the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Multi-Point In-Situ Measurements of Turbulent Flow in a Wind Turbine Wake and Inflow with a Fleet of UAS
Tamino Wetz
Norman Wildmann
Abstract. The demand on wind energy for power generation will increase significantly in the next decade due to the transformation towards renewable energy production. In order to optimize the power generation of a wind farm, it is crucial to understand the flow in the wind turbine wake. The flow in the near wake close downstream of the wind turbine (WT) is complex and highly three-dimensional. In the present study, for the first time, the SWUF-3D fleet of multirotor UAS is deployed for field measurements on an operating 2~MW WT in complex terrain. The UAS-fleet has the potential to fill the meteorological gap of observations in the near wake with high temporal and spatial resolution wind vector measurements plus temperature, humidity and pressure. During the experiment, the flow up- and downstream of the WT is measured simultaneously. Various flight patterns are used to investigate the near wake of the WT. The velocity deficit and the turbulence profile in different downstream distances are measured by distributed UAS which are aligned perpendicular to the flow in the near wake. The results show the expected double-Gaussian shape in the near wake under nearly stable atmospheric conditions. However, measurements in unstable atmospheric conditions with high turbulence intensity levels lead to single Gaussian-like profiles at equal downstream distances (< 1D). Additionally, horizontal momentum fluxes and turbulence spectra are analyzed. The turbulence spectra of the wind measurement at the edge of the wake could reveal that tip vortices can be observed with the UAS.
Tamino Wetz and Norman Wildmann
Status: closed
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RC1: 'Comment on wes-2023-6', Anonymous Referee #1, 17 Feb 2023
General comments
This study employs a fleet of UAS to measure the flow around a 2 MW wind turbine under various configurations and atmospheric conditions. The velocity and turbulence intensity in the wake are quantified. Exciting and convincing results characterizing the momentum fluxes in the wake and the effect of atmospheric stability on the wake shape are presented. Overall, the manuscript is well-written and provides novel contributions to the field of in-situ wind turbine wake measurements. The study is well within the scope of Wind Energy Science, and I strongly support its publication.
Specific comments
- Table 1 could benefit from the inclusion of some additional information. In the “pattern” column, it would be helpful to know at which longitudinal distance the lateral patterns were located for each case. What was the duration of each flight? Also, adding the lapse rate for each case would provide a sense of how strongly stable or convective the boundary layer was during each measurement. Finally, the region of operation of the turbine (i.e., above or below rated power) will substantially affect the strength of the wake at any given time, so this must be included as well.
- Page 13-14: The discussion of the tip vortex measurements is not very strong. I agree that all of the factors listed will weaken the peak in the frequency spectrum. However, is the signature of the tip vortices visible in any of the time series taken at the wake edge location, even for just a short time? Is this what is meant by the line “approximately seven tip vortices could be observed between the measurement position and the WT”? Especially because tip vortex breakdown is discussed throughout the manuscript, it would be beneficial to elaborate a bit here.
- Page 15: It would be nice to have a discussion of 1-2 more examples from Fig. 9a. For example, in case #708 the velocity shows a strong increase between 1D and 2D. Do you think this is because the large value of γ is causing the centerline of the wake to be deflected away from the measurement location? Or does the large value of Ix indicate that the wake is already starting to recover?
- Page 17, lines 361-362: If future researchers are to use this UAS method to measure wind turbine wake flows, it would be helpful to have some elaboration on the difficulties. What do you think are the sources of uncertainty in the lateral velocity measurements and how could they be addressed in future studies? Do you think there could be vertical wake deflection caused by the terrain? Or is this due to some uncertainty in the lateral velocity measurements themselves or the positioning of the UAS? This information could help future researchers decide whether this method is appropriate for their investigations. This elaboration could also be added to the conclusion section of the manuscript.
- Figures 9 and 10: For the longitudinal flight patterns, did the authors consider applying conditional averaging? Since the wind direction is changing, you could take the average of all velocities that are measured when the instantaneous misalignment angle β is less than a certain value (see references below).
- Page 20: A couple other field studies have observed the double Gaussian shape of the near wake. These should be included for completeness:
- Abraham, A., Dasari, T., & Hong, J. (2019). Effect of turbine nacelle and tower on the near wake of a utility-scale wind turbine. Journal of Wind Engineering and Industrial Aerodynamics, 193, 103981.
- Keane, A. (2021). Advancement of an analytical double-Gaussian full wind turbine wake model. Renewable Energy, 171, 687-708.
Technical corrections
- Why are Δx and Δy used rather than x and y throughout the manuscript? If they are different, make sure they are clearly defined.
- Figure 1: The numbers on the elevation contours are not clearly visible.
- Page 7, lines 175-176: “laps” should be “lapse”.
- Figure 4 legend: There is no black bar in the figure.
- Figures 4, 8-14, and A1: Please make the symbols bigger so the different cases can be differentiated more easily.
- Figure 7: Does the dotted line represent k-5/3? Also based on the caption, the y-axis label should be Su.
