the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Flutter Behavior of Highly Flexible Two- and Three-bladed Wind Turbine Rotors
- UTD Center for Wind Energy, Department of Mechanical Engineering, The University of Texas at Dallas, Richardson, Texas, USA
- UTD Center for Wind Energy, Department of Mechanical Engineering, The University of Texas at Dallas, Richardson, Texas, USA
Abstract. With the progression of novel design, material, and manufacturing technologies, the wind energy industry has successfully produced larger and larger wind turbine rotor blades while driving down the Levelized Cost of Energy (LCOE). Though the benefits of larger turbine blades are appealing, larger blades are prone to aero-elastic instabilities due to their long, slender, highly flexible nature, and this effect is accentuated as rotors further grow in size. In addition to the trend of larger rotors, new rotor concepts are emerging including two-bladed rotors and downwind configurations. In this work, we introduce a comprehensive evaluation of flutter behavior including classical flutter, edgewise vibration, and flutter mode characteristics for two-bladed, downwind rotors. Flutter speed trends and characteristics for a series of both two- and three-bladed rotors are analyzed and compared in order to illustrate the flutter behavior of two-bladed rotors relative to more well-known flutter characteristics of three-bladed rotors. In addition, we examine the important problem of blade design to mitigate flutter and present a solution to mitigate flutter in the structural design process. A study is carried out evaluating the effect of leading edge and trailing edge reinforcement on flutter speed and hence demonstrates the ability to increase the flutter speed and satisfy structural design requirements (such as fatigue) while maintaining or even reducing blade mass.
Mayank Chetan et al.
Status: final response (author comments only)
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RC1: 'Comment on wes-2021-132', Anonymous Referee #1, 05 Feb 2022
Dear authors, thank you for a nicely written article on a very relevant topic. The paper reads well and I recommend publishing it, pending some key improvements. The biggest feedback is that the article falls short of what I was hoping to find in it. First, the paper is a little blurry on what is novel in terms of theory and models. I am confused whether the models presented in section 2 are any different than the models available in literature and correctly cited in the bibliography. This should be made clearer. In addition, the exploration of the design space is very limited. Why only LE and TE reinforcement thickness? What about outer shell skin thickness, or spar cap placement, or relative placement of LE and TE? Overall the reader is left with many unanswered questions, and this is a pity, since you have all the tools to answer these questions. Finally, I would like to see what it takes to get the designs up to an acceptable flutter margin. It would be good to define this acceptable limit rather than increasing the margin to an arbitrary amount. Then it would be good to see the feasibility of the blade from a design/manufacturing/cost perspective.
Below you find a longer list of comments pointing to specific parts of the text that have margin for improvements:
Title: the title does not seem to be the most accurate: none of the 3-bladed rotors is highly flexible. On the contrary, the 3-bladed designs are actually fairly stiff
Figure 2: This is a nice image, but it is slightly confusing. It reports the WindPACT rotor, which is not listed in 3.1. The IEA10 is instead listed in 3.1, but is not included in Figure 2 nor in Table 1
Table 1: columns on the right miss units
Table 2: wouldn’t it be more appropriate to use the digital twin described in https://doi.org/10.1002/we.2636?
Page 8
line 168: please quantify “slightly higher”
Line 170: I think it's a little hard to make this claim without evidence, especially since the flap and torsion are coupling in this instability.
Line 172: don’t all these references use the same exact formulation? I find this misleading throughout the paper, see points above
Table 3: It might be nice to add the rated rpm for each design
Figure 6 and 7: please add torsional stiffness
Page 12
Line 227: solidity of the rotor, not of the turbine
Line 228: “larger margins between the EA and CG locations in the chordwise direction.” This is not necessarily true, rather an artifact of some simple scaling steps
Table 5: 13A and 13B have a super low margin. Perhaps these (and the SUMR50) cases would be most suitable for additional analysis in the next section since they have the lowest margin. Also, the flutter margin of SUMR50 is much too low to be a reasonable design. The authors should make some note of this.
Page 14
line 262: what about the aerodynamic center as an important design driver?
Line 272: “real designs”, This is confusing. These aren't real designs except for the SUMR-D blade, which is a blade designed with scaling laws, so not at all close to industrial products.
Page 15
Line 273: I question the ability of the SUMR-50 blade to meet these requirements given the low flutter margin.
Page 16
Section 5.1 would benefit a revision. This study doesn't use any of the (very nice) features listed in the text, while it would be more useful to know how the blade structures and aerodynamics are modeled
Line 298: I wonder if these analyses consider the blades to be rotating
Figure 9: please focus on the aspects related to this study. For example, how have you been computing blade stiffnesses? With PreComp? Or BPE? Or maybe Becas?
Page 17
Line 308: I was hoping to understand much more about the solution space. For example, what about the effect of chordwise placement of spar caps and shear webs. And what about chordwise placement of the blade axis? This is only one of many design choices that blade designers need to take. The feeling is that you've not moved the needle on this design exploration to a sufficient amount. Much can and should be done.
Line 309: Related to the point above, outer and inner skins are much more important for the torsional behavior than LE and TE reinforcements.
Line 317: One tricky aspect here is that you're likely using shell elements in modeling of the blade structure. Shell elements accept any thickness, although fin-shape trailing edges cannot physically fit many composite layers
Line 320: point #1 is kind of obvious... because of the airfoil shape, moving away from the LE moves you from the neutral axis faster than at the TE. LE is therefore better suited to increase flap stiffness, although this is probably not very relevant. You don't add material at the LE to increase flap stiffness
Line 322: flapwise frequencies are more (not mostly) sensitive
Line 323: point #4 seems just a natural consequence of points #1 and #3
Line 326: please explain the sentence “for the flutter speed, this occurs at lower TE and higher LE reinforcement”
Figure 11 in the top left corner I’d recommend plotting flutter margin. In the bottom right, it’s unclear why the space is so non linear
Page 20
Line 339: I have the feeling that what's truly happening here behind the hood is a change in the positions of EA and COG. I'd find that analysis much more informative than a parametric study on LE and TE thicknesses. It’s hard to draw any general conclusion out of this parametric study
Line 346: please spell out acronym MAC
Line 349: The analysis seemed to show some correlation for "longer" blades, should use that instead of "larger"
Line 350: rotor instead of turbine
Line 352: The difference between points 1 and 2 is subtle and having a single point would improve readability
Line 359: flutter margins are reduced as well, which is likely more important than flutter speeds
Line 366: This was already said in line 355. The whole paragraph seems convoluted and could be shortened focusing on the key conclusions, which I understand as:
1) Flutter margins decrease with blade length
2) 3-bladed rotors show edge instabilities at lower rpm than flap-torsion flutter
3) The opposite holds true for 2-bladed thanks to larger chords generating higher edge stiffness
Page 21
Line 380: All these works use the same theory. This seems to me a point of great confusion. The methods for flutter prediction used in this work seem no different than the works cited from literature. It is certainly reassuring to see that the results presented in this work match older results, but the paper should make clear that the theory is not a novel contribution, only the parametric studies and the comparison 3-vs 2-bladed rotors.
Line 382: I don't think your paper supports this conclusion, aside from the findings already available in literature
- RC2: 'Reviewer comment on wes-2021-132', Anonymous Referee #2, 23 May 2022
Mayank Chetan et al.
Mayank Chetan et al.
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