Helmholtz (K-H) instability for both cases. However, a secondary inner disturbance mode was only found for the non-drooped airfoil, which triggers a much earlier transition and accelerates breakdown to 3D turbulence within 1-2 wavelengths of the primary disturbance mode, while the drooped airfoil shows a delayed transition onset and a more gradual breakdown to 3D turbulence. Unsteady and dynamic reattachment followed by a stable turbulent separation near the trailing edge is identified for both configurations. In contrast, the drooped NLF-416 airfoil sees a negligible improvement in aerodynamic performance, which is attributed to the gradual transition inception that occurs on the base NLF-416 airfoil. The physical basis for these differences is explored in the context of linear stability theory to guide further optimization of leading-edge protective droops.