the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
01 Nov 2021
01 Nov 2021
CFD-based curved tip shape design for wind turbine blades
- 1Aero- and Fluid Dynamics (AFD) section, DTU Wind Energy, Lyngby Campus, Nils Koppels Allé, building 403, 2800 Lyngby, Denmark
- 2Airfoil and Rotor Design (ARD) section, DTU Wind Energy, Risø Campus, Frederiksborgvej 399, 4000 Roskilde, Denmark
- 1Aero- and Fluid Dynamics (AFD) section, DTU Wind Energy, Lyngby Campus, Nils Koppels Allé, building 403, 2800 Lyngby, Denmark
- 2Airfoil and Rotor Design (ARD) section, DTU Wind Energy, Risø Campus, Frederiksborgvej 399, 4000 Roskilde, Denmark
Abstract. This work presents a high-fidelity shape optimization framework based on computational fluid dynamics (CFD). The presented work is the first comprehensive curved tip shape study of a wind turbine rotor to date using a direct CFD-based approach. Preceeding the study is a thorough literature survey particularly focused on wind turbine blade tips in order to place the present work in its context. Then follows a comprehensive analysis to quantify mesh dependency and to present needed mesh modifications ensuring a deep convergence of the flow field at each design iteration. The presented modifications allow the framework to produce up to 6 digit accurate finite difference gradients which are verified using the machine accurate Complex-Step method. The accurate gradients result in a tightly converged design optimization problem where the studied problem is to maximize power using 12 design variables while satisfying constraints on geometry as well as on the bending moment at 90 % blade length. The optimized shape has about 1 % r/R blade extension, 2 % r/R flapwise displacement, and slightly below 2 % r/R edgewise displacement resulting in a 1.12 % increase in power. Importantly, the inboard part of the tip is de-loaded using twist and chord design variables as the blade is extended ensuring that the baseline steady-state loads are not exceeded. For both analysis and optimization an industrial scale mesh resolution of above 14 · 106 cells is used which underlines the maturity of the framework.
Mads Holst Aagaard Madsen et al.
Status: open (until 12 Feb 2022)
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CC1: 'Comment on wes-2021-115', Malo Rosemeier, 05 Nov 2021
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This academic study presents well a blade tip shape design optimization from a purely aerodynamic point of view using the IEA 10 MW reference blade. The optimization is constrained not to exceed the flap-wise design bending moment at 90% rotor radius. The final tip shape extends the reference blade by 1% of the relative radius (r/R) while introducing a luvward prebent and a backward sweep of 2% r/R, respectively. The AEP increases by 1.12%.
In the introduction, the business case for tips as optimized in this study is stated to be a load neutral AEP increase for already existing blades in field. The optimized blade tip shall be mounted as a retrofit, which could look as illustrated in the work by Rosemeier and Saathoff (2020), for example. Now, the principle of the work is applied to the final tip shape of this study: A new blade tip of, e.g., 10% r/R, is newly manufactured, a piece of the original blade tip must be cut away (due to the curved shape of the tip extension) in field at the installed rotor, and the new tip is pulled over the existing blade and is re-mounted. A structural connection between the existing blade and the tip is achieved through a bonding connection within an overlap region. This extended and curved blade tip itself, the overlap, and additional adhesives or ribs are expected to increase the mass at the blade tip when compared to the reference. Additional mass at the tip introduces an increase of the design-dimensioning lead-lag bending moment at the blade root. The lead-lag load collective is driven by the number of rotor revolutions during the lifetime of say 20 years. To compromise the load increase, the lifetime would need to be reduced. The reduced lifetime decreases the total energy yield, which would need to be outweighed with the AEP increase of the optimized tip shape. These aspects are part-wise highlighted in the work by Rosemeier and Saathoff (2020), wherein it was concluded that the tip retrofit's AEP increase minus the additional cost for development, certification, mounting, and manufacture need to be opposed to an easy to "implement" lifetime extension of the turbine at a wind site that is weaker than assumed during the design.
In the conclusions, you mentioned another possible business case, i.e., the implementation of site-specific blade tips in the framework of a modular blade design. This means that for a new blade design, a standard root segment and a variety of different tip sigments optimized to site-specific condition categories is manufactured. If not manufactured from one mould, the blade root and tip segment would need to be joined. Such a joint adds mass to the blade and the standard root segment would need to be designed to carry the loads of tip segment with the largest load envelope. Finally, the modular blade root segment is expected to be increased in mass compared to the non-modular reference. The consequences are similar to what is decribed in the previous paragraph.
