This article reports about a wind-tunnel experiment carried out in the ONERA F2 low-speed wind tunnel on a model of the DU 97-W-300Mod airfoil designed for wind turbine application. The wind tunnel, the airfoil model, and experimental techniques used are presented, with special emphasis on the data processing and corrections required to derive airfoil forces and pressure distribution. To better document the flow physics at play, the results are illustrated by infrared thermography and surface oil flow visualization. The test allowed investigating Reynolds number effects between 1 and 3.8 millions. To ameliorate the understanding of the benefits and limitations of such airfoil testing, one section is devoted to the comparison of present results with previous experiments in other wind tunnels. Some of the difficulties arising in airfoil testing are evidenced and discussed to contribute to the improvement of test methods.

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This chapter focuses on airfoils for wind turbine blades and their characteristics. The use of panel codes such as XFOIL and RFOIL and CFD codes for the prediction of airfoil characteristics is briefly described. This chapter then discusses the requirements for wind turbine blade airfoils and the effect of leading edge roughness and Reynolds number. After a description of how airfoils can be tested, this chapter discusses methods to represent airfoil characteristics at high angles of attack. A number of methods for correcting characteristics for the effect of three-dimensional flow on the blade are presented. This chapter then discusses ways to establish a data set for blade design and concludes with a view on future research in the field of wind turbine blade airfoils.

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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.@en

At the 1998 Nagano Winter Olympic Games, zigzag tape was introduced on the race suit lower legs and cap of speed skaters. Application of these zigzag devices on live skaters and cylinders in the wind tunnel showed large improvements in the aerodynamic drag. These wind-tunnel results were unfortunately not widely published, and the impact of the zigzag strips in a real skating environment was never established. This paper aims to show the background of the application of the zigzag tape and to establish the impact it may have had on speed-skating performance. From comparisons of 5000 m races just before, during and just after the Nagano Olympics and an analysis of historic world record data of the 1500 m men’s speed skating, the impact of the zigzag tape turbulators on average lap times on 1500 and 5000 m races is calculated to be about 0.5 s.

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A relatively simple method is presented to predict the maximum two-dimensional drag coefficient of an airfoil only using its shape. The method is based on a contribution related to the leading edge thickness in terms of the y/c coordinate at x/c=0.0125 and a contribution related to the trailing edge flow angle which appears also to be sensitive to the leading edge thickness. The relations were deduced from measurements in the Delft low-turbulence wind tunnel. The first contribution was established using 3 airfoil models with systematically varying leading edge y/c coordinates and a zero trailing edge angle. The second followed from measurements of one of these airfoils equipped with sheet metal flaps of various flap deflections. Compared to measurements found in the public domain differences are found up to ? 2.3% with an average of about-0.2%.

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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.

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The accuracy of airfoil polar predictions is limited by the usage of imperfect turbulence models. Can machine-learning improve this situation? Will airfoil polars teach the effect of turbulence on skin-friction? We try to answer these questions by refining turbulence treatment in the Rfoil code: boundary layer closure relations are learned from airfoil polar data. Two turbulent closure relations, for skin friction and energy shape factor, are parametrized with a class-shape transformation. An experimental database is then used to define code inaccuracy measures that are minimized with an interior point gradient algorithm. Results show that airfoil polars contain exploitable information about turbulent phenomena. Inferred closures agree with direct numerical simulation results of skin friction and the new code predicts drag more accurately. Maximum lift remains under-predicted but Rfoil maintains its robustness and suitability for optimization of wind energy airfoils.

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Aerodynamic experiments have been executed in the wind tunnel and on a wind turbine blade to measure the impact of roughness on the airfoil characteristics and the associated effect on rotor performance and to establish the transition location on a rotating blade. The wind tunnel tests have been performed in the low-speed, low-turbulence wind tunnel of TUDelft. The wind turbine tests were carried out at ECN's Wind Turbine Test Site. Roughness simulation material has been installed on the airfoil leading edge to measure the impact on airfoil performance. Microphones were mounted on the airfoil surface to detect the boundary layer laminar to turbulent transition position both on the wind tunnel model and on the wind turbine blade.

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In modern large wind turbine blades thick flat back airfoils are often used in the root part of the blades due to their structural advantages. Although in the root structural properties are more important than aerodynamic performance, thick flat back airfoils do have higher drag which limits their applicability. Numerical simulations demonstrated that a previously introduced non-conventional flat back airfoil concept, called “swallow tail”, has potential showing lower drag for similar or slightly improved lift values. In the present study the swallow tail concept is evaluated experimentally in the TU Delft low-speed wind tunnel. An existing wind tunnel model of airfoil DU97-W-300 was modified to obtain a flat back airfoil with a 10% trailing edge thickness. The model was tested in the wind tunnel with and without the swallow tail for various conditions. The results of the experiment showed that the swallow tail reduced the drag up to 40% at various angles of attack. Moreover, the maximum glide ratio increased up to 45% while both the stall and post stall behavior remained almost identical. The results suggest that the swallow tail can help to increase power output of the wind turbine without modifying the turbine’s operating conditions. Due to the lower drag thick flat back airfoil sections could possibly be located at more inboard parts of the blades.

