Abstract
<title>Abstract</title> <p>In the aerodynamic performance of wind turbine blades and other airfoil based systems, accurate prediction of roughness effects is essential for reliable performance assessment and long-term power loss estimation in wind energy applications. Surface roughness disrupt the viscous sublayer, enhance near-wall turbulence production, and alter the mean velocity profile, resulting in increased skin-friction coefficients and modified boundary layer development, directly affecting aerodynamic loads and overall performance. Despite this established understanding, most experimental studies and model calibrations are based on canonical configurations, such as zero-pressure-gradient boundary layers, pipe flows, or channel flows. While these simplified cases provide valuable insight, they do not represent the complex flow physics encountered in practical applications, where significant favourable and adverse pressure gradients are common. In CFD simulations, surface roughness effects are typically represented through the equivalent sand-grain roughness height , although experimental roughness conditions are often defined in terms of geometric characteristics or surface coverage. A procedure is proposed to estimate the equivalent roughness height by establishing a relationship between the percentage of surface area covered by particles and the equivalent roughness height used in turbulence models, for the ”standard roughness” applied at the leading edge of airfoils in the experimental results reported in Abbott and Doenhoff’s classical reference book, ”Theory of Wing Sections”. Turbulence models, including k − ε, k − ω SST, and Spalart- Allmaras, combined with various roughness corrections, are evaluated in three NACA airfoils to quantify model accuracy and identify the conditions under which current roughness modeling approaches remain valid or require further development.</p>