A Novel, Coupled CFD-DEM Model for the Flow Characteristics of Particles Inside a Pipe
Round 1
Reviewer 1 Report
The paper is well-written and well-organised. I would suggests proof reading by a professional English proof reader as there are language issues here and there - however not to the extent that it affect the quality of the paper to any significant degree.
My primary comment concerns the novelty of this study and the motivation for it. The novelty is as I understand solely based on the fact that this study couple OpenFOAM and a commercial DEM code. As the author's clearly describe in the paper, the CFD-DEM-coupling is nothing new. They give a thorough background to what is done in this field. So, why is it important to show that the coupling between an open source CFD code and a commercial DEM code work?
There is an obvious practical value of course, but one could argue that that could be equally well published as a tutorial in OpenFOAM (and belonging websites, blogs etc.).
So, I would like the authors to put into a more rigid scientific context the value of being able to do the present coupling they do.
Author Response
Response to Reviewer 1’s Comments
Dear Reviewer:
Thank you for your comments on this manuscript. According to your advice, we corrected the relevant parts of the manuscript. The answers to your questions are provided below.
Point 1: I would suggest proof reading by a professional English proof reader as there are language issues here and there – however not to the extent that it affect the quality of the paper to any significant degree.
Response 1: We agree that the language quality of the article needed to be improved. Therefore, we commissioned American Journal Experts (AJE) to edit the language of the manuscript once more. The revised manuscript is the proofread version.
Point 2: My primary comment concerns the novelty of this study and the motivation for it. The novelty is as I understand solely based on the fact that this study couple OpenFOAM and a commercial DEM code. As the author’s clearly describe in the paper, the CFD-DEM-coupling is nothing new. They give a thorough background to what is done in this field. So, why is it important to show that the coupling between an open source CFD code and a commercial DEM code work?
Response 2: Thank you for identifying this problem. We believe that open source CFD, especially OpenFOAM, is an important part of CFD. Due to its code universality and low cost, it has gradually become the mainstream method in CFD research. Thus, open source CFD is essential in the development of CFD. Meanwhile, currently, PFC3D is the most mature and widely used commercial DEM. PFC3D has a powerful DEM computing function, a built-in Fish language and embedded C++ and Python environments, which can accomplish user-defined programming and coupling with a third-party software. In addition, PFC3D is inexpensive, allows for the easy customization of commercial software, and facilitates the programming of open source software. Therefore, we feel that the coupling between the open source CFD code and the commercial DEM code is very important
Point 3: There is an obvious practical value of course, but one could argue that that could be equally well published as a tutorial in OpenFOAM (and belonging websites, blogs etc.).
Response 3: We completely agree with your suggestion.
Point 4: So, I would like the authors to put into a more rigid scientific context the value of being able to do the present coupling they do.
Response 4: In the revised manuscript, we added and modified the following discussion in Section 2.2 to further explain the coupling scheme:
The coupling program is written in C++ and integrated in Python. Then, the p2pLink classes in the Python module of PFC3D were employed to exchange data between OpenFOAM and PFC3D in the Python environment. The PISO algorithm is used to solve the discretized pressure-velocity coupling equations in OpenFOAM.Special thanks to you for your excellent comments.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear Editor,
In this paper, a 3D CFD-DEM model is developed by coupling two software programs, OpenFOAM and PFC3D. The article is well organized and contains some novelties. However, before the acceptance, there are some issues that need to be better clarified and some corrections that should be implemented.
In Table 2, a reference should be added for each correlation. The quality of Fig. 2 is very low and also, the data for each case is not distinguishable. I suggest the authors to change the thickness and type of the lines for this graph. In Fig. 9, as the experimental data is used and also because the data are discrete, I suggest to use symbols instead of lines. In the “model description” section, the meshed geometry should be depicted and discussed. Also, mesh independence study should be carried out for this study. There is no mention of how the geometry is meshed and what are the type and size of elements. Maybe refereeing to a recent review paper in the field would help the readers to understand the challenges.https://www.sciencedirect.com/science/article/abs/pii/S0045793016301773
Author Response
Response to Reviewer 2’s Comments
Dear Reviewer:
Thank you for your comments on this manuscript. According to your advice, we have corrected the relevant parts of the manuscript. The answers to your questions are provided below.
