Model for Wall Shear Stress from Obliquely Impinging Planar Underexpanded Jets
Round 1
Reviewer 1 Report
Review of a manuscript titled “ Model for Wall Shear Stress from Obliquely Impinging Planar Underexpanded Jets” by Patrick et al.
The work is about developing a correlation for maximum wall shear stress for a Obliquely Impinging Planar Underexpanded Jets with angles varying from 0 to 60. The work was presented well and validated the experimental work also, but certain aspects are not clear and need to be addressed.
1. In line 60, Impingement angles were varied from 60 to 90 and table 1 says it is varied from 0-60. Please make sure that statement is consistent.
2. How did the authors made sure y+ is less than 1. It would better if the authors specify the first grid length
3. The computational domain used in the work is not presented. It would be better to state nature computational domain used and show the figure with mesh. At the same time, no need to present equations 1 –3, refer to the Fluent manual.
4. Why do the authors fix the range of angle 0-60?
5. Based on the previous work in your group, with lower H/W , the flow near the wall is supersonic and the trend is non-linear and the fit does not work. Is the reason why data from H/W = 15 is removed from correlation?
6. It is essential to state the range over which the relationship developed is valid. Also, in generally state the parameter space where deviation is more like lower H/W and 60 degree angle etc.
Minor comments:
Color mapping in the Figures 5 and 6 is difficult to read, making the colour variation better to distinguish.
Author Response
We thank the reviewer for their thoughtful response, we have addressed the concerns point by point below:
- In line 60, Impingement angles were varied from 60 to 90 and table 1 says it is varied from 0-60. Please make sure that statement is consistent.
This error has been corrected, thank you for pointing it out.
- How did the authors made sure y+ is less than 1. It would better if the authors specify the first grid length
The authors used data from our previous published work on normally impinging to evaluate the maximum wall shear stress at 3.4 NPR at each h/w ratio for the normal impingement case (which has the maximum wall shear stress of all impingement angles) to then calculate at what distance from the wall y+=1 would be for that maximum wall shear stress. The structured prism layer mesh was then generated for each h/w ratio with the first node at the distance for y+=1 at the maximum shear stress. As such first grid length varies in each geometry and it is not practical to list them all in the manuscript. Further explanation of this has been included in the manuscript.
- The computational domain used in the work is not presented. It would be better to state nature computational domain used and show the figure with mesh. At the same time, no need to present equations 1 –3, refer to the Fluent manual.
The computational domain is represented by a schematic in figure 1. As the geometry is relatively trivial, and the mesh small enough that details would not be visible when viewing the whole domain, the authors do not feel it is helpful to include.
- Why do the authors fix the range of angle 0-60?
A range of 0-60 degrees was selected as after 60 degrees the characteristics of the system are more similar to an angled wall jet than that of an impinging jet, and as such outside of the scope of the work.
- Based on the previous work in your group, with lower H/W , the flow near the wall is supersonic and the trend is non-linear and the fit does not work. Is the reason why data from H/W = 15 is removed from correlation?
That is correct. H/W below 30 will lead to supersonic flow in the boundary layer and as such these cases are not included in the anlalysis. This has now been made clear in the manuscript.
- It is essential to state the range over which the relationship developed is valid. Also, in generally state the parameter space where deviation is more like lower H/W and 60 degree angle etc.
We have now made clear in the manuscript that the relationships will only hold for subsonic boundary layer flow, corresponding to H/W > 30 and NPR between 1.0 and 3.4. There should be know limit with regard to maximum stand off height.
Minor comments:
Color mapping in the Figures 5 and 6 is difficult to read, making the colour variation better to distinguish.
The color scheme was chosen to be friendly to those with color blindness and to work in gray scale.
Reviewer 2 Report
Title: Model for Wall Shear Stress from Obliquely Impinging Planar Underexpanded Jets
Manuscript ID: applsci-1820442
The authors presented a parametric study of obliquely impinging, underexpanded planar jets. Impingement angles from 30° to 90° are examined for jets standoff 60 height to nozzle width ratios of 15-30 and nozzle pressure ratios from 1.0-3.4. The computational data is used to develop a simple relationship for maximum wall shear stress as a function of only the jet parameters: impingement angle, jet width, jet standoff height, and jet nozzle pressure. The brief report covers a very interesting paper; however, I have the following concerns related to CFD modelling, which I would like the authors to address in their revised manuscript.
