Discharge Flow of Spherical Particles from a Cylindrical Bin: Experiment and DEM Simulations

Round 1

Reviewer 1 Report

The manuscript entitled „Discharge flow of spherical particles from a cylindrical bin: experiment and DEM simulations“ by Kobyłka et al. report on theoretical and experimental studies on the discharge flow of wooden spheres. The objectives of this work is to validate a theoretical DEM model of the discharge process, to shed light on the influence of the many variables of the model, as well as interpret dynamic mechanisms of flow commencement and arrest.

 

The text is very well written and include novel insights in this interesting and likewise challenging topic. The authors might consider to following minor comments in order to improve their manuscript.

 

p. 3, l. 102: Since particle shape and properties are essential for the discharge behavior, more information about the wooden spheres is needed. Particle diameter distribution, at least some photos, sphericity, etc. are additional information necessary for further interpretation of the obtained results.

 

p. 3, l. 105: How was the number of particles N determined and why is this number different from table 2?

 

p. 4, l. 139: For at least some material friction coefficients are tabulated. Please have a look at the literature (or perform standard measurements) for better interpretation of the choice of the exact numbers.

 

Figure 2 shows force over time for different load cells. What do the authors mean by “load cell”?

 

p. 6, l. 157: Please clarify how you determined the outflow rate experimentally and in the case of the DEM simulations.

 

Figure 3: This reviewer thinks that a "classical" diagram showing discharge rate vs. hole diameter would be beneficial including both DEM and experiments.

 

p. 12, l. 285: Can you show the distribution of the coordination number before opening and at the end of the simulation?

 

p. 17, l. 397: Shape of the dome. You could show the shape of the dome by plotting the centroid of the particles azimuthally averaged in a radial, axial 2D diagram. Figure 11 looks nice, but the information content is rather limited. Plus, what is the color code showing?

 

p. 20, l. 468: Trajectories of particles: Please compare your results with Alonso-Marroquin, F., Mora, P. Beverloo law for hopper flow derived from self-similar profiles. Granular Matter 23, 7 (2021). https://doi.org/10.1007/s10035-020-01067-1

Author Response

Responses to Reviewer's comment attached

Author Response File: Author Response.docx

Reviewer 2 Report

I have reviewed the manuscript “Discharge flow of spherical particles from a cylindrical bin: experiment and DEM simulations” which has been submitted to the journal Processes.

In this work, the authors combine experiments and DEM simulations of the discharge of grains from a cylindrical bin to investigate the role of several variables on the global dynamics (flow rate and clogging).

Overall, I think that the manuscript could be meaningful as it deals with an important problem (the silo discharge) approaching different topics that are still poorly understood. Nevertheless, there is still lot of room for improvement, so major revision is necessary.

Below I list the most important concerns I have:

1. The section that focuses on the shape of the dome is completely empty of results. Nothing is said apart from referring to previous works. I understand the difficulty of performing an analysis of 3D domes in a systematic way but, to be honest, I think that removing this section it is better than trying to fill it with words (not results).
2. In page 7, the authors state: “In the experiments, reliable flow was observed for an orifice diameter higher than 60 mm, 164 i.e. for D0/dp>4.0, while reliable flow in simulations was observed for D0/dp≥4.2”. In my opinion, “reliable flow” does not seem very scientific. A definition should be given of what is considered reliable flow: how much time of uninterrupted flow? How many times is this tested?...
3. Table 2 shows intriguing results for the effect of particle-particle friction coefficient. Nevertheless, it is not clear to me if these results correspond to a single event; i.e. a single silo discharge. In that case, the authors should note that the presented results are not really meaningful as the occurrence of clogging is known to resemble a Poison process, so the avalanche size distribution (given some experimental conditions) is an exponential decay. Therefore, a number of avalanches (or realizations) are necessary to estimate how likely a system is to clog.
4. In the same way, I would urge the authors to demonstrate the robustness of the results displayed in Fig 6. Is the behavior reported always similar, independently on the arch considered? As the authors show in Fig 11, a given orifice can be obstructed by many different arches with dissimilar properties. Therefore, it will be interesting to see how the coordination number changes among different arches developed in the same conditions.
5. The two sentences: “No predictable pattern of clogging of the material in the deposit was observed when Young’s modulus increased or decreased by two orders of magnitude. However, it was observed that the softer particles exhibited higher tendency to clog.” sound contradictory. Moreover, I recommend the authors showing evidence of this with a figure.
6. I think that some extra important references about silos and particle shape can be added. At least these two:
   1. Hafez, A., Liu, Q., Finkbeiner, T., Alouhali, R. A., Moellendick, T. E., & Santamarina, J. C. (2021). The effect of particle shape on discharge and clogging. Scientific reports, 11(1), 1-11.
   2. Goldberg, E., Carlevaro, C. M., & Pugnaloni, L. A. (2018). Clogging in two-dimensions: effect of particle shape. Journal of Statistical Mechanics: Theory and Experiment, 2018(11), 113201.

Author Response

Responses to Reviewer's comment attached

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear authors.

Having read the new version of the manuscript and the authors reply, I regret I cannot accept the manuscript. In a stochastic process like the arch formation in silos, presenting results of a single dome per experimental condition is a completely wrong strategy.

Therefore, I recommend the authors to completely redesign the experiment and focus in the effect of a single variable (or two maximum) but implementing many repetitions of the same experiment (of course, with slightly different initial positions of particles).

Sincerely yours,

The referee 

Author Response

Please, see attachement.

Author Response File: Author Response.docx