New Insight into the Atmospheric Conditions for Wind Turbine Aerodynamics and Aeroelastics
The DANAERO MW experiment was initiated because scientists and wind turbine designers in Denmark realised that more knowledge about the aerodynamics for megawatt wind turbines was required to further optimise the design of these wind turbines. In 2007 the DANAERO MW project was given a grant by the Danish Energy Agency. Risø DTU was made the coordinator, and DONG Energy, LM Wind Power, Siemens Wind Power and Vestas Wind Systems agreed on carrying out a series of experiments. The project was finalised at the end of 2009.
By Christian Bak (Senior Scientist) and Helge A. Madsen (Research Specialist), Risø DTU National Laboratory for Sustainable Energy, Denmark
The Experiments
The overall objective of the project was to provide an experimental dataset that can improve the knowledge of a number of fundamental aerodynamic, aeroelastic and aeroacoustic issues and in general improve the design basis for megawatt rotors. Three types of measurements were performed:
Much information can be extracted from the measurements, but only two examples from the measurements will be presented here. These two examples should be considered as preliminary since the task of post-processing the very large amount of data has only just begun.
Inflow Measurements on the Siemens 3.6MW Turbine
In March 2007 a five-hole Pitot tube was installed at radius 36 metres at the 53.5-metre-long blade on the Siemens 3.6MW turbine at the Høvsøre test site in Jutland, for measurement of local inflow angle and relative velocity. The Høvsøre test site, Risø DTU Test Station for Large Wind Turbines, is the Danish centre for testing megawatt wind turbines and is situated a few kilometres from the west coast of Jutland. In total five turbines are installed along a row at the test site and the Siemens turbine is situated in the middle of this row so the turbine will operate in the wake of other turbines for some wind directions. The signals from the Pitot tube have been sampled at 35Hz together with a number of signals from the turbine such as electrical power and rotor speed, and from a meteorology mast.
One of the mechanisms in the atmospheric flow with considerable effect on the performance and loads on megawatt turbines is the development of strong variation of the wind speed as a function of the height due to stable flow conditions, typically developing during night-time and then disappearing during daytime. A typical example of this has been measured with the five-hole Pitot tube on the Siemens 3.6MW turbine (Figure 1). As shown in the plot in Figure 2, the time trace of the inflow angle measured during night-time at 4am shows a considerable variation for every revolution of the rotor, but otherwise the inflow is very regular. It should be noted that the pitch of the blade is changed by 1 degree every minute, which is clearly seen in the time trace. During daytime at 3pm the variation for each rotor revolution in the time trace has almost disappeared and the signal has become more stochastic.
Aerodynamic and Aeroelastic Measurements on the 2MW NM80 Turbine
The test turbine is situated in a small wind farm at Tjæreborg close to the west coast of Jutland about 1 kilometre from the North Sea. In total the wind farm has eight turbines placed in two rows, which gives different single and multiple wake situations with the closest spacing about 3.5 rotor diameters.
For the NM80 with hub height 57 metres and 80-metre diameter rotor a new LM38.8 metre blade was manufactured, and during the production process equipment for measuring surface pressure profiles at four radial stations was placed inside the blade as shown in Figure 3. Additionally, the most outboard blade section was instrumented with around 50 microphones to measure high frequency surface pressure fluctuations (Figure 4). These data are used for determination of position of transition and for aeroacoustic characterisation. At a radial position close to the microphones, high frequency inflow has also been measured with a hot wire probe and with a Pitot tube.
Eleven measurement campaigns were conducted from late June 2009 to mid September 2009. The campaigns were carried out covering many different wind conditions including free inflow and wake operation.
As well as the pressure, inflow and microphone sensors, a considerable number of strain gauges and accelerometers mounted on the blades, the shaft and the tower were monitored. Also, a meteorology mast of height 90 metres was erected and this measured wind speeds and wind directions at five different heights. The sensors on the turbine and in the meteorology mast were sampled with 35Hz for 10 minutes, while the pressure taps were sampled with 100Hz for 9 minutes and 30 seconds and started together with the 35Hz measurements. The microphones were sampled with 50kHz in 10-second periods, starting each minute.
