Airfoil optimization for stall regulated vertical axis wind turbines

Abstract

Multi-megawatt Vertical Axis Wind Turbines (VAWTs) have inherent design and operational advantages that make them relevant for floating offshore applications. Many offshore VAWT concepts use stall for power regulation. Stall regulation imposes several constrains in the aerodynamic, structural and generator design. Due to the azimuthally varying and unsteady aerodynamics experienced by a VAWT, designing an airfoil for stall regulation is still a significant challenge. In this work, a family of airfoils for stall regulated VAWT is defined through numerical airfoil optimization, based on the original work of Simao Ferreira and Geurts [20]; the optimization is multi-objective, optimizing structural and aerodynamic performance. The control performance is not yet implemented in the function. The optimization is performed at Reynolds numbers representative of multi-megawatt VAWT. The performance of the VAWT, including operation in dynamic stall, is evaluated with an unsteady double-wake viscous-inviscid panel method and CFD simulations. The performance of the optimized airfoils is compared against a conventional airfoil, namely the NACA 0018 airfoil. The stall regulatory performance is assessed to provide insight for future optimizations. The aerodynamic performance is evaluated using three different numerical models: a single wake panel model coupled to airfoil polar data; a viscous-inviscid double wake panel model, capable of simulating dynamic stall; and an eulerian RANS CFD model. The airfoils are also design for a robust performance in the case of surface roughness. The preliminary results show that the design for surface roughness conflicts with the design for dynamic stall control. An extensive study of multi-objective optimisations with different weights of the different elements of aerodynamic performance is presented.