Improved RANS methodology to quantify the aerodynamic impact of leading-edge roughness on thick wind turbine blades
Authors:   , Gutiérrez Amo Rubén 
Published: February 2024

The current economic situation and the urgency of decarbonization are significantly influencing the design of wind-turbine blades. As a cost-reduction approach, the blade thickness can be increased in its outer part to reduce the number of composite layers while the blade stiffness is enough to keep blade deflections and loads within limits. While implementing the mentioned strategy may result in a lower overall lift-to-drag ratio for the blade compared to a relatively thinner blade, this reduction can be considered acceptable for blades that are not affected by roughness. However, the aerodynamic performance of thick blades is relatively more sensitive to blade roughness due to the associated adverse pressure gradient (APG) of their sections. Hence, a relatively higher reduction in the annual energy production (AEP) of the wind turbine occurs in the presence of blade roughness. This roughness varies randomly during the blade lifetime since wind turbines are installed worldwide in multiple environmental conditions. Thus, the aerodynamic performance of thick blades should be robust enough to keep the AEP for multiple surface conditions. On one side, the research on the roughness effect on airfoil performance is mainly based on thin airfoils used at the blade tips. Even for these airfoils, a non-standardized methodology for both experimental and numerical assessments is found in the literature. On the other side, the knowledge on the effect of leading-edge roughness (LER) on thick airfoils is scarce and based on traditional methods that do not consider all the aerodynamic impacts. Hence, this Ph.D. thesis assesses the validity of these methods on thick airfoils and researches their necessary adaptations to capture the LER effects. A 30%thick airfoil of industrial relevance has been selected in thiswork as a first step in the research. Wind-tunnel measurements are performed for this airfoil at a Rec of 3×106. These measurements constitute the basis of the experimental validation carried out in the Ph.D. thesis. Two methods to emulate LER, zig-zag (ZZ) tapes and sandpaper, have been employed, and a different impact on lift and drag coefficients has been quantified for positive angle of attacks (AoAs). The sandpaper causes a higher loss in airfoil performance than the ZZ tapes and amplifies the lift and drag fluctuations once the flow separates at high AoAs. The promotion of flow separation due to LER is the main aerodynamic impact that occurs on the airfoil suction side (SS) for high AoAs and on the airfoil pressure side (PS) for low AoAs. Furthermore, the region of flow separation on the airfoil PS remains nearly at the same location for both experimental-LER methods which results in a similar-linear variation of CL with the AoA for both methods. With respect to the infrared-thermography (IR) images, the flow-separation line is concluded to be mainly 2D. Subsequently, the numerical procedure to estimate the airfoil performance with LER is investigated. The most recent developments of the equivalent sand-grain approach in k−ω−SST are applied by means of 2D-Reynolds-Average Navier-Stokes (RANS) simulations tomodel the effects of LER. This Ph.D. thesis extends the use of these developments for an airfoil that is thicker than those researched in the literature. First, the suitability of the boundary-condition implementation on the computational fluid dynamics (CFD) code has been verified through flat plate cases. Later, the focus of the research is shifted to the application of Aupoix’s boundary conditions (BCs) on airfoil cases. Two airfoil cases from the literature reveal the failure of these BCs at high Rec and AoAs. Finally, the 30% thick airfoil is simulated, showing a substantial failure in the flow-separation prediction for all the studied BCs and different values of the equivalent sand grain. The open topic of 2D-RANS limitations for detached flows is concluded to become even more relevant for cases with LER if compared with smooth surface assumptions. Thus, a dedicated experimental campaign of the 30% thick airfoil has been carried out to understand the mentioned failure. Particle-image velocimetry (PIV) measurements on the airfoil PS reveal a difference with numerical predictions in the peak location of all the Reynolds shear stresses components. This offset increases with the freestream velocity and results in an excess of momentum near the airfoil surface and the consequent delay of flow separation. Two eddy-viscosity corrections fromthe literature, which were only applied to smooth surfaces, are considered in the numerical procedure developed in this Ph.D. thesis. The peak location of the Reynolds-shear-stresses is shifted by these corrections and validated with PIV measurements. Besides, the PIV measurements demonstrate that the Bradshaw’s constant (a1), defined as the ratio a1 = u′v′/k within the boundary layer, is lower than the default value assumed by the turbulence model and varies along the airfoil PS. A constant value of 0.29 in the numerical simulations is concluded to provide a suitable prediction of flow separation. Finally, the turbulent viscosity (μt ) corrections previously validated for only one AoA of 0◦, are researched for an AoA range from −20◦ to 20◦ and two additional airfoils with maximum relative thickness t/c of 25% and 18 %. First, a lack of robustness of the damping factor ( fb) and a1 methods can be found on the 30% thick airfoil once they are applied at the mentioned AoA range. Subsequently, the a1 value is estimated for each airfoil side and AoA whose later variation demonstrates that a specific a1 value is required for each one. However the literature is currently using a constant value of a1 for all AoAs and airfoils. As a result of the Ph.D. research, a new methodology is suggested to differentiate the a1 value per airfoil side and AoA which states the need for future research to find a relation of a1 with a specific flow magnitude. After employing the suggested methodology in an aeroelastic model of a wind-turbine, the shape of the power and loads distributions is replicated. Nevertheless, the methodology prior to this Ph.D. research provides wrong loads and power distributions as well as an underestimation of AEP loss of 2.5 %.