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Akther, Najiba

University of British Columbia

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 0.5×10⁶and an angle of attack of 11.19°. Second, a Natural Laminar Flow (NLF-416) airfoil is simulated for a chord Reynolds number of 0.5 × 10⁶ and an angle of attack of 10.18°. 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.

Text

2023-05-29

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/84714

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/