How leading edge roughness influences rain erosion of wind turbine blades?

Rain erosion of wind turbine blades is observed very commonly and is a concerning issue. Damage on the leading edge of blades leads to loss of aerodynamic efficiency. The mechanisms of damage initiation are not yet fully understood. This study focuses on the effect of existing surface damage or roughness of the leading edge of blades on impact stresses and erosion rates for the protective coating layer. The main mechanisms of interaction between impacting rain droplets and surface irregularities of a polyurethane-based coating are examined through finite element simulations and high-stress areas are identified. The results of the simulations reveal that areas inside erosion pits experience larger von Mises stresses than other undamaged areas. This could be explained by increased velocity as the water flows into erosion pits, observed in the simulations. At the same time, impacts on rough and damaged areas cause large stress values over a larger area than impacts on flat surfaces, due to the interaction with surface damage. These two observations could be the cause for faster erosion rates near damaged areas, as observed in experimental images during rain erosion testing. The predicted elevated stresses due to initial roughness or pre-existing damage could lead to faster damage growth, and to erosion pits growing wider and deeper.