Water Hammer Simulation Using Simplified Convolution-Based Unsteady Friction Model

Round 1

Reviewer 1 Report

I consider that this research is suitable to be published since the authors have included some useful simplification for determining the evolution of the friction factor during transient flow occurrence.  This research is also supported with experimental measurements, which were made by other authors.  

Some minor comments are:

- A notation section should be included. Please check all variables and parameters. 

- A numbering of equations should be performed in the Appendix A.

- Improve the presentation of Figure 12.  In addition, this Figure contains in the y-axis a comma (",") as decimal symbol, but in the x-axis the decimal symbol is a ".". Similar comments for Figure 9.

Author Response

The authors would like to thank the reviewer for his/her fruitful comments and suggestions. Changes in the manuscript are highlited and commented below. We did our best to address all the excellent comments and suggestions as much as possible.

I consider that this research is suitable to be published since the authors have included some useful simplification for determining the evolution of the friction factor during transient flow occurrence.  This research is also supported with experimental measurements, which were made by other authors.

Thank you for your kind words.

Some minor comments are:

1. The reviewer’s comment:

A notation section should be included. Please check all variables and parameters.

1. The authors' answer:

Thank you for your valuable comment. The notation section has been added to the revised version after the Conflicts of Interest statement.

2. The reviewer’s comment:

A numbering of equations should be performed in the Appendix A.

2. The authors' answer:

Thank you for your valuable comment. The numbering of equations presented in the Appendix A has been added.

3. The reviewer’s comment:

Improve the presentation of Figure 12.  In addition, this Figure contains in the y-axis a comma (",") as decimal symbol, but in the x-axis the decimal symbol is a ".". Similar comments for Figure 9..

3. The authors' answer:

Thank you for your valuable comment. As suggested new versions of Figures 9 and 12 have been prepared, this time not in Excel which gave us no possibility to change the comma to dot, but in Matlab from which all the comparison graphs have been inserted.

Again, the authors would like to thank the reviewer for his/her excellent comments and suggestions.

Author Response File: Author Response.pdf

Reviewer 2 Report

Review of “Water Hammer Simulation Using Simplified Convolution-based Unsteady Friction Model”

The paper addresses the water hammer 1D numerical models considering an improved numerical model based on, the convolution-based unsteady friction model however, the paper needs to be a reviewed to cope with the high standards of Water.

General considerations

1.       The manuscript is well structured.

2.       The english language and style must be reviewed.

Line 45 – please consider removing “all symbols are defined as they first apper in the text”, since this is common.

Line 48 – please consider ilustrating the sentence “the speed propagation speed of the increased pressure wave is about three to four times greater…” using some numbers and references.

Lines 49 – 53 – please consider rephrasing and please provide references.

Lines 54-56 – this is valid for transient events/hydraulic transients, please add this explanation.

Lines 86 – the Johnston procedure will also be evaluated, please consider rephrasing.

Line 87 – please consider clearify all the objectives of the paper.

Eq4 – correct these equations.

Line 112 – left boundary and right boundary is confusing.

Line 133 – Brunone et al. [4] model, please consider correcting it.

Eq13 – please correct the equation according to reference.

Line 142 – authors of this paper – please consider adding the references and specify which authors.

Lines 159 – 162 – please consider adding the explanation by the researcher.

Line 261 – when refering to experimental facility, please provide references, and describe the experimental facility main features – pipe supports, hydropneumatic size,

Line 283 – please consider adding some examples, comparison with a well known approach (classical approach), long time is very confusing.

Line 285 – bulk pressure amplitudes – please consider using the characteristic time.

Line 289 – rarest is not common, coarse mesh? Also densest, more refined, please consider using other terms.

Figure 5 – the peak seems like an FSI issue, please consider analysing literature, and provide a clear explanation with references. Were the test repeated? Is it possible to compare same discharges and the pressure peaks? Are they consistent ?

Line 308 –consider replacing “they”.

