Impact of Different Nose Lengths on Flow-Field Structure around a High-Speed Train

Round 1

Reviewer 1 Report

This work presents numerical studies on the aerodynamics of HSTs with different nose lengths. The research design is rigorous and the results are sufficient and of interest to researchers and engineers in this field. The only suggestion the reviewer would like to make is that the motivation part of the introduction needs to be strengthened. In other words, the authors need to improve their literature review to lay out the rationale of their work, the innovations they are making, and the potential significance of the research.  

Author Response

Response to Reviewer 1 Comments

Dear Editor:

First of all, the authors would like to thank the reviewers’ comments concerning our manuscript. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewer's comments are as flowing:

 

Point 1: The only suggestion the reviewer would like to make is that the motivation part of the introduction needs to be strengthened. In other words, the authors need to improve their literature review to lay out the rationale of their work, the innovations they are making, and the potential significance of the research. 

Response 1: Thanks to the reviewer for suggestion about improving the introduction in this study. The authors revised the introduction according to the reviewer's comments and suggestion. Among them, the 2 to 4 paragraphs of it mainly introduce the previous research on the influence of nose shape on aerodynamic performance. The motivation part of it is mainly in 5 and 6 paragraphs. The introduction is modified as follows:

  The aerodynamic performance of a high-speed train (HST) is closely related to its nose shape [1], such as wake flow, train surface pressure, and aerodynamic force. According to this characteristic, the aerodynamic performance of the train can be improved by optimizing the nose shape of the train, especially to reduce the aerodynamic drag force of the train, which will be beneficial to the increase of train speed and the reduction of energy consumption [2,3]. Therefore, the research on the influence of nose shape on train aerodynamic performance has attracted the attention of a large number of scholars.

  With the improvement of computing power, some scholars have studied the evolution mechanism of the flow field with different nose shapes by numerical simulation and revealed the basic law of the influence of train nose shape on flow field. For example, Niu et al. [3] used the IDDES method to study the aerodynamic performance of 8 and 12 m nose trains. It was found that nose length has a great influence on train drag, lift force, development of the boundary layer, pressure and velocity around the train, and the wake vortex. Hassan et al. [4] used the LES method to study the time-averaged flow and instantaneous flow of trains with different nose lengths under crosswind conditions. The results showed that the flow on the leeward side and top of the train is different with different nose lengths. Chen et al. [5] also studied the aerodynamic performance of trains with different nose lengths under crosswind conditions. It was found that the change of nose length had effects on train surface pressure, horizontal slice pressure, and vortex structure on the leeward side of the train. The vortex intensity on the leeward side and tail can be weakened by increasing the nose length of the train. Xie et al. [6] studied the pressure on the rail side of different nose shapes (bulge-wide, ellipsoid, spindle, and flat-wide types), and found that the peak value of trackside pressure caused by the bulge-wide shape was the largest.

  Numerical simulation is one of the important methods used to study the influence of train nose shape on train aerodynamic performance. However, there are many factors that affect the train nose shape, and the numerical simulation of each will increase the cost of nose shape design. Therefore, some scholars have used some optimization methods to improve the efficiency of nose shape optimization. For example, Krajnović [7] and Yao et al. [8] used the response surface method (RSM); Vytla [9], Howe [10], and J. Muñoz-Paniagua et al. [11] used the genetic algorithm (GA), and Jakubek and Wagner [12] and J. Muñoz-Paniagua et al. [13] utilized the adjoint method (AM). After, the numerical simulation of the optimized model is carried out with these methods, and the results of the numerical simulation are used to judge whether the optimization goal is achieved, including improving the drag, lift force, and lateral stability of the train, among other characteristics.

  In addition to the numerical simulation method, some scholars have used wind tunnels to study the influence of nose shape on the aerodynamic performance of trains. Cheli et al. [14] studied the EMUV250 train model with different nose shapes by wind tunnel testing, and it was found that the lift force, lateral force, and overturning moment of the new model were reduced. Zhang and Zhou [15] used wind tunnel tests to compare the aerodynamic forces and moments of different nose shapes under different yaw angles, and the results showed that the nose shape has a significant impact on aerodynamic forces and moments. Bell et al. [16] studied the slipstream and wake flows of simplified trains with different roof slant angles by wind tunnel experiments, and it was found that the nose length had a great influence on the separation of wake vortex. Through the wind tunnel tests, it was found that because of the different nose shapes of a train, the flow-field structure around the train is different, which leads to the change in the distribution of the train surface pressure, especially the pressure distribution of the streamlined head, which, in turn, leads to differences in the aerodynamic forces and moments of the train.

