the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jan 2025
On the wake re-energization of the X-Rotor vertical-axis wind turbine via the vortex-generator strategy
Abstract. Wake losses are a significant source of inefficiencies in wind farm arrays, hindering the development of high-energy density wind farms offshore. Studies have demonstrated the potential of vertical-axis wind turbines (VAWTs) to achieve high-energy density configurations due to their increased rate of wake recovery compared to their horizontal-axis counterparts. Recent works have demonstrated a wake control technique for VAWTs that utilizes blade pitch to accelerate the wake recovery, hereinafter referred to as the "vortex-generator" method. The present work is an experimental investigation of the wake topology using this control technique for the novel X-Rotor VAWT. The time-averaged wake topology of the X-rotor has been measured by stereoscopic particle-image velocimetry at three fixed-pitch conditions of the top blades, namely a pitch-in, pitch-out, and a baseline case with no pitch applied. The results demonstrate the wake recovery mechanism linked to the streamwise vorticity system of the rotor and the mechanisms that lead to a streamwise momentum recovery, where the pitched-in case injects high momentum flow from above the rotor while ejecting the wake from the sides. In contrast, the pitched-out case operates in a mirrored fashion, with high momentum flow injected into the wake from the sides while low-momentum flow is ejected out axially above the rotor. These modes of operation demonstrate a significant increase in the available power for hypothetical downstream turbines, reaching as high as a factor of 2.2 two rotor diameters downstream compared to the baseline case. The pitched-in case exhibits a higher rate of momentum recovery in the wake compared to the pitch-out configuration.
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RC1: 'Comment on wes-2025-3', Anonymous Referee #1, 14 Apr 2025
This paper presents an experimental investigation regarding vortex generator (i.e. blade pitch based) wake recovery strategies for an X-Rotor type Vertical Axis Wind Turbine (VAWT). Stereoscopic PIV measurements are conducted in an open jet wind tunnel up to 6D downstream of the X-rotor on cross-stream planes within the wake. Data are obtained for three different blade pitch configurations (for the upper blades of the X rotor only) and the impact on wake characteristics, wake recovery and wake vortex dynamics are discussed. The paper is well written in general. Few points that might be considered for improving the paper are listed below:
1) Please provide a trade-off analysis quantifying the power loss from the upstream turbine when it's operated at different pitched cases versus the potential energy gains obtained within the wake.
2) In addition to thrust coefficients, please provide a table showing the power coefficient levels at different pitch angles as well.
3) Please discuss scaling issues. This turbine operates at a very low Reynolds number when compared to its full scale counterpart. How would this affect the wake vortex dynamics, which is discussed in detail for the scaled turbine.
4) How would this control strategy be implemented on a real full size turbine? What would the authors recommend regarding the necessity of dynamic pitch actuation since a static pitch control would be impractical? Would a dynamic pitch actuation result in similar wake vortex dynamics and recovery?
5) Also please comment on the expected effects of freestream turbulence and wind shear on the observed vortex dynamics within the wake. Current experiments are performed at a very low freestream turbulence level and uniform inflow conditions, which is not realistic in actual wind farms.
6) Adding some wake turbulence related information would be nice. I understand the authors have used only 300 vector maps for averaging, which is not enough to obtain converged statistics, but it would still provide a good supporting information for the arguments presented in the paper, since turbulence has a major effect in wake recovery and the wake dynamics.
7) Please place the coordinate axes exactly at the origin in Figure 4 to clearly show where the origin is. Also, y-axis is defined as a lateral coordinate and z-axis is referred to as an "axial" coordinate. In reality z-axis is not an axial coordinate. The x-axis, which shows the main streamwise direction, is an axial coordinate so I would recommend not referring to z-axis as an axial coordinate.
8) Regarding presentation of results, the beta=0 baseline case is always presented as a middle figure but in the text it's always discussed first since that is the baseline reference case. I understand the current layouts showing beta=-10 deg first followed by beta=0 deg and beta=10 deg, but this layout makes it difficult for the reader to follow the figures when reading the text.
9) Freestream wind speed of 2.7 m/s is quite low, and therefore difficult to measure with low uncertainty. What is the estimated uncertainty level for this parameter?