1. What is the element size for the mesh?
2. What are the initial boundary conditions? Is this only a steady case study with no initial conditions?
3. What are the fluid properties? For example, Line 77: What are the values for these: density and velocity at the nozzle exit?
4. Add a Figure showing the computational domain, either 2D or 3D. Also, illustrate the boundary conditions on that figure.
5. Any reason for using ANSYS Fluent 17?
6. Lines 136-137 “The CFD simulations show good agreement in the shape and magnitude of the pressure profile.” Again, I need a value compared to the literature to accept this statement.
7. Line 147: Do you mean Figure 3?
8. Line 157: This line and the entire paper required intensive proofreading. Same for Line 174
9. Many of this paper's results got the same outcomes for the authors' article title: Wall Jet Similarity of Impinging Planar Underexpanded Jets-Ref [24] https://0-doi-org.brum.beds.ac.uk/10.1016/j.ijheatfluidflow.2019.108516 therefore, the authors need to state what is novel in this brief report and cite these figures 2 and 3.
Overall, it was a good report and I would like the authors to address the above points.
Author Response
We thank the reviewer for their thoughtful response, we have addressed the concerns point by point below:
- What is the element size for the mesh?
The authors used data from our previous published work on normally impinging to evaluate the maximum wall shear stress at 3.4 NPR at each h/w ratio for the normal impingement case (which has the maximum wall shear stress of all impingement angles) to then calculate at what distance from the wall y+=1 would be for that maximum wall shear stress. The structured prism layer mesh was then generated for each h/w ratio with the first node at the distance for y+=1 at the maximum shear stress. As such first grid length varies in each geometry and it is not practical to list them all in the manuscript. The range of at the wall ranges from 150-1500 nm across all cases. Further explanation of this has been included in the manuscript.
- What are the initial boundary conditions? Is this only a steady case study with no initial conditions?
This is a steady state simulation with ambient pressure boundary conditions at the outlets.
- What are the fluid properties? For example, Line 77: What are the values for these: density and velocity at the nozzle exit?
The relations in this work will hold for any ideal gas, but the simulations were conducted with an isentropic choked nozzle of air exiting into ambient air at 1 atmosphere and 300K. The density and velocity at the exit are defined by isentropic nozzle relations and not direct inputs to the simulation. The boundary condition is defined by the pressure ratio and the total temperature.
- Add a Figure showing the computational domain, either 2D or 3D. Also, illustrate the boundary conditions on that figure.
The computational domain is relatively trivial and is well represented in figure 1, boundary condition info has been added to this figure.
- Any reason for using ANSYS Fluent 17?
Analysis was conducted a couple years ago and the high powered computer we used for simulations was only compatible with Fluent 17
- Lines 136-137 “The CFD simulations show good agreement in the shape and magnitude of the pressure profile.” Again, I need a value compared to the literature to accept this statement.
There is no literature to compare to, this is an experiment we conducted ourselves, with further explanation provided in [24] https://0-doi-org.brum.beds.ac.uk/10.1016/j.ijheatfluidflow.2019.108516
- Line 147: Do you mean Figure 3?
Yes thank you for catching the mistake.
- Line 157: This line and the entire paper required intensive proofreading. Same for Line 174
We have conducted a thorough proofreading and adjusted these lines to be more clear.
- Many of this paper's results got the same outcomes for the authors' article title: Wall Jet Similarity of Impinging Planar Underexpanded Jets-Ref [24] https://0-doi-org.brum.beds.ac.uk/10.1016/j.ijheatfluidflow.2019.108516 therefore, the authors need to state what is novel in this brief report and cite these figures 2 and 3.
The results in this paper are novel as they present relationships for an obliquely impinging jet, while the previous paper focused only on normal impingement. The overlap is only in validation, where the same validation was used in each manuscript. We will cite figures 2 and 3.
Round 2
Reviewer 1 Report
All the queries were answered well. The manuscript is good for publication.
Reviewer 2 Report
The authors have addressed most of my concerns.
