Experimental Assessment of the Flow Resistance of Coastal Wooden Fences

Round 1

Reviewer 1 Report

Review of

Experimental assessment of the flow resistance of coastal wooden fences

by Zou, Hofland, Stive and Mai

 

Submitted to Water

 

 

This manuscript reports on a small scale experimental study of drag loss induced by wooden fences. The overall topic of the paper is quite interesting, with increasing applications. However, I list below a series of major and minor modifications and questions to be adressed before I can recommend publication.

 

Major remarks

 

1- A distinction is made between Eqs. 1 and 2, but those drag formulations are basically the same excepted that the Darcy-Forcheimer equation also includes linear effects. When the latters are negligible or small, both expressions should contain the same physics and therefore converge to the same kind of formulations, apart from some variations in the empirical coefficients. Can the authors provide a more general presentation in the Introduction ? This is evoked in the Discussion section but I think it will be a good starting point to introduce the paper.

 

2- I do not really understand how the non-linear inertial term in the DF equation (Eq. 2) can be related to laminar friction.

 

3- Details should be given on the scaling strategy and associated limitations. How do you choose the model-scale dimensions ? What are the similtude conditions ? What is the effect of walls in both model- and full-scale cases ? Is the full-scale model thickness sufficient to consider a representative medium ? Why the model-scale Cd are not porosity-dependent ?

 

4- Illustrative picture from typical systems used in the field would be helpful, together with additional informations in the legend of Figs 2 and 4 (top view vs side view, etc).

 

5- l. 153-154 : the frame effect is indeed probably small but the authors should give at least a simple quantitative estimation.

 

6- Maybe « Inhomogeneous » is not a correct term for describing such a regular arrangement. Does this configuration correspond to a classical one used in the field ?

 

7- I found the term « filter velocity » inappropriate. Use rather flux, discharge per unit area or Darcy velocity.

 

8- The authors should include the specific surface of each tested system in Table 1 and mention the potential effects of specific surface in the analysis of the results, the Discussion section, and probaly also in the Introduction when presenting the drag coefficient (see e.g. Arnaud et al. "Wave propagation through dense vertical cylinder arrays: Interference process and specific surface effects on damping." Applied Ocean Research 65 (2017): 229-237.)

 

9- l.179. I do not understand how is estimated and corrected the extra pressure loss ? What was the steel mesh size ? Can the authors provide an estimation of the head loss generated by the steel meshes ?

 

10- Fig. 7. More details should be given to explain the differences with existing studies. Why such variations in their Cd and not in the present experiments ?

 

11- l.269 (and related to my first question) : Neglecting the linear term would be ok only if I/u² is independent on u (constant in Fig 6). This is true only for the larger velocities.

 

12- l.324. I feel the discussion ends abruptly, much more can be said, in particular about the difference between models and full-scale results, the applicability to real-world configurations, the contribution of laminar and turbulent drags, the comparison with existing results, etc

 

13- I think the organisation between Discussion and Conclusion sections has to be modified. Conclusion should be reduced to more general synthesis and perspectives.

 

14- Nothing is said about the measurement uncertainties, or I missed it.

 

 

Minor corrections

 

l.56-57 : The authors should be more explicit, which phenomema ?

 

l.57-58 : I do not see why the wake of the upstream cylinder and the distance and position of neighboring cylinders should be taken into account only for randomly arranged array

 

Eq. 2 and later on : the empirical coefficients in the Darcy-Forcheimer equation should rather be named drag coefficients than friction.

 

B should rather be named thickness and so on in the paper with thickness instead of width. Width usually refers to dimension transerve to the main flow

 

Avoid to cite references as « the authors of [X] », use names or different formulations.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

In the paper entitled “Experimental assessment of the flow resistance of coastal wooden fences” the authors investigate the resistance of a brushwood fence by determining the hydraulic gradient over a fence sample under stationary conditions. Fences are studied both in model and full-scale samples with homogeneous and staggered configurations. Results of hydraulic gradients show that the varying behavior of the samples strongly depends on the fences’s porosity, the minimum spacing ratio configuration, and the flow velocity. The paper also investigates the fences’ resistance considering the bulk drag coefficient and the friction coefficient. The paper is interesting, and its results and conclusions seem mostly sound. However, the quality of the paper should be improved, by considering and addressing the following comments:

1) The authors should try to explain, at least in their introductory section, what are the main innovations of their work with respect to current literature. In the current form of the paper, it is unclear what is its main contribution to existing knowledge on assessing the hydraulic gradient of different configurations of coastal wooden fences and determining their resistance to different flow conditions. Including a flow chart of the paper’s methodology in the introductory section is also highly recommended.

