New Poul la Cour Tunnel at DTU Risø
There are many tools available to the wind turbine engineer in the pursuit of the understanding and optimisation of wind turbine blade aerodynamics. One of the most important (and sought after) is a high Reynolds number wind tunnel, which is an invaluable tool for validating computational models and cost-effectively simulating complex aerodynamic problems. In April 2018, the Technical University of Denmark (DTU) inaugurated the Poul la Cour Tunnel at its Risø campus. The tunnel is one of the largest university-owned tunnels in the world and is dedicated to wind energy research. Together with the Danish aerodynamic upgrade company Power Curve, DTU is carrying out fundamental research into the impact of blade contamination on turbine performance and the mitigation impact of aerodynamic devices such as vortex generators.By Nicholas Gaudern, Chief Technical Officer, Power Curve, Denmark
The specifications of the Poul la Cour Tunnel (PLCT) will be discussed along with the research and development possibilities. The results of initial testing in collaboration with Power Curve will be presented and their impact on aerodynamic upgrade development discussed.
Impact of Blade Surface ContaminationA key focus of Power Curve is to continually drive an increase in the understanding of the impact of blade surface contamination on wind turbine power output and how to effectively mitigate this impact. It is now widely accepted in the wind industry that blade surface contamination, including erosion of the surface and accumulation of dirt/insects, can reduce turbine power output by 2 to 4% even on modern pitch-regulated turbines. Power Curve and the Technical University of Denmark have been partners in several nationally funded research projects in this field, and in both projects wind tunnel testing has been a key component as it allows the measurement of complex aerodynamic phenomena that can be difficult and costly to model computationally using, for example, computational fluid dynamics.
Poul la Cour Tunnel
The PLCT is of the closed-return type and is capable of both aerodynamic force and acoustic measurements. The centrepiece of the tunnel is its 4.7-metre diameter, 2.4MW fan that is capable of producing a maximum test section wind speed of 105m/s, which is more than three times that required to be classified as hurricane force. The huge power of the fan requires the tunnel to have an active cooling system to maintain a stable test section temperature. The test section is 3 metres wide, 2 metres high, and 9 metres long, which gives the tunnel an impressive maximum Reynolds number (Re) of 7 million per metre of model chord. This high maximum Re is critical to be able to test the aerodynamic performance of aerofoils used on the most modern large wind turbines.
The PLCT is of the closed-return type and is capable of both aerodynamic force and acoustic measurements. The centrepiece of the tunnel is its 4.7-metre diameter, 2.4MW fan that is capable of producing a maximum test section wind speed of 105m/s, which is more than three times that required to be classified as hurricane force. The huge power of the fan requires the tunnel to have an active cooling system to maintain a stable test section temperature. The test section is 3 metres wide, 2 metres high, and 9 metres long, which gives the tunnel an impressive maximum Reynolds number (Re) of 7 million per metre of model chord. This high maximum Re is critical to be able to test the aerodynamic performance of aerofoils used on the most modern large wind turbines.
Aerodynamic Measurement Capability
To allow high quality measurements it is important that aerofoil models do not significantly deflect under load; the high wind speeds in the PLCT can subject the aerofoil to over 3 tonnes of lift force! The aerodynamic behaviour of aerofoils will be measured using a large number of pressure sensors – up to 384 in total. A typical configuration would include 160 pressure measurements around the aerofoil surface, 128 along the tunnel walls, and 96 in a traversing wake rake. For measuring the very low drag values associated with aerofoils with large amounts of laminar flow, a wake rake is essential. The wake rake is able to traverse the height and width of the tunnel to capture the pressure loss in the wake, which can be converted to a drag value.
