the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jul 2026
Bio-Inspired Outer-Span Hybridization of the NREL Phase VI Rotor for Enhancing Aerodynamic Performance and Flow-Field Stability
Abstract. The presenting work evaluates a selective bio-inspired outer-span hybridization of the NREL Phase VI wind turbine rotor. An owl-inspired airfoil (OA-T), obtained through controlled thickness scaling of a reconstructed owl-wing section, is integrated over the outer 20 % of the blade span while preserving the original chord and twist distributions. The objective is to assess whether localized geometric modification in the torque-dominant tip region can improve aerodynamic performance under increasing loading. A validated CFD framework is employed at both airfoil and rotor scales. Transition SST simulations reproduce the experimental lift behavior of the owl-inspired airfoil, while steady RANS–MRF simulations capture the torque response of the baseline Phase VI rotor. Using the same modeling setup, the hybrid configuration demonstrates a systematic increase in peak power coefficient from 0.38 to 0.42 at a tip speed ratior λ = 5.41, reaching 0.448 at higher tip-speed ratios. Flow-field analysis reveals outward redistribution of tangential loading, a shift toward a more chordwise-distributed pressure response, delayed separation progression, and a more confined wake structure at elevated wind speeds. The results show that controlled outer-span hybridization within a benchmark rotor can enhance torque retention and aerodynamic stability without full-blade redesign.
- Preprint
(3010 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 19 Aug 2026)
- RC1: 'Comment on wes-2026-111', Anonymous Referee #1, 31 Jul 2026 reply
-
RC2: 'Comment on wes-2026-111', Anonymous Referee #2, 06 Aug 2026
reply
The manuscript replaces the airfoil sections over the outer 20% of the NREL Phase VI blade span with a thickness-scaled owl-derived airfoil (OA-T) while preserving chord and twist, and reports a peak power coefficient increase from 0.38 to 0.42 at λ = 5.41 (0.448 at λ = 7.58) based on steady RANS–MRF simulations. The topic is of interest and the use of a well-documented benchmark rotor is appropriate. However, the manuscript suffers from a fundamental mismatch between the low-Reynolds-number premise of the airfoil design/validation and the operating conditions of the rotor to which it is applied, from an inconsistent turbulence-modeling strategy that undermines the physical interpretation of the results, and from a modeling approach (steady MRF) that cannot support the wake and turbulence claims made. There are also unresolved inconsistencies with the authors' own TORQUE 2026 conference paper, incorrect citations for the experimental reference data, and numerous presentation deficiencies.
Prior to recommending the paper for publication there are several things to be addressed:
Major Comments:
- The authors state several times in the literature that the OA-T excels in low to moderate Re numbers. How is this relevant to the high-Re number that typically characterizes Wind Turbines (WT) Rotors. This is relevant to Sections 2.1.2 and 3.1.1
- Regarding the validation in section two there are two issues: a) the difference in Re between the 2D airfoil simulations and the 3D WT simulations b)Drag comparison is missing. Is the data available?
- The 2D airfoil simulations employ transition model (γ-Reθ). It is not clear what the authors do for the 3D one. They mention that he k–ω SST model "was adopted for all rotor simulations due to its robustness in resolving adverse pressure gradients and separation phenomena typical of wind-turbine blade operation," “but then in the analysis they suggest the existence laminar separation bubbles (LSB) etc. (again on this below)
- Are the steady state MRF simulations adequate in the deeply stalled region of NREL VI rotor?
- The conference paper (Qamch et al., 2026) replaces the outermost 30% of span; the present manuscript states 20%. Note that Fig. 10 (torque–velocity) appears essentially identical to Fig. 11 of the conference paper despite different replacement extent; if the geometry changed, the torque response should differ. The validation figures (Figs. 7 and 8) are also reused from the conference paper.
- How is the geometric transition at r/R = 0.80 between the S809 (t/c = 20.95%) and the OA-T (t/c = 11.5%) handled.Please describe and show the transition region.
- The authors compare sectional pressure-coefficient distributions for the baseline and hybrid blades and attribute the "small-amplitude fluctuations" observed in the hybrid Cp curves to "laminar separation bubbles in the mid- to aft-chord region" (lines 330–331). This interpretation cannot be sustained with the numerical setup described in Section 2.1.3:
-All rotor simulations employ the fully turbulent k–ω SST model(?).
