Study of distributed roughness effect over wind turbine airfoils performance using CFD

Abstract

CFD predictions have been carried out to study the aerodynamic behavior of wind turbine airfoils with distributed roughness over their surface. Wind turbines blades work in variable roughness surface conditions during their operational life, new or washed blades with very low roughness levels and blades that are contaminated by insects, dirt, dust or erosion. The existence of roughness over the blade surface generates a performance loss in the airfoil aerodynamics which understanding and accurate prediction is very important for wind turbine blade designers. In this paper, CFD calculations using the in-house compressible code WMB for several Reynolds numbers and roughness sizes are presented. Numerical data are compared to OSU experimental results for the NREL S809 wind turbine airfoil with locally distributed roughness over the airfoil leading edge. The study is completed with computations for the NACA0012 airfoil and a study of the boundary layer evolution with distributed roughness over the airfoil surface. WMB (Wind Multi Block) is a CFD method developed and validated by CENER and the University of Liverpool for wind turbine aerodynamics analysis (2D and 3D). It is capable of analysing compressible, RANS or URANS equations. In this study RANS equations have been solved. In addition, distributed roughness can be simulated using WMB. WMB has the capacity to simulate distributed roughness elements spread over a chosen area of the airfoil surface (upper or lower area, the whole airfoil or only an isolated zone). Nevertheless, this work is focused on airfoils with leading edge roughness calculations with turbulent flows. Roughness is included in WMB using two different approaches: Hellsten-Laine model and Knopp et al model. In Hellsten-Laine model the boundary condition for the specific dissipation rate is modified to account for the roughness layer that replaces the viscous sublayer. On the other hand in Knopp model a law of the wall is used to obtain values for k (turbulent kinetic energy) and ω at the wall. The main goal of this work is to calculate airfoil aerodynamics when roughness elements are distributed over its surface for different types of airfoils at high Reynolds numbers and including a sensitivity study to roughness parameters. Special attention will be paid to stall area prediction. In this work global airfoil magnitudes and local boundary layer magnitudes are studied numerically with WMB and compared with experiments. The final conclusion obtained from the study presented in this paper is that WMB is a valid tool to study airfoil aerodynamics when there are rough elements distributed over its surface.