Co-authors​ :

 Antonios Tempelis (DENMARK), Leon MISHNAEVSKY JR. (DENMARK) 

Composite structures are responsible for maintaining the structural integrity of wind turbine blades under harsh operational conditions. Apart from structural loads that are generated by wind and rotation, environmental loads in the form of rain droplets and solid particles also pose as a threat for the surface of blades. Impacts of such particles can occur at speeds equal to the tip speeds of blades (80-100 m/s) and are responsible for a phenomenon called leading edge erosion[1]. Erosion of the blade surface is caused by gradual failure of the protective coating material and over time, erosion progresses into the underlying composite laminate. An image of an eroded blade surface can be observed in Figure 1. The first effect of leading edge erosion is the reduced aerodynamic efficiency of blades that in turn reduces the energy production of turbines[2]. If no measure is taken for the repair of the protective coating layer before erosion reaches the composite substrates, repair costs could also increase significantly[3].
This work focuses on dynamic rain droplet impact simulations on the protective coating layer, often made from polymeric materials such as polyurethane[4], and on the development of a computational model that predicts surface erosion over time and the point when erosion reaches the composite substrate. The impact simulations are carried out with the finite element software ABAQUS, using a coupled Eulerian-Lagrangian framework to account for the large deformations of the water drop. The free surface of the raindrop is tracked with the volume of fluid method and an equation of state is used to consider compressibility of water. Special focus is placed on impacts with damaged coating surfaces or initial surface defects, in an effort to understand how damaged surfaces affect the erosion process. The output of the simulations, in the form of time-dependent stress values, is used in a computational model which calculates fatigue damage over the surface of the blade and predicts the depth of erosion through time of operation[5].
The main findings from impact simulations are that surface roughness can increase the stresses in the coating material due to local flow accelerations and interaction with surface anomalies, which lead to elevated erosion rates in damaged areas. Experimental data in the form of images from rain erosion test samples confirm the observations from the simulations. The current stage of development for the erosion prediction model is presented and the effect of different drop sizes and impact velocities on surface degradation is explored.