the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Influence of rotor blade flexibility on the near wake behavior of the NREL 5MW wind turbine
Leo Höning
Laura J. Lukassen
Bernhard Stoevesandt
Iván Herráez
Abstract. High fidelity computational fluid dynamics simulations of the NREL 5MW wind turbine rotor are performed, comparing the aerodynamic behavior of flexible and rigid blades with respect to local blade quantities as well as the wake properties. The main focus has been set on rotational periodic quantities of blade loading and fluid velocity magnitudes in relation with the blade tip vortex trajectories describing the development of those quantities in the near wake. The results show that the turbine loading in a quasi-steady flow field are mainly influenced by blade deflections due to gravitation. Deforming blades change the aerodynamic behavior, which in turn influences the surrounding flow field, leading to non-uniform wake characteristics with respect to speed and shape.
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Leo Höning et al.
Status: final response (author comments only)
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RC1: 'Comment on wes-2023-78', Anonymous Referee #1, 01 Sep 2023
The paper “Influence of rotor blade flexibility on the near wake behavior of the NREL 5MW wind turbine” provides further insight into the aerodynamic response of the NREL 5MW wind turbine using both rigid and flexible simulations. In particular, the work focuses on the effect of a flexible structure on the near wake, both in terms of wake deficit and tip vortex trajectory. The paper is well structured and represents valuable work worthy of publication in WES, however some comments need to be addressed before publication. In addition, the clarity of the manuscript could be improved in some points. I have tried to mark the most difficult passages, where I believe the authors should try and improve the presentation of the work.
Major:
Lines 50-54: It is unclear whether the authors are referring to the centrifugal forces, the gravitational forces, or something else. Please clarify.
Section 2.2. Since the work is focused on the behavior of the wake, additional information should be provided on the boundary conditions of the simulation, especially on the inflow conditions, as these could alter the wake response of the turbine significantly. Was a laminar inflow used? Otherwise, which values were used for the inflow turbulence?
Section 2.4. No information is provided about the discretization of the rotor region and boundary layer. The discretization of the blade is crucial for correctly capturing the aerodynamic response of the rotor and the behavior of the tip vortices. The authors cited the PhD thesis by Dose (2018), however I could not find it and it does not seem easily accessible for the interested reader. Nevertheless, further details should be included anyway, at least concerning the number of elements used to discretize the boundary layer and the subsequent value of the y+.
Line 198: There are no vertical red lines in Figure 2 (b). I imagine the authors are referring to the black dashed lines in the same Figure. Please clarify if this is not the case, and modify the Figure if necessary.
Line 226: I think the constant \omega expresses the rotational speed and not the rotational frequency.
Line 226: “This sinusoidal torsion behavior is superimposed by deformation due to the eigenfrequencies of the blade with a much smaller magnitude. Although barely visible in the three investigated deformation components (Figure 3), it can be observed in the resulting forces acting on the blade especially in the frequency domain (Figure 2(b)).” If I understand this correctly, the authors are pointing out the small-amplitude and high-frequency oscillations observed in the torsional deformation in Figure 3 (c). Then “barely visible” is referred indeed to such deformations. If this is the case, the authors should clarify this section of the text as it can be cause of confusion. Maybe it should be highlighted first that the torsional deformations described previously are clearly visible in Figure 3 (c). Then, it should be pointed out that there are also higher frequency components in the time-series which are due to the eigenfrequencies of the blade.
Minor:
Line 5: change are to is.
Lines 29- 32: I believe the sintax of this sentence could be simplified for clarity.
Line 33: I believe a preposition and article are missing. The text should read “[…] based [on a] non linear […]”
Line 37: I believe the sentence could be improved: “This study showed that the averaged aerodynamic power output is reduced on the one hand, due to the blades bending towards a smaller rotor diameter and therefore less area to extract energy from, and on the other hand, [due to the] twisting [of] the blades towards lower angles of attack (AoA), which leads to a lower gliding ratio”.
Throughout the text, Sect. should be used rather than Sec., as per WES recommendation.
Lines 123 & 126: Two times the term “mio” appears, I believe referring to million. I think this might be a typo.
Line 276 typo: therefor
Line 325: typo: “shows the non-dimensionalized radial location of [the] tip vortex trajectory […]”
Line 336: The expression “the flexible blades induce more velocity at 270° and induce less velocity in the axial component at 90°” could be improved, as an increase in induction factor means a reduction in wake velocity and vice-versa.
Citation: https://doi.org/10.5194/wes-2023-78-RC1 -
RC2: 'Comment on wes-2023-78', Anonymous Referee #2, 12 Sep 2023
Influence of rotor blade flexibility on the near wake behavior of the NREL 5MW wind turbine
This manuscript compares the results of high-fidelity simulation of a reference wind turbines with rigid and flexible blades. Results shows the difference in blade loading and near wake features, such as velocity deficit, vortex trajectories, etc between the different settings. Due to the increase of rotor size, the blades will become more flexible. I believe the results are interesting to wind energy community.
I have some general comments regarding the manuscript:
- The motivation to study the near wake using a coupled FSI model needs more elaboration.
- Some detail regarding the simulation setup is required.
- Uncertainty due to the data extraction method needs to be quantified.
Please refer to the attached pdf file for some additional comments.
Good luck with the revision!
Leo Höning et al.
Leo Höning et al.
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