Proof of concept for wind turbine wake investigations with the RPAS SUMO

Joachim Reuder1, Line Båserud1, Stephan Kral1, Valerie Kumer1, Jan-Willem Wagenaar2

1Geophysical Institute, University of Bergen2Energy Research Centre of the Netherlands (ECN)

joachim.reuder@uib.no

REFERENCES• Båserud, L.,J. Reuder, M. O. Jonassen, S. T. Kral, M. Bakhoday Paskyabi, and M. Lothon,

Proof of concept for turbulence measurements with the RPAS SUMO during the BLLAST campaign, submitted to Atmospheric Measurement Technique Discussions, 2016

• Kumer, V., J. Reuder, B. Svardal, C. Sætre, and P. Eecen,Characterisation of single wind turbine wakes with static and scanning WINTWEX-W LiDAR data, Energy Procedia, 80, 245-254, DOI: 10.1016/j.egypro.2015.11.428, 2015.

• Reuder, J., P. Brisset, M. Jonassen, M. Müller, and S. Mayer, The Small Unmanned Meteorological Observer SUMO: A new tool for atmospheric boundary layer research, Meteorol. Z., 18, 2, 141-147, 2009.

• Reuder, J., M. Jonassen, and H. Olafsson, The Small Unmanned Meteorological Observer SUMO: Recent developments and applications of a micro-UAS for atmospheric boundary layer research, Acta Geophysica, 60, 5, 1454-1473, DOI: 10.2478/s11600-012-0042-8, 2012

• Reuder, J., L. Båserud, M. O. Jonassen, S. T. Kral, and M. Müller, Exploring the potential of the RPA system SUMO for multi-purpose boundary layer missions during the BLLAST campaign, submitted to Atmospheric Measurement Technique Discussions, 2016

ACKNOWLEDGEMENTSThe work presented in this study was performed in the collaboration project”Qualification of SUMO technology for wind turbine wake assessment”between the Geophysical Institute at the University of Bergen and Statoil ASA.The SUMO measurements were embedded in the joint ECN-NORCOWEmeasurement campaign WINTWEX-W. The Norwegian Centre for OffshoreWind Energy (NORCOWE) is funded by the Research Council of Norway(RCN1938211560). Part of the instrumentation deployed during the campaignhas been provided by the National Norwegian infrastructure project OBLO(Offshore Boundary Layer Observatory) also funded by RCN (project 227777).We would like to thank the Energy Research Centre of the Netherlands (ECN)and all involved staff for their collaboration during and after hosting theWINTWEX-W campaign, and in particular for facilitating the SUMO flightspresented here. The authors are also grateful to Martin Müller and ChristianLindenberg for the preparation and operation of the SUMO system during thecampaign.

The measurements

The SUMO flights presented in this study were embed-ded in the joint measurement campaign WINTWEX-W(Kumer et al., 2015) performed in collaboration bet-ween NORCOWE and ECN at the ECN test siteWieringermeer in the Netherlands. This campaign wasspecifically dedicated to the full-scale investigation ofstructure and dynamics of single turbine wakes andlasted from November 2013 to May 2014.On May 10, 2014 the SUMO system performed 5 flightmissions in the vicinity of the 5 NORDEX N80 2.5 MWturbines. The racetrack pattern chosen provides foreach flight horizontal turbulence transects parallel tothe row of wind turbines at two different distances. Witha wind from SW of about 12-15 m/s the track positionsA and B were located ca. 5 and 1.5 rotor diameter (D)downstream.

The SUMO system

SUMO (Small Unmanned Meteorological Observer) is a micro-RPAS (Remotely Piloted Aircraft System) with a wingspan and length of 80 cm and a take-off weight of 700 g (Reuder et al., 2009). In addition to sensors for the basic atmospheric parameteres temperature, humidity and pressure it is now also equipped with a commercially available turbulence measurement system based on a 5-hole flow probe (5HP) that allows measurements of the 3-dimensional wind vector with 100 Hz (Reuder et al., 2012, 2016; Båserud et al., 2016).

