Numerical Simulations on the Performance of Two-Dimensional Serpentine Nozzle: Effect of Cone Mixer Angle and Aft-Deck
Round 1
Reviewer 1 Report
In this paper, a computational fluid dynamics investigation of the effect of different geometrical configurations on a two-dimensional serpentine nozzle is described. The parametric study is conducted by varying the exhaust mixer cone angle, the aft-deck length and the aft-deck shape; in addition, a comparison between results from the nozzle with and without the aft-deck is presented. The paper include a detailed discussion of the results, which consists mainly of contour plots and profiles of velocity and temperature. I found the study interesting and worth publishing, but there are some areas that need attention.
The biggest concern I have is that nowhere in the study the authors mention anything about the effect on the aerodynamic characteristics or on thrust force, by changing these geometrical parameters. There may be indeed a reduction in velocity and temperature, which is beneficial from the detectability standpoint, but how about thrust penalties. The authors should address this point.
It is surprising to see that the results from different mesh resolutions in figure 4 are identical. I would expect some differences when the mesh is refined. Also, the refinement may not be sufficient; the authors only doubled the total number of mesh points, which means that the cell side was decreased by 2^(1/3)=1.26 (assuming a cubical cell). The authors should decrease the cell side by at least 2, which translates in an increase of the total number of points by 8.
It is also surprising to see that the results for the trapezoidal and triangular aft-deck configurations in figures 27d, 28 and 29 are almost identical. I would regard the trapezoid as a shape that converges to a rectangle as the smaller angle goes to 90 degrees (the triangle being the starting point). At what angle of the trapezoid do the results start to show some differences?
In the numerical simulations, it is mentioned that a no-slip condition was used at the wall. What was y+ at the wall?
Other minor issues/typos:
-p.3, start of second paragraph: “in the open literature”
-p. 3, same paragraph: rephrase “The results indicated that decreasing mixer diameter increasing bypass ratio and decreased the temperature inside the nozzle and at the external jet.”
-p. 3, same paragraph: rephrase “Flow characteristics include total and static pressure, wall shear stress, and Mach number streamlines were investigated.”
-p.5, line 1: “Sun”
-p. 5, lines 9-10: replace “is existed” by “exists”
-p. 5, line 19: remove “this of”
-Section 2, first line: replace “considers” by “represents”
-p.8, line 2: “by the length”
-figure 3: I would expect a smooth variation of the area. Is it because of the low resolution?
-p.12: Explain what is the physical interpretation of “… increasing the mixer cone angle decreased the velocity inside the nozzle.” Is it because the flow separation point moves upstream?
-p.13, line 4: “in Fig. 10”
-p.14, line 6 from the bottom: replace “entertained” by “entrained”
-p.16, line 5 from the bottom: replace “and improving” by “, thus improving”
-figure 17: why isn’t the upper contour plot symmetric?
-p. 21: split the second phrase; it’s too long and confusing
-I don’t think figure 20 and the associated discussion are necessary; you can remove them without compromising the content
-p. 29, line 5: replace “composes” by “consists”
Author Response
Thank you for reviewing our manuscript we try to answer all your comments in a good manner and apply all the recommendations.
-The biggest concern I have is that nowhere in the study the authors mention anything about the effect on the aerodynamic characteristics or on thrust force, by changing these geometrical parameters. There may be indeed a reduction in velocity and temperature, which is beneficial from the detectability standpoint, but how about thrust penalties. The authors should address this point.
- As mentioned in the manuscript the main goal of this study is investigating the effect of some geometric parameters on the flow characteristics mainly temperature and velocity so we don’t consider other parameter such as thrust or other aerodynamic characteristics. However, based on the reviewer request, the effect of cone angle on the thrust is added to the manuscript in Fig. 10. We concern on the effect of cone angle only because the aft-deck has little effect on the internal flow characteristics.
- It is surprising to see that the results from different mesh resolutions in figure 4 are identical. I would expect some differences when the mesh is refined. Also, the refinement may not be sufficient; the authors only doubled the total number of mesh points, which means that the cell side was decreased by 2^(1/3)=1.26 (assuming a cubical cell). The authors should decrease the cell side by at least 2, which translates in an increase of the total number of points by 8.
- Actually, for grid independency test many grid sizes were tested rather than mentioned in the manuscript and there was a little change in the results. We agree with the reviewer opinion so the other mesh sizes and other refinements are used. The used meshes are 1, 200,150 cells, 5, 420,193 cells, 12, 360,253 cells refers to coarse, medium and fine mesh, respectively. The obtained results indicate decrease in cell size results in change in results at certain value then the reduction in cell size doesn’t affected by cell size. Most modeling studies used model insensitive to the grids in order to have no effect on the solution.
- It is also surprising to see that the results for the trapezoidal and triangular aft-deck configurations in figures 27d, 28 and 29 are almost identical. I would regard the trapezoid as a shape that converges to a rectangle as the smaller angle goes to 90 degrees (the triangle being the starting point). At what angle of the trapezoid do the results start to show some differences?
