the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Sep 2023
Synchronised WindScanner Field Measurements of the Induction Zone Between Two Closely Spaced Wind Turbines
Abstract. Field measurements of the flow interaction between the near-wake of an upstream wind turbine and the induction zone of a downstream turbine are scarce. Measuring and characterising these flow features in wind farms for various operational states can be used to evaluate flow models and design control systems at the windfarm level. In this paper, we present induction zone measurements of a utility-scale 3.5 MW turbine with a rotor diameter of 126 m in a two-turbine wind farm operating under waked and un-waked conditions. The measurements were conducted with two synchronised continuous-wave WindScanner lidars that could resolve longitudinal and lateral velocities by dual-Doppler reconstruction. An error analysis was performed to quantify the uncertainty in measuring complex flow situations with two WindScanners by simulating the measurement setup, WindScanners sensing characteristics, and inflow conditions in a Large-Eddy Simulation. The flow evolution in the induction zone of the downstream turbine was characterised by performing horizontal planar dual-Doppler scans at the hub height for four different inflow cases, varying from undisturbed inflow to full and partial wake scenarios. The measurements revealed more evidence of horizontal asymmetry of the induction zone owing to vertical wind shear under undisturbed inflow conditions. Evaluation of the engineering models of the undisturbed induction zone showed good agreement along the rotor axis. In the full wake case, the measurements indicated a deceleration of the upstream turbine wake due to the downstream turbine induction zone as a result of the very short turbine spacing. We observed that the downstream turbine induction zone during wake steering depended on the direction of the wake steering while the lateral movement of the deflected wake could be measured.
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RC1: 'Comment on wes-2023-114', Anonymous Referee #1, 28 Sep 2023
The authors present measurements of the induction zone of a wind turbine. The measurements were acquired by two Doppler lidars, that scanned in a synchronous mode a horizontal plane, at the hub height of a 3.5 MW wind turbine, that extended up to 0.8 D upstream. The conduction of field campaigns using multiple Doppler lidar is a challenging task. The authors provide a thorough description of the installation and the alignment of the wind lidars, as well as an investigation of the magnitude of the errors in the measuring setup. Using the acquired wind measurements, the spatial characteristics of the induction zone are examined during four periods when the inflow of the wind turbine examined was characterised by free flow, fully waked or partially waked conditions. The overall manuscript is well written and well structured. However, I think that there are some parts that should be clarified and explained better, before the article is ready for publication. I am highlighting these points in the list below.
Major comments
- The authors present four data sets of induction zone measurements, which correspond to cases where the inflow of a utility-scale wind turbine is characterised by free, waked and partially waked conditions. There is a thorough description of the observed spatial features of the flow, but there is a lack of scientific conclusions. The interaction of the induction zone with the wake flow is something to be expected. My recommendation to the authors is to present what do we learn from the results of this study. In the last sentence of the Conclusions sections they write “A preliminary evaluation of the engineering models of the induction zone indicates that the models do not completely capture the complex flow behaviour and turbine interactions”. I could not find a discussion of this topic in the manuscript for the case of the waked inflow. Figure 12 shows the induction zone for a free flow, but when the authors write “turbine interactions” I understand they did an evaluation of engineering models of the induction zone in waked conditions.
- Figure 1 presents a photograph of the landscape where the measurement campaign took place. There we can see that the topography of the area between the two wind turbines is not homogeneous. There is a tree fence which is located in the vicinity of the WT1 wind turbine and extending towards the meteorological mast, and furthermore a crop field between the wind turbines WT1 and WT2. The authors don’t describe in detail the physical characteristics of these features and don’t discuss what if there is, or not, an impact of these features on the flow. I think that this is important, since it can partially explain the features of flow presented in Figs 6, 10, 13 and 15. For example, in Fig. 10 (c) the induction zone is seen to be symmetric right in-front of the rotor plane, but it gets asymmetric as the upwind distance increases. Why should this happen? Have the authors acquired any measurements or have they performed an LES study of the flow (where the variations of the terrain features were taken into consideration) while both wind turbines were not operating?
- Following the comment above, in the abstract and the conclusions it is stated that the measurements presented in this article reveal a horizontal asymmetry of the induction zone which the authors claim that it is due to the vertical shear. This statement is based on the observed characteristics of one case (Case 1), with a shear exponent equal to 0.21. On which basis the authors support that this shear exponent is strong enough in order to induce a horizontal asymmetry? And how can they decouple the observed horizontal asymmetry from potential spatial variations of the horizontal flow due the heterogeneity of the terrain?
- I think that the error values that are used in Sect 3.1, concerning the line-of-sight (0.1 %) and the pointing accuracy (0.1 deg) are rather low. Regarding the line-of-sight the authors use as a reference the work of Pedersen and Courtney 2021 to support the choice of the 0.1 % value. First, the value reported in that study concerns a cw Doppler lidar, but not the Doppler lidars used here. And second, I guess that the line-of-sight error is dependent also on the probe length. Regarding the pointing accuracy, this will be dependent on the scanning speed. I think that the authors should address these points in the “Discussion” section.
- Section 3 presents the results of the virtual Windscanner evaluation using LES. According to the LES the largest values of the “w” component are equal to +/- 0.8 m/s (by taking into consideration the contours of Fig.6 and the inflow free wind speed at the hub height (7.7 m/s). However, for the estimation of the propagated uncertainty a constant vertical component is assumed, with a varying magnitude for each of the cases. Specifically, in Sect. 3.2.1 “w” is equal to 0.2 m/s while in Sects. 3.2.2 and 3.2.3 “w” is equal to 1.0 m/s. Can the authors comment on the selection of those values?
