Study of the Drag Reduction Characteristics of Circular Cylinder with Dimpled Surface
Round 1
Reviewer 1 Report
The manuscript deals with an interesting phenomenon, but is affected by several flawnesses and cannot be accepted in the present form. It must be revised according to the following points.
- The aim of the manuscript is not clearly stated.
- Two different symbols are used for the average velocity u and U in equations (1-5). Please correct.
- The Governing equations and turbulence model is not adequately presented. Is the considered setup 2D or 3D? If it is 2D, as I assume, I am puzzled because it is known that if the flow becomes turbulent, becomes 3D too. Then considering 2D simulations makes sense if the flow is laminar. I really invite the authors to consider the possibility to perform at least one 3D simulation, to check the validity of thir results, in order to motivate the choice of a 2D setup.
- In equation (1), (2) the turbulence model is missing. There should be the Reynolds stress term suitably expressed by means of the turbulent viscosity. The latter should be calculated by means of the turbulence model.
- Coupling among equations (6-9) is not clear at all.
- It seems that Cd and Clrms are not defined before being used in table 2. I think that they are the drag and lift coefficient. How have they been calculated?
- How the velocity field analysis is realted to the main focus of the manuscript, which is the determination of the drag force? Have the velocity profiles been used to calculate the drag force?
Author Response
The author read the comments carefully and made corrections and replies.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Study on the drag reduction characteristics of circular cylinder with the dimpled surface
This paper studies the effect of small dimples on the surface of the circular cylinder and the dimples are installed aiming reduction of the vortex induced vibration. The authors made numerical simulation and experiment both. However, I cannot recommend this paper to publish at this stage.
Figure 1: The schematic diagram of the cylinder with dimples is shown. However, it is difficult to understand and image the dimples by this figure.
Line 55: To set the first point of the computational mesh over the cylinder, the author uses y^+ and it is determined by the Eq.(10). However, the plus is the wall unit which is determined by the wall-shear stress. I cannot understand why the wall unit is determined by Eq.(10).
Figure 2(b): The zoom-up view of the computational mesh is shown. I cannot understand the distribution of the mesh in the dimple and I cannot judge the grid resolution in the dimple is enough or not.
Table 2: The verification is done in this table. However, there is no verification for the flow filed around the cylinder with dimples. Since the dimples are very small, sufficiently fine grid resolution is required and it should be checked. Without verification for the cylinder with dimple, all the numerical results are doubt for me at least.
Line 216 "The mean skin-friction coefficient" What is the "mean" in this context and what is the definition of the skin-friction coefficient? Since the cylinder has dimples, how do you teat it when the wall-shear stress is computed?
Figures 4 and 5: The skin-friction coefficient and pressure drag are shown, These figures are not clear. Please explain why the author judged that this profile is true. In the dimpled case, the dimple may disturb the flow and turbulence may occurs. Therefore, it is expected that the skin-friction drag may significantly increase. But from the figures, the increment is not enough. Why?
Line 269: How and what do you define or measure vortex shedding frequency f.
Line 272: "indicating that the dimpled structure can reduct the strength of vortex shedding" why?
Line 284: "The console controls..." This information is not enough to explain the base flow and experimental apparatus.
Figure 10: Why the profile is not symmetry? In case a-1, the unexpected velocity is measured at Y/d=7. What is this?And the authors should discuss the turbulent component, since the dimple promotes the turbulence and suppresses the flow separation.
Figure 11 (and 12): The flow direction should be shown from left to right. And this diagram is for the low Reynolds number flow at Re=40. In the Reynolds number, the Karman vortex street should appear. Therefore, I think that the following discuss is meaningless.
Author Response
The author read the comments carefully and made corrections and replies.
Author Response File: Author Response.pdf
Reviewer 3 Report
The comments are attached.
Comments for author File: Comments.pdf
Author Response
The author read the comments carefully and made corrections and replies.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The manuscript can be accepted as all the issues have been addressed.
Author Response
Thank you for all your comments and suggestions on my paper, your recognition of my reply, and accepting the views of this paper.
Reviewer 2 Report
Firstly, please highlight or point out where is the modified part in the manuscript according to the comment rise from the reviewers.
Point 6 in the rebuttal: The author should show that the profile is correct or not. Using Fluent dose not mean that the results is correct.
The author shows the result in another fine mesh. Therefore, please compare these cf profiles. I do not think that the the sharp (or sudden drop) profile is physically correct one.
And, relating to point 3, I think that computational mesh of the side wall of the dimple is not enough.
Point 7, The author employed formula to calculate f, which is a function of Reynolds number only. It means that the frequency is not changed, is not it? Otherwise, the f is meaningless.
Point 11, Fig. 14 is the instantaneous profile, but discussion in the manuscript is in the time averaged. I cannot understand why twin vortical structure appears just behind the cylinder? Because the vortical structure is meandering behind the cylinder and they will shed, it is expected that the twin vortex does not appears under the time averaged flow field.
Author Response
We are very grateful for this review’s detailed comments that have helped us improve significantly this manuscript. The major changes to the manuscript have been highlighted in red. Following are actions we have taken to address specific issues.
Author Response File: Author Response.pdf
Reviewer 3 Report
The paper has been somewhat improved compared to the previous version.
I still do not feel comfortable with authors declaring something about VIV while no research has been done in that direction. The paper is concerned with the drag reduction by modifying the surface with dimples.
The authors ignored this comment in my previous letter.
Please remove:
...then suppress the vortex-induced vibration (VIV),...
from the introduction.
Please remove:
...suppress VIV by placing dimples...
from the last paragraph of the introduction.
etc
Author Response
We are very grateful for this review’s detailed comments that have helped us improve significantly this manuscript. The major changes to the manuscript have been highlighted in red. Following are actions we have taken to address specific issues.
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
For readers' understanding, the flow direction in figures at Sec. 4 should be inverted left and right.
