Numerical and Experimental Studies on the Aerodynamics of NACA64 and DU40 Airfoils at Low Reynolds Numbers

Round 1

Reviewer 1 Report

Manuscript Number: applsci-2067296

Full Title: Numerical and experimental studies on the aerodynamics of NACA64 and DU40 airfoils at low Reynolds numbers

 

I – General Comments

The manuscript deals with aerodynamics aerodynamic loads of scaled airfoil sections, firstly, tested in a wind tunnel. In general, because of the limited space of wind tunnels, the mismatch of Reynolds numbers may lead to different aerodynamic loads. As contribution, the authors have investigated the Reynolds number effects by using numerical simulations and wind tunnel experiments to report the aerodynamics of a thin airfoil (NACA64) and a thick airfoil (DU40). The investigations have been conducted at two Reynolds numbers (i.e., 4,000 and 60,000). A version of the two-dimensional vortex particle method with varying-sized particles has been employed to simulate the unsteady flow and compared to the steady-state simulations by XFOIL. There are important recommendations/questions addressed to the authors, before to accept their manuscript to be published as Applied Science’s paper.

 

II - Specific Comments

(i) In “Abstract”, the present state of the art concerning the investigated topic has been reasonably introduced and contextualized. However, the main contribution of the present work must be better clarified aiming to justify their publication into the specialized literature context.

(ii) In general, the Introduction of a technical paper must help the reader understand why a research is important and what it is contributing to the research field. Put in other words, the context of the Introduction a scientific paper: a) must state the motivation for doing a research and what it will contribute to the research field; b) must present a brief overview of the current state of research; c) must present some more detailed information on the specific topic of the research; d) must present a description of the exact question or hypothesis that the paper will address; and e) must properly summarize the adopted approach to solve the investigated problem. Therefore, it is necessary to present more technical discussions concerning these key points including examples of past models through a critical review. As consequence, the contributions of the manuscript can better explained and contextualized in this part of the paper. Therefore, it is recommended a new wording by authors to make adjusts the “Introduction” of the present manuscript.

(iii) Still with respect the present “Introduction”: a) it is not recommended the use of Tables or Figures (particularly, Table 1 and Figure 1 can be removed from section 1 to section 2, in the latter they can be easily contextualized);  b) the manuscript did not cite papers early published in Applied Sciences. Why? (that key point needs to be clarified by authors to justify their publication in Applied Sciences too); and c) it is interesting the authors observe that the use of experimental investigations and numerical simulations blended with surface roughness modeling to simulate the problem is very applicable. Two recent papers, recently published in the literature can be cited by authors as reference (including some short comment), i.e.:

[1] https://doi.org/10.1016/j.matpr.2022.02.153.

[2] https://doi.org/10.3390/en14248237.

Are the surface roughness effects important for the investigated problem? That key point can be better discussed and contextualized by the authors.

(iv) The section 2 is weak, because the authors omitted the assumed hypothesis, governing equations and boundary conditions for the chosen problem. Figure 1 can be properly linked with the general formulation of the problem. The recommended section and so called “General formulation of the problem” must be separated of the present section already titled as “Numerical and experimental methods”.

(v) On page 5, line 146, the unitary values adopted for mainstream flow, chord length and fluid density need to be better explained. The authors have chosen two Reynolds numbers of 4,000 and 60,000 to conduct the numerical simulations by using the VPM. How does the transition to turbulence manifest in wake and boundary layer (e.g. in Figure 5)? What is expected when including surface roughness effects into the numerical simulations?

(vi) What was the wind turbulence intensity during the experimental investigations? How was the saturation state of a typical numerical simulation attained in terms of aerodynamic forces (e.g. in Figure 4)? A short comment about experimental data uncertainties is welcome too.

(vii) It is recommended to substitute the term “vorticity convection” by “advection vorticity” into the manuscript. Unfortunately, the manuscript did not introduce the fundamentals of the VPM to clarify the numerical approach to solve (separately?) the vorticity advection and the vorticity diffusion. What was the criterion to establish near the airfoil, the particle radius of the order of the viscous length scale? Is the particle a vortex blob? The manuscript should discuss more precisely about Eulerian, Lagrangian (meshless) and hybrid description to better introduce the present VPM. What is the sensitivity of the present VPM for Reynolds number increase?

(viii) The aerodynamic loads computation and formulation needs to be introduced into the manuscript. On page 6, line 190, there is an insufficient comment about them.

(ix) In Figure 5, please, make sure about the viscous drag and pressure drag computations. How does the numerical approach lead with those two computations?

(x) It is recommended to difference “vortical structures” of “discrete vortices (or cluster of vortex blobs)” into the manuscript (e.g. on page 10, line 285).

