Citation: Yassin, K.; Kassem, H.; Stoevesandt, B.; Klemme, T.; Peinke, J. Numerical Simulation of Roughness Effects of Ice Accretion on Wind Turbine Airfoils. Energies 2022, 15, 8145. https://doi.org/10.3390/ en15218145 Academic Editor: João Carlos de Campos Henriques Received: 15 June 2022 Accepted: 28 August 2022 Published: 1 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Numerical Simulation of Roughness Effects of Ice Accretion on Wind Turbine Airfoils Khaled Yassin 1, * ,† , Hassan Kassem 2 , Bernhard Stoevesandt 2 , Thomas Klemme 3,‡ and Joachim Peinke 1 1 ForWind-Centre for Wind Energy Research, Institute of Physics, University of Oldenburg, 26129 Oldenburg, Germany 2 Fraunhofer Institute for Wind Energy Systems, Küpkersweg 70, 26129 Oldenburg, Germany 3 Senvion GmbH, Überseering 10, 22297 Hamburg, Germany * Correspondence: khaled.yassin@uni-oldenburg.de † Current address: Institute of Energy and Climate Research (IEK-14), Forschungszentrum Juelich GmbH, 52425 Juelich, Germany. ‡ Current address: Nordex Energy SE & Co., KG, Service Product Management, Langenhorner Chaussee 600, 22419 Hamburg, Germany. Abstract: One of the emerging problems in modern computational fluid dynamics is the simulation of flow over rough surfaces. A good example of these rough surfaces is wind turbine blades with ice formation on its leading edge. Instead of resolving the airflow field using a fine computational grid near the wall, rough wall functions (RWFs) can be used to model the flow behavior in case of the presence of roughness. This work aims to investigate the performance of state-of-the-art RWFs to show which of these models can provide the most accurate results with the lowest computational cost possible. This aim is achieved by comparing coefficients of lift and pressure resulting from CFD simulations with wind tunnel results of an airfoil with actual ice profiles collected from the site. The RWFs are used to simulate airflow field over the airfoil profiles with ice profile attached to its leading edge using OpenFOAM CFD framework. The comparison of the numerical simulations and the wind tunnel measurements showed that the Colebrook RWF provided the best agreement between simulation and experimental results while using about 20% of the number of cells used with smooth RWF. Keywords: ice accretion; cold climate; wind turbine 1. Introduction For the last few decades, energy generation from wind has increased rapidly as a renewable source of energy. Because of this rapidly increasing demand for wind energy, wind turbine manufacturers have been trying to increase the size of the turbine rotors. By increasing rotor diameter and tower height, the single turbine can now harvest more power from wind in almost the same horizontal area occupied by old, small size wind turbines. However, some of the best wind energy sites such as in Europe and North America suffer from icing atmospheric conditions. These conditions can decrease the annual energy production by up to 40% as shown by Sailor et al. [1]. This drop in blade aerodynamic performance occurs due to several detachments and reattachment areas on the surface due to the presence of a rough ice surface. Since the 1930s, different experiments were conducted to understand the effect of roughness on fluid flow in general. One of the pioneers in this field was Nikuradse [2] who investigated turbulent flow in pipes with different relative roughness values and Reynolds numbers between 10 4 to 10 6 . He noticed that the boundary layer follows the log law as in the case of a smooth surface. However, in the case of rough surfaces, there is a clear velocity shift (Δu). A few years later, Schlichting [3] studied the internal flow of a square-section channel with one rough wall. This rough wall had spherical segments, cones, Energies 2022, 15, 8145. https://doi.org/10.3390/en15218145 https://www.mdpi.com/journal/energies