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Passive vane-type vortex generators (VGs) are commonly used on wind turbine blades to mitigate the effects of flow separation. However, significant uncertainty surrounds VG design guidelines. Understanding the influence of VG parameters on airfoil performance requires a systematic approach targeting wind energy-specific airfoils. Thus, the 30%-thick DU97-W-300 airfoil was equipped with numerous VG designs, and its performance was evaluated in the Delft University Low Turbulence Wind Tunnel at a chord-based Reynolds number of 2×10⁶. Oil-flow visualizations confirmed the suppression of separation as a result of the vortex-induced mixing. Further investigation of the oil streaks demonstrated a method to determine the vortex strength. The airfoil performance sensitivity to 41 different VG designs was explored by analysing model and wake pressures. The chordwise positioning, array configuration, and vane height were of prime importance. The sensitivity to vane length, inclination angle, vane shape, and array packing density proved secondary. The VGs were also able to delay stall with simulated airfoil surface roughness. The use of the VG mounting strip was detrimental to the airfoil's performance, highlighting the aerodynamic cost of the commonly used mounting technique. Time-averaged pressure distributions and the lift standard deviation revealed that the presence of VGs increases load fluctuations in the stalling regime, compared with the uncontrolled case.

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We describe a probabilistic approach to design airfoils for wind energy applications. An analytical expression is derived for the probability of perturbations to the operational blade-section angle of attack. It includes the combined influence of wind shear, yaw-misalignment, and turbulence intensity. The theoretical fluctuations in angle of attack are validated against an aero-structural simulation of a 10 MW horizontal axis wind turbine, operating under different inflow conditions. Finally we incorporate the probabilistic approach into a multi-objective airfoil optimization problem, which is solved with a genetic algorithm. The results illustrate the compromise between airfoil performance for a specific angle of attack and robustness of airfoil performance over a large range of angle of attack fluctuations@en

This study describes a methodology for designing airfoils suitable to employ actuation in a wind energy environment. The novel airfoil sections are baptized wind energy actuated profiles (WAP). A genetic algorithm-based multi-objective airfoil optimizer is formulated by setting two cost functions: one cost function for wind energy performance and the other representing actuation suitability. The wind energy cost function compares the candidate airfoils' performance with 'reference' wind energy airfoils, considering a probabilistic approach to include the effects of turbulence and wind shear. The actuation suitability cost function is developed considering horizontal axis wind turbines active stall control, including two different control strategies designated by 'enhanced' and 'decreased' performance. Two different actuation types are considered, namely, boundary layer transpiration and dielectric barrier discharge plasma. Results show that using WAP airfoils provides much higher control efficiency than adding actuation on reference wind energy airfoils, without detrimental effects in non-actuated operation. The WAP sections yield an actuator employment efficiency that is two to four times larger than those obtained with reference wind energy airfoils, at equivalent wind energy performance. Regarding geometry, and compared with typical wind energy airfoils, WAP sections for decreased performance display an upper surface concave aft region, while for increased performance, a convex upper surface aft region is obtained. The present study emphasizes that there is much to gain in designing airfoils from the beginning to include actuation effects, especially compared with employing actuation on already existing airfoils. The results demonstrate the potential of including actuation effects in the airfoil design process, thus enabling novel horizontal axis wind turbines control strategies.

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We explore integral boundary layer approximations for shear layer flows with vortex generators. The flow field is decomposed to highlight two phenomena: shear over the wall and vortex-driven mixing of the shear layer. The Navier-Stokes Equations are normalized to identify a new adimensional parameter: the vortex strength number (Vg). Usual boundary layer scales are valid when the vortex strength number (Vg) is of order one or smaller. New Boundary Layer Equations comprising the effect of streamwise vortex filaments are obtained and integrated accross a periodic vortex cell. The new integral equations share their structure with the original Von Karmann Integral Equations but use different variables. The deduction concludes with an approximate interaction equation for the construction of generalized closures from the classic set of Swafford turbulent closure relations. The new formulation is solved numerically and it is compatible with future integration in the Xfoil or Rfoil viscous-inviscid airfoil analysis codes.@en

There are many ways to learn from data. Our first experiment consisted in reproducing the way aerodynamicists work [2] with a genetic optimizer. The data pool was too narrow and asymptotic tendencies were unreliable. Our 2nd Experiment, a simple version of [4], had a virtually unlimited data pool and used neural networks. Results were better, but computationally expensive. Data assimilation approaches used in EO [ 7] could yield better results..@en

Bobsleighs races nowadays are decided upon hundredths of a second. As the margins are that small the aerodynamics of a bobsleigh can play a significant role in a race outcome. This paper investigates the influence of the gap between the nose and the rear cowling of a two-man bobsleigh. The paper especially focusses on the misalignment between both cowling parts due to lateral rotation in a track bend. CFD analyses and wind tunnel tests have shown that the rotation of the nose increases the drag due to an enlarged frontal area and due to adverse effects of the airstream flowing into the cavity. The results are used to define an area of investigation to alter the gap geometry which may lead to reduced drag when the bobsleigh parts are misaligned.@en

Experimental results and complimentary computations for airfoils with vortex generators are compared in this paper, as part of an e_ort within the AVATAR project to develop tools for wind turbine blade control devices. Measurements from two airfoils equipped with passive vortex generators, a 30% thick DU97W300 and an 18% thick NTUA T18 have been used for benchmarking several simulation tools. These tools span low-to-high complexity, ranging from engineering-level integral boundary layer tools to fully-resolved computational uid dynamics codes. Results indicate that with appropriate calibration, engineering-type tools can capture the e_ects of vortex generators and outperform more complex tools. Fully resolved CFD comes at a much higher computational cost and does not necessarily capture the increased lift due to the VGs. However, in lieu of the limited experimental data available for calibration, high _delity tools are still required for assessing the e_ect of vortex generators on airfoil performance. @en