- In our study we consider only one angle of trapezoid shape with 45o that gives results close to those of nozzle with triangle shape. Therefore, it is difficult to determine the angle at which the results will differs from those of triangular shape. We think this can be used as a new parameter can be investigated in a separate study.
- In the numerical simulations, it is mentioned that a no-slip condition was used at the wall. What was y+ at the wall?
- STAR CCM + provides different types of wall treatment; high y+, low y+, and ally+. In this study, the all-y+ wall treatment was selected and give the best results compared to other wall treatments. This treatment is a hybrid treatment that uses a blended wall function that emulates the low-y+ wall treatment for a fine mesh and highy+ wall treatment for a coarse mesh. consequently, the all-y+ wall treatment is relevant for a wide range of near-wall grid conditions. An explanation is added to section 3.1 in the manuscript.
Other minor issues/typos:
-p.3, start of second paragraph: “in the open literature”
- The phrase “in the open literature” phrase is added to the manuscript.
-p. 3, same paragraph: rephrase “The results indicated that decreasing mixer diameter increasing bypass ratio and decreased the temperature inside the nozzle and at the external jet.”
- Under your recommendation, the sentence is rephrased into “The results indicated that decreasing the mixer diameter increased the cold bypass flow and improved the mixing between the hot core flow and cold bypass flow that resulted in decreasing the temperature inside the nozzle and at the external jet. ”
-p. 3, same paragraph: rephrase “Flow characteristics include total and static pressure, wall shear stress, and Mach number streamlines were investigated.”
- Under your recommendation, the sentence is rephrased into “The Flow characteristics such as total and static pressures, wall shear stress, and streamlines of Mach number are investigated.”
-p.5, line 1: “Sun”
- The name is corrected
-p. 5, lines 9-10: replace “is existed” by “exists”
- The verb “is existed” is replaced by the verb “exists”
-p. 5, line 19: remove “this of”
- The phrase “this of” is removed.
-Section 2, first line: replace “considers” by “represents”
- The verb “considers” is replaced by “represents”
-p.8, line 2: “by the length”
- This phrase is modified
-figure 3: I would expect a smooth variation of the area. Is it because of the low resolution?
- Yes, we agree with you the variation of area seems to not smooth may be due to the low resolution of the figure.
-p.12: Explain what is the physical interpretation of “… increasing the mixer cone angle decreased the velocity inside the nozzle.” Is it because the flow separation point moves upstream?
- Flow separation point moves upstream may be one reason of decreasing the velocity inside the nozzle. But in the manuscript, we mentioned another reason which is increasing cone angle decreased the amount of core flow (as shown in Table 3) that has high velocity resulted in decreasing the velocity inside the nozzle. The two reasons are mentioned in the manuscript.
-p.13, line 4: “in Fig. 10”
-The phrase “in Fig. 10” modified
-p.14, line 6 from the bottom: replace “entertained” by “entrained”
- The verb “entertained” is replaced by “entrained”
-p.16, line 5 from the bottom: replace “and improving” by “, thus improving”
- The phrase “and improving” is replaced by “thus improving”
-figure 17: why isn’t the upper contour plot symmetric?
- The upper figure indicates the velocity contour for the nozzle without aft-deck. Removing aft-deck decreased the wetted perimeter area at the exit of the nozzle so the mixing process with ambient air was weak. This is result in some unstability in jet propagation hence unsymmetry of contour plot.
-p. 21: split the second phrase; it’s too long and confusing
- The phrase is split.
-I don’t think figure 20 and the associated discussion are necessary; you can remove them without compromising the content
- Under your recommendation, figure 20 and the associated discussion are removed.
-p. 29, line 5: replace “composes” by “consists”
The verb “composes” is replaced by “consists”
Reviewer 2 Report
1. Is the following sentence your own claim or is it sourced from other research articles. In case, if it is sourced from some other research papers, kindly please cite them - “About 90% of infrared radiation is provided by the exhaust nozzle and turbine section”.
2. Reference numbers are random. For instance, after reference [5], reference [2] is quoted. Kindly please arrange the paper in a logical fashion with corresponding reference.
3. The following sentence does not make clear sense – “The results indicated that decreasing mixer diameter increasing bypass ratio and decreased the temperature inside the nozzle and at the external jet.” Kindly rephrase
4. The boundary conditions used in the bypass and the core can be made available for creating better understanding to the readers. Is the values of total pressure, temperature and turbulence intensity (Table 1 does not include the TI value) considered at bypass and core is randomly chosen or based on some research articles. In case, if its based on the available literature, kindly cite the same.
5. Grid cell numbers can be expressed in million for better understanding to the international readers.
6. Figure 4 shows no deviation with respect to the form of grids. Can we construe that the model is insensitive to the grids or the values of grids considered were different by less values and hence there is no change.
7. Validation part is not sufficiently written. A mere representation of graphs will not be enough. The author(s) can both quantitatively and qualitatively explain the validation part for better understanding to the readers.