Much information can be extracted from these measurements, and characteristics concerning the airfoil, the inflow and the aeroelastic response can be analysed. However, here only one example will be shown, which represents the newest development in the measurement technology and information that has never been published for a wind turbine in real, atmospheric conditions. In this example the signals from the 50 microphones in the wind tunnel and on the turbine at the section at 37 metres radius are briefly analysed. Three different situations are compared: the flow over the turbine blade in atmospheric conditions (1), and the flow over the corresponding blade section in the wind tunnel at a level of low turbulence (2) and at a level of higher inflow turbulence generated by a turbulence grid upstream of the blade section (3). Investigations of the energy distribution as a function of frequencies in the blade section boundary layer showed much higher turbulence or unsteadiness close to the leading edge in the atmospheric inflow than in the wind tunnel flow for frequencies up to about 500Hz. This is also clear when 10-second time traces of the surface pressure fluctuations are compared as shown in Figure 5. The scale in the figures is the same and also the inflow velocity on the rotor and in the wind tunnel is almost the same. It is clear from the comparison in the figure that the scales of the turbulence caused by the grid in the wind tunnel are too small to represent atmospheric conditions. In the future analysis of the pressure fluctuation data, a key question will be to investigate what influence the high energy content in the inflow below 500Hz has on transition when compared with wind tunnel conditions.
Further Work
The results from the DANAERO project that are shown here are preliminary and represent a small fraction of the collected data. A new project is now ongoing where a database of all the data is being formed and where many measurements will be analysed. The output from the ongoing project will include a validation of the existing aerodynamic and aeroelastic design complex and formulations of new models and tools. Even though many operational conditions will be analysed in the ongoing project, it is believed that the measurements will be analysed for many years to come because the measurements were carried out in such detail and therefore carry much information.{/access}
The DANAERO MW experiment was initiated because scientists and wind turbine designers in Denmark realised that more knowledge about the aerodynamics for megawatt wind turbines was required to further optimise the design of these wind turbines. In 2007 the DANAERO MW project was given a grant by the Danish Energy Agency. Risø DTU was made the coordinator, and DONG Energy, LM Wind Power, Siemens Wind Power and Vestas Wind Systems agreed on carrying out a series of experiments. The project was finalised at the end of 2009.By Christian Bak (Senior Scientist) and Helge A. Madsen (Research Specialist), Risø DTU National Laboratory for Sustainable Energy, Denmark
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}The experiments agreed as part of the DANAERO project should decrease the uncertainty of several mechanisms characterising wind turbine rotor flow. They were set up to reveal mechanisms such as:
- the three-dimensional flow that is introduced, when the flow is separating on the blades and exposed to the centrifugal forces on the rotating blades
- transition of laminar flow to turbulent flow in the boundary layer of the blade, especially in three dimensions
- the turbulent scales in the inflow to the blade, especially vortices at a size of around 50 centimetres and below
- the aeroacoustic emission
- the variation of the mean wind speed from the bottom to the top of the rotor
- the operation of wind turbines in the wake of upstream wind turbines.
The Experiments
The overall objective of the project was to provide an experimental dataset that can improve the knowledge of a number of fundamental aerodynamic, aeroelastic and aeroacoustic issues and in general improve the design basis for megawatt rotors. Three types of measurements were performed:
- Measurement on two-dimensional airfoil sections in three wind tunnels; at Delft University (NL), at LM Glasfiber (DK) and at Velux (DK).
- Measurement of inflow characteristics on the 3.6MW Siemens wind turbine at the Høvsøre test site (DK).
- Aerodynamic and aeroelastic measurements (including high frequency kilohertz data) on one of the blades (LM 38.8 metre blade) on the NM80 2MW turbine at the small Tjæreborg wind farm in Jutland (DK).
Much information can be extracted from the measurements, but only two examples from the measurements will be presented here. These two examples should be considered as preliminary since the task of post-processing the very large amount of data has only just begun.
Inflow Measurements on the Siemens 3.6MW Turbine
In March 2007 a five-hole Pitot tube was installed at radius 36 metres at the 53.5-metre-long blade on the Siemens 3.6MW turbine at the Høvsøre test site in Jutland, for measurement of local inflow angle and relative velocity. The Høvsøre test site, Risø DTU Test Station for Large Wind Turbines, is the Danish centre for testing megawatt wind turbines and is situated a few kilometres from the west coast of Jutland. In total five turbines are installed along a row at the test site and the Siemens turbine is situated in the middle of this row so the turbine will operate in the wake of other turbines for some wind directions. The signals from the Pitot tube have been sampled at 35Hz together with a number of signals from the turbine such as electrical power and rotor speed, and from a meteorology mast.
