Influences of High-Speed Train Speed on Tunnel Aerodynamic Pressures
Round 1
Reviewer 1 Report
The aim of the paper is to determine the influence of a high-speed train’s speed on the aerodynamic pressures when passing through a tunnel using CFD. For the analysis of the results, the tunnel passage is split into three stages: before the train is entering the tunnel, during the train passing through the tunnel and after the train has exited the tunnel. For each of the stages, maximum, minimum and peak-to-peak pressures are evaluated for cross-sections along the tunnel.
The main contributions are the assessment of the whole tunnel passage, i.e. considering not only the train running inside the tunnel, but also the pressure series before the train enters the tunnel and after it has exited the tunnel using 3D-CFD. Although these pressures are smaller than during the train running inside the tunnel, they may be important to assess fatigue damage.
Strengths of the paper include the clearly presented methodology and detailed description of the wave diagrams.
For Stage II and Stage III, the maxima/minima of the aerodynamic pressures and their location along the tunnel are dependant on the tunnel length (and train length). Therefore, they might also affect the correlations with train speed (at least spatially). Here a simulation with a tunnel of different length might be helpful to assess these dependencies.
Additionally, a recommendation / some ideas for a practical approach would be desirable. Would a combination with a 1D model help to explore the parameter space? Is a 1D model sufficient for the wave propagation and reflection? Would you assess every tunnel on a case by case basis or are your results transferable to other tunnel geometries?
The correlation formulas are such that y is in kPa and x in km/h. It might be helpful to mention this before Fig. 14.
Looking at Fig. 18 for measurement positions 1-V & 19-V, one might assume that also a linear interpolation would be adequate. Here it might be helpful to mention that using linear correlation the boundary condition 0 Pa at 0 km/h will not be satisfied with a good fit.
Line 144 / 148: What is the resulting boundary condition of the bottom surface? Is it a sliding wall (no-slip wall moving with train) boundary condition inside part 2 and a fixed no-slip wall boundary condition for the remaining bottom surfaces?
Line 151: Should be “stationary part 1 and sliding part 2” instead of “sliding part 1 and stationary part 2”
Line 179: As you are investigating different train speeds, you could add a comment if the y+ values refer to the whole train speed range or a single speed.
Line 189: Here it would be helpful to know, what the differences between the meshes are. Did you increase the cell size or the ratio of the layers? Did you increase the cell size of the wall nearest cells? Additionally, the difference in mesh resolution is small for a grid sensitivity study.
Line 211: Should be “Roman numerals” instead of “Greek numerals”
Author Response
Point 1: For Stage II and Stage III, the maxima/minima of the aerodynamic pressures and their location along the tunnel are dependant on the tunnel length (and train length). Therefore, they might also affect the correlations with train speed (at least spatially). Here a simulation with a tunnel of different length might be helpful to assess these dependencies.
Response 1: Thanks for the reviewer's professional suggestion.
The aim of this manuscript is to investigate the characteristics of aerodynamic pressures on the tunnel wall caused by the whole tunnel passage of the high-speed train at different speeds.
The other parameters, such as tunnel length, whole train length, and blockage ratio of train-tunnel, will be further explored in subsequent research.
Point 2: Additionally, a recommendation / some ideas for a practical approach would be desirable. Would a combination with a 1D model help to explore the parameter space? Is a 1D model sufficient for the wave propagation and reflection? Would you assess every tunnel on a case by case basis or are your results transferable to other tunnel geometries?
Response 2: Thanks for the reviewer's professional suggestion.
The three-dimensional characteristics of the aerodynamic pressure on the tunnel at the entrance and exit of the tunnel, as well as the moment of the train travelling through the monitoring section, can not be simulated by the 1D model. Additionally, the influences of the train geometry on the aerodynamic pressure can not be studied using the 1D model.
However, the qualitative assessment of the pressure wave propagation and reflection is sufficient based on the 1D simulation model.
Point 3: The correlation formulas are such that y is in kPa and x in km/h. It might be helpful to mention this before Fig. 14.
Response 3: Thanks for the reviewer's professional suggestion.
The explains of 'x' and 'y' have been added before Fig. 14 in this manuscript. The detailed descriptions are as follows:
The 'x' and 'y' show the train speed and the maximum peak pressure, respectively.
Point 4: Looking at Fig. 18 for measurement positions 1-V & 19-V, one might assume that also a linear interpolation would be adequate. Here it might be helpful to mention that using linear correlation the boundary condition 0 Pa at 0 km/h will not be satisfied with a good fit.
