AbstractsPhysics

This thesis addresses the process of airfoil optimization for vertical axis wind turbines (VAWT). The airfoils are designed for large scale turbines above 5 MW. The VAWT concept is relevant for offshore floating wind energy, because of its low center of gravity (stability) and their simplicity (low maintenance). An optimal tip speed ratio of 4-4.5 is chosen with an average Reynolds number of 5 million. The solidity c/R of the turbine is 0.1. These operation conditions are representative for the new generation VAWT. The goal of this thesis is to develop an optimization process for VAWT airfoils and to demonstrate it by designing an airfoil, while taking into account airfoil soiling. A literature review presents the previous research in VAWT airfoil design, showing that no consensus has been previously reached about VAWT airfoil design. From the literature review an optimization objective derived by Simão Ferreira [30] is chosen. The airfoil is optimized for aerodynamic and structural performance. The aerodynamics is assessed on airfoil level according to the objective of lift slope over drag. Structurally, the airfoil will be optimized for flapwise bending stiffness. Airfoil soiling is simulated on the airfoil by using turbulent transition. A genetic optimization tool for airfoils coupled with RFOIL, an airfoil analysis tool, is used to generate VAWT airfoils. The objective function values are calculated using the aerodynamic coefficients from RFOIL and the geometric properties of the airfoils. The optimization process is validated by analyzing the results with three different models for full VAWT analysis. These models are: 1) an inviscid panel model coupled with RFOIL, 2) a double wake panel model and 3) a CFD model. Three airfoils resulting from the optimization are tested using the aerodynamic models. The performance of the airfoils validates the objective functions, but performance for the soiled case is not satisfactory. These preliminary findings were presented at the 33rd Wind Energy Symposiumat the AIAA SciTech conference [32], the full paper can be found in appendix B Five different strategies are developed to optimize airfoils. The results are analyzed using a double wake panel model. The optimization strategy in which airfoils are optimized for soiled conditions results in the best performing airfoils. The RK2-27 is a demonstration airfoil resulting from this optimization strategy. The CP of this airfoil for a tip speed ratio of 4 and a solidity of 0.1 is 0.53 in the clean case and 0.45 in the soiled case. This was determined by both the inviscid panel model and the double wake panel model. The RK2-27 has an increased CP compared to the NACA 0018 of 0.04 in the clean case at the design operating conditions. The CP in the soiled case is only 0.02 lower than the NACA 0018. The maximum thickness of the airfoil increased by 50% from 18% to 27%. The RK2-27 has similar aerodynamic performance compared to the traditionally used NACA 0018, while structurally it performs significantly better.