Aerodynamic Characteristics of Different Airfoils under Varied Turbulence Intensities at Low Reynolds Numbers

Round 1

Reviewer 1 Report

The paper discusses the results of several URANS computations on two different types of air -foils. Turbulence intensity and Reynolds number are varied within ranges that are typical of UAVs wing, the effects of such variations are discussed within the paper.

The paper relies on CFD that has been validated at very low Reynolds number. In my opinion, the paper has two major limits:

The computations are validated against experiments just in terms of lift and drag coefficients for a very low Reynolds number case. The validation (Fig. 3) highlights limits in the prediction of the correct lift coefficient. There is no validation or comparison of the static pressure distribution over the airfoil with experiments. Since all results are based on URANS, it is difficult to consider them meaningful without a proper validation. Furthermore, since the test case is a well-known NACA0012, I think there should be several experimental works to be used as efficient validation cases. This is compulsory to consider the paper for journal publication. The result’s section is too long and verbose, this section should be completely revised in order to accept the paper for publication. This section considers all cases run by the authors, they should bring the attention to just the most meaningful ones. Furthermore, there are no comparisons with the literature on this subject (that is very extensive), and the main conclusion discussed by the author (i.e, the increase of the Reynolds number has a similar effect like an increase in turbulence, lines 516-520) is quite trivial and well reported in literature. There are very recent publications on Experiments in fluids on the effects of turbulence and Reynolds number on laminar flow separation that discuss these effects in much more details. The increase of the Reynolds number alter the state of the boundary layer at separation reducing the shear layer thickness and consequently its stability characteristics. The increase of turbulence intensity leads to an earlier transition because of the higher velocity fluctuations that penetrate within the boundary layer. Namely, the result’s section should be shorter (less figures), more concise and much better discussed with respect to recent literature on separation bubble in order to accept the paper for publication.

There are also some minor comments, several typos and unclear sentences:

Line 61, increase the number of reference works, there are more recent papers related to effect of Reynolds number and turbulence intensity (see recent Experiments in Fluids papers) The word “obvious” is often used, I suggest to remove it from the text, it is not typical of scientific English Lines 217-219: a laminar separation that does not reattach is not a separation bubble (please revise the text here and afterward when this is mentioned, e.g., in lines 467-468) Line 242 correct “is shown” to “are shown” Line 261-262 “… of the airfoil move towards…” the sentence does not sound correct to me, please revise i Comment to Fig. 9 is quite confused, please define Cd,f and Cd,p and revise lines 265-271 Line 314-316: the comments on this case are not clear to me, if the laminar separation does not reattach, it is not a separation bubble. Line 367-368: “the pressure distribution curve appears a slight increase in negative pressure”, this sentence is not clear

Author Response

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Reviewer 2 Report

On the whole, this is an interesting paper addressing a topical subject. The English language is throughout of acceptable quality with minor grammatical incorrections.  The topic is relevant and the presentation of the results is, on the whole, thorough.  My critique of this paper is mostly on the weak description of the methodology; for example, the authors mention that the numerical calculations were based on the unsteady Reynolds Averaged Navier-Stokes equations.  However, no discussion or presentation, or even mention, of the time-step employed is included.  Likewise, the convergence criteria is never discussed and this is equally true for the transient simulations as it is for the steady state ones.  There is also no information regarding the way in which the results from the transient simulations are presented.  The reason one runs transient simulations is because the flow has an unsteady pattern which is challenging for steady state simulations.  The authors ought to have presented a time history of the variations of some relevant computed quantity, for example Cl and their results ought to have related to the time-average of the transient simulation results.  If this is what the authors mean by the comments in lines 184 and following, then it ought to be better explained.  There are a number of instances when the authors make qualitative assessments of data, for example line 179 refereing to Figure 3 with which it is difficult to agree.  Finally, the  paper contains no nomenclature, although it is conceded that a reasonable number of variables are described in the text.  However, many are not and although the readers are directed to the appropriate sources, it is felt that , if an equation is worth including, its component variables ought to be listed.  I suggest that you employ the expression 'grid' to describe the computational mesh and 'nodes' to describe the discrete grid elements.  This set of comments ought not to detract from the work carried out and its presentation, which is on both accounts thorough and with real engineering usefulness.

Author Response

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Reviewer 3 Report

Dear author, please find attached my comments.

 

Regards

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Author Response

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Round 2

Reviewer 1 Report

I congratulate the authors for the effort done to improve the papers, in the present form the paper is about ready for publication. The English has been improved, the text organization is improved as well, and recent works by other research group are now properly mentioned.

I have just a small remark concerning the experimental-numerical validation, in the worst case there is about 15% variation of numerical with respect to experimental result, this has to be better acknowledge while discussing the results. Particularly, when you mention the difference at small angle of attack (lines 325 to 327), they might be also due to some numerical inaccuracy.

I also found a small typo:

line 243: figure 4: do you mean figure 6? 

Author Response

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Reviewer 3 Report

All my comments have been successfully addressed. English grammar and style deep revision is recomended.

Author Response

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