In the past decade, the power output of wind turbines has increased significantly. This increase has been primarily achieved through manufacturing larger blades resulting in high blade tip velocities and increased susceptibility to rain erosion. This paper is the first part in a two-part paper that presents a framework for the analysis of rain erosion in wind turbine blades. Two ingredients of the framework are presented. A stochastic rain texture model is developed to generate three-dimensional fields of raindrops consistent with the rainfall history at the turbine location by integrating the micro-structural properties of rain, i.e. raindrops size and spatial distribution with its integral properties such as the relationship between the average volume fraction of raindrops and rain intensity. An in-house GPU-accelerated computational fluid dynamics model of free-surface flows and a multi-resolution strategy are used to calculate the drop impact pressure as a function of time and space. An interpolation scheme is finally proposed to find the time evolution of impact pressure profile for any given drop diameter using the high fidelity simulation results, significantly reducing the computational cost. Other ingredients of the framework pertaining to drop impact-induced stresses and the blade coating fatigue life are presented in part II.

With the current trend in wind energy production, the need to manufacture larger wind turbine blades is on the rise. The high blade tip velocities associated with large blades subject them to various damages due to high speed impact with foreign objects such as raindrops. This paper is the second part in a two-part paper that presents a framework for rain erosion prediction in wind turbine blades. In part I, a stochastic rain texture model and multi-resolution simulation of raindrop impact on solid object were discussed. In part II, the predicted impact pressure profiles are imported into a finite element model of the wind turbine blade shell to analyze the raindrop impact-induced transient stresses within the blade coating and the ensuing fatigue damage pattern. The analysis is complemented with a fatigue stress-life estimation process that integrates elements of fatigue life calculation with 3D fields of raindrops generated from the stochastic rain texture model to relate damage accumulation rates to rain intensities. These accumulation rates, together with the statistics of rainfall history, provide a means for estimating the expected fatigue life of the blade coating as an indication of the onset of surface roughening or the end of the incubation period.

We present a computational study on the dynamics and freezing of micron-size water droplets impinging onto super-hydrophobic surfaces, the temperatures of which are below the freezing point of water. Icing poses a great challenge for many industries. It is well known that increasing hydrophobicity can make a surface ice-phobic. Experiments show that millimeter size water drops landing on super-hydrophobic surfaces bounce off even when the surface temperature is well below the freezing point. However, it has been reported that the ice-phobicity feature of such surfaces can vanish due to frost formation on the surface, or when small micro-droplets begin to freeze and stick to the surface. Using an in-house, 3D, GPU-accelerated computational tool, we investigated the impact dynamics and freezing of a 40 μm water droplet impinging at 1.4 m/s onto two different super-hydrophobic surfaces chosen from [1]. The advancing and receding contact angles are 165° and 133°, respectively, on one surface, and 157° and 118°, respectively, on the other. The surface and initial droplet temperatures were varied from −25 to 25°C and from 0 to 25°C, respectively. On each surface a “transition” surface temperature was found, at which the drop behavior transitions from bouncing off the surface to sticking. The time between drop landing and bounce-off as well as the contact diameter between the stuck drop and the surface both increase with decreasing the surface temperature. The simulations also show that at some surface temperatures a thin ice layer forms during droplet spreading and then remelts as the droplet recoils.