One of the mechanisms in the atmospheric flow with considerable effect on the performance and loads on megawatt turbines is the development of strong variation of the wind speed as a function of the height due to stable flow conditions, typically developing during night-time and then disappearing during daytime. A typical example of this has been measured with the five-hole Pitot tube on the Siemens 3.6MW turbine (Figure 1). As shown in the plot in Figure 2, the time trace of the inflow angle measured during night-time at 4am shows a considerable variation for every revolution of the rotor, but otherwise the inflow is very regular. It should be noted that the pitch of the blade is changed by 1 degree every minute, which is clearly seen in the time trace. During daytime at 3pm the variation for each rotor revolution in the time trace has almost disappeared and the signal has become more stochastic.
Aerodynamic and Aeroelastic Measurements on the 2MW NM80 Turbine
The test turbine is situated in a small wind farm at Tjæreborg close to the west coast of Jutland about 1 kilometre from the North Sea. In total the wind farm has eight turbines placed in two rows, which gives different single and multiple wake situations with the closest spacing about 3.5 rotor diameters.
For the NM80 with hub height 57 metres and 80-metre diameter rotor a new LM38.8 metre blade was manufactured, and during the production process equipment for measuring surface pressure profiles at four radial stations was placed inside the blade as shown in Figure 3. Additionally, the most outboard blade section was instrumented with around 50 microphones to measure high frequency surface pressure fluctuations (Figure 4). These data are used for determination of position of transition and for aeroacoustic characterisation. At a radial position close to the microphones, high frequency inflow has also been measured with a hot wire probe and with a Pitot tube.
Eleven measurement campaigns were conducted from late June 2009 to mid September 2009. The campaigns were carried out covering many different wind conditions including free inflow and wake operation.
As well as the pressure, inflow and microphone sensors, a considerable number of strain gauges and accelerometers mounted on the blades, the shaft and the tower were monitored. Also, a meteorology mast of height 90 metres was erected and this measured wind speeds and wind directions at five different heights. The sensors on the turbine and in the meteorology mast were sampled with 35Hz for 10 minutes, while the pressure taps were sampled with 100Hz for 9 minutes and 30 seconds and started together with the 35Hz measurements. The microphones were sampled with 50kHz in 10-second periods, starting each minute.
Much information can be extracted from these measurements, and characteristics concerning the airfoil, the inflow and the aeroelastic response can be analysed. However, here only one example will be shown, which represents the newest development in the measurement technology and information that has never been published for a wind turbine in real, atmospheric conditions. In this example the signals from the 50 microphones in the wind tunnel and on the turbine at the section at 37 metres radius are briefly analysed. Three different situations are compared: the flow over the turbine blade in atmospheric conditions (1), and the flow over the corresponding blade section in the wind tunnel at a level of low turbulence (2) and at a level of higher inflow turbulence generated by a turbulence grid upstream of the blade section (3). Investigations of the energy distribution as a function of frequencies in the blade section boundary layer showed much higher turbulence or unsteadiness close to the leading edge in the atmospheric inflow than in the wind tunnel flow for frequencies up to about 500Hz. This is also clear when 10-second time traces of the surface pressure fluctuations are compared as shown in Figure 5. The scale in the figures is the same and also the inflow velocity on the rotor and in the wind tunnel is almost the same. It is clear from the comparison in the figure that the scales of the turbulence caused by the grid in the wind tunnel are too small to represent atmospheric conditions. In the future analysis of the pressure fluctuation data, a key question will be to investigate what influence the high energy content in the inflow below 500Hz has on transition when compared with wind tunnel conditions.
Further Work
The results from the DANAERO project that are shown here are preliminary and represent a small fraction of the collected data. A new project is now ongoing where a database of all the data is being formed and where many measurements will be analysed. The output from the ongoing project will include a validation of the existing aerodynamic and aeroelastic design complex and formulations of new models and tools. Even though many operational conditions will be analysed in the ongoing project, it is believed that the measurements will be analysed for many years to come because the measurements were carried out in such detail and therefore carry much information.{/access}
