In conjunction with the increasing interest in renewable energy as an alternative to fossil fuels, researchers have continually focused on wind energy industry in a bid to increase the power output of wind turbines. This has prompted production of more wind turbines with higher power output. Increasing the blade size is a primary method for enhancing the turbine’s power output which has resulted in blade tip velocities of up to 120m/s.
With the high blade tip velocities, high susceptibility to erosion comes in, particularly in harsh environments such as regions with heavy rain and hail. Rain erosion of the blades initiates with a consistent rise in the blade’s surface roughness until small pits form close to the leading edge. With time, the density of the resulting pits increase until gouges form. An increase in surface roughness of the blades translates to a rise in aerodynamic drag coefficient that consequently leads to low performance as well as energy loss.
University of Massachusetts Dartmouth researchers, led by Dr. Mazdak Tootkaboni from the department of civil engineering, have recently published a two-part research paper on predicting rain erosion in wind turbine blades. The aim of the research was to integrate well-established theories as well as computational models to come up with a framework that could approximate the expected erosion lifetime of a selected blade, given rainfall history at a particular region, blade shell attributes, and operational conditions of the wind turbine. These papers are published in the Journal of Wind Engineering and Industrial Aerodynamics.
As a first step, the research team developed a stochastic model of rain texture that was capable of relating the integral attributes of rain, for instance, rain intensity and average volume of water per unit volume of air to its micro-structural attributes including raindrop sizes as well as their spatial distribution. The model allowed for the reproduction of three-dimensional fields of raindrops.
Temporal and spatial variations of the impact pressure developed in droplet-surface collision were then computed using a GPU accelerated CFD model of free surface flows. The authors proposed a multiresolution method in a bid to minimize computational cost and an interpolation scheme to compute the impact pressure profile for any drop size efficiently and accurately.
The final component of the proposed framework entailed the computation of fatigue damage for every raindrop by undertaking a stress analysis of its collision with the coating surface and probabilistically integrating these fatigue damages to estimate the “expected” erosion life time of the coating. The authors computed the stresses through a finite element modeling of the drop impact, where the droplet impact pressure, already calculated from CFD analysis of rain drop impact on the coating surface, was applied on the surface as a spatially varying time dependent external load.
Amirzadeh, A. Louhghalam, M. Raessi, M. Tootkaboni. A computational framework for the analysis of rain-induced erosion in wind turbine blades, part I: Stochastic rain texture model and drop impact simulations. Journal of Wind Engineering & Industrial Aerodynamics, volume 163 (2017), pages 33–43.Go To Journal of Wind Engineering & Industrial Aerodynamics
Amirzadeh, A. Louhghalam, M. Raessi, M. Tootkaboni. A computational framework for the analysis of rain-induced erosion in wind turbine blades, part II: Drop impact-induced stresses and blade coating fatigue life. Journal of Wind Engineering and Industrial Aerodynamics, Volume 163, (2017), Pages 44–54.Go To Journal of Wind Engineering & Industrial Aerodynamics