Author Response

The authors would like to thank the reviewer for his/her fruitful comments and suggestions. Changes in the manuscript are highlighted and commented below. We did our best to address all the excellent comments and suggestions as much as possible.

The paper addresses the water hammer 1D numerical models considering an improved numerical model based on, the convolution-based unsteady friction model however, the paper needs to be a reviewed to cope with the high standards of Water.

General considerations

1. The manuscript is well structured.

       Thank you for your kind words.

2. The English language and style must be reviewed.

Thank you for your suggestion. We did our best to improve the paper as suggested by the reviewer. We have asked a proficient English writer to check the paper.

3. The reviewer’s comment:

Line 45 – please consider removing “all symbols are defined as they first apper in the text”, since this is common.

3. The authors' answer:

Thank you for your suggestion. We have removed the sentence.

4. The reviewer’s comment:

Line 48 – please consider ilustrating the sentence “the speed propagation speed of the increased pressure wave is about three to four times greater…” using some numbers and references.

4. The authors' answer:

Thank you for your valuable comment. As suggested new sentences are introduced in the Introduction section, as well a new reference is cited as follows: “An interesting practical example can be drawn using the dependency graph for pressure wave speeds in water flows presented in Pothof and Karney’s Chapter 1 of Guidelines for Transient Analysis in Water Transmission and Distribution Systems [1]. It is shown that a typical pressure wave speed in a steel pipe with elasticity modulus  N/m², inner diameter to wall thickness ratio (D/e) equal to  is about 1300 m/s, while for the same ratio of D/e in a PVC pipe ( N/m²) c is about 400 m/s and in a HDPE pipe ( N/m²) c is just about 200 m/s. These values show that the pressure wave speed in metal pipes is more than three times larger than in PVC pipes and more than six times than in HDPE ones.”.

5. The reviewer’s comment:

Lines 49 – 53 – please consider rephrasing and please provide references.

5. The authors' answer:

Thank you for your valuable comment. The text has been corrected to: “During water hammer events several accompanying phenomena may occur including: cavitation [2, 3, 4] (when the pressure drops to the vapour pressure of the liquid), unsteady friction [5, 6, 7] (resistance of the liquid during unsteady flow against the pipe wall), and fluid-structure interaction [8, 9, 10] (interactions of movable or deformable pipe structure with an internal or surrounding fluid flow).”

Nine new references [2-10] have been cited in the corrected manuscript according to this improvement.

6. The reviewer’s comment:

Lines 54-56 – this is valid for transient events/hydraulic transients, please add this explanation.

6. The authors' answer:

Thank you for your valuable comment.  We have modified this sentence according to the suggestion: “Assuming the adequate restraint of the pipe elements and pressure above the liquid vapor pressure, then the modelling of the unsteady friction remains the greatest challenge.”

7. The reviewer’s comment:

Lines 86 – the Johnston procedure will also be evaluated, please consider rephrasing.

Line 87 – please consider clarify all the objectives of the paper.

7. The authors' answer:

Thank you for your valuable comments.  In the revised version of our paper we have rephrased and clarified the sentences to: “The objective of this paper is aimed to further test a computationally effective and accurate CBM model developed by Urbanowicz [31]. In an earlier work, this approach was verified only for the case of unsteady flows with cavitation [31], therefore, in this paper, we validate the model against the experimental results without cavitation [29]. Second objective is verification of the effectiveness of the Johnston’s lumped friction model [32], according to which the unsteady friction can be concentrated only at the boundary nodes of the numerical grid.”

8. The reviewer’s comment:

Eq4 – correct these equations.

8. The authors' answer:

Thank you for noticing the error in Eq. 4 which has been corrected.

9. The reviewer’s comment:

Line 112 – left boundary and right boundary is confusing.

