General comments

Dear Authors,

I have reviewed your article and would like to share my feedback.

The article presents an in-field test campaign on a 1.5 MW wind turbine that was converted from an upwind to a downwind rotor configuration. Downwind rotors have not been extensively studied. However, as the authors rightly point out, they could play an important role in accommodating highly flexible blades in future turbine designs.

The article offers several contributions that are valuable to the wind energy research community. These include the safety checks and planning required for converting the rotor configuration, the measurement and comparison of performance, loads, and noise for the downwind and upwind configurations, and the validation of an aero-servo-elastic model against field measurements for both configurations.

The research objectives and hypotheses are clearly presented, and the discussion of the methodology and results is sufficiently detailed. The paper is generally well-structured.

For these reasons, I believe the article merits publication in Wind Energy Science. However, I recommend that the authors address the following comments to strengthen the manuscript.

One major point: while the results are interesting and valuable, they are mostly presented without interpretation. The paper would benefit from a discussion that attempts to explain the trends or, at the very least, provides guidance on how to interpret the results.

Specific comments

-    Abstract: The final sentence is overly specific. I recommend focusing instead on the broader potential of downwind rotor technology for future wind turbines. References to floating wind should be avoided, as this topic is not addressed in the article.

-    R51: “and increased vertical entrainment allows downstream turbines to increase their power output.” Please explain what "vertical entrainment" refers to and why it increases when the wake is deflected downward.

-    R78: “The rotor, however, spins in the opposite direction with respect to the nacelle.” Could you briefly clarify whether this has any implications for the wind turbine generator?

-    R79: “the pitch actuators need to operate between 180° and 270°.” Please note here that this required a change to the blade home position and some physical modifications to turbine components. Clarify that this is not a problem for the pitch actuators.

-    R86: Please remove “The next sections describe the design, planning, execution, and results of the experiment.”

-    Table 1: Some details, such as the wind turbine manufacturer’s address or the turbine serial number, may be unnecessary for understanding the article. Please review and remove any unnecessary information.

-    R142: “reversed aerodynamic thrust”. Clarify that in the downwind configuration, the thrust force during normal operation is directed outward from the nacelle, pulling the transmission rather than pushing it.

-    R168: “that was generated with a 3D scanner.” Please add the accuracy specification of the scanner.

-    R175: “The measurements show a range of 0.5° and a dependency on the yaw angle, which might suggest that the tower is itself not perfectly vertical.” Could this also be due to tower bending from gravitational loads?

-    R195: “ultimate thrust of 250 kN.” How was this value determined?

-    R241: “with a photogrammetry process relying on photos shot from the ground while pointing vertically up” Please comment on the accuracy of this process in estimating pitch angles.

-    R245: “to avoid the risk of falling into the deadband of the vane instrument.” This is unclear. Please briefly explain what the risk is.

-    R249–252: This section is not clear. Did you modify the generator, the gearbox, or both?

-    R293: “only the day of 13 April 2024…construction being stopped on a Saturday”

-    Earlier it’s mentioned that construction was postponed. Please clarify this apparent contradiction.

-    R315: “average wind speed, average turbulence intensity” Are these values measured at hub height? Were they recorded by the wind vane?

-    R366: “Note also that the OpenFAST data refer to the simulations that model the inflow recorded in the field”. In the results section, please calrify that OpenFAST simulations used inflow conditions recorded in the field (for model validation) and also tested identical inflow for both rotor configurations (to isolate configuration effects). This dual approach should be emphasized.

-    R370: “oscillate around the 0% line”. Note that this applies only for wind speeds greater than 8 m/s.

-    R374: “This prediction” Specify whether this refers to the small drop or the higher power.

-    R379: “underpredicting this quantity by as much as 100 %” Could you discuss why the downwind configuration shows higher power, why OpenFAST underpredicts it, and why the uncertainty is larger in the downwind case?

-    R382–383: “The prediction of increasing DEL matches with both the inflow from the field and the inflow from DLC-1.1”. This agreement is especially evident at wind speeds above 7 m/s, where data is more abundant—please specify this.

-    R424: “The downwind dataset is compared to the upwind dataset collected during an IEC noise test…” This is unclear. It may be due to the dataset not being clearly marked in the figure. Consider clarifying or labeling the dataset better.

