the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Sep 2023
Performance characterization of a laminar gas-inlet
Da Yang
Margarita Reza
Roy Mauldin
Rainer Volkamer
Abstract. Aircraft-based measurements enable large-scale characterization of gas-phase atmospheric composition, but these measurements are complicated by the challenges of sampling from high-speed flow. Under such sampling conditions, the sample flow will likely experience turbulence, accelerating | mixing of potential contamination of the gas-phase from the condensed-phase components on walls and reduced vapor transmission due to losses to the inner walls of the sampling line. While a significant amount of research has gone into understanding aerosol sampling efficiency for aircraft inlets, a similar research investment has not been made for gas sampling. Here, we analyze the performance of a forward-facing laminar flow gas inlet to establish its performance as a function of operating conditions, including ambient pressure, freestream velocities, and sampling conditions. Using computational fluid dynamics (CFD) modeling we simulate flow inside and outside the inlet to determine the extent of freestream turbulent interaction with the sample flow and its implication for gas sample transport. The CFD results of flow features in the inlet are compared against measurements of air speed and turbulent intensity from full-sized high-speed wind-tunnel experiments. These comparisons suggest that the Reynolds Averaged Navier-Stokes (RANS) CFD simulations using the Shear Stress Transport (SST) modeling approach provide the most reasonable prediction of the turbulence characteristics of the inlet.
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Da Yang et al.
Status: final response (author comments only)
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RC1: 'Comment on amt-2023-196', Anonymous Referee #1, 10 Oct 2023
The authors present results about the characterization of an inlet for sampling air into an aircraft. As the authors state, the focus of characterizing such inlets has been on the sampling of particles and less on gas-phase species. In this paper, the authors focus on the characterization of turbulence of a forward-facing laminar flow gas inlet that is an improved version of a previously used inlet. CFD modelling is compared with some measurements in a wind tunnel showing that some parameters are better described by CFD modelling using the shear tress transport model. There is little to no experimental characterization of the gas-phase inlet concerning the transmission of gas-phase species, which would be of high interest for the atmospheric community and what I had expected to see after reading the abstract. Therefore, this paper is mainly describing the engineering aspects of CFD modelling and the model results accompanied with wind tunnel experiments. It is not very clear if there are results that can be generalized or if results only apply for the specific inlet described in this work. As non-expert on CFD modelling and the descriptive character of the paper, I cannot judge, if the modelling on its own is worth being published in AMT. Overall, the paper reads to me like an engineering report that is certainly needed during the development of such an inlet rather than a research paper in atmospheric sciences.
Specific comments:
Introduction: It is not clear what the authors mean with “gas-phase transport efficiency” mentioned in line 44. There should be a clear definition.
Discussion: I would have expected to read much more about the consequences of the results concerning air sampling / loss of species and experimental characterization. In my opinion this would be needed for a paper in AMT. There is no discussion with results reported in literature as gas-phase sampling using such inlets have been applied in numerous previous aircraft campaigns.
Line 33: “the” is missing between “inside” and “aircraft”
Line 265: “understanding” instead of “understand”
Citation: https://doi.org/10.5194/amt-2023-196-RC1 -
CC1: 'Reply on RC1', Suresh Dhaniyala, 14 Oct 2023
The authors thank for the reviewer for their comments. Please find our responses below.
Comment: The authors present results about the characterization of an inlet for sampling air into an aircraft. As the authors state, the focus of characterizing such inlets has been on the sampling of particles and less on gas-phase species. In this paper, the authors focus on the characterization of turbulence of a forward-facing laminar flow gas inlet that is an improved version of a previously used inlet. CFD modelling is compared with some measurements in a wind tunnel showing that some parameters are better described by CFD modelling using the shear tress transport model. There is little to no experimental characterization of the gas-phase inlet concerning the transmission of gas-phase species, which would be of high interest for the atmospheric community and what I had expected to see after reading the abstract.Therefore, this paper is mainly describing the engineering aspects of CFD modelling and the model results accompanied with wind tunnel experiments. It is not very clear if there are results that can be generalized or if results only apply for the specific inlet described in this work. As non-expert on CFD modelling and the descriptive character of the paper, I cannot judge, if the modelling on its own is worth being published in AMT. Overall, the paper reads to me like an engineering report that is certainly needed during the development of such an inlet rather than a research paper in atmospheric sciences.
Response:
We appreciate the reviewer’s comments on the paper. The reviewer correctly points out that the paper is focused on engineering evaluation of a gas inlet using CFD and wind-tunnel measurements. We, however, disagree with the reviewer’s comment about the extent of applicability of the paper to the AMT audience. A reading of the mission statement of AMT listed below (relevant words highlighted in bold) clearly suggests that the objectives of this paper fits perfectly with the mission of AMT.
The mission statement from the AMT journal website:
The main subject areas comprise the development, intercomparison, and validation of measurement instruments and techniques of data processing and information retrieval for gases, aerosols, and clouds. Papers submitted to AMT must contain atmospheric measurements, laboratory measurements relevant for atmospheric science, and/or theoretical calculations of measurements simulations with detailed error analysis including instrument simulations. The manuscript types considered for peer-reviewed publication are research articles, review articles, and commentaries.