Specific comments
- Lines 10 – 12: The authors write: “The measurements revealed more evidence of horizontal asymmetry of the induction zone owing to vertical wind shear under undisturbed inflow conditions”. Why do the authors write “revealed more evidence” here, since it is the first time where they mention the horizontal asymmetry of the induction zone?
- Lines 14 – 15: The authors write: “We observed that the downstream turbine induction zone during wake steering depended on the direction of the wake steering…”. Isn’t this an obvious statement?
- Lines 105 – 106: What was the purpose of equipping the mast with a gas analyser?
- Line 110: What was the spatial resolution of the inflow lidar?
- Line 146, Eq.5: Please describe what is denoted by “x”.
- Line 151: The effective radius is an important parameter for the operation of a cw Doppler lidar since it determines the spatial resolution of the lidar. How is the value stated here (56 mm) determined?
- Line 169: What is meant with the term “greedy controller”?
- Line 172 – 173: How were the group of horizontal planes averaged? Where the measurements grouped based on their position in a grid?
- Line 175, Table 2: The authors present in Table 2 a list of different types of errors along with a qualitative characterisation of their impact. I think that this qualitative characterisation rather than general, it depends a lot on the measuring configuration and features of the measured flow. Therefore, I suggest that the authors should either discuss why the impact of these errors are general or mention that this characterisation concerns the specific measuring campaign. For example, the assumption of zero vertical component is probably not high over offshore areas. Furthermore, maybe this table is more part of the “Results” than of the “Methods”.
- Line 186: Why is the maximum distance of the cw lidars used equal to 300 m?
- Line 189: “This effect is most severe for measurements at the wake edges, …” Please add that this statement concerns this study.
- Lines 202 – 203: What is meant with the term “effective intersection diameter”?
- Lines 213 – 218: I think that the authors should refer here to already published articles that have investigated the impact of the measuring position to measuring errors in a dual lidar measuring configuration, such as:
- Peña A, Mann J. Turbulence Measurements with Dual-Doppler Scanning Lidars. Remote Sensing. 2019; 11(20):2444. https://doi.org/10.3390/rs11202444
Please note that I am neither the author nor the co-author of the above publication.
- Lines 231 – 234: Eqns. 7 and 8 assume that the uncertainty terms are uncorrelated. Please add this assumption.
- Line 244. What are the unfavourable conditions that the authors refer to? Please elaborate. And what was the impact of the lower availability on the spatial distribution of the measurements? Were they certain areas with systematically lower data availability values than others?
- Line 261. What was the agreement between the power law model and the measurements?
- Line 306, Figure 6 (top row, right), why does the distribution of the “w” component is tilted in respect to the wind turbine rotor?
- Line 310: The authors state that there is “an excellent agreement between the LES and the virtual WindScanner…” I think that this is subjective statement. The author should explain why they think that there is a good agreement between the two. Especially when in the next sentence it is stated that there are deviations between the virtual lidars and the LES.
- Lines 316 – 335. The authors state that Fig 7. shows that error in “u” is larger the WT2 rotor plane. However, from the contour plot this is not obvious. For example, the error in P2 is similar to the one at P5 and higher than the P6. Furthermore, the maximum error in the “v” component when y/D>0 looks that is higher than 14%.
- Figure 8. It is very difficult from the colour of the bars to identify the contribution of each error. Can you please choose better colours?
- Line Figure 12 presents the velocity deceleration of the longitudinal wind speed along centre of the rotor. The figure presents that the model based on the 1D Vortex sheet theory fails to reproduce the observed wind characteristics. I am wondering to which extent this is observed due to the model or due to a non-optimum selection of the free wind speed and of the induction zone factor. Have the authors tried to estimate the free wind speed and the induction zone from the Windscanner measurements?
- Line 450. Figure 15 presents the longitudinal component of the wind measured for two different yaw misalignment angles. I can understand the spatial distribution of the “u” component, where the trace of the wake propagates mainly at the region where y/D<0. One the other hand when the yaw misalignment of the WT1 is equal to 12.8 degrees then wake tract is in the region where y/D>0. However the direction of the propagation of the wake seems strange. I would expect that the wake should move upwards, but from the measurements it looks that it is moving downwards. Can the authors comment on this?
Minor comments
Line 5. Please change the ”The measurements were conducted with…” with ”The measurements were acquired by…”
Lines 96 – 97: Please add in this sentence that WT1 is the upstream wind turbine and WT2 is the downstream.
Line 103: Please replace the verb “outfit” with the verb “equip”
Lines 104-105: Please add the names of the Theis products.
Line 140: What is meant with the “Without generalisation …”? Please clarify.
Line 167: Remove the dot after “Table”
Line 168: Table 1. Label: Is it Fig. 2(b) or Fig. 2(a)?
Line 183: Figure 4. Replace “The doted lines…” with “The dashed lines…”
Line 187: Please delete the second “averaging”
Line 252, Table 3. Please add a description of each column in the label of the table.
Line 317. Replace Figure 7 with Fig. 7
Line 325. Replace Figure 7 with Fig. 7
Citation: https://doi.org/10.5194/wes-2023-114-RC1 - AC1: 'Reply on RC1', Anantha Padmanabhan Kidambi Sekar, 18 Dec 2023
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RC2: 'Comment on wes-2023-114', Anonymous Referee #2, 05 Oct 2023
- AC2: 'Reply on RC2', Anantha Padmanabhan Kidambi Sekar, 18 Dec 2023
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