9. The authors' answer:

Thank you for your valuable comment. We have clarified these sentences as follows:

“When at the i = 1 node (cross-section) of the characteristics grid the flow velocity v is determined (quickly closing valve) for the time t > 0, and at the i = N + 1 node the pressure p is known (reservoir pressure), then:”

and

“Conversely, if the pressure p was determined as the boundary condition at the i = 1 node of the characteristics grid (reservoir section), and the value of the flow velocity v at the i = N + 1 node (valve section), then:”

10. The reviewer’s comment:

Line 133 – Brunone et al. [4] model, please consider correcting it.

Eq13 – please correct the equation according to reference.

10. The authors' answer:

Thank you for noticing an error in Eqs. 11, 12 and 13 which has been corrected - 8 was changed to 4.

11. The reviewer’s comment:

Line 142 – authors of this paper – please consider adding the references and specify which authors.

11. The authors' answer:

Thank you for your valuable comment. The sentence has been revised as suggested.

12. The reviewer’s comment:

Lines 159 – 162 – please consider adding the explanation by the researcher.

12. The authors' answer:

Thank you for your valuable comment. As we have corrected all the respective IAB models and to avoid confusion, this part was excluded from the revised version of our paper, as well as the cited reference.

13. The reviewer’s comment:

Line 261 – when referring to experimental facility, please provide references, and describe the experimental facility main features – pipe supports, hydropneumatic size,

13. The authors' answer:

Thank you for your valuable comment. The reference from what we have noticed is provided correctly – reference [19] from 2006. We have significantly expanded the description of this Adamkowski-Lewandowski test rig as suggested by the reviewer.

14. The reviewer’s comment:

Line 283 – please consider adding some examples, comparison with a well known approach (classical approach), long time is very confusing.

14. The authors' answer:

Thank you for your valuable comment. We do not fully understand this comment. Does the reviewer mean additional graphical comparisons, i.e. for other initial Reynolds numbers? If so, the obtained results are very similar to those presented in Figs. 7, 8 and 10, 11. This is due to the fact that the parameters of this system, and in particular the resulting water hammer number, have similar values. Hence, we do not see the need to present them. The simulation in a very fine grid of characteristics was not carried out because the sufficient density was selected based on the Computational Compliance Criteria (please see the paper Urbanowicz 2017, https://doi.org/10.1051/e3sconf/20171903021).

15. The reviewer’s comment:

Line 285 – bulk pressure amplitudes – please consider using the characteristic time.

15. The authors' answer:

Thank you for your valuable comment. This sentence has been changed according to the suggestion: “This time covers eighteen water hammer periods (t/(4L/c)), more than enough for an adequate comparison study.”

16. The reviewer’s comment:

Line 289 – rarest is not common, coarse mesh? Also densest, more refined, please consider using other terms.

16. The authors' answer:

Thank you for your valuable comment. We have changed the words “rarest” to “coarse mesh” and “densest” to “very fine mesh” in this sentence.

17. The reviewer’s comment:

Figure 5 – the peak seems like an FSI issue, please consider analysing literature, and provide a clear explanation with references. Were the tests repeated? Is it possible to compare same discharges and the pressure peaks? Are they consistent?

17. The authors' answer:

Thank you for your valuable comment. The revised version of our paper discusses this problem in detail, these new conclusions are drawn after reading additional papers.

“These short-duration peaks at the first pressure amplitude plateau are most probably the result of undesired mechanical vibrations produced by the valve closing drive [45]. They are quickly damped out for all types of supports - they are present only at the first pressure pulse and do not influence further water hammer pressure oscillations. Other reason of these peaks (initial disturbances) can be probably linked to the system response due to the excitation from the step load induced by the fast-closing valve [46]. Other source of such peaks can be explained to be the result of the type of the valve used [47, 48]. The use of the globe valve instead of the ball valve allows to eliminate their presence in experiments.”

18. The reviewer’s comment:

Line 308 –consider replacing “they”.

18. The authors' answer:

Thank you for your suggestion. Corrected in the revised version of the paper as suggested.

Again, the authors would like to thank the reviewer for his/her excellent comments and suggestions. We did our best to address all the excellent comments and suggestions as much as possible.

Author Response File: Author Response.pdf