Aircraft inlet studies are commonly published in AMT (e.g. Sanchez-Marroquin, et al., Atmos. Meas. Tech., 12, 5741–5763, 2019) and AMT publications of such studies are dependent on CFD simulations (e.g. Moharreri, A., et al. Atmospheric Measurement Techniques, 2014). Thus, related work on evaluation of aircraft inlets in wind-tunnels would be very relevant to the AMT audience. More importantly, the role of turbulence on transport of particles and gases in aircraft inlets is known to be important but often ignored by the community (including in the papers above) because of the challenges of getting turbulence right. This paper provides important guidance on that front – demonstrating the significant differences between model approaches, and the accuracy of certain CFD models over others (unfortunately the commonly used models are often less accurate, as seen here, but more widely used because they more easily converge). While this paper could be sent to a fluids-related journal, our choice in publishing this in AMT is driven by the need to have validated modeling approaches relevant to the atmospheric community be visible in a journal relevant to the community.
Specific comments:
Introduction: It is not clear what the authors mean with “gas-phase transport efficiency” mentioned in line 44. There should be a clear definition.
Response:
The gas-phase transport efficiency is defined as the mass fraction of water at any cross-section compared to the ambient mass fraction of water. This is clarified in the paper now.
Discussion: I would have expected to read much more about the consequences of the results concerning air sampling / loss of species and experimental characterization. In my opinion this would be needed for a paper in AMT. There is no discussion with results reported in literature as gas-phase sampling using such inlets have been applied in numerous previous aircraft campaigns.
Response:
This paper uses the validated CFD model to demonstrate two important findings: 1) the model challenges the very widely made assumption in the atmospheric community that laminar flow in inlet lines, and core sampling from such lines is preferrable over turbulent sampling; and 2) the use of an incorrect model (widely used k-e model) results in prediction of 10-20% higher losses than predicted by the validated k-w SST model. These findings are described in pages 13 and Figure 6b of the original paper. These findings support that the benefit of minimizing the residence time by accepting turbulence far outweigh attempts to minimize wall losses by laminar core sampling. We will expand on this aspect in the revised manuscript.
We are in the process of finishing experiments to fully characterize sampling losses of species in the inlet under high-speed wind-tunnel conditions. As might be expected, wind-tunnel measurements of gas transport efficiencies are quite challenging and that work will merit its own paper. The current paper stands on its own merit, as the validated calculation of turbulence characteristics in the inlet is relevant for any gas and aerosol sampling. About results of gas-sampling inlet efficiency in the literature, we are unaware of such published studies that we could take advantage of for our validation. We would greatly appreciate any pointers in this regard.
Line 33: “the” is missing between “inside” and “aircraft”
Response: The typo is now fixed.
Line 265: “understanding” instead of “understand”
Response: The typo is now fixed.
We appreciate the opportunity to respond and look forward to any additional comments there might be.
Citation: https://doi.org/10.5194/amt-2023-196-CC1
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CC1: 'Reply on RC1', Suresh Dhaniyala, 14 Oct 2023
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RC2: 'Comment on amt-2023-196', Anonymous Referee #2, 08 Nov 2023
Yang and coauthors describe a novel inlet for measuring gas-phase species on aircraft platforms. Using a computational fluid dynamics (CFD) model, they simulate the behavior of sampled air inside this inlet, and complement their simulations with observations taken of a prototype inlet inside a high-speed wind tunnel. They report flow speeds and turbulence intensities for several combinations of inlet and sampling parameters. Finally, they estimate the throughput efficiency of the inlet.
This is a well written report that describes an inlet that often isn’t characterized as well as aerosol-phase inlets. I believe it is a valuable contribution to the field, and I would recommend publication, following some minor modifications.
General: The authors have spent considerable time engineering an inlet that maintains as close to laminar flow as possible, by reducing turbulence. However, their simulations in Section 4 indicate that laminar flow might not necessarily be the factor that reduces losses in the inlet. The authors should spend more time exploring this finding, as I’m concerned it partially undercuts their results. If laminar flow isn’t the key factor in reducing losses, then is this really the ideal inlet configuration for gas-phase sampling?
Some additional calculations may be helpful which estimate losses when a given chemical species has a theoretical loss probability that is less than 100% upon collision with the wall. Reading that section, I think the authors are assuming that as soon as a molecule collides with the wall then it is lost. In that case, another water molecule will come take its place, and will also be lost when it collides with the wall surface. However, if the loss probability is less than 100%, then wouldn’t a laminar flow will have a very thin layer of molecules that are repeatedly interacting with the walls, but not necessarily being lost? I would expect that this simulation would reduce the estimated losses, and would be more physically reasonable for most chemical species.
Specific comments:
Line 12: Replace “|” with “, and”
Line 68: “Using elliptical cross-sections for the leading edge…” is unclear. Which dimension is elliptical in Fig 1?
Line 70: Can you describe in more detail what “flow straightening” means in this context. How do you quantify this?
Section 2.1: You describe in detail how a chemical calibration has been done in the past, leading me to wonder why it wasn’t done in this paper. Why is it “beyond the scope of this paper”?
Line 217: What is the basis for the 0.5 – 3% range? Is that based on physical parameters or is that the range needed to span the observed turbulence intensities?
Citation: https://doi.org/10.5194/amt-2023-196-RC2
Da Yang et al.
Da Yang et al.
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