Experimental Investigation of a Roughness Element Wake on a Hypersonic Flat Plate
Round 1
Reviewer 1 Report
Review of “Experimental scrutiny on hypersonic flat plate with attached isolated roughness element acted upon by a wake flow field” by Junhao Han, Lin He, Xiwang Xu, and Zhengbang Wu, submitted to Aerospace
General comments:
This paper describes an experimental investigation of roughness-induced instability on a flat plate with NPLS. The experiment is novel, significant, and well described. The paper is overall readable, but there are various small errors in English usage that ought to be cleaned up, e.g. articles, punctuation, verb tenses, word selection, and just general clarity of expression. I recommend the paper be accepted after these minor revisions are implemented.
Specific comments:
1. p 1 l 1: The title is a bit of a mouthful. Perhaps “Experimental Investigation of Roughness Element Wake on a Hypersonic Flat Plate”.
2. p 1 l 20: This sentence is an example of imperfect article usage. “Due to laminar-turbulent transition in a hypersonic boundary layer, surface friction…”
3. p 1 l 32: Reference 7 seems to be low-speed work. Can you cite a more applicable source?
4. p 3 l 144: Are you saying the flow is dried, or merely “should be” (but isn’t)? Instead of “dusted”, do you mean “filtered”?
5. fig 1: The part labeled “Value” is probably a Valve.
6. p 4 l 152: At this low of a freestream static temperature, are you sure you don’t have flow condensation?
7. p 4 l 156: Specify “freestream pressure fluctuation level…”.
8. figs 6 and 7: These aren’t helpful to the reader and can be omitted.
9. fig 8: This figure also isn’t helpful and can be omitted. The sentence in lines 209--211 is perfectly adequate (“After calibration, the maximum error is…”).
10. p 7 l 238: Wording such as “image gray is low” doesn’t make sense. Image brightness is low? Please clarify in a few places.
11. p 7 l 241: I think you mean “at the bottom of the image”, not “below the image”.
12. p 9 l 288 ff: Past work has used surface measurements to study the spanwise wake structure in a configuration similar to yours. See, for example, Wheaton & Schneider AIAA J 2012, Wheaton & Schneider JSR 2014, and Lash et al. AIAA 2017-0760. How do your findings compare with theirs?
13. table 2: “vortex”, not “vertex”. The caption and text has it right.
14. p 13 l 439: You report speeds to the nearest 0.01 m/s. Do you truly have that much precision? An uncertainty analysis needs to be included.
15. p 13 l 442: You state “the velocity…is lower than the mainstream velocity”. By how much? What is the mainstream (boundary-layer edge?) velocity, anyway? It’d be helpful to report the vortex convective velocities normalized by the edge velocity. You attribute the slower-thanedge speed to the fact that you’re looking lower than the “real boundary layer thickness”, but it could be that these structures convect at less than the mass velocity. Does any of the literature you reviewed shed light on this?
16. p 13 l 469: What does “should be” mean? You want it to be? Is expected to be? (Based on what?)
17. fig 17: Font size too small.
18. p 15 l 494 ff: More statistics than just mean, minimum, and maximum of your 50 samples would be welcome. Perhaps box-and-whiskers plots for Figure 17?
19. p 15 l 536 ff: A plot of the transition location as a function of k/delta would be most welcome.
20. p 15 l 536 ff: I suggest you use the common terms “critical” and “effective” roughness height in this discussion.
21. references: Some citations are incomplete, e.g. 1 and 14.
22. references: You seem to report AIAA paper numbers as page numbers, e.g. references 3 and 15.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Aerospace_1917942_Review
Experimental scrutiny on hypersonic flat plate with isolated roughness element acted upon by a wake flow field
Junhao Han, Lin He, Xiwang Xu, Zhengbang Wu
The authors experimentally visualize and track the evolution of coherent structures in the wake region of a cylindrical isolated roughness element (RE) on a flat plate placed in hypersonic flow with Ma = 6. The hair-pin vortices evolving from the larger horse-shoe vortex that emanates from a bluff body the cylinder is characteristic to this flow. The authors employ the NPLS technique to visualize the wake structures of an RE placed on a flat plate. Eight inline-placed CCD cameras capture nanoparticle laden flow illuminated by a pulsed laser. The authors visualize the structures in the perpendicular x-z and x-y planes in the wake region of the RE. The three-dimensional structures are deduced based on the two perpendicular planar images. The qualitative three-dimensional evolution along the streamwise direction is presented. Finally, the effect of aspect ratios (k/d) of RE on the wake structures is presented. They characterize the wake structures geometric attributes of the structures i.e, location of the transition and the wavelength of the wavy vortices after transition. With increase in k/d, the transition advances closer to the RE and results in finer structures.
- 1. It is essential to refer to wind tunnel design report in section 2.1. what is nozzle design used in the wind tunnel, pressure storage system etc
- 2. what is make of Nd.YAG laser used in this work.
- 3. The authors present the results in a more qualitative way with minimal quantification of the structures. They have present results qualitatively as well.
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4.In lines 134 onwards the author mention that the time evolution characteristics is examined using time-resolved flow. Though the authors claim that their measurements are time resolved, it is not clear what time resolution of the image acquisition is. It appears that the flow evolution is explained based on instantaneous images. The authors need to emphasize in the introduction section.
Author Response
please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors have taken care of all the comments well. The manuscript can be accepted in its current form.