- Page 22, line 435: I believe the authors are referring to a different case, as #206 occurs under unstable atmospheric conditions per Table 1.
Citation: https://doi.org/10.5194/wes-2023-6-RC1 -
AC1: 'Reply on RC1', Tamino Wetz, 16 Mar 2023
The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2023-6/wes-2023-6-AC1-supplement.pdf
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RC2: 'Comment on wes-2023-6', Stefano Letizia, 28 Feb 2023
The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2023-6/wes-2023-6-RC2-supplement.pdf
-
AC2: 'Reply on RC2', Tamino Wetz, 16 Mar 2023
The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2023-6/wes-2023-6-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Tamino Wetz, 16 Mar 2023
Status: closed
-
RC1: 'Comment on wes-2023-6', Anonymous Referee #1, 17 Feb 2023
General comments
This study employs a fleet of UAS to measure the flow around a 2 MW wind turbine under various configurations and atmospheric conditions. The velocity and turbulence intensity in the wake are quantified. Exciting and convincing results characterizing the momentum fluxes in the wake and the effect of atmospheric stability on the wake shape are presented. Overall, the manuscript is well-written and provides novel contributions to the field of in-situ wind turbine wake measurements. The study is well within the scope of Wind Energy Science, and I strongly support its publication.
Specific comments
- Table 1 could benefit from the inclusion of some additional information. In the “pattern” column, it would be helpful to know at which longitudinal distance the lateral patterns were located for each case. What was the duration of each flight? Also, adding the lapse rate for each case would provide a sense of how strongly stable or convective the boundary layer was during each measurement. Finally, the region of operation of the turbine (i.e., above or below rated power) will substantially affect the strength of the wake at any given time, so this must be included as well.
- Page 13-14: The discussion of the tip vortex measurements is not very strong. I agree that all of the factors listed will weaken the peak in the frequency spectrum. However, is the signature of the tip vortices visible in any of the time series taken at the wake edge location, even for just a short time? Is this what is meant by the line “approximately seven tip vortices could be observed between the measurement position and the WT”? Especially because tip vortex breakdown is discussed throughout the manuscript, it would be beneficial to elaborate a bit here.
- Page 15: It would be nice to have a discussion of 1-2 more examples from Fig. 9a. For example, in case #708 the velocity shows a strong increase between 1D and 2D. Do you think this is because the large value of γ is causing the centerline of the wake to be deflected away from the measurement location? Or does the large value of Ix indicate that the wake is already starting to recover?
- Page 17, lines 361-362: If future researchers are to use this UAS method to measure wind turbine wake flows, it would be helpful to have some elaboration on the difficulties. What do you think are the sources of uncertainty in the lateral velocity measurements and how could they be addressed in future studies? Do you think there could be vertical wake deflection caused by the terrain? Or is this due to some uncertainty in the lateral velocity measurements themselves or the positioning of the UAS? This information could help future researchers decide whether this method is appropriate for their investigations. This elaboration could also be added to the conclusion section of the manuscript.
- Figures 9 and 10: For the longitudinal flight patterns, did the authors consider applying conditional averaging? Since the wind direction is changing, you could take the average of all velocities that are measured when the instantaneous misalignment angle β is less than a certain value (see references below).
- Page 20: A couple other field studies have observed the double Gaussian shape of the near wake. These should be included for completeness:
- Abraham, A., Dasari, T., & Hong, J. (2019). Effect of turbine nacelle and tower on the near wake of a utility-scale wind turbine. Journal of Wind Engineering and Industrial Aerodynamics, 193, 103981.
- Keane, A. (2021). Advancement of an analytical double-Gaussian full wind turbine wake model. Renewable Energy, 171, 687-708.
Technical corrections
- Why are Δx and Δy used rather than x and y throughout the manuscript? If they are different, make sure they are clearly defined.
- Figure 1: The numbers on the elevation contours are not clearly visible.
- Page 7, lines 175-176: “laps” should be “lapse”.
- Figure 4 legend: There is no black bar in the figure.
- Figures 4, 8-14, and A1: Please make the symbols bigger so the different cases can be differentiated more easily.
- Figure 7: Does the dotted line represent k-5/3? Also based on the caption, the y-axis label should be Su.
- Page 22, line 435: I believe the authors are referring to a different case, as #206 occurs under unstable atmospheric conditions per Table 1.
Citation: https://doi.org/10.5194/wes-2023-6-RC1 -
AC1: 'Reply on RC1', Tamino Wetz, 16 Mar 2023
The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2023-6/wes-2023-6-AC1-supplement.pdf
-
RC2: 'Comment on wes-2023-6', Stefano Letizia, 28 Feb 2023
The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2023-6/wes-2023-6-RC2-supplement.pdf
-
AC2: 'Reply on RC2', Tamino Wetz, 16 Mar 2023
The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2023-6/wes-2023-6-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Tamino Wetz, 16 Mar 2023
Tamino Wetz and Norman Wildmann
Tamino Wetz and Norman Wildmann
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