-The simulations are steady-state.; chordwise oscillations of the kind visible in Figs. 16(d)–(f) are, in steady fully turbulent conditions. The authors should state explicitly whether the plotted Cp remain the same throughout the simulations. The fluctuations are reported already at U = 5 m/s (line 330), where the flow is described by the authors themselves as fully attached. Also these fluctuations are not evident in the contour plots. This is also the case for the (LSB) claimed in the Cp discussion that do not appear in any of the velocity/streamline figures (Figs. 13–15), and no skin-friction or wall-shear distributions are shown.
Minor Comments:
- Line 154–155: grid independence is assessed at a single angle of attack (10°). Justify why this angle is chosen and why the conclusion of grid independence automatically transfer across higher Reynolds numbers.
- Fig. 5 / lines 174–178: describe how the rotor mesh is refined between the six levels (uniform scaling? local refinement zones? prism-layer growth held fixed?). Grid-convergence is not interpretable without the refinement strategy
- Table 1 caption: "time dependent Lift Coefficient" — the simulations are steady; correct the caption.
- Eq. (2) symbols are not clearly explained.
- Terminology: sectional pressure-coefficient distributions are repeatedly called "Cp loops" and their "evolution" (lines 323–338). This is non-standard; clarify what "chordwise evolution" means.
- Figure ordering and referencing: Figs. 13–14 (5 and 15 m/s) are discussed, then Fig. 15 (10 and 13 m/s) is introduced afterwards, which breaks the wind-speed progression and is confusing. Reorder figures to follow the narrative (or vice versa). Fig. 14 is referenced before Fig. 15 but placed inconsistently relative to the discussion.
- Naming: "Model A" vs "Hybrid Model A" are used interchangeably (e.g., Figs. 9, 10, and text); use one designation throughout.
- Legends and colorbars are unreadable in Figs. 13, 14, 17, 18, and 19; Fig. 1 lacks a legend. All must be regenerated at readable size.
- Fig. 19: the 3D streamline views as presented do not clearly demonstrate the claimed wake differences; Quantitative wake cuts (e.g., velocity-deficit or vorticity contours at downstream planes, tip-vortex trajectories would clear things up
- Line 17: "ratior" → "ratio"; line 11: "The presenting work" → "The present work"; general language editing is required throughout
Citation: https://doi.org/10.5194/wes-2026-111-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 143 | 37 | 13 | 193 | 12 | 11 |
- HTML: 143
- PDF: 37
- XML: 13
- Total: 193
- BibTeX: 12
- EndNote: 11
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
From the point of view of this reviewer, the manuscript does not comply with the quality expected for the Wind Energy Science journal. The authors are encouraged to keep working in this fascinating field. The research line fits the interests of the journal and of the community around it.
MAJOR TECHNICAL COMMENTS
Why a 3D simulation for the airfoil?
Reynolds for Table 1?
Reynolds and lambda for Figure 5? pitch?
The manuscript is about tip design, but Figure 9 clips the tip
Figure 11: effect of pitch is missing for this comparison
OTHER MAJOR COMMENTS
Fixable by re-reading the manuscript:
- Changes in font size inside the same paragraph
- Heterogeneity in size and style of figures, which are generally small
- References without year (6-9 june)
- RANS defined two times
- Check how Equation 2 is written. What is "u"?
Potential AI-abuse signs:
- Excessive use of "-" (e.g. wind-turbine)
- Numbers sometimes with ",", sometimes without
Figure 1 is very confusing
Font size in Figure 2
Figure 6 does not serve its purpose
MINOR COMMENTS
Figure 8: y axis range
One cannot see clearly the mesh from Figure 4
NREL Phase VI blade mentioned in Section 2.1.4, after results have been presented
reynold -> Reynolds
Potential abuse of subsubsections
Could be interesting to include wall BC for Figure 3
Figure 14: it the plot is essentially 2D, one could remove the 3D effect we see for the geometry. Velocity legend is hard to read
Uncommented figures in the Appendix
Line 123: (())
There are many acronyms in the abstract (not defined), that unnecessary limit its readability
ADDITIONAL REMARKS
How could this idea scale to other wind turbine models?
The proposed loading redistribution may also come with some risks, as aerodynamic performance is not the only design driver for horizontal axis wind turbines. There seems to be no mention about it in the manuscript
While k-w SST has been established as the canonical turbulence modeling approach in the field, it is still RANS. The phenomena studied are very linked to separation, and it could be premature to firmly conclude about the potential of the technology simply based on the computations. Could be a future work