To allow high quality measurements it is important that aerofoil models do not significantly deflect under load; the high wind speeds in the PLCT can subject the aerofoil to over 3 tonnes of lift force! The aerodynamic behaviour of aerofoils will be measured using a large number of pressure sensors – up to 384 in total. A typical configuration would include 160 pressure measurements around the aerofoil surface, 128 along the tunnel walls, and 96 in a traversing wake rake. For measuring the very low drag values associated with aerofoils with large amounts of laminar flow, a wake rake is essential. The wake rake is able to traverse the height and width of the tunnel to capture the pressure loss in the wake, which can be converted to a drag value.
Acoustic Measurement Capability
As wind turbines become more numerous and have increasingly larger rotor diameters with a trend towards higher tip speeds, it is important to continually
reduce their aerodynamic noise. Of course, this must be achieved without reducing aerodynamic efficiency. The PLCT enables simultaneous measurement of both aerodynamic forces and acoustic emissions. This is achieved using microphone arrays installed in a large anechoic chamber surrounding the test section. The anechoic chamber measures 11.5 × 11.0 × 13.0 metres and is effective in the frequency range of 100Hz to 10kHz. To maintain a high quality flow during acoustic measurements, the test section walls that are normally solid for pure aerodynamic tests are replaced by Kevlar walls that constrain the flow while being suitably porous to the transmission of sound waves. Power Curve will utilise the tunnel in this configuration to continuously develop its trailing edge serration product.
As wind turbines become more numerous and have increasingly larger rotor diameters with a trend towards higher tip speeds, it is important to continually
reduce their aerodynamic noise. Of course, this must be achieved without reducing aerodynamic efficiency. The PLCT enables simultaneous measurement of both aerodynamic forces and acoustic emissions. This is achieved using microphone arrays installed in a large anechoic chamber surrounding the test section. The anechoic chamber measures 11.5 × 11.0 × 13.0 metres and is effective in the frequency range of 100Hz to 10kHz. To maintain a high quality flow during acoustic measurements, the test section walls that are normally solid for pure aerodynamic tests are replaced by Kevlar walls that constrain the flow while being suitably porous to the transmission of sound waves. Power Curve will utilise the tunnel in this configuration to continuously develop its trailing edge serration product.Benchmarking of Tunnel Measurements
All wind tunnels are different and to put any measurements in context it is important to compare the test results of well-documented aerofoils from different established facilities. Some of the first tests in the PLCT used the National Advisory Committee for Aeronautics (NACA) 63-418 aerofoil, which is widely used on wind turbines. Figure 4 presents the lift and drag data obtained in the PLCT compared with other facilities and shows very good agreement, which gives confidence that the flow quality of the tunnel and the measurement methodology used are of a high quality. The data is compared at a Re of 3 million as not all of the commercially available tunnels have the ability to achieve higher Re.
All wind tunnels are different and to put any measurements in context it is important to compare the test results of well-documented aerofoils from different established facilities. Some of the first tests in the PLCT used the National Advisory Committee for Aeronautics (NACA) 63-418 aerofoil, which is widely used on wind turbines. Figure 4 presents the lift and drag data obtained in the PLCT compared with other facilities and shows very good agreement, which gives confidence that the flow quality of the tunnel and the measurement methodology used are of a high quality. The data is compared at a Re of 3 million as not all of the commercially available tunnels have the ability to achieve higher Re.
Wind Tunnel Testing of Contaminated Blades
The high Re capability of the PLCT and excellent flow quality enable precision measurement of the impact of blade contamination on aerofoil performance. There are many methods available to simulate blade contamination with the most common being zig-zag tape, which is a thin plastic tape cut in a zig-zag pattern that forces the boundary layer to rapidly transition to a turbulent state, and sandpaper of different grain sizes wrapped around the leading edge to mimic more widely distributed contamination. The most recent test campaign in the PLCT as part of an Energy Technology Development and Demonstration Program (EUDP) project with Power Curve used the NACA 63-418 aerofoil model with an exchangeable leading edge that allowed the installation of different three-dimensional (3D) printed inserts to investigate the impact of more realistic 3D contamination geometries.
