Parametric slat design study for thick-base airfoils at high Reynolds numbers

Abstract

Abstract. Standard passive aerodynamic flow control devices such as vortex generators and gurney flaps have a working principle that is well understood. They increase the stall angle and the lift below stall and are mainly applied at the inboard part of wind turbine blades. However, the potential of applying a rigidly fixed leading-edge slat element at inboard blade stations is less well understood but has received some attention in the past decade. This solution may offer advantages not only under steady conditions but also under unsteady inflow conditions such as yaw. This article aims at further clarifying what an optimal two-element configuration with a thick main element would look like and what kind of performance characteristics can be expected from a purely aerodynamic point of view. To accomplish this an aerodynamic shape optimization procedure is used to derive optimal profile designs for different optimization boundary conditions including the optimization of both the slat and the main element. The performance of the optimized designs shows several positive characteristics compared to single-element airfoils, such as a high stall angle, high lift below stall, low roughness sensitivity, and higher aerodynamic efficiency. Furthermore, the results highlight the benefits of an integral design procedure, where both slat and main element are optimized, over an auxiliary one. Nevertheless, the designs also have two caveats, namely a steep drop in lift post-stall and high positive pitching moments.

Reply on RC1

“…This observation might be affected by the accuracy of the drag measurements, especially when the wake rake is used. However, these trends are in line with the CFD results of Steiner et al (2020). For completeness, Fig.…”

“…As expected, this increase is further amplified when 155 the contribution of the lift force on the slat is added (coloured dots). Again, the slat has a negligible effect on the drag force experienced by the main airfoil and the lift-to-drag ratios are consistent with those obtained by CFD simulations in similar conditions Steiner et al (2020).…”

“…It is also expected that reducing the gap width h slat , while avoiding confluent boundary layers, increases the positive effect brought by the slat. However, the results obtained by Steiner et al (2020) with a 2% gap width (i.e. h slat /c main = 0.02) led to smaller lift coefficients, smaller lift-over-drag ratios, and a similar stall angle than with a 4% gap width.…”

“…The model was mounted vertically in the test section on two aluminium attachment plates flush with rotating discs in the upper and lower tunnel wall that provide changes in angle of attack. The slat profile is a custom, cambered airfoil with a chord length equal to 0.3c main , as this was shown to lead to good performances in Steiner et al (2020). The coordinates of the slat element are available in LeBlanc et al (2021).…”

“…The set of configurations investigated here, and their associated labels, is presented in Table 1. The values of the slat angle β slat are chosen based on previous works (Steiner et al, 2020;Schramm et al, 2016;Jaume and Wild, 2016), which show that a slat angle of around 20 • leads to the best performance. In order to assess the sensitivity of the results to β slat , three different values separated by 5 • are considered here, namely β slat = 16.4 • , 21.4 • , 26.4 • .…”