Induced stalled flow due to roughness sensitivity for thick airfoils in modern wind turbines

Abstract

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.

On the extension of k−ω−SST corrections to predict flow separation on thick airfoils with leading‐edge roughness

“…As a result, a relatively stronger APG is given at an AoA of 0°, which challenges the validation even more. The experiments of Gutierrez et al 5 constitute the reference for this study because they offer the framework to understand the steady 2D‐RANS failure and also quantify the impact of flow separation on the power production of a modern wind turbine 6 . They will be referred to as LSLT in the next part of the paper since they have been conducted at the low‐speed low‐turbulence wind tunnel of TU Delft.…”

“…30 The simulations failed in predicting the flow separation independently of the boundary condition used in the LE to emulate roughness. Later, aeroelastic calculations 6 revealed that the operational AoA was 5 at the mid-span region where thick airfoils can be located in case of thick blades. The numerical study 30 demonstrated that the steady 2D-RANS approach mainly fails in predicting the flow separation over the airfoil PS for this AoA of 5 .…”

“…Gutierrez et al 5 demonstrated that LE roughness could induce flow separation on the pressure side (PS) of a 30% thick airfoil for low positive AoAs (AoA 5°). Additionally, Gutierrez et al 6 showed that this flow separation varies the lift and drag more substantially than for 25 % and 18 % thick airfoils. Hence, the blade conditions for which the flow separates must be suitably defined to avoid blade‐loading problems and abrupt power losses.…”

“…Although several RANS studies focused on predicting LE-roughness effects for thin airfoils, 10,11 also the numerical prediction of flow separation on thick airfoils remains inaccurate, even for smooth surfaces. Sezer-Uzol et al 12 extensively reviewed CFD studies from 2001 to 2020 for wind-turbine airfoils at a Reynolds number of Re > 10 6 . Most of these studies were performed under a steady 2D-RANS approach, obtaining a suitable prediction of force coefficients when the flow was attached but also an underprediction of the flow separation beyond the maximum lift.…”