  At present, research on the influence of nose shape on train aerodynamic performance is conducted more to analyze the influence of train nose shape change on aerodynamic force, aerodynamic moment, train surface pressure, pressure distribution around a train, and wake vortex structure. However, few scholars have paid attention to the relationship between wake and slipstream when the nose length changes. According to previous research, in the near-wake region, the slipstream will have a large peak value, which is due to the wake of twin counter-rotating vortices continuing to develop outward and downward beyond the width of the train. It is under this condition that the largest slipstream velocities are measured [15,17-18]. In addition, when the train passes, for static people and objects along the railway, a gust will be produced accompanied by pressure and velocity transients [19, 20]. As a result, by studying the relationship between the wake and slipstream under different nose lengths, it contributes to design the train nose shape and reduce the harm to static people and objects around the train.

  The main purpose of this study was to study the flow-field structure around a HST with different nose lengths, including the time-averaged and instantaneous slipstream velocity and the time-averaged pressure and wake flows. Furthermore, the aerodynamic drag and lift force of the head and tail of a train with different nose lengths will be compared and analyzed. Finally, the basic laws of aerodynamic performance with different nose lengths are obtained. Four nose lengths are used in this study: 4, 7, 9, and 12 m. The research structure of this study is divided into the following five sections: Section 2 covers methodology, including model geometry, computational domain, boundary conditions, numerical method, mesh strategy, and mesh sensitivity. The algorithm validation is presented in Section 3. Qualitative and quantitative results are given in Section 4. Conclusions are summarized in Section 5.

Author Response File: Author Response.docx

Reviewer 2 Report

Authors present a paper about: "Impact of different nose lengths on flow-field structure around a high-speed train".

I suggest the revision of the Abstract: Acronyms without definition (as an example: IDDES) - correct also that along the paper. Also, all that quantitative results in an Abstract is not correct in my opinion.

Figures with graphic results have poor quality. If possible improve that figures.

Author Response

Response to Reviewer 2 Comments

 

Dear Editor:

First of all, the authors would like to thank the reviewers’ comments concerning our manuscript. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewer's comments are as flowing:

 

Point 1: Acronyms without definition (as an example: IDDES) - correct also that along the paper. 


Response 1: According to the reviewer's suggestion, it has been defined where it first appears in the paper.

IDDES: Improved delayed detached-eddy simulation.

RANS: Reynolds-averaged Navier-Stokes.

LES: Large eddy simulation.

ΔC_{P max}: Maximum pressure coefficient.

ΔC_{P min}: Minimum pressure coefficient.

ΔC_P: Pressure change coefficient.

ΔC_{P max}, ΔC_{P min}, ΔC_P are standard in the train aerodynamic [1].

CEN European Standard. Railway Applications-Aerodynamics. Part 4: Requirements and Test Procedures for Aerodynamics on Open Track. CEN EN 14067-4, 2013.

 

Point 2: All that quantitative results in an Abstract is not correct in my opinion.

Response 2: According to the reviewer's opinion, the authors revised quantitative results in an abstract. In this study, the time-averaged and instantaneous slipstream velocity, time-averaged pressure, wake flows, and aerodynamic force of a high-speed train (HST) with different nose lengths are compared and analyzed using improved delayed detached-eddy simulation (IDDES) method. Four train models were selected, with nose lengths of 4, 7, 9, and 12 m. To verify the accuracy of the numerical simulation results, they were compared with wind tunnel test results. The comparison results show that the selection of the numerical simulation method is reasonable. The research results show that with increasing nose length, the peak values of the time-averaged slipstream velocity of the trackside position (3 m from the center of track and 0.2 m from the top of rail) and the platform position (3 m from the center of track and 0.2 m from the top of rail) decrease continuously, and show a trend of rapid reduction at first, and then a slow decrease. As the nose length increased from 4 to 12 m, the time-averaged slipstream velocity at the trackside position and platform position are decreased by 57% and 19.5%, respectively. At a height of 1.6 m from the top of the rail, ΔC_{P max} (Maximum pressure coefficient), |ΔC_{P min}| (The absolute value of minimum pressure coefficient), and ΔC_P (Pressure change coefficient) decrease with increasing nose length, which is similar to the peak value of time-averaged slipstream velocity, decreasing rapidly at first and then slowly. As the nose length increased from 4 to 12 m, decreases of ΔC_{P max}, |ΔC_{P min}|, and ΔC_P by 26.5%, 58.5%, and 44.8%, respectively. Different nose lengths also have a significant impact on wake flow.

 

 

Point 3: Figures with graphic results have poor quality. If possible improve that figures.

Response 3: According to the reviewer's suggestion, some figures quality improvements are as follows (In word):

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have addressed the issue raised by the reviewer.

Reviewer 2 Report

Authors improved the paper.