2) The introductory section of the paper should mainly focus on aims and objectives, innovative aspects and methodologies followed therein, and on an extensive literature review on the different subjects of the research. Including basic mathematical formulas or structural parts of the methodology followed in the paper, is therefore not recommended. Though, the authors could try to reformulate their manuscript accordingly, separating their introductory section from the description of the paper’s methodology and the equations it is based on.

3) In Section 2.1. the authors should try to include more details on the set-up of the experimental models described in Figure 1. In its present form, the scheme is quite difficult to follow.

4) In Sections 2.1 and 2.2 describing the experimental set-up of the fences in the tubes under study, it is not clear how basic quantities of the experiments were fixed. This mainly focuses on discharges used in the experiments (line 126), the time for keeping a constant discharge (line 126), the sampling frequency of the experiments (line 127), and the duration used to calculate the pressure head (line 131). How were these quantities selected? Were these selections based on existing literature? If so, please provide respective references.

5) In Section 3 of the manuscript (lines 217-218) it is mentioned that “the turbulence contribution for large-scale cases appears at every stage of the flow.”. How can this be extracted from Figure 6? Perhaps, replacing the existing figures with colour ones will significantly assist understanding of the main conclusions of the work.

6) Please show the values of C_D mentioned in lines 233-235 of Section 3.2. in Figure 7.

7) In Section 3.2. (lines 239-240) the authors mention that for large Reynolds numbers, the bulk drag coefficient becomes constant, and is only a function of fence characteristics. But is this true for all cases? Results for case 5 (Figure 7) seem to not follow this.

8) In Figure 7 the difference in porosity of the fences can be highly distinguished for Re/n<1000. Could the authors provide a possible physical explanation on this matter? Could the authors provide further explanations on large differences between the results of this study and previous works of Ozeren et al. (2013) and Hu et al. (2014), especially for Re/n <1000?

9) Could the authors provide some physical explanations on differences between their results and those in Ergun’s study? Please elaborate further on this matter, if possible.

10) How were values of α and β (Table 3) extracted? What do they represent?

Some minor comments are given below:

• Introduction – lines 43-50: Please include the units (in SI) for variables, where missing i.e. for u, ρ, and ν.
• Introduction – line 47 : “… cylinder characteristics…”
• Introduction – line 50 : “… with ν the kinematic viscosity.”
• Introduction – line 53 : “Schewe [15] measured …”
• Introduction – line 56 : “Nepf [12] also …”
• Introduction – line 83: Please include the units (in SI) for pressure difference (δp)
• Laboratory Experiments – Figure 1 - Pressure Sensors PS3-PS5 can not be distinguished.
• Laboratory Experiments – lines 119-120: “… there is one sensor located in the middle of the surface to measure pressure loss.” How is it shown in Figure 1?
• Figure 2 caption – “… Figures (c) and (d)…”
• Laboratory Experiments – lines 128-129: Please include the units (in SI) for n, and A.
• Laboratory Experiments – lines 128-129: “…[m/s], where n is the porosity), which are considered …”
• Laboratory Experiments – line 133: “…where the subscript “*” indicates…”
• Laboratory Experiments – line 136: “… tested using stiff plants, …”
• Laboratory Experiments – line 143: Does the diameter range 2,6-4.5mmm correspond to just one-fifth of the total samples? Please check Figure 3a again.
• Laboratory Experiments – line 144: “…prototype diameters range from …”
• Laboratory Experiments – line 153: “Furthermore, Albers and Von Lieberman [1]…”
• Laboratory Experiments – line 165: “… that is similar to…”
• Experimental Results – line 197: “…cases 1 to 5, is different …”
• Experimental Results – line 204: “…to larger s₂/D_m than …”
• Experimental Results – Figure 5: Horizontal axis title should be u_n[m/s]?
• It is better to replace Figures 5, 6, 7, 8, 9 and 10 with colour ones? In the present form of the Figures, results for the different cases are really hardly distinguished.
• In Figure 6, the y axis could range in [0, 20] s²/m²
• Experimental Results – line 226 – Please include units (in SI) for I and g.
• Experimental Results – line 239 – “Thus, the hydraulic behavior of the flow inside the fence for the two cases might be similar.”?
• Experimental Results – line 249 – “…confirmed by Nepf [12].”
• Experimental Results – line 277 – “A linear … in Figure 8.”
• Experimental Results – line 282 – “The lower the porosity, the lower the β-value.”?
• Experimental Results – line 287 – “In Figure 9, …”
• Experimental Results – line 289 – Perhaps f_ν should be replaced with f_t?
• Experimental Results – line 301 – Please rephrase “…, causing scatters to be less scattering,..”
• Experimental Results – line 311 – Perhaps Eq. 12(b) should be replaced with Eq. 13(b).
• Experimental Results – lines 313-314 – Please rephrase “…are more…than C_D.”
• Experimental Results – lines 316 – “Hereafter, the link between …”
• Experimental Results – line 317 – “This link is obtained from combining Eqs. (11) and (12).
• Experimental Results – line 319 – “Moreover, Ergun [21]…”
• Experimental Results – line 321– “.., with the subscript “E” used for …”
• Conclusions – line 336 – “… is equal to the one estimated for the lowest porosity…”
• Conclusions – line 343 – “… the lowest porosity…”
• Conclusions – line 343 – “… an inhomogeneous arrangement [1-6].”