Having these thoughts in the back one's mind your described business cases in the introduction/conclusions should be overthought or reformulated to address the above. Moreover, the practical relevance of such purely aerodynamic optimizations can be increased if they would consider the additional impact of mass loads, e.g., by constraining the bending moment at 90% r/R to be also mass load neutral. The additional mass could be modeled as a function of volume and a "smeared" density, for example, plus some constant masses for the overlap or the joint. Do the authors think it would be possible to implement such a constraint in the optimization?
Reference:
Rosemeier, M. and Saathoff, M.: Assessment of a rotor blade extension retrofit as a supplement to the lifetime extension of wind turbines, Wind Energ. Sci., 5, 897–909, https://doi.org/10.5194/wes-5-897-2020, 2020.-
AC1: 'Reply on CC1', Mads H. Aa. Madsen, 09 Nov 2021
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Dear Malo Rosemeier,Thank you for your comment.It is correct that both the sleevelike business case mentioned in the Introduction and the modular blade business case mentioned in the Conclusion will result in a mass increase when also considering joints, additional adhesives, etc. Implementing such considerations into a more realistic compound objective function could bring about a more application-oriented study and it should be included in future work.However, we feel it is outside the scope of the current work.We will take your thoughts into consideration during the awaiting first review of the paper.Best regards,Mads H. Aa. Madsen
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AC1: 'Reply on CC1', Mads H. Aa. Madsen, 09 Nov 2021
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RC1: 'Comment on wes-2021-115', Anonymous Referee #1, 29 Nov 2021
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Review: CFD-based curved tip shape design for wind turbine blades – Madsen, HA et al.
Opening Comments:
This manuscript presents an optimization framework for tip designs in wind turbines using CFD. The authors are attempting to come up with a load neutral tip shape for wind turbines using a direct high-fidelity CFD based approach. Only aerodynamic effects are considered. The authors begin with an extensive literature review that places the current work in context with archival literature. They divide recently published literature into parametric studies and optimization studies. They point to the need for higher model fidelity as cited by several previous studies, as well as the need for optimizations with unifying design optimization problems. These are the drivers for this study.
The optimization method involves a straightforward process of determining design variables, calculating a deformed surface mesh, subsequently calculating a flowfield, functions of interest and constraints using EllipSys3D. This cycle is repeated until an optimized shape is obtained. The IEA 10MW is used as the baseline, and the design optimization problem is performed on this turbine. The results are presented in terms of optimal step size, and finally the optimized tip shape compared to the baseline. The authors conclude by pointing out that mesh deformation and setting up the finite difference method carefully are critical in a CFD based approach, but ease of use may mean surrogate based approaches may be more viable currently.
There are a few specific concerns with methods and results that have been outlined below:
Specific concerns:
- The complex step method is used to compute reference gradients. However, it is also mentioned that there is a lack of robustness in the authors’ implementation of the complex step method. This statement will need to be clarified to build confidence on the accuracy of reference gradient and the results presented here.
- The variables in the design optimization process are not clearly explained. I assumed θ1,2,3,4 and c1,2,3,4 are twist and chord for four points along the span in the tip region (outer 10%), but this is not stated clearly and locations not given.
- It is not clear how c is scaled. Is it with respect to chord at 0.9r/R?
- Recommend using the term merit function when discussing design optimization problem for consistency, rather than just in the results.
- An iso-view of the blade in comparison to Zahle (2018) may be more informative than current tile 3 of figure 15.
Technical concerns:
While this manuscript has been written well for most part, there are areas where typing errors have crept in or word choices cause confusion. A few examples: line 75 (also), line 241 (I’m not sure unimodal is the right word here), line 309, line 348, line 479, line 629, line 819 and so on. Recommend careful proofreading once again and rewording the unclear sentences.
Closing comments:
In general, this is a well-written and researched paper and will advance the literature on CFD based design approaches for blade tip devices. The current study can be considered a companion study to the ones by Zahle et al. (2018) and Barlas et al. (2021) where the authors also tackle the problem of load neutral blade tip extensions. The paper addresses questions relevant to the scope of WES and presents a framework for using a CFD based design optimization process. I recommend that with minor revisions (addressing the concerns listed above), the paper be accepted for publication in WES.
Mads Holst Aagaard Madsen et al.
Mads Holst Aagaard Madsen et al.
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