-    R447: “When the turbine is operating at a rated speed of 18.3 rpm”. Consider adding rotor speed (rpm) as a secondary y-axis in the corresponding figure.

-    R451: “which corresponds to a rotor speed below the minimum operational rotor speed of the turbine”. Is the turbine fully or partially shut down in this condition? Please clarify.

Technical corrections

-    The paper refers to “the team” throughout. While not incorrect, scientific writing is typically impersonal. If this style was not a deliberate choice, consider revising for a more standard scientific tone.

-    Figure 2: Move details from the caption (e.g., blue dot location, microphone positions, meteorological tower) to the legend.

-    Figure 5: Clarify in the caption that the numbers in the matrices represent the count of 10-minute periods.

-    R307: Remove the word “loads”.

-    R320: Replace “, and” with a period.

-    R365: Confirm whether “binned” or “raw” is correct. 

-    R367: Correct “show” to “shows”.

-    R372: Change “numerically” to “in OpenFAST simulations”.

-    R437: Replace “audible” with “present”.

-    R447: Change “rotor speed of the turbine” to “blade passing frequency”.

-    R460: Change “+0.4%” to “0.4%”.

Dear Authors,

I have reviewed the article and summarized my feedback below. 

The article has clear research objectives, well-presented hypotheses, and a detailed discussion of methodology and results. Its overall structure is good.

The article contributes to the wind energy research community. It explains the procedure of operating an upwind rotor in a downwind configuration and gives the details and analysis of the possible risks.  Performing both the measurements and numerical simulations helps compare the downwind and upwind cases as well as validate the numerical method. 

My suggestions for improving the manuscript are listed below.

General comments:

- The term "team" is used frequently, which is not common for a technical article.

- Too much detail is given about the turbine and its manufacturer.

The first sentence of the abstract states: "This paper discusses the motivation, preparation, risk mitigation, execution, and results of a full-scale experiment where the rotor of a 1.5 MW wind turbine was operated in a downwind configuration"

- It would be more clear if it is mentioned that the 1.5 MW wind turbine is an upwind wind turbine.

Lines 56, 183, ..: +40%, +20%, ...: + sign can be left out.

Line 77: "For the same wind and the same observer, the rotor keeps spinning clockwise when viewed from upwind. The rotor, however, spins in the opposite direction with respect to the nacelle."

- Does it mean "the same wind direction and observer position" here? Also, the second sentence is not clear. 

Line 100: "During the experiment, the team made sure to minimize sources of background noise."

- Although the details are given later, this statement sounds subjective in its own. Please add here that the details of these efforts are given below.

Line 121 and caption of Figure 2: Microphone locations are explained in the text and in the caption of the figure. First of all, the detailed explanations in the caption should be moved to the text. Secondly, these measures should be given in rotor diameter (D). 

Line 168: "...3D computer-aided design model that was generated with a 3D scanner"

- It is not clear what kind of design model was used and what was exactly generated. Could you please elaborate on this 3D design model? 

Line 170: "However, the masses listed in the technical documentation include the fixture used for shipping..."

- Isn't it possible to find out the (approximate) weight of the mentioned fixture.

Table 2: Does the "Blade" refer to "Blade mess"?

Line 191: "In upwind operations, thrust at the main bearing is received by a shoulder in the housing and then transmitted to the bedplate. There is no shoulder, however, in the upwind direction."

 - Shouldn't the last sentence refer to "downwind direction"?

Line 195: "The value was set 50 kN higher than the ultimate thrust of 250 kN."

- How is the ultimate thrust determined?

Line 205: "Converting a downwind rotor from upwind to downwind...".

- The second word, "downwind", seems unnecessary.

Line 242: "Table 3 lists the three pitch angles for both upwind and downwind operations, reconstructed via photogrammetry."

- Please indicate the accuracy of these measurements.

Line 315: "...the average wind speed, average turbulence intensity, and average exponential shear exponent."

- Please explain how these data are averaged.

Line 370: "... both OpenFAST predictions and experimental recordings oscillate around the 0 % line ..."

This is not the case for wind speeds below 7m/s.

Lines 414-418: From the results presented in section 5.1, it is clear that OpenFAST is not as accurate in predicting the downstream configuration as in the upstream cases. Could you please elaborate on the possible reasons for this difference in predictions? 

Figure 13: Does this figure refer to a single rotor speed (18.3 rpm) as indicated in text line 448? Please make this clear.