Abstract. Ice accretion on wind turbine blades causes both a change in the shape of its sections and an increase in surface roughness. These lead to degraded aerodynamic performances and lower power output. Here, a high-fidelity multi-step method is presented and applied to simulate a 3 h rime icing event on the National Renewable Energy Laboratory 5 MW wind turbine blade. Five sections belonging to the outer half of the blade were considered. Independent time steps were applied to each blade section to obtain detailed ice shapes. The roughness effect on airfoil performance was included in computational fluid dynamics simulations using an equivalent sand-grain approach. The aerodynamic coefficients of the iced sections were computed considering two different roughness heights and extensions along the blade surface. The power curve before and after the icing event was computed according to the Design Load Case 1.1 of the International Electrotechnical Commission. In the icing event under analysis, the decrease in power output strongly depended on wind speed and, in fact, tip speed ratio. Regarding the different roughness heights and extensions along the blade, power losses were qualitatively similar but significantly different in magnitude despite the well-developed ice shapes. It was found that extended roughness regions in the chordwise direction of the blade can become as detrimental as the ice shape itself.

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Wind turbines in cold climates are likely to suffer from icing events, deteriorating the aerodynamic performances of the blades and decreasing their power output. In this work, a 3-hour rime ice accretion event is numerically simulated on five significant sections of a wind turbine blade operating in steady wind using a high-fidelity procedure based on the Blade Element Momentum Theory. The onshore NREL 5MW reference wind turbine is studied. Ice accretion is simulated through a fine multi-step process, adding ice layers approximately 0.5 mm thick; each step consists of the successive coupling of a CFD simulation, a Lagrangian particle-tracking of the cloud droplets, an ice accretion step, and re-meshing of the new geometry. Ice roughness is modelled with an equivalent sand-grain approach. After computing the aerodynamic coefficients of ice-contaminated airfoils, power losses are obtained considering the aeroelastic response of the wind turbine in turbulent winds as defined by the Design Load Case 1.1. The effect of the extension of roughness on the surface of the blade is also assessed. In the considered operating conditions and accretion times, a strong dependence between the decrease in power output and the tip-speed ratio and a small dependence on surface roughness are found.

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