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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.

SUMMARYWe present an implementation of Hysing's (Int. J. Numer. Meth. Fluids 2006; 51:659-672) semi-implicit method for treating surface tension, for finite volume models of interfacial flows. Using this method, the surface tension timestep restriction, which is often very stringent, can be exceeded by at least a factor of 5 without destabilizing the solution. The surface tension force in this method consists of an explicit part, which is the regular continuum surface force, and an implicit part which represents the diffusion of velocities induced by surface tension on fluids interfaces. The surface tension force is applied to the velocity field by solving a system of equations iteratively. Since the equations are solved only near interfaces, the computational time spent on the iterative procedure is insignificant.

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