There is continuous effort to try and improve the aerodynamic performance of wind turbine blades. This experimental study focusses on the addition of a passive slat on a thick airfoil typically used in the inboard part of commercial wind turbine blades. Nine different slat configurations are considered, with both a clean and tripped main airfoil. The results are compared with the performance of the airfoil without slat, as well as the airfoil equipped with vortex generators. It is found that, when the airfoil is clean, the increase in lift-to-drag ratio due to the presence of a slat is larger than when vortex generators are used. This is also true for the tripped airfoil but only at small angles of attack. As expected, in all configurations, the presence of the slat delays flow separation and stall. Finally, for a clean airfoil and small angles of attack, the slat decreases the lift-to-drag ratio of the main airfoil only. By contrast, as the angle of attack increases, it seems that the slat changes the flow field around the main airfoil in such a way that its lift-to-drag ratio becomes larger than for the airfoil without slat. These effects are less pronounced when the airfoil is tripped. This work helps to better understand the role of a slat in improving the aerodynamics of blade sections. It can also be used to validate simulation tools in the field.
The development of innovative add-ons and their combination are topics of high interest for wind turbine manufacturers. Such devices can increase the energy yield of a wind turbine by a few percent, leading to potentially significant reductions in levelised cost of energy. Add-on devices are already commonly used on commercial wind turbine blades. The type of add-ons and their location along the blade depend on the target objective, which is often to improve the aerodynamic performance of the rotor. A good example of such devices is low-drag vortex generators (VGs), which are typically used at the inboard or mid-board sections of the blade. At these locations, the airfoil sections are rather thick with a high maximum lift in order to allow for the chord length to be reduced, without penalising greatly the overall energy yield. Thick airfoils inboard are also useful to reduce standstill loads under extreme conditions. However, the inboard part of the blade typically operates under large angles of attack. Hence, large flow separation may occur at these locations. In order to mitigate this and increase the lift-to-drag ratio, vortex generators can be positioned in arrays in front of the separation line. These devices trigger the formation of small vortices in the boundary layer that re-energise the near-wall flow, hence preventing the flow from separating
This work focusses on passive flow control devices that increase aerodynamic performance. In this context, aside from VGs, inboard devices include Gurney flaps, which increase the lift force on the blade section by increasing the effective camber when placed on the pressure side and close to the trailing edge
Although slats have been widely used in the aircraft industry, they are currently not used on commercial wind turbine blades. They could however be beneficial in the inboard part of the blade in order to delay the onset of stall, which is susceptible to occur due to the large angles of attack at these locations. The effect of a leading-edge slat on the flow field is however complex
The purpose of the present work is to explore experimentally the effectiveness of using a leading-edge slat on a thick base airfoil commonly used in commercial wind turbine blades. It also complements the numerical results obtained by
The experiment is conducted in the low-speed low-turbulence tunnel (LTT) of Delft University of Technology
Set of configurations and associated labels.
The main element is a composite DU00-W2-401 airfoil model with a chord length of
Illustration of the slat parameters of interest in this work: (i) gap width
CAD design of the slat cross-section
In order to mitigate interference effects due to the tunnel wall boundary layers, pairs of vortex generators (VGs) are installed on the main profile close to the walls, on both the pressure and suction sides of the main airfoil, as shown by Fig.
Photographs of the experimental setup: main profile with slat element
Illustration of the geometrical characteristics of the vortex generators.
Finally, all the tests are performed with both a clean and a tripped main airfoil. For the tripped cases, a zig-zag turbulator is placed on the main airfoil, as shown on the right photograph in Fig.
Total lift coefficient
The lift and drag coefficients on the airfoil and slat can be determined by using a combination of pressure measurements on the airfoil surfaces and using a wake rake. On the airfoil surfaces, the normal force coefficient
Data are recorded using an electronic data acquisition system and are online reduced to show corrected force and moment coefficients and pressure distributions. A thermal camera enables us to visualise the location of flow transition along with the pressure distribution. Wool tufts are also placed on the base airfoil to have a measure of flow separation and identify possible three-dimensional wall effects during testing.
Example of tuft visualisation obtained for a clean airfoil (i.e. no tripping) with a slat at
Total drag coefficient
Lift-to-drag ratio
Pressure coefficient
Infrared images of the suction side of the clean main airfoil (i.e. no tripping) at an angle of attack
Figure
Figure
This is mimicked by the associated infrared images presented in Fig.
Total lift coefficient
Lift coefficient
In order to assess the sensitivity of the results to surface roughness, the boundary layer on the main airfoil is tripped using a zig-zag turbulator tape placed at the 10 % chord location on both the pressure and suction sides. The tape height is calculated according to
The drag coefficients are presented in Fig.
Total drag coefficient
Lift-to-drag ratio
Figure
Infrared images of the suction side of the tripped main airfoil at an angle of attack
Pressure coefficient
Figures
Lift coefficient
Drag coefficient
Lift-to-drag ratio
This paper summarises the main results obtained from wind tunnel experiments on a DU00-W2-401 airfoil, equipped with either a slat or vortex generators, in both clean and tripped conditions. The results suggest that the use of a slat can significantly increase the aerodynamic performance of the system. For a clean airfoil and small angles of attack, the presence of a slat decreases the lift-to-drag ratio of the main airfoil only. This is in line with the slat effect described in the literature
Based on these results, the present study shows that using a slat on a wind turbine blade could be beneficial provided that the slat angle is not too large and the gap width not too small. It is however important to bear in mind that the present conclusions hold for the geometries and parameters investigated here and do not consider any structural challenges that arise when attaching a slat to a wind turbine blade. In particular, the effect of the slat on the overall blade mass and aeroelastic responses, as well as the logistics of attaching the slat to the blade, are aspects that would need further analysis. Future work should therefore focus on incorporating these aspects in the overall assessment of the feasibility and potential of using slats on commercial wind turbine blades.
Table
Values of angles of attack below which the wake-rake drag is used.
The data set and post-processing codes are publicly available in
AV wrote the original manuscript, post-processed and interpreted the results, acquired funding for the project, and was the responsible supervisor of the research. BL was the scientist in charge of the experiment (preparation, run, post-processing), interpreted the results, and revised the manuscript. JS advised on the model design and choice of parameters, interpreted the results, and revised the manuscript. NT advised on the preparation of the experiment, interpreted the results, and was responsible for the wind tunnel facility.
The contact author has declared that neither they nor their co-authors have any competing interests.
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The authors would like to acknowledge Rijksdienst voor Ondernemend Nederland (RVO) through the TSE Hernieuwbare Energie funding scheme (ABIBA project). We are also grateful for the technical support of Stefan Bernardy and Emiel Langendijk of the Low-speed Wind Tunnel Laboratory in helping with the design and preparation of the model setup. We also would like to thank Wind Tunnel Services. Finally Nicholas Balaresque of Deutsche WindGuard is acknowledged for giving us the opportunity to use their DU00-W2-401 airfoil model as basis for the measurements.
This research has been supported by the Rijksdienst voor Ondernemend Nederland (grant no. TEHE116332).
This paper was edited by Joachim Peinke and reviewed by Christian Navid Nayeri and one anonymous referee.