Response 4: Thanks for the reviewer's helpful suggestion.
The related sentences have been added according to the reviewer's comment. The detailed descriptions are as follows:
There is an approximately linear relationship between each of the three types of maximum peak pressure value and the train speed raised to the power from 1.12 to 2.33 except for the boundary condition 0 kPa at 0 km/h.
Point 5: Line 144 / 148: What is the resulting boundary condition of the bottom surface? Is it a sliding wall (no-slip wall moving with train) boundary condition inside part 2 and a fixed no-slip wall boundary condition for the remaining bottom surfaces?
Response 5: Thanks for the reviewer's helpful suggestion.
The bottom surface of the stationary part 1 is a fixed no-slip wall. Relative to the stationary part 1, the bottom surface of the sliding part 2 is a sliding wall. Relative to the sliding part 2, the bottom surface of the sliding part 2 is a fixed no-slip wall. Relative to the train, the bottom surface of the sliding part 2 also is a fixed no-slip wall.
Point 6: Line 151: Should be “stationary part 1 and sliding part 2” instead of “sliding part 1 and stationary part 2”
Response 6: Thanks for the reviewer's helpful suggestion.
The sentence 'sliding part 1 and stationary part 2' has been modified to 'stationary part 1 and sliding part 2' in this manuscript.
Point 7: Line 179: As you are investigating different train speeds, you could add a comment if the y+ values refer to the whole train speed range or a single speed.
Response 7: Thanks for the reviewer's professional suggestion.
The related comment has been added according to the reviewer's comment. The detailed descriptions are as follows:
The y+ values of train surface and tunnel wall in our simulation are controlled between 30 and 180 for the train speed varies from 275 km/h to 400 km/h, which meets the requirements of the standard.
Point 8: Line 189: Here it would be helpful to know, what the differences between the meshes are. Did you increase the cell size or the ratio of the layers? Did you increase the cell size of the wall nearest cells? Additionally, the difference in mesh resolution is small for a grid sensitivity study.
Response 8: Thanks for the reviewer's professional suggestion.
The main difference between the fine mesh and the coarse mesh is the mesh density (the number of the mesh). For different mesh densities, the smallest cell size is the same, and the number and the ratio of the layers of the train nose and the tunnel wall nearly also are the same. However, the number of layers in the far-field and the longitudinal axis of the numerical model are different.
Point 9: Line 211: Should be “Roman numerals” instead of “Greek numerals”
Response 9: Thanks for the reviewer's helpful suggestion.
The phrase 'Greek numerals' has been modified to 'Roman numerals' in this manuscript.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this article, the authors carried out a series of three-dimensional numerical simulations to study the influences of high-speed train speed on tunnel aerodynamic pressures. The results will be helpful to understand the complex pressure wave interactions related to HSR.
If you can add some thresholds to fig. 17 and 20 from the viewpoint of the tunnel structural safety during operation it will be beneficial. But it may be beyond the scope of this article.
The minor typo is shown on page 25.
CRediT > Credit
Author Response
Point 1: If you can add some thresholds to fig. 17 and 20 from the viewpoint of the tunnel structural safety during operation it will be beneficial. But it may be beyond the scope of this article.
Response 1: Thanks for the reviewer's professional suggestion.
The influences of the operating high-speed train on the tunnel structural safety will be investigated in subsequent research.
Point 2: The minor typo is shown on page 25.
CRediT > Credit
Response 2: Thanks for the reviewer's careful check.
The minor typo has been modified in this manuscript.
Reviewer 3 Report
The paper presents an interesting study about the aerodynamic pressures inside tunnels when a high-speed train passes through. Numerical simulations using ANSYS Fluent are carried out and the results are presented. Design and meshing processes are well described. Although the paper is well written and structured, some work must be done before considering it for publication:
- The novelty of the paper must be highlighted both in the abstract and the introduction. The manuscript is interesting, but I couldn’t find what is the original contribution to the state of the art after reading it.
- The abstract should be amended. Numeric data should be removed because the reader hasn’t enough information to evaluate them yet, and the novelty must be included.
- Regarding Figure 5, I wonder if it is correct. The naming of nodes in both meshes seems to not match the names of the common interface: green “A” matches green “a”, green “B” matches magenta “b”… is this right?
- In general, figures and tables must be smaller so their font size coincides with the font size of the text. In addition, the format of figures should be standardized.
- In Figure 7, the vertical label states “Pressire” instead of “Pressure”.
- What is the length of the simulated tunnel? On page 12, line 244 it is stated that is 100 m, but the calculations seem to be made with a length of 1000 m.