(xi) It is suggested (and would be welcome) that the authors explore/summarize the “stall” phenomenon linked with the temporal history of the drag and lift coefficients under attack angle increase.

(xii) In “Concluding remarks”, it is necessary to include comments with respect the numerical results behavior as compared as previous works (specially experimental data). In closing, it is important to complete the manuscript with perspectives for a future research. Finally, the main contribution of the present manuscript should be clarified aiming to justify its publication in Applied Sciences.

  

III - Recommendation for the Applied Sciences´ Editor

In my opinion, the present manuscript needs attend all topics above presented. Upon consideration of all points above, I think the paper could be considered for publication in Applied Sciences.

Author Response

Thank you for your careful review. The responses to your valuable suggestions, as well as the responses to other reviewers' suggestions, are attached. 

Author Response File: Author Response.pdf

Reviewer 2 Report

More details on the wind tunnel test would be interesting: corrections applied, efficacy of the 2D-simulation with endplates

Author Response

Thank you for your careful review. The responses to your valuable suggestions, as well as the responses to other reviewers' suggestions, are attached. 

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript discusses the effect of Reynolds number on the aerodynamic performance of two airfoils NACA64 and DU40 using Vortex Particle Method (VPM). The authors also compared their VPM results with XFOIL and their experiment.

 

While the work sounds interesting, I cannot recommend its publication due to the following shortcomings.

1. The formulation of the VPM

1. I presume the number of particles grows automatically in the simulations when certain criteria are satisfied. Could the author address this issue?

2. Quality of the VPM results

1. What is the time range used to obtain the time-averaged coefficients?

2. From Fig. 2, it seems that the number of particles just reached the steady number for Re_c = 60000 before the simulation stopped. If the time averaged coefficients are obtained using this time (0 - 9 for DU40, 0-12 for NACA 64), the results include the transient range that induce errors in the estimation of the coefficients. The simulations for Re_c = 60000 should be run with more time just like those Re_c = 4000, and use the range of steady particle number for the estimation of the coefficients 

3. Comparison with experimental data

1. While the experimental data are obtained at Re_c = 60000, focus of comparison should be at Re_c = 60000, rather than 4000. At least more XFOIL and VPM data at Re_c = 60000 (only a few data) should be shown and compared.

2. Since the effect of Reynolds number on aerodynamic performance is large at low Reynolds number range. More intermediate Reynolds numbers should be simulated to “understand the mechanism of the Reynolds number effects on the airfoil aerodynamics in the low Reynolds number region.” (Line 105)

 

By the way, could the authors provide the proper number of the airfoils  e.g. NACA 64-018? They can serve as one of the aerodynamic data sources for future references. 

Author Response

Thank you for your careful review. The responses to your valuable suggestions, as well as the responses to other reviewers' suggestions, are attached. 

Author Response File: Author Response.pdf

Reviewer 4 Report

Add the units to the Nomenclatures table. 

Line 56, do not put any abbreviations without a previous introduction please. 

Highlight the gap between the current and the previous studies please. 

No information about the software that utilized in the analysis. 

Too long conclusion's section, please remember, it is a conclusion. However, if necessary, you can split it to two sections, conclusion n recommendations. 

Author Response

Thank you for your careful review. The responses to your valuable suggestions, as well as the responses to other reviewers' suggestions, are attached. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Manuscript Number: applsci-2067296-v2

Full Title: Numerical and experimental studies on the aerodynamics of NACA64 and DU40 airfoils at low Reynolds numbers

 

The authors have carefully replied all the questions I raised before. I am satisfied with their responses and the corresponding corrections. The current version can be accepted for the publication in Applied Sciences.

Author Response

Thanks for your insightful review, which helps a lot on our writing. 

Reviewer 3 Report

I would like to thank the authors for taking my considerations into account in the manuscripts. The manuscript is significantly improved. However, I still cannot recommend its publication due to the quality of the presented results.

Regarding the case at Re = 60000 for NACA64, the time-averaged quantities are determined from Ut/C = 2 to 12. Nevertheless, the calculation of this case only reached steady-state after Ut/C ~ 10 based on Figure 3. Transient solutions are also included in obtaining the time-averaged quantities, which can greatly affect the accuracy of these quantities. This may be the reason of VPM results deviating from the experimental results more than XFOIL results do at Re = 60000. Such inclusion also leads to the questioning about the usefulness of VPM in this case. Thus, the time-averaged quantities must not include the transient solutions to improve the quality of the presented results.

Author Response

Thanks for your careful review. Please see the attachment of our responses. 

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

I would like to thank authors for addressing my concerns and recommend its publications.