Effect of Chordwise Struts and Misaligned Flow on the Aerodynamic Performance of a Leading-Edge Inflatable Wing
Round 1
Reviewer 1 Report
This paper performs steady-state Reynolds-Average Navier-Stokes simulations with the help of the CFD technique to study a leading-edge inflatable wing for airborne wind energy applications. Generally, the reviewer thinks this is a successful work with a good contribution to improving the understanding of the flow behaviour and aerodynamics of the leading-edge inflatable wings. The whole paper is well written and the structure is reasonable. Some minor issues can be considered before a final publication.
1) In the abstract, some texts could be included to give a brief introduction on the background of this work. It looks too sudden to describe your work from the first sentence.
2) Please pay more attention to the format of the equation. all the variables should be italic. For instance, Re is a variable, which should be italic. In C_S, C is a variable but not S. So S should not be italic.
3) Can the authors give any pieces of evidence to prove the numerical accuracy of the simulation in this work. Almost all the works are performed via CFD. But CFD sometimes is not very reliable if they are not validated with the wind tunnel experimental results.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
- Did the author perform grid independence for the mesh solutions? What about Grid Convergence Index?
- How is implemented laminar-turbulent transition model using OpenFoam software.
- Figure 6 hasn’t got units
- Has the influence of the wing shape factor on the work efficiency can be analized?
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
The presented paper is a prolongation of aerodynamic research of leading-edge inflatable (LEI) wing. Authors added inflatable strut tubes to the model of LEI wing and executed Steady-state Reynolds-Averaged Navier-Stokes (RANS) simulations with the transition Langtry-Menter turbulence model for different flight regimes. Calculated aerodynamic characteristics were compared with ones for LEI wing model without strut tubes. The comparison reveals a minor influence of strut tubes on overall aerodynamic performance of the LEI wing.
Authors sufficiently justified the goal and method of the research and provided a clear and complete overview of previous works and relevant references.
Specific comments.
Figure 4: It's not clear where is structured mesh and unstructured one (blue lines also delimit tip sections).
Line 143: It would be good to have a bit more information about mesh (first prismatic layer height; maximum number of prismatic layers; prismatic layers grow ratio; maximum value of cells aspect ratio). Also mesh convergence was executed on 10.9 and 5.67 millions mesh sizes. What was the corresponding number of cells on the wing surface?
Line 228 and Figure 11: An interesting phenomenon. Re=1e+06 gives higher critical CL than Re=3e+06 and 15e+6. Normally the higher the Reynolds number the thinner the boundary layer in the rear part of the wing and the better it should resist the adverse pressure gradient. Could you describe what is the physics behind such behavior?
Suggestion. In the text between line 235 and 236 you proposed to improve post-stall accuracy of the calculations by applying an unsteady formulation or LES turbulence model for 3D LEI wing. I would also suggest testing the PISO solver with Langtry-Menter model for 2D airfoil cases including linear part in vicinity critical AoA. From personal experience of exploiting Langtry-Menter model SIMPLE and PISO algorithms can provide quite different results. It's very interesting if the effect of decreasing critical CL with increasing Reynolds number will hold for unsteady formulation.
Kind regards!
Author Response
Please see file attached.
Author Response File: Author Response.pdf
Reviewer 4 Report
The authors investigate the effect of struts and the sidewards flow due to turning on the forces acting on a leading edge inflatable kite for airborne wind energy. The methodology is identical to previous work, only summarized here and was already validated at that stage. The results show limited effect of the struts, but large effect of the turning on the lift/drag ratio. The limitations such as the effect of steady RANS and the undeformed shape despite the turning are clearly mentioned.
The quality of the research is good and the text is clear. Some remarks:
- The focus is on lift and drag, and also sidewards force is mentioned, but moments are not considered. Can this be added, as this is also important for the trajectory? Or doesn’t that provide interesting data?
- The authors are using steady RANS for a geometry with tip vortices and also local vortices near the struts. They also mention that in future work unsteady RANS should be considered. Didn’t they experience convergence problems with the steady RANS? Is the solution always truly steady or was some stabilization required?
- On page 7, the authors mention that the transition from a laminar to a turbulent boundary layer is different in these smooth models compared to the real case. Do the authors have sources or more precise data to support these statements?
- On page 8 (line 228), the authors show that the lift coefficient increases with Re up to a maximum, and then decreases for further increases of Re. The results support this statement, but can this also be explained physically? What is happening for Re > 10^6?
Minor comments:
- Page 9, equation 1: This quantity is actually already used earlier, so mention it at first use.
- Page 12, line 271: radio => ratio
- Page 12, line 271-272: These 2 lines belong more in the previous paragraph, they are not related to the side force which is discussed in this paragraph.
Author Response
Please see file attached.
Author Response File: Author Response.pdf
