Experimental and numerical study on the impact force of water droplets in the medium speed regime

Abstract. This study investigated the impact force exerted by a water droplet colliding with a solid surface at medium velocities. Raindrop impacts on wind turbine blades can cause erosion damage, reducing the power efficiency. Addressing this issue is of practical importance for wind energy. Previous studies have revealed that the droplet impacts can be scaled with inertial force, expressed as ρD2V2 , where ρ, D, and V are water density, droplet diameter, and impact velocity, respectively. This study examined whether this scaling can be applied under conditions representative of raindrop impacts on wind turbine blades, where the impact velocity is relatively high, ranging from 35 to 126 m/s. To this end, the impact force of millimeter-sized droplets was measured using a whirling arm rain erosion tester and a pressure sensor. Computational fluid dynamics simulations were conducted to model the experimental conditions. The simulation results, assuming an ideal spherical droplet, showed the scaling of ρD2V2 for the impact force. The experimental data generally followed the scaling law; however, the peak impact forces tended to be slightly higher. Images of the droplets before impact indicate that the droplets were significantly deformed by the high-speed airflow in the vicinity of the rapidly rotating blade. To accurately evaluate impact forces at moderate-to-high velocities, it is necessary to account for aerodynamic effects on the droplets rather than assuming a spherical shape.