the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Oct 2025
Detailed experimental investigation of the aerodynamics and blade/tower interaction of a 1.5 MW wind turbine in a downwind configuration
Abstract. We present a detailed experimental investigation of the flow details of the blade/tower interaction on a 1.5 MW wind turbine operated in a downwind configuration. The study aims to address claims of shortcomings in the downwind turbine concept, such as impulsive blade loading and the generation of low-frequency noise. The measurement campaign was part of a comprehensive project conducted by the National Renewable Energy Laboratory and its partners exploring the feasibility, reliability, and performance of wind turbines whose rotors are oriented downwind instead of the more traditional upwind configuration. The campaign measured blade pressure distributions at two radial positions on one blade and pressure on the tower surface at the position corresponding to the blade path of the outboard instrumented section on the blade. Pressure distributions on the blade and tower were measured using add-on pressure belts, and local inflow was measured on the blade with two five-hole probes. Despite challenging weather conditions, two measurement campaigns of different durations were successfully completed. The analysis of the datasets indicates the data are of good quality and highlights the importance of dynamically reconstructing the tower pressure measurements that used pressure belts up to 10.7 m in length. Impulse loading is found to be 100–150 N m−1 on the tower and 200–500 N m−1 on the blade in a time span of approximately 0.3 s. The detailed pressure measurements make the dataset well suited for use in validating high-fidelity models such as full three-dimensional computational fluid dynamics rotor simulations that include tower flow.
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Status: open (until 18 Dec 2025)
- RC1: 'Comment on wes-2025-204', Anonymous Referee #1, 23 Nov 2025 reply
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General comments:
A nice article about the application of an innovative measurement technique to gain new insights on the topic of blade/tower interaction for downwind rotors. The article is rather lengthy for the message it brings and some parts could be streamlined better to make it more concise. Although the purpose of the measurement dataset is to progress modeling and validation, there is very few about this in the paper. As such the novelty is mainly about the challenges faced during the experimental campaign and the corresponding solutions instead of the intended application of the results. However the results do provide an unique insight in the aerodynamic pressure on blades and tower in a downwind configuration, although the result interpretation could benefit from a more thorough study. From the viewpoint of novel measurements, the article provides new and valuable insight and is worthy of publication.
It is recommended to follow up on the below suggestions for improvement:
-p2 intro. Although later on in the paper in section 4.5.1 the NREL UAE from 2000 is mentioned, which featured wind tunnel testing of a downwind configuration, it is not mentioned here, while it is believed to be applicable.
-p5 line 113. It is indicated that one of the pressure ports of the belt is used as a reference. Although reference is made to a 2022 publication, please make clear where and how the reference pressure is obtained, both in the case of blade and tower application? For the blade application, is the pitot used? From section 3.5 about calibration of the 5 hole probe, line 237 at p12 seems to indicate the static pressure from the pitot is used as reference to the belt? If this is the case, it is unclear whether a physical connection by means of a tube exists?
-p7 fig .4 The layout is difficult to read in spanwise direction and can be ambiguously interpreted. Consider using different colours and/or a legend to fix this.
-p7 section 3.3 The dynamic pressure reconstruction is validated by generating rapid pressure pulses, which is very encouraging to see. Could the authors comment on the applicability of the reconstruction method, acknowledging real life pressure variations consist of a mixture of different frequencies?
-p9 section 3.5. Please clarify the significance of this calibration (also refer to the above comment about reference pressure)
Can the authors comment on the applicability of the wind tunnel calibration in comparison to operation in a turbulent environment with flow curvature?
-p11 fig. 8 Please clarify the significance of the variables in horizontal and vertical axes in reference to Fig 6b and eq. (7) to (10)
-It is recommended to investigate to what extent Fig 10 to 13 can be reduced/removed without detriment to the message of this paper.
-p17/18 line 320-331 and beyond. Provide a reference to the 2000 work (see also intro comment).
Can the authors comment on what would be a realistic/representative distance between tower and blade for a downwind configuration, in relation to the difference between the 2000 experiment and here? Besides this difference and the absence of tower measurements in 2000, what was the outcome of the downwind measurements carried out in 2000 against this work, e.g. are the physics comparable?
-p16/17 section 4.3 and 4.4 fig 14/15/16. It is hypothesized that the differences in pressure coefficient are largely related to uncertainty in the estimated AOA from the five-hole probe. Acknowledging this reasoning, what angle of attack in the 2D simulations would be needed to match the pressure distributions? I suppose that would be easy to verify. Are the lift coefficient deviation shown in Fig. 16 in line with this hypothesis?
-Fig 17 Can the wind speed variation be included in this plot as well to observe the correlation with loads?
-p19 section 4.5.2 and fig 18 and 19. A qualitative description of the observed load variation on tower and blade are given for a high and low wind speed. Can the authors comment on the different physics behind the observed different load variations between low and high wind speeds cases (e.g. occurrence of dips instead of peaks)?
p26 section 4.5.5 last paragraph
Acknowledging the small distance between tower and blade, in how far is it justified to model tower flow and its effect on the blade flow separately, in other words is the interaction between the two negligible?
How is the tower incident inflow speed defined in the OpenFAST implementation, is it impacted by the blade induced velocity and if so in an iterative way? If not accounted for would its effect reduce the difference in the magnitude of the drop between simulations and measurements? As suggested it would be nice to compare the five hole probe measured speed to the tower model predicted tower wake velocities, which should not be difficult to do?