8. Figure 9, the qualitative flow characteristics about the TKE seems logical, however the quantitative values are so strong to agree. For all X/L values, TKE for 10 degree and 15 degree is well within 100 J/kg, whereas for 20-degree mixer angle, the value of TKE spikes up to 250 J/kg. Please address the same. We might miss some
9. Figure 10, for 15- and 20-degree angle, a dip in the temperature at around 0.1 X/L is evident from the figure. Discussion about the same cannot be found in the text. The author(s) may miss out some important finding.
10. Figure 13, is there a possibility for the flow streamlines to change over time.
11. Figure 16, is a mere representation of various parameters to show the importance of aft-deck and does not hold any significant contribution in terms of knowledge addition. The reviewer understands the in-depth work of the author(s), however, the author feels that the aft-deck part of the paper can be organized in a much better way for better understanding to the reader. The overall length of the paper can be reduced a bit.
12. Conclusion can be written in points to make it clutter free.
Author Response
Thank you for reviewing our manuscript we hope to answer all your comments in agood way.
- Is the following sentence your own claim or is it sourced from other research articles. In case, if it is sourced from some other research papers, kindly please cite them - “About 90% of infrared radiation is provided by the exhaust nozzle and turbine section”.
- The above sentence is from a research article and it was cited in the manuscript.
- Reference numbers are random. For instance, after reference [5], reference [2] is quoted. Kindly please arrange the paper in a logical fashion with corresponding reference.
- Under the reviewer recommendation, the references in the manuscript are arranged in a logical fashion.
- The following sentence does not make clear sense – “The results indicated that decreasing mixer diameter increasing bypass ratio and decreased the temperature inside the nozzle and at the external jet.” Kindly rephrase.
- The sentence is rephrased as “The results indicated that decreasing the mixer diameter increased the cold bypass flow and improved the mixing between the hot core flow and cold bypass flow that resulted in decreasing the temperature inside the nozzle and at the external jet .”.
- The boundary conditions used in the bypass and the core can be made available for creating better understanding to the readers. Is the values of total pressure, temperature and turbulence intensity (Table 1 does not include the TI value) considered at bypass and core is randomly chosen or based on some research articles. In case, if its based on the available literature, kindly cite the same.
- The boundary conditions are applied to the bypass and core regions are based on the experiment’s tests carried out by FIRSTEC Co., Ltd, Republic of Korea and I don’t have the permission to mention that in the manuscript. The values of turbulent intensity are added to the manuscript.
- Grid cell numbers can be expressed in million for better understanding to the international readers.
- Under your recommendation, the grid cell number is expressed in million (one million and 200,150 cells, five million and 420,193 cells, twelve million and 360,253 cells)
- Figure 4 shows no deviation with respect to the form of grids. Can we construe that the model is insensitive to the grids or the values of grids considered were different by less values and hence there is no change.
- For modeling studies, the solution should be independent of number of grid cells so the used model was constructed insensitive to the grids. As shown in the manuscript the decrease in cell size results in change in results until certain value then the grid cell doesn’t affect the simulation results.
- Validation part is not sufficiently written. A mere representation of graphs will not be enough. The author(s) can both quantitatively and qualitatively explain the validation part for better understanding to the readers.
- The validation part is modified and more details are mentioned to give better understanding of this part.
- Figure 9, the qualitative flow characteristics about the TKE seems logical, however the quantitative values are so strong to agree. For all X/L values, TKE for 10 degree and 15 degree is well within 100 J/kg, whereas for 20-degree mixer angle, the value of TKE spikes up to 250 J/kg. Please address the same. We might miss some.
- We agree with the reviewer comment, the values of TKE is higher for 20-degree mixer angle compared to TKE for 10 degree and 15 degree. But this higher increase is still match with the figure trend as increasing cone angle resulted in increased TKE. Increasing cone angle may increase the projected area so the mixing process and turbulent motion improved results in higher increase in turbulent kinetic energy.
- Figure 10, for 15- and 20-degree angle, a dip in the temperature at around 0.1 X/L is evident from the figure. Discussion about the same cannot be found in the text. The author(s) may miss out some important finding.
- The values at X/L =0.1 is recalculated and the figure is modified. The temperature is increased gradually until the end of the mixer then it gradually decreased up to nozzle exit.
- Figure 13, is there a possibility for the flow streamlines to change over time.
- There is a possibility for the flow streamlines to change with time. But in our case the results were analyzed after the solution was stable and the results didn’t change with time.
- Figure 16, is a mere representation of various parameters to show the importance of aft-deck and does not hold any significant contribution in terms of knowledge addition. The reviewer understands the in-depth work of the author(s), however, the author feels that the aft-deck part of the paper can be organized in a much better way for better understanding to the reader. The overall length of the paper can be reduced a bit.
- We agree with you the aft-deck part may be long but we try to discuss the most available obtained results to give a complete description for the effect of aft-deck on the flow characteristics. But under the reviewer request this part is organized again and some figures are compressed or removed.
- Conclusion can be written in points to make it clutter free.
- The conclusion is modified and the main findings are written in points
Round 2
Reviewer 1 Report
I am satisfied.
Reviewer 2 Report
The author(s) have addressed all the review comments properly. The paper can be accepted in its present form and can be published.
The number of highlight points in the conclusion can be minimized/ merged to have a minimum number of notable points to attract readers' attention.
