Experimental Study of Cavitation Development and Secondary Circulation Flow between Two Eccentric Cylinders

Round 1

Reviewer 1 Report (Previous Reviewer 1)

The authors have addressed all the questions I raised. I think it is now ready for publication.

Reviewer 2 Report (Previous Reviewer 2)

The authors have answered all of my questions and i have no further comments.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

In this work, cavitation inception and development at low Reynolds numbers between two eccentric cylinders were investigated. It is an important and interesting topic, but I feel it is difficult to follow the experimental part.

1. More information about the pressure measurements need to be provided, e.g. what is the effect of the port on fluid flow? Error bar for the pressure measurements? Location of the pressure sensor?

2. The quality of the images could be further improved.

3. Details about the image processing need to be provided. It is not clear how to generate the conclusions based on the images provided.

4. Due to curvature of the cylinder, how to determine the actual size of the bubbles?

5. Fig 5 shows the results using aluminum powder, the information about the powder is missing. It is not clear how to use the rig mentioned in the paper to measure the fluid flow with aluminum powder.

Author Response

Dear reviewer,

We appreciate a lot your time  in reviewing our paper and providing valuable comments. The authors have carefully considered the comments and tried our best to address every one of them:

1. 

More pressure measurements details were added to the text.  Pressure sensor, which was located outside of the cylinders, was connected to a port, in the inner cylinder, in the middle of generatrix, through 15mm long tube with the same diameter (0.5mm). Fluid did not enter the tube. Each measurement was done for 1min, numerous measurements with the same flow parameters showed total uncertainty of 0.05 mmHg. We also added one more figure (Figure 3) which contains additional pressure measurements results.

2.

Our images show dynamics of the bubbles inception and development which is more challenging process than stationary case. Unfortunately we do not have access to high quality speed camera and had to capture this process at 30fps under microscope with the resolution of 640х 480.

3.

All frames are shown without any additional corrections in photo editing software.

4.

The size of the cavitation bubble on frame 5 (Figure 3) is about 0.13 mm. The physical size of the bubble was determined from a calibration strip, which after the experiment was placed into the gap between the cylinders. The diameter of the outer cylinder made of organic glass is 100 mm, and the size of the cavitation bubble is 0.5 mm or less, which makes it possible to consider the distortions introduced by the outer cylinder to be insignificant due to the small aperture angle of observation. 

5.

In experimental studies, flow patterns have been mostly visualized by means of anisotropic aluminum powder. In our case reverse flow near cavitation bubbles chain was discovered by adding a small concentration of aluminum powder to the flow and a recording of the motion on video camera. When location of reverse flow was defined we were adding separated particles to the flow to track the particle motion. Knowing frame rate and cavitation bubbles sizes we could estimate velocity of observed particle.

Reviewer 2 Report

This manuscript presents an experimental study on cavitation development and secondary circulation flow between two eccentric cylinders. Cavitation is a complex multiphase flow phenomenon and it has many industrial and engineering applications.  Experimental study is an important methodology in understanding bubble motions and cavitation development. The experiment presented in this manuscript looks good. However, the analysis of the result looks insufficient in general. Therefore, I suggest the following revisions.

1 There is almost no quantitative analysis except the snapshot of experiment. The pressure in Figure 2 is the only quantitative measurement presented, but not well linked with the analysis on cavitation and secondary circulation.

2 The discussions on the secondary circulation looks insufficient.

3 There are many published results on bubble dynamics and secondary flows in recent years. More citations had better be added in the literature review, and point out the new contribution of this paper.

4 Usually cavitation occurs in flow at relatively high Reynolds number, which is much higher than the Reynolds number presented in this manuscript. How do you generate bubbles at such low Reynolds number?

5 More details about the experiment shall be added. For example, how do you take the photo of the bubbles? What’s the resolution of the snapshot?

6 English language shall be further improved.

7 The organization of the manuscript can be improved as introduction, methodology, results and discussion, conclusion. The results and discussion can be further divided into sub-sections.

 

 

 

 

Author Response

Dear reviewer,

We appreciate a lot your time in reviewing our paper and providing valuable comments. The authors have carefully considered the comments and tried our best to address every one of them:

1.

More pressure measurements details were added to the text.  Pressure sensor, which was located outside of the cylinders, was connected to a port, in the inner cylinder, in the middle of generatrix, through 15mm long tube with the same diameter (0.5mm). Fluid did not enter the tube. Each measurement was done for 1min, numerous measurements with the same flow parameters showed total uncertainty of 0.05 mmHg. We also added one more figure (Figure 3) which contains additional pressure measurements results.

2.

Some more results discussion was added. 

3.

More papers were added to review.

4.

A slight steady decrease in the local pressure in the liquid may be sufficient for the occurrence of gaseous cavitation (when the pressure drops under the vapour pressure). Pressure is increasing in the converging region of the flow and dropping down in the diverging region. A pressure drop of only 2% results in cavitation inception and development, which was confirmed in many flow variations by our group and other researchers. Reynolds number is defined by a gap size and tangential velocity of the outer cylinder. Inception and development of such cavitation process is a very interesting phenomenon and definitely different from conventional development of cavitation on pump blades or in the mixing region of cavitating jets and can't be described with cavitation number (which is quite vague parameter). 

6,7

One more round of proof reading was done and more details were added to the paper.