The high Re capability of the PLCT and excellent flow quality enable precision measurement of the impact of blade contamination on aerofoil performance. There are many methods available to simulate blade contamination with the most common being zig-zag tape, which is a thin plastic tape cut in a zig-zag pattern that forces the boundary layer to rapidly transition to a turbulent state, and sandpaper of different grain sizes wrapped around the leading edge to mimic more widely distributed contamination. The most recent test campaign in the PLCT as part of an Energy Technology Development and Demonstration Program (EUDP) project with Power Curve used the NACA 63-418 aerofoil model with an exchangeable leading edge that allowed the installation of different three-dimensional (3D) printed inserts to investigate the impact of more realistic 3D contamination geometries.
Benefits of Using Vortex Generators
Reliable computational modelling of the aerodynamics of vortex generators can be difficult and costly. The use of a wind tunnel like the PLCT can enable the measurement of large numbers of different configurations with great precision. Using the same vortex generator panels that Power Curve has installed on over 700 turbines worldwide, a test was carried out in the PLCT to measure the potential performance gains that are possible if these vortex generators are applied to aerofoils with contaminated surfaces. Figure 5 shows a lift polar for the NACA 63-418 aerofoil in three different configurations: clean surface, contaminated surface (modelled with sandpaper), and contaminated surface with Power Curve vortex generators. It is clear to see that the vortex generators will restore much of the lift (especially around the operating region of 5 to 10 degrees) that is lost when the aerofoil is contaminated. On a typical turbine with some contamination, this restoration of lift could increase annual energy production by 2 to 3%.
Reliable computational modelling of the aerodynamics of vortex generators can be difficult and costly. The use of a wind tunnel like the PLCT can enable the measurement of large numbers of different configurations with great precision. Using the same vortex generator panels that Power Curve has installed on over 700 turbines worldwide, a test was carried out in the PLCT to measure the potential performance gains that are possible if these vortex generators are applied to aerofoils with contaminated surfaces. Figure 5 shows a lift polar for the NACA 63-418 aerofoil in three different configurations: clean surface, contaminated surface (modelled with sandpaper), and contaminated surface with Power Curve vortex generators. It is clear to see that the vortex generators will restore much of the lift (especially around the operating region of 5 to 10 degrees) that is lost when the aerofoil is contaminated. On a typical turbine with some contamination, this restoration of lift could increase annual energy production by 2 to 3%.
Future Research
The PLCT has already demonstrated its very high flow quality and precision measurement capabilities through testing campaigns such as the one in collaboration with Power Curve to investigate the benefit of vortex generators on contaminated aerofoils. In the future, many other exciting research projects are planned in collaboration with Power Curve including other aerodynamic add-ons such as Gurney flaps and trailing edge serrations, active and passive trailing edge flaps, laminar–turbulent transition, dynamic stall behaviour, aeroelastic effects, and standstill loads (those critical for blade stability). For further information about the PLCT please see www.plct.dk.
The PLCT has already demonstrated its very high flow quality and precision measurement capabilities through testing campaigns such as the one in collaboration with Power Curve to investigate the benefit of vortex generators on contaminated aerofoils. In the future, many other exciting research projects are planned in collaboration with Power Curve including other aerodynamic add-ons such as Gurney flaps and trailing edge serrations, active and passive trailing edge flaps, laminar–turbulent transition, dynamic stall behaviour, aeroelastic effects, and standstill loads (those critical for blade stability). For further information about the PLCT please see www.plct.dk.
Biography of the Author
Nicholas Gaudern has worked in the wind industry for 10 years since graduating from the University of Cambridge. He has a broad technical background covering the entire wind turbine system and specialises in blade aerodynamics including aerofoil design, add-on design, and the understanding and mitigation of leading edge erosion and contamination.
Nicholas Gaudern has worked in the wind industry for 10 years since graduating from the University of Cambridge. He has a broad technical background covering the entire wind turbine system and specialises in blade aerodynamics including aerofoil design, add-on design, and the understanding and mitigation of leading edge erosion and contamination.
