Lidars for Wind Tunnels - an IRPWind Joint Experiment Project

Measurement campaigns with continuous-wave Doppler Lidars (Light detection and ranging) developed at DTU Wind Energy in Denmark were performed in two very different wind tunnels. Firstly, a measurement campaign in a small icing wind tunnel chamber at VTT in Finland was performed with high frequency measurements for increasing the understanding of the effect of in-cloud icing conditions on Lidar signal dynamics. Secondly, a measurement campaign in the relatively large boundary-layer wind tunnel at NTNU in Norway was performed in the wake of a scaled test turbine in the same configuration as previously used in blind test comparisons for wind turbine wake modelers. These Lidar measurement activities constitute the Joint Experiment Project” L4WT - Lidars for Wind Tunnels, with applications to wakes and atmospheric icing in a prospective Nordic Network” with the aim of gaining and sharing knowledge about possibilities and limitations with lidar instrumentation in wind tunnels, which was funded by the IRPWind project within the community of the European Energy Research Alliance (EERA) Joint Programme on Wind Energy

Eksperimentelle undersøkelser av den turbulente vakestrømningen bak vindturbiner

Interactions of wind turbine wakes with downstream turbines can reduce a wind farm’s power production and increase loads on the individual turbines. For the purpose of wind farm optimization, different aerodynamic approaches to modify the performance and wake flow of one or two model wind turbines have been tested in a number of wind tunnel experiments. In a first set of measurements, different modifications of the rotor design to limit wake effects are studied. Herein, the effect of the blade number on the wake development is studied by comparing the wake properties behind 2- and 3-bladed model wind turbines. Also, the influence of the rotational direction is investigated by comparing the performance of an aligned two-turbine array with co- and counter-rotating rotors. Moreover, the effect of winglets on the performance and vortex interaction in the wake is assessed. For this purpose, a new rotor with aerodynamically optimized winglets has been designed. The performance of the rotor is compared to a reference rotor without winglets and effects on the vortex interaction and velocity recovery in the wake are investigated. The second set of measurements investigated the control of the model wind turbines by intentional yaw misalignment. Therefore, the wake flow behind a yawed turbine exposed to different inflow conditions is measured, while also the power and loads on a two-turbine array are analyzed for varying separation distances, lateral offsets and yaw angles. Selected test cases are furthermore provided for validation purposes of CFD codes. In a Blind test experiment, performance and wake data are compared to computational results from external groups. All the experiments have been carried out in the closed-loop wind tunnel at NTNU in Trondheim. The wakes were investigated for uniformly distributed and sheared inflow velocity profiles with different turbulence intensities ranging from 0.23% to 10.0%. During the project different rotor designs from 2- to 3-bladed rotors, all with a diameter of D = 0.9 m, are investigated. The velocities in the wake are measured using a 2-component laser Doppler velocimetry system or a Cobra probe, which is used to extract phase-averaged information from the wake flow. The potential of the blade number and opposite rotational directions in turbine array are found not to have a significant potential for the optimization of a wind farm. While not affecting the mean velocity distribution, the blade number is observed to influence to turbulence peak levels in the wake. An opposite rotation of the downstream turbine is assessed only to be effective for very small turbine separation distances, where the energy contained in the wake swirl of the upstream turbine can be extracted. The design of aerodynamically optimized winglets could rise the power coefficient CP of a single rotor by 8.9%, whereas the thrust coefficient CT only increased by 7.4%. Winglets are furthermore found to accelerate the tip vortex interaction in the wake, leading to a local shear layer enlargement and earlier wake recovery. In a wind farm, rotors with winglets extract more energy and leave a similar amount of kinetic energy in the wake for potential downstream turbines. Yaw control is found to have the largest potential for the optimization of wind farms. The total power of an aligned two-turbine array is assessed to increase up to 11% by deflecting the upstream turbine’s wake laterally though an intentional yaw misalignment. However, yaw moments on yawed turbines and turbines operating in a partial wake are observed to increase, showing the importance of considering loads for yaw control. Finally, the comparison of experimental data to numerical predictions in the Blind test confirmed the strength of codes based on Large-Eddy Simulations (LES) in predicting mean velocity and turbulent kinetic energy levels in the wake precisely