SUMO vs. sonic anamometer

In a first step we compared the wind velocity components averaged over theSUMO flight legs, measured at altitudes between 80 and 120 m, with thecorresponding data from a sonic anemometer at 108 m height on the meteoro-logical mast MM3 (left figure). Both data sets represent upstream wind con-ditions. The black line shows the sonic anemometer measurements in 32 Hzresolution, the gray line denotes a 10 min running mean. The SUMO measure-ments are given as average values over each straight leg and marked asorange stars. The data shown are for the eight legs of flight #4.The main results are:• excellent agreement for the East-West component u• slight underestimation for the North-South component v, most likely due to

the neglection of the yaw angle deviations in-flight• slight overestimation in the vertical component w, most likely due to an offset

in the angle of attack of the 5 HP

A comparison of the spectral behaviour, between the 100 Hz measurementsfrom the 5HP of SUMO and the 32 Hz data from the sonic anemometer is inthe figure to the right. It shows the instantaneous and averaged energy spectraof the wind velocity components u, v, and w.• both systems agree in general reasonably well and show a clear inertial

subrange• the peak in the SUMO data for u and v around 1 Hz is most likely induced by

aircraft motion triggered by the internal control loops of the autopilot• the obvious lack of energy in the v component for the SUMO data for the

lower frequencies requires further investigations• SUMO spectra for the vertical component are following the -5/3 slope of the

inertial subrange also for the highest frequencies, while the sonic data areshowing signs of spectral attenuation in this region; potentially a signature offlow distortion by the mast.

SUMO wake measurements

The second step was the analysis of the collected data sets with respect to the wakecharacterization. The left figure presents the East-West wind component u measuredalong the flight track given in UTM coordinates for the positions B (ca. 1.5 rotordiameter downstream) and D (upstream). The overall length of the x-axis presented is1 km and the ticks are labeled every 100 m. The thin gray lines show the data from theindividual legs (10 in the case of B and 4 in the case of D), the colored lines a 10 m binaverage along the UTM East-West coordinate.In the flight legs at position B the wind turbines WT6 and WT7 create a clear signature:• the reduction in wind speed reaches 3-4 m/s• the wake deficit extends over a horizontal distance of about 150 m• both wind speed deficit and extension compare well with corresponding measure-

ments from static and scanning wind lidar systems (Kumer et al., 2015)

The turbulent kinetic energy (TKE) distribution over the flight legs, calculated from theSUMO flow measurements gives deeper insight into the flow and wake structure:• higher level of TKE and TKE variability at A and B downstream compared to C and D

upstream, indication the turbulence induced by the wind turbine row• highest TKE level in the flanks of the wake (indicated by the black arrows in the right

figure), while the TKE in the center of the wake is only slightly enhanced comparedto the background TKE.

Location of the 5 NORDEX N80 2.5 MW turbines(WT5-WT9) and the 108 m meteorological mast at theECN test site Wieringermeer, together with the tracksof the 5 SUMO flights performed on May 10, 2014.The wind was coming from Southwest, placing theflights # 1-3 and 5 downstream and the flight # 4upstream of the row of wind turbines..

Instantaneous (left) and averaged (right) energy spectra of the wind velocitycomponents u (top), v (middle) and w (bottom) for leg 2 during flight # 4 at14:52 UTC. The SUMO data are in orange, the sonic anemometer data inblack. The gray lines indicate the expected -5/3 slope of the inertialsubrange.

Comparison of wind speed measurements taken by SUMO andthe sonic anemometer at 108 m on the meteorological mastduring flight # 4. The sonic anemometer data are given by theblack lines, the SUMO data are marked by the orange stars.The panel show from top to bottom: East-West wind componentu, North-South wind component v, horizontal wind speed vh, andvertical wind speed w.

above: East-West wind speed component u measured by SUMO inpositions B (ca. 1.5 rotor diameter downstream the row of windturbines) and position D (upstream).

right: Turbulent kinetic energy (TKE) as function of the horizontalposition derived from the SUMO 5HP turbulence measurements for the4 different positions with respect to the wind turbine row.