Numerical Modeling of a Short-Dwell Coater for Bio-Based Coating Applications

Round 1

Reviewer 1 Report

The paper concerns with a numerical study on the flow field structure in a Short Dwell 14 Coater (SDC) by using a bio-based coating material, namely a non-Newtonian liquid. Four different rheological models were applied. Numerical methods applied are described in detail. Simulation results of the flow field, such as velocity streamline, strain rate distributions, etc. are analyzed in detail. It is interesting to see the significant differences of velocity streamline distributions inside the SDC Coater by using different viscosity models. However, there is no experimental validation, which is the weak point of the paper. Nowadays, the flow visualization is not difficult in such a simple geometry.  In the regions close to the blade, both shear thinning and elastic play an important role, the Giesekus model sounds good, however the model parameter should be validated/adjusted according to the applied materials. There is no explanation on the choice of the parameter, such as a in the paper, which is another weak point for the paper.   

Following are some remarks that might help the authors to improve the presentation:

1. Line 173: (Figure 5 6(b and c) )
2. Vertical title of Figure 9(c): should be Shear stress
3. Fig 9 (b) is the same as Fig.(10). Suggestion: Fig.9 Pressure distribution. Fig. 10 (a) – (c) Stress distribution. Then descriptions on stress distributions can be put together. 9 is too small. The size like Fig.10 would be nice.
4. How much is the relaxation time constant used in model 3 and 4?
5. if possible flow visualization in a quasi-2d model (SDC) would be very useful for the validation of the results in Fig.7 

Author Response

Dear Reviewer 1:

We are thankful to the constructive comments given to our manuscript, we really appreciate the reviewers for their thoughtful suggestions, corrections and concerns. We have considered all the comments, addressed all concerns, made corrections, and revised our manuscript accordingly. We believe that our revised manuscript meets the expectations of reviewers and the standards of your reputed journal for publication

The following are the response to reviewers’ comments made on our manuscript.

Reviewer 1: Comments and Suggestions for Authors

The paper concerns with a numerical study on the flow field structure in a Short Dwell 14 Coater (SDC) by using a bio-based coating material, namely a non-Newtonian liquid. Four different rheological models were applied. Numerical methods applied are described in detail. Simulation results of the flow field, such as velocity streamline, strain rate distributions, etc. are analyzed in detail. It is interesting to see the significant differences of velocity streamline distributions inside the SDC Coater by using different viscosity models. However, there is no experimental validation, which is the weak point of the paper. Nowadays, the flow visualization is not difficult in such a simple geometry.  In the regions close to the blade, both shear thinning and elastic play an important role, the Giesekus model sounds good, however the model parameter should be validated/adjusted according to the applied materials. There is no explanation on the choice of the parameter, such as a in the paper, which is another weak point for the paper.   

Following are some remarks that might help the authors to improve the presentation:

1. Line 173: (Figure 5 6(b and c))

Response: Thank you for identifying the typo error, we have now corrected it. Now it refers to Figure 6(a and b), instead of Figure 5(a and b).(see line 218)

1. Vertical title of Figure 9(c): should be Shear stress

Response: Thank you for the corrections: We have now changed the vertical title (y axis title) from normal stress to shear stress, and the figure is now presented in Figure 10 b as per suggestion given in the following comment. (See Figure 10 b, Line 383)

1. Fig 9 (b) is the same as Fig.(10). Suggestion: Fig.9 Pressure distribution. Fig. 10 (a) – (c) Stress distribution. Then descriptions on stress distributions can be put together. 9 is too small. The size like Fig.10 would be nice.

Response: Thank you reviewer for this constructive suggestion, we have now assigned the pressure distribution graph in Figure 9 separately, ( See Figure 9 Line 331), whereas the three stress distributions graphs are assigned to Figure 10 with captions (a-c). Moreover, we have presented the graph separately to have a better visibility as suggested ( See Figure 10 382-385).  We have also merged some texts describing the figures presented previously as Figure 9b and Figure 10.(see lines 333-343, and 368-380)

1. How much is the relaxation time constant used in model 3 and 4?

Response: We have used two relaxation times have been used for both Model 3 and Model 4, corresponding to the Deborah numbers De=2 (l=3x10^-5 sec) and De=200 (l=3x10^-3 sec). We have also indicated the use of these relaxation time constants in the manuscript’s (see Lines 210-211)

1. if possible flow visualization in a quasi-2d model (SDC) would be very useful for the validation of the results in Fig.7 

Response: Unfortunately, our actual experimental setup does not allow for flow visualization as it is practically an industrial piece of equipment. This poses further challenges as the exact geometry cannot be replicated due to confidentiality issues. For these reasons, we have used a simplified SDC geometry as our study domain to investigate the effects of colour viscoelasticity under the paper coating conditions. We take this comment very seriously and we will look into the development of an experimental setup that will allow flow visualization and further validation of our approach. The parameter alpha has been chosen based on previous studies in the field [17].

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have developed a new methodology for prediction of coating process with application for biomaterials. A 2D computational fluid dynamics approach has been used for simulation of coating process. The work is novel and well presented. It can be accepted subject to the minor revision:

1. The Introduction is not strength enough. It needs to be revised by reviewing similar models in this field as well as adding more details about the practical application of the coating in biotechnology.
2. The flow regime is not properly discussed. Please add more details.
3. Please comment on the validation of simulation results. How would you validate the results?
4. Have the authors used scaling in the CFD simulations?

Author Response

Dear reviewer 2:

We are thankful to the constructive comments given to our manuscript, we really appreciate the reviewers for their thoughtful suggestions, corrections and concerns. We have considered all the comments, addressed all concerns, made corrections, and revised our manuscript accordingly. We believe that our revised manuscript meets the expectations of reviewers and the standards of your reputed journal for publication

The following are the response to reviewers’ comments made on our manuscript.

Reviewer No.2

Open Review

Comments and Suggestions for Authors

The authors have developed a new methodology for prediction of coating process with application for biomaterials. A 2D computational fluid dynamics approach has been used for simulation of coating process. The work is novel and well presented. It can be accepted subject to the minor revision:

1. The Introduction is not strength enough. It needs to be revised by reviewing similar models in this field as well as adding more details about the practical application of the coating in biotechnology.

Response: Thank you for the valuable suggestion, We have added, few lines in the first paragraph to address the practical application of biopolymers in coating (See Lines 31-36), plus, we have provided a brief review on the similar models and comparisons, emphasizing the choice of constitutive models to describe the rheology of the coating materials (see third and fourth paragraphs of the introduction section (Line 54-80)

1. The flow regime is not properly discussed. Please add more details.

Response: Thank you for the constructive suggestion, we have included additional details describing the flow regimes in section 3, (See Lines 249-253, 258-260, 276-281, 284-287, 290-293).

1. Please comment on the validation of simulation results. How would you validate the results?

Response: We have used the experimental data reported in the literature (Oliveira et al[40]) to cross validate the models in flow through axisymmetric 12:1 contraction channel. We have indicated this in lines 205-206 in the text. As also mentioned in our Response to Reviewer 1, our actual experimental setup does not allow for flow visualization as it is practically an industrial piece of equipment. This poses further challenges as the exact geometry cannot be replicated due to confidentiality issues. Hence, direct validation of the simulated flow fields is not possible at this stage. However, we will look into the development of an experimental setup that will allow flow visualization and further validation of our approach

1. Have the authors used scaling in the CFD simulations?

Response: No, we have not used scaling in CFD simulations; instead we have used the actual dimensions of the simplified industrial short dwell coater.

Author Response File: Author Response.docx