Modern wind turbines employ thick airfoils in the outer region of the blade with strong adverse pressure gradients and high sensitivity to flow separation, which can be anticipated by leading-edge roughness. However, Reynolds average Navier-Stokes simulations currently overpredict the Reynolds shear stresses near the surface, and the flow separation is not correctly predicted. Hence, these methods are not representative enough to optimize the blade design to avoid flow separation, which becomes relevant for rough blades. While several eddy-viscosity corrections in the (Figure presented.) turbulence model have been previously studied to predict flow separation over smooth airfoils, the present study aims to extend their applicability to airfoils with leading-edge roughness. Two corrections, whose effect on flow physics has not been empirically quantified, are addressed. Particle image velocimetry measurements have been performed on a 30% thick airfoil to quantify the impact of these corrections. The reduction of the eddy viscosity introduced by the corrections leads to a shift of the peak location of the Reynolds shear stresses away from the surface, which, in turn, promotes flow separation and improves the prediction of the mean velocity and the pressure-coefficient distribution. Besides, the ratio between the main turbulent shear stress and turbulent kinetic energy is demonstrated to be lower than the standard value used in the (Figure presented.) turbulence model at the boundary-layer outer edge. Adjusting this ratio for an angle of attack of 0° decreases the error on the predicted lift and drag coefficients from 75% to 3% and from 58% to 39%, respectively.@en

An experimental study focusing on the change of the aerodynamic performance of a wind turbine with the employment of trailing-edge serrations is presented. The design procedure which starts from the serration design, together with the experimental wind tunnel testing and the installation in an already operating wind turbine is discussed. A prediction methodology to estimate the aerodynamic performance change of the machine with the blade add-on is validated by in-field measurements. Wind tunnel experiments for the validation of the original serration design have been carried out on a Nordex ADO30 airfoil with a relative thickness of 30% and on a NACA643418 airfoil with 18% thickness. The two airfoils are respectively used in this study due to their peculiar characteristics. While the first has been designed for relatively high Cl/Cd performance versus structural integrity (i.e. relatively high thickness), the second one is a typical reference airfoil used in wind turbines for its interesting Cl/Cd performance in both laminar and rough conditions. Clean and rough conditions have been tested in order to prepare a database for the analysis of the full turbine in several wind conditions. Aerodynamic forces with the serrated trailing-edge extensions are measured for different angles of attack and serration flap angles. The results are further discussed and employed for the analysis of the full rotor. Results already show that an increase in the flap angle is typically associated with an increment in lift, but not necessarily in drag. This has a beneficial effect on the operational regime of the machine when taken into account. The influence of the trailing-edge serrations on an operating wind turbine has been quantified in terms of total loads and energy production. The power curves with and without the trailing-edge installations are further analyzed and compared with the theoretical predictions. • Experimental tests show big impact of serrations on airfoil's aerodynamic properties • Predictive law used for extending the serrations influence to the whole blade • Theoretically, serrations have positive impact on power production of wind turbines • Theoretically, serrations increase wind turbine loads and reduce its life-time • Positive impact of serrations on power production of real machines has been probed@en

A study of the aerodynamic performance of a NACA 643418 airfoil with trailing edge serrations is presented. For the prediction of the changes in lift due to the serration installation, an empirical law is derived that can be extended to typical cambered airfoils for wind turbine applications. The law is deduced from 2D and 3D Reynolds‐averaged Navier–Stokes simulations (RANS) of the flow over the airfoil. Lift and drag together with the changes in the wake flow due to the presence of the serrated edges are investigated. An additional study of the sensitivity of the results at Rec = 3·106 with respect to the turbulence modeling is carried out by using three different RANS models: Spalart–Allmaras, k‐omega SST, and Transition SST. Results show that the changes in lift due to trailing‐edge extensions are approximated by the effect of a split plate with reduced length.@en

A computational fluid dynamics study is carried out to model the effects of distributed roughness at the airfoil leading-edge using the equivalent sand grain approach and Reynolds-averaged Navier–Stokes equations. The turbulence model k - ω-shear stress transport (SST) is selected to emulate a fully turbulent flow. Three k and ω boundary conditions are studied to model roughness effects. One refers to Wilcox’s boundary condition and the other two refer to Aupoix’s boundary conditions. Besides, Hellsten’s correction is used to ensure Wilcox’s boundary condition compatibility with the shear stress transport limiter. After validating the implementation of these boundary conditions, they are applied to three different airfoils. One of them is a thick airfoil with industrial relevance. For this airfoil, Wilcox’s boundary condition significantly underestimates the roughness impact on aerodynamic coefficients. The pressure gradient simplification in Wilcox’s boundary condition formulation is the driving factor behind this effect. The pressure gradient effect on Aupoix’s boundary condition is minimal.@en

The mid-span region of wind turbine blades can be thickened to fulfil the structural requirements of the blade. Hence, thick airfoils, that were designed to operate at the root region of the blade, are moved to the mid-span region. This could not imply remarkable variations of the blade performance once its surface is smooth. However, the sensitivity of thick airfoils to roughness could cause significant aerodynamic impacts such as flow separation. This research aims to quantify the impact of the blade thickness, under smooth and rough conditions, in the annual energy production and the fatigue loads of the blade. Ten blade designs, linearly interpolated in thickness, are studied employing aero-elastic computations. The results reveal that the thickest blade increases the annual energy production by 5% with respect to the thinnest blade under rough conditions. Whereas this increase is less than 1% under smooth conditions. The loss of annual energy production varies with the blade thickness linearly for thin blades while it varies exponentially for thick blades up to 22%. Fatigue loads assessment confirmed a reduction of the damage equivalent load under smooth conditions, whereas the thickest blade increased it 28% under rough conditions.@en

A numerical and experimental analysis of the trailing edge serrations effect on the aerodynamic performance of the NACA643418 airfoil is presented. 3D Reynolds-averaged Navier-Stokes simulations (RANS) of the flow over the airfoil with and without trailing edge serrations have been carried out with the Transitions SST turbulence model. From these calculations an empirical law for the prediction of the aerodynamic behaviour of the airfoil with serrations is derived. The law is validated through wind tunnel experiments. The NACA643418 airfoil has been tested with trailing edge serrations installed with different flap angles to analyze the effect of the parameter on the performance of the airfoil. Results show that the trailing edge serrations cause a significant increment in lift coefficient with low penalties in drag.@en

In this study, the aerodynamic performance of a thick airfoil is analysed, after installing leading-edge roughness to emulate a severe state on the airfoil surface. The impact on aerodynamic coefficients has been quantified using two roughness methods: zig-zag tape and sandpaper. Wind tunnel tests are carried out at a Reynolds number of 3•106. At low angles of attack, zig-zag tape and sandpaper provide comparable lift and drag coefficients but significant variations of these coefficients are obtained for high angles of attack. Stalled flow is the cause of the most significant variation on the airfoil performance between smooth and rough surface states. Vortex generators are adapted to recover the lift coefficient value previously given by the airfoil under smooth conditions. As a result, vortex generators are able to reduce the loss of lift and the sensitivity of the airfoil to the rough state.@en