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Citation: https://doi.org/10.5194/wes-2025-3-RC1 -
RC2: 'Comment on wes-2025-3', Anonymous Referee #2, 15 May 2025
In the paper „On the wake re-energization of the X-Rotor vertical-axis wind turbine via the vortex-generator strategy“ the authors present results from PIV measurements at different distances to the turbine model investigated. The results show very good agreement with theoretical expectations. The paper is well written and the authors do a good job to explain the complex aerodynamic situations. Nevertheless, one important point is missing, which the authors mention but do not give an answer to: what is the power loss of the turbine when operated with these pitch angels? One finding is the increase in available power for a second in.lien turbine, but that could also be increased by just shutting the first turbine down (extreme measure).  I totally agree that it is important to understand the underlying phenomena resulting from the pitched blades and the impact on the wake recovery, but that has to be set in context with the performance of the turbine itself. I highly recommend that the authors measure the power output of the turbine for these three blade pitch angles. If the first turbine suffers too much, what would be the consequence — would a smaller pitch angle also have the same impact on the wake or would that effect be negligible for smaller pitch angles?
In line 50 the authors write that the X_rotor concept has been further advanced to the XROTOR concept. In the following they explain that in that XROTOR design there are HAWT at the tips of the X-rotors. Unfortunately, at this point it is not clear which design they used for the investigations in the paper. In line 51 they write „the X-Rotor is an innovative VAWT design …“, but isn’t that innovative design called XROTOR? They describe primary and secondary blades, but never use XROTOR again in the text. In figure 3, where the model is explained, there are also no HAWT at the blade tips — I was very confused and to not really the point in mentioning the XROTOR concept and explaining it especially since it is not clear when that concept is not of interest any more.Â
Figure 4 is hard to understand, but I think the authors already did a very good job to explain it. Nevertheless, it is not explained: a) why the blades on the lower side are shorter? and b) why are they not pitched in this investigation. The authors should explain briefly why this is the case instead of just stating that it is like that. Why did the authors chose +-10° for the pitch angles ?
In figure 3 it looks like the sketches of the pitched blades are int he wrong oder — it looks like the the upper right one belongs to the inward blade pitch (10°) and lower right one to the outward blade pitch (-10°)
I think figure 5 is not needed since the differences in the planes are not that significant and they have no real consequence for the following analysis. Just mentioning that the planes have been slightly shifted upwards to capture the more interesting areas is enough.Â
The results presented in figure 6 show nicely the effect of the different blade pitch on the wake development. This also raises the question, why the lower blades are not pitched to increase that effect. At the same time, it would be interesting to see how the c_p of the turbines behaves with the pitched blades. I assume that it will go down, but by how much is the question, which is important for optimising the total output of a potential wind farm. Can the authors comment on that ?
In figure 8 the authors should remove (or at least clearly mark) the points for which the area and perimeter could not be calculated since the wake was not completely covered by the PIV data. The points are not representative and are misleading. The text should be adapted accordingly. This also applies to all other data in which part of the information is missing, e.g. trajectory of CVP cores — but this only the authors can decide.Â
The discussion in 4.2 (available power) is a little misleading since it only concentrates on the available power in the wake purely based on the u_x component of the wind. Like mentioned before, the X-Rotor will have a decreased power coefficient with the pitched blades, which is totally neglected here. Also, the inflow conditions for the second X-Rotor are totally different since cross flow must be increased dramatically, which in turn must have an impact on the performance of the second turbine. This should be discussed in more detail since the overall producible power should increase and not only the theoretically available power.
It is really impressive how well the measured data follows the theoretical behaviour sketched in figure 1 (or vice-versa). I can only imagine the experimental effort put in these experiments.
The translucent lines are really hard to see, the authors should increase the contrast a bit.Â
Some minor and general comments:
It could help to label the figures with a), b) and so on, it makes the referencing easier.Â
In figure 17 please also use the nomenclature A, A_x, A_y,…., etc. to be consistent with figure 16.
line 24 : HWAT is not explained (even though it should be clear, it would be good just to have all abbreviations explained)
line 109: What is „AC“ ?
line: 148. After Figure 2 a space is missing.
line 319: I believe it should be „the projected frontal area“ and not the „the protected“.
Citation: https://doi.org/10.5194/wes-2025-3-RC2
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