Downwind Rotor: Studies on yaw Stability and Design of a Suitable Thin Airfoil

Abstract

The advantages of the downwind rotor for a wind turbine are explored as design solution for large wind turbines offshore. One of the key solutions of decreasing the cost of offshore wind energy is increasing rotor size. Large conventional rotors could cause structural problems to the blades themselves and the other turbine components. A downwind turbine configuration could allow the use of more slender and hence lighter blades.

A general assessment of the benefits of large size wind turbines for offshore application is initially investigated. The advantages of the downwind configuration for large rotors in terms of improved yaw stability and weight saving are presented.

Initially the yaw stability of the downwind rotor is compared to the more classic upwind configuration. Experimental and computational results of yaw moments are presented for fixed yaw angles of ±10;±20 and ±30 degrees, for all operational tip speed ratios.

In the second part of the thesis the design of a new and thin airfoil is presented. The method used was an inverse design using the routine of Xfoil. The airfoil is intended to be used for the outer part of a slender blade. The main design objectives were: high lift-to-drag ratio for a wide range of angles of attack, smooth stall and leading edge roughness insensitivity of maximum lift coefficient. These characteristics should ensure high power output.

A model of the airfoil was later built and tested in the wind tunnel at NTNU. The wing was shaped as a two-dimensional model, hence with constant chord spanning the whole wind tunnel width. The results are corrected due to wind tunnel boundary effects and a uncertainty analysis is included. The results are compared with the prediction obtained with Xfoil.

The insensitivity to leading edge roughness was tested by applying stripes of sand of different grain sizes at several chord locations. The effect of inflow turbulence was tested by introducing a grid in the wind tunnel test section.