Viscous-inviscid interaction technique for prediction of aerodynamic characteristics of HAWT blade S809 airfoil

Predicting the HAWT blades loads accurately is one of the most important parts in the wind turbine aerodynamics calculation, and is based on an accurate evaluation of the aerodynamic coefficients and of the upstream velocity field. However, these aerodynamic loads estimations remain a complicated task to perform due to the complex nature of the flow around the blades, and to their cyclical fluctuations, which can be a significant source of fatigue and vibration, resulting from the yaw, stall or turbulent conditions where the HAWT operate mostly. The main aim of the present paper is to develop a sophisticated computational model that can predict aerodynamic coefficients without using great amounts of computer time. This model is based on a strong viscous-inviscid interaction technique using the blowing velocity concept. The inviscid air flow around S809 is modeled using the potential theory of arbitrary wing sections and Prandtl-Glauert compressibility correction, the 2-D pressure potential coefficients calculations for any orientation and Reynolds number, were made. Whereas the viscous part is modeled by using the Von Karman momentum integral equation, which is resolved according to the flow regime, laminar or turbulent, the onset of flow transition governed by the viscous effects was determined by using Michel's criterion, and the boundary layer separation point location is determined from the shape factor by using Head's criterion. The numerical results of aerodynamic characteristics of HAWT blade S809 airfoil have been benchmarked against experiments for different angle of attack and Reynolds numbers, and generally a good agreement is obtained.