An experimental study on rotor-wake interactions of wind turbines

Interactions of wind turbine wakes with downstream turbines can reduce a wind farm’s power production and increase loads on the individual turbines. For the purpose of wind farm optimization, different aerodynamic approaches to modify the performance and wake flow of one or two model wind turbines have been tested in a number of wind tunnel experiments. In a first set of measurements, different modifications of the rotor design to limit wake effects are studied. Herein, the effect of the blade number on the wake development is studied by comparing the wake properties behind 2- and 3-bladed model wind turbines. Also, the influence of the rotational direction is investigated by comparing the performance of an aligned two-turbine array with co- and counter-rotating rotors. Moreover, the effect of winglets on the performance and vortex interaction in the wake is assessed. For this purpose, a new rotor with aerodynamically optimized winglets has been designed. The performance of the rotor is compared to a reference rotor without winglets and effects on the vortex interaction and velocity recovery in the wake are investigated. The second set of measurements investigated the control of the model wind turbines by intentional yaw misalignment. Therefore, the wake flow behind a yawed turbine exposed to different inflow conditions is measured, while also the power and loads on a two-turbine array are analyzed for varying separation distances, lateral offsets and yaw angles. Selected test cases are furthermore provided for validation purposes of CFD codes. In a Blind test experiment, performance and wake data are compared to computational results from external groups. All the experiments have been carried out in the closed-loop wind tunnel at NTNU in Trondheim. The wakes were investigated for uniformly distributed and sheared inflow velocity profiles with different turbulence intensities ranging from 0.23% to 10.0%. During the project different rotor designs from 2- to 3-bladed rotors, all with a diameter of D = 0.9 m, are investigated. The velocities in the wake are measured using a 2-component laser Doppler velocimetry system or a Cobra probe, which is used to extract phase-averaged information from the wake flow. The potential of the blade number and opposite rotational directions in turbine array are found not to have a significant potential for the optimization of a wind farm. While not affecting the mean velocity distribution, the blade number is observed to influence to turbulence peak levels in the wake. An opposite rotation of the downstream turbine is assessed only to be effective for very small turbine separation distances, where the energy contained in the wake swirl of the upstream turbine can be extracted. The design of aerodynamically optimized winglets could rise the power coefficient CP of a single rotor by 8.9%, whereas the thrust coefficient CT only increased by 7.4%. Winglets are furthermore found to accelerate the tip vortex interaction in the wake, leading to a local shear layer enlargement and earlier wake recovery. In a wind farm, rotors with winglets extract more energy and leave a similar amount of kinetic energy in the wake for potential downstream turbines. Yaw control is found to have the largest potential for the optimization of wind farms. The total power of an aligned two-turbine array is assessed to increase up to 11% by deflecting the upstream turbine’s wake laterally though an intentional yaw misalignment. However, yaw moments on yawed turbines and turbines operating in a partial wake are observed to increase, showing the importance of considering loads for yaw control. Finally, the comparison of experimental data to numerical predictions in the Blind test confirmed the strength of codes based on Large-Eddy Simulations (LES) in predicting mean velocity and turbulent kinetic energy levels in the wake precisely

Experiments in the wind turbine far wake for the evaluation of an analytical wake model

publishedVersio

Wind tunnel study on power output and yaw moments for two yaw-controlled model wind turbines

publishedVersio

A Detached-Eddy-Simulation study: Proper-Orthogonal-Decomposition of the wake flow behind a model wind turbine

publishedVersio

An experimental study on the effects of winglets on the tip vortex interaction in the near wake of a model wind turbine

An experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight-cut wing tip and a downstream-facing winglet shape are compared on the same two-bladed rotor operated at its design tip speed ratio. Phase-averaged measurements of the velocity vector are synchronized with the rotor position, visualizing the downstream location of tip vortex interaction for the two blade tip configurations. The mean streamwise velocity is found not to be strongly affected by the presence of winglet tip extensions, suggesting an insignificant effect of winglets on the time-averaged inflow conditions of a possible downstream wind turbine. An analysis of the phase-averaged vorticity, however, reveals a significantly earlier tip vortex interaction and breakup for the wingletted rotor. In contradistinction, the tip vortices formed behind the reference configuration are assessed to be more stable and start merging into larger turbulent structures significantly further downstream. These results indicate that an optimized winglet design can not only contribute to a higher energy extraction in a rotor's tip region but also can positively affect the wake's mean kinetic energy recovery by stimulating a faster tip vortex interactio

Vortex interaction in the wake of a two- and three-bladed wind turbine

The vortex interaction in the wake behind a two- and three-bladed model scale wind turbine is investigated. The two rotors have equal solidity, and produce similar power and thrust at the design tip speed ratio. Phase-averaged quantities of the wake flow from one to four rotor diameters behind the turbines are measured in a wind tunnel. It is found that the two-bladed turbine has slower wake recovery than the three-bladed turbine, and a larger velocity deficit is produced in the far wake. The tip vortices behind the two-bladed turbine is more stable than behind the three-bladed turbine, and the vortex structures exist further downwind. In a wind farm, this could reduce the power production and increase fatigue loads for the turbines operating in the wake flow, especially during stable atmospheric conditions

Vortex interaction in the wake of a two- and three-bladed wind turbine

The vortex interaction in the wake behind a two- and three-bladed model scale wind turbine is investigated. The two rotors have equal solidity, and produce similar power and thrust at the design tip speed ratio. Phase-averaged quantities of the wake flow from one to four rotor diameters behind the turbines are measured in a wind tunnel. It is found that the two-bladed turbine has slower wake recovery than the three-bladed turbine, and a larger velocity deficit is produced in the far wake. The tip vortices behind the two-bladed turbine is more stable than behind the three-bladed turbine, and the vortex structures exist further downwind. In a wind farm, this could reduce the power production and increase fatigue loads for the turbines operating in the wake flow, especially during stable atmospheric conditions