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The author has clarified all the comments I concerned. I think it could be received.

Author Response

Many thanks for your time and effort to review our manuscript. 

Reviewer 2 Report

The manuscript appears significantly improved, compared to its previous version. The authors tried to address all review comments and made the necessary amendments in their manuscript. 

Some suggested minor syntax and grammar changes to the newly added text are given below:

• Introduction - Lines 39-41 - “Installed in front …are assembled…The frame … has the role of … small amounts of …”
• Introduction - Line 44 - “… concludes that …”
• Caption of Figure 1 - The word “construct” could be replaced with “construction” or “structure”?
• Introduction - Lines 64-66 - Recheck the sentence “The feasible … I.” because its meaning is not clear.
• Introduction - Line 69 - “In many years … is common …”
• Introduction - Lines 72-74 – “For the inner part of the wooden fence, bamboo …straight, leading to …”?
• Introduction - Line 77 - “… influenced by …”
• Introduction – Line 83 – “…, concluding that higher values of SSA led to greater wave dissipation.”? Include an explanation of SSA the first time it appears in the text.
• Methodology - Line 111 - “… a rapid decrease of…”
• Methodology - Lines 112-114 - “This phenomenon …. array increases, leading to a high…”. In this sentence it is unclear where “those” refers to.
• Methodology - Line 118 - “… randomly related.”
• Methodology - Line 137 - “… representing the friction terms and…”?
• Methodology - Lines 140-141 - Recheck the consistency of the sentence added.
• Methodology - Lines 155-156 - “The water level … in order to avoid direct pressure …”
• Methodology - Line 160 - “… can be subsequently estimated by …”
• Methodology - Lines 169-171 - “Tested thicknesses … from the smallest (B = 0.30m) to the largest (B = 0.60m), depending … and based on …”?
• Methodology - Lines 171-172 - “When the fence sample was set, a set of discharges … was imposed …”?
• Methodology - Line 185 - “… and with one opening per…”?
• Methodology - Lines 185-186 - “To investigate the effect of steel meshes on the…”?
• Methodology - Lines 190-191 - “When … the atmosphere and the water…”
• Methodology - Lines 196-198 - “The uncertainties related to water level and pressure head measurements …, where …”
• Methodology - Lines 204-207 - “For the two discharges included in Table 1, … of measured pressure head … Also, … outside decreased…”
• Methodology - Line 244 - The word “ration” should be replaced with “ratio”.
• Experimental Results - “Reynold’s number” should be replaced with “Reynolds number”
• Experimental Results - Line 329 - “… of all cases …”
• Experimental Results - Lines 332-333 - “The narrow … were below …, and this is …”
• Experimental Results - Line 334 - “…. less effectively … 3.0, which caused …”
• Discussion - Lines 374-375 - Please rephrase “The results …. thicknesses.”, because the meaning is not clear.
• Discussion - Lines 395-396 - “The laminar …, described in …”
• Discussion - Lines 400-401 - Please rephrase “This extra … case 2.”, because the meaning is not clear.
• Discussion - Lines 403-404 – “For the bulk … reduction resulting in …”?
• Discussion - Line 406 - “… to the highest value … However, … influenced by …”
• Discussion - Lines 428-429 - Please rephrase, because the meaning is not clear.
• Conclusions - Lines 442-443 - Please rephrase, because the meaning is not clear.
• Conclusions - Line 451 - “…, usually causing the space between bamboo branches to be irregular.”?
• Conclusions - Line 452 - “…, which depends on…”

Author Response

Please see the attachment.

Author Response File: Author Response.docx