When wind passes through the rotor of a wind turbine, the velocity is decreased while turbulence is increased. The region of decreased wind speed behind the rotor is known as the wind turbine wake and is bounded by a complex structure of helical vortices. This structure occurs to be more stable in low ambient turbulence and low tip speed ratio conditions, leading to a delayed recovery of the wake. The diminished wind speed in the wake leads to a decline in power output for downstream wind turbines, with this loss scaling proportionally to the cube of the velocity. This study uses field tests and simulations to evaluate enhanced wake recovery with segmented Gurney flaps on a 3.8-MW research wind turbine. Four Gurney flaps were attached at regions near the tip of each blade. This configuration is hypothesized to induce turbulence that destabilizes the vortex system, resulting in faster wake recovery. Field tests using a scanning LiDAR were conducted to quantify the wind turbine wake recovery between the baseline and the retrofitted configuration in various atmospheric conditions. The results show a consistent increase in wake recovery for the Gurney flap configuration, generally at all downstream distances. This was illustrated by a reduction of axial velocity deficits of roughly 10% at hub height, at five diameters downstream distance. The influence of retrofitting on turbine power and loads was limited. Summarizing, a very successful field test campaign was executed, which demonstrated the use of segmented Gurney flaps as a promising add-on to promote enhanced wind turbine wake recovery for improved overall wind farm performance.

This study presents results from a long-term measurement campaign on a research wind turbine in the field. Pressure measurements are conducted at 25% blade radius over several months. Together with inflow measurements provided by a LiDAR system, they form an extensive dataset, which is used in the validation of numerical aerodynamic models. The model validation is conducted based on both ten-minute average data as well as time-resolved unsteady data. Initially, it is investigated how representative ten-minute average pressure measurements are of the underlying unsteady aerodynamics. Binned ten-minute average pressure distributions are then analysed together with their numerical counterpart, consisting of a combination of rotor and airfoil level aerodynamic/aeroelastic simulation results using average environmental and operating conditions as input. Finally, time-resolved measurements and simulation results are compared, validating the aeroelastic tools' capability to reproduce unsteady aerodynamics. Overall, reasonable agreement is found between numerical simulations and field experiment data showcasing two aspects: Numerical tools based on blade element momentum theory and panel methods with viscous-inviscid interaction remain relevant for simulating modern multi-megawatt wind turbines, and long-term pressure measurements provide invaluable means for validating such tools.

BEM (Bladed Element Momentum) models have shown to be inaccurate in predicting loads in the case of yawed flow [1-3]. Actuator disk momentum theory is the basis for BEM codes, following Glauert’s auto-gyro theory [4]. Therefore, a first step to improve BEM in yawed flow is to assess momentum models for the actuator disk in yaw and investigate possibilities for improvement. In the past, several models have been developed for an actuator disc under yawed conditions, but they are subject to various assumptions. In this paper, different yaw models for the actuator disk will be reviewed and compared against higher fidelity models: fixed and free [5] wake vortex models so that the earlier made assumptions can be assessed. The comparison considers the case of a uniformly loaded actuator disk at varying yaw angle (0°~90°) and thrust coefficient (0.1~0.9). From the models which have been assessed, Øye’s correction [6] performs best, however, this model too suffers from deficiencies. This indicates that an improved momentum model for yawed flow is necessary.

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.

This paper presents final results from the EU project AVATAR in which aerodynamic models are improved and validated for wind turbines on a scale of 10 MW and more. Special attention is paid to the improvement of low fidelity engineering (BEM based) models with higher fidelity (CFD) models but also with intermediate fidelity free vortex wake (FVW) models. The latter methods were found to be a good basis for improvement of induction modelling in engineering methods amongst others for the prediction of yawed cases, which in AVATAR was found to be one of the most challenging subjects to model. FVW methods also helped to improve the prediction of tip losses. Aero-elastic calculations with BEM based and FVW based models showed that fatigue loads for normal production cases were over predicted with approximately 15% or even more. It should then be realised that the outcome of BEM based models does not only depend on the choice of engineering add-ons (as is often assumed) but it is also heavily dependent on the way the induced velocities are solved. To this end an annulus and element approach are discussed which are assessed with the aid of FVW methods. For the prediction of fatigue loads the so-called element approach is recommended but the derived yaw models rely on an annulus approach which pleads for a generalised solution method for the induced velocities.

This paper presents the most recent results from the EU project AVATAR in which aerodynamic models are improved and validated for wind turbines on a scale of 10 MW and more. Measurements on a DU 00-W-212 airfoil are presented which have been taken in the pressurized DNW-HDG wind tunnel up to a Reynolds number of 15 Million. These measurements are compared with measurements in the LM wind tunnel for Reynolds numbers of 3 and 6 Million and with calculational results. In the analysis of results special attention is paid to high Reynolds numbers effects. CFD calculations on airfoil performance showed an unexpected large scatter which eventually was reduced by paying even more attention to grid independency and domain size in relation to grid topology. Moreover calculations are presented on flow devices (leading and trailing edge flaps and vortex generators). Finally results are shown between results from 3D rotor models where a comparison is made between results from vortex wake methods and BEM methods at yawed conditions.

ECN with its partners TU DelŌ, Avent LiDAR Technologies and XEMC Darwind executed the four-year TKI Wind op Zee project LAWINE (LiDAR ApplicaƟon for WInd Energy Efficiency). In this project the applica Ɵon of LiDAR technology has been developed and validated so that it can be used to improve the operaƟon of offshore wind farms with the goal to further reduce the cost of offshore wind power plants. The planned deliverables in the project have been met within Ɵme and budget requirements. Gathering a project team that includes a variety of competences has resulted in a fruiƞul cooperaƟon leading to the following interesƟng project results: 1. It has been verified that ground based LiDAR can be applied for wind resource assessments as well as power and loads assessment campaigns. This way wind turbine performance can be verified without the requirement of expensive masts. 2. Power performance assessments can be performed more accurately by using the wind profile measurements of the LiDAR. 3. Wake characterizaƟon by LiDAR measurements has been demonstrated, which will assist the opƟ- mizaƟon of lay-outs of offshore wind farms. 4. In the project it has been demonstrated that nacelle LiDARs are suitable to be used as basis in power performance assessments. 5. It has been demonstrated that LiDAR measurements in combinaƟon with advanced controllers have significant benefits for rotor speed regulaƟon and reducƟon of faƟgue loads.6. It has been demonstrated that LiDARs can determine yaw misalignment accurately, which is an important aspect to implement the ECN wind farm controllers. 7. ECN developed the LiDAR calibraƟon facility where industry can calibrate its LiDARs in a very effecƟve manner. Such uƟlity is crucial for the successful applicaƟon of LiDARs in wind resource assessments and power performance verificaƟon. These results have been disseminated with researchers and industry over various conferences and workshops.