Point 1: In Table 2, a reference should be added for each correlation.
Response 1: In the revised manuscript, we added a reference for each correlation.
Point 2: The quality of Figure 2 is very low and also, the data for each case is not distinguishable. I suggest the authors to change the thickness and type of the lines for this graph.
Response 2: In the revised manuscript, we replotted Figure 2 and improved its quality. We replotted Figure 2 and changed the thickness and type of lines to make it easier to distinguish between different cases.
Point 3: In Figure 9, as the experimental data is used and also because the data are discrete, I suggest to use symbols instead of lines.
Response 3: In the revised manuscript, we replotted Figure 9 and used symbols to represent the experimental data.
Point 4: In the “model description” section, the meshed geometry should be depicted and discussed.
Response 4: In the revised manuscript, we added and modified the following discussion in Section 3.1 to further explain the meshed geometry:
The entire fluid element that contains the entire pipe is a parallelepiped with dimensions of 0.18 m × 1.2 m × 0.18 m. Along the three axes of the fluid element, the 6 × 40 × 6 fluid cells are divided so that each fluid cell is a cube with a side length of 0.03 m, as shown in Figure 10.Point 5: Also, mesh independence study should be carried out for this study.
Response 5: Thank you for identifying this problem. In our study, we always used the following inequality [Equation (14)] where is the minimum width of the flow domain and is the fluid cell length.
In my opinion, when the inequality is satisfied, we can assume that the numerical computation is mesh-independent, which confirms that the research satisfies the condition of mesh independence.
Point 6: There is no mention of how the geometry is meshed and what are the type and size of elements.
Response 6: We added and modified the following discussion in Section 3.1 to describe how the geometry is meshed and the type and size of the elements:
The entire fluid element that contains the entire pipe is a parallelepiped with dimensions of 0.18 m × 1.2 m × 0.18 m. Along the three axes of the fluid element, 6 × 40 × 6 fluid cells are divided so that each fluid cell is a cube with a side length of 0.03 m, as shown in Figure 10.Point 7: Maybe refereeing to a recent review paper in the field would help the readers to understand the challenges.
Response 7: In the Introduction section of the revised manuscript, we added four recent review papers as references [3-5,12].
Special thanks to you for your excellent comments.
Author Response File: Author Response.docx
Reviewer 3 Report
This article introduced a novel coupled CFD-DEM model for the flow of particles inside a pipe, which is an important subject in many industrial fields. The authors present a coupling methodology combining the existing techniques and numerical results show the superiority of their approach. The paper is written well so this article is acceptable for publication. However, for the completeness of this article, this referee cannot help giving several comments.
Even if the methodology can be chosen from the existing one, the details for discretization will be important for readers. If possible, please present details for numerical schemes for coupling. Even if the fluid inside the pipe problem is very important in many fields, the article only introduced particles inside the pipe problems. For the completeness, do some survey and add several references related fluid inside the pipe problem to Introduction section. (Ex. Core-Annular Flow(Experimental, Numerical)Author Response
Response to Reviewer 3’s Comments
Dear Reviewer:
Thank you for your comments for this manuscript. According to your advice, we corrected the relevant parts of the manuscript. The answers to your questions are provided below.
Point 1: Even if the methodology can be chosen from the existing one, the details for discretization will be important for readers. If possible, please present details for numerical schemes for coupling.
Response 1: In the revised manuscript, we added and corrected the following discussion in Section 2.2 to further explain the coupling scheme:
The coupling program is written in C++ and integrated in Python. Then, the p2pLink classes in the Python module of PFC3D were employed to exchange data between OpenFOAM and PFC3D in the Python environment. The PISO algorithm is used to solve discretized pressure-velocity coupling equations in OpenFOAM.Point 2: Even if the fluid inside the pipe problem is very important in many fields, the article only introduced particles inside the pipe problems. For the completeness, do some survey and add several references related fluid inside the pipe problem to Introduction section. (Ex. Core-Annular Flow (Experimental, Numerical)).
Response 2: Thank you for identifying this problem. In the revised manuscript, we added two citations (i.e., references [22] and [23]) on the core-annular flow using the CFD-DEM model.
Special thanks to you for your excellent comments.
Author Response File: Author Response.docx