- In Figures 16, 19 and 22, I recommend the authors change the colour of the green lines to a darker one for better visibility.
- What is the meaning of the two red dots in Figure 19?
Author Response
Point 1: The novelty of the paper must be highlighted both in the abstract and the introduction. The manuscript is interesting, but I couldn’t find what is the original contribution to the state of the art after reading it.
Point 2:The abstract should be amended. Numeric data should be removed because the reader hasn’t enough information to evaluate them yet, and the novelty must be included.
Responses 1 and 2: Thanks for the reviewer's professional suggestion. The first and second comments are replied together.
Considerable researchers have studied the characteristics of tunnel aerodynamic pressures by field monitoring, model experiment, and numerical simulation, but they mostly focused on the behaviours during the high-speed train running inside a railway tunnel. The behaviours before the high-speed train enter the tunnel and after it exited the tunnel have not been further explored.
Therefore, the behaviours of the whole tunnel passage of the high-speed tarin are assessed by the authors, i.e. considering not only the train running inside the tunnel, but also the other two series before the train nose entering the entrance and after the train tail leaving the exit based on a series of three-dimensional numerical simulations.
The novelty of this manuscript has been modified both in the abstract and the introduction according to the reviewer's comment.
Point 3: Regarding Figure 5, I wonder if it is correct. The naming of nodes in both meshes seems to not match the names of the common interface: green “A” matches green “a”, green “B” matches magenta “b” is this right?
Response 3: Thanks for the reviewer's helpful suggestion. The Fig. 5 has been modified according to the reviewer's comment. The modified Figure is as follows:
Point 4: In general, figures and tables must be smaller so their font size coincides with the font size of the text. In addition, the format of figures should be standardized.
Response 4: Thanks for the reviewer's helpful suggestion.
The format of figures throughout this manuscript has been standardized according to the reviewer's comment.
Point 5: In Figure 7, the vertical label states “Pressire” instead of “Pressure”.
Response 5: Thanks for the reviewer's careful check.
The authors are very sorry for this clerical mistake. The word 'Pressire' has been modified in Fig. 7.
Point 6: What is the length of the simulated tunnel? On page 12, line 244 it is stated that is 100 m, but the calculations seem to be made with a length of 1000 m.
Response 6: Thanks for the reviewer's careful check.
The authors are very sorry for this clerical mistake. The length of the simulated tunnel should be 1000 m. The clerical mistake has been modified in this manuscript.
Point 7: In Figures 16, 19 and 22, I recommend the authors change the colour of the green lines to a darker one for better visibility.
Response 7: Thanks for the reviewer's helpful suggestion.
The colour of the green lines in Fig. 16, 19 and 22 have been changed the dark cyan.
Point 8: What is the meaning of the two red dots in Figure 19?
Response 8: Thanks for the reviewer's professional suggestion.
The two red dots represent the maximum positive peak and the maximum negative peak in Fig.19, respectively. The detailed descriptions are as follows:
As shown in Fig. 19, when the train speed is 350 km/h, the aerodynamic pressure of the point near the tunnel middle section achieves the maximum positive peak at 14.1 s (the first red dot). After three consecutive rarefaction waves pass through the monitoring section, the aerodynamic pressure achieves the maximum negative peak at 16.9 s (the second red dot).
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
I would like the authors for their effort in amending the paper. They have comprehensively answered all my questions and I believe the paper is suitable for publication now.
However, small changes must be done before publishing the paper: the right word for “rarefaction” is “refraction”. This mistake is repeated several times in the text.
Author Response
Point 1:However, small changes must be done before publishing the paper: the right word for “rarefaction” is “refraction”. This mistake is repeated several times in the text.
Response 1: Thanks for the reviewer's careful check.
The phrase for 'rarefaction wave' is a terminology.
When the high-speed train nose suddenly enters a tunnel from open air, due to the confinement of the airspace by the tunnel wall, the air in front of the train is compressed. As a result, a compression wave is generated at the tunnel entrance and propagates towards the tunnel exit at sonic speed. When the compression wave arrives at the tunnel exit, part of it is reflected into the tunnel in the form of the rarefaction wave (or expansion wave).
Similarly, when the high-speed train tail enters the tunnel, the original compressed air between the tunnel wall and the train body surface is released. As a result, a rarefaction wave is generated at the tunnel entrance and propagates towards the tunnel exit at sonic speed. When the rarefaction wave arrives at the tunnel exit, part of it is reflected into the tunnel in the form of the compression wave.
