the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jul 2025
Improving turbulent airflow direction measurements for fiber-optic distributed sensing using numerical simulations
Abstract. This study investigates the impact of microstructure geometry on the thermal and turbulence responses of fiber-optic (FO) cables under varying flow conditions and turbulence intensities. The underlying measurement principle is the directionally sensitive heat loss from electrically heated FO cables with imprinted microstructures exposed to turbulent airflows resembling a long hot-wire anemometer. Using the COMSOL Multiphysics 6.0 finite-element software, this study explores a wider range of different configurations of filled and hollow filled-coned microstructures of varying size compared to existing studies. The research identifies optimal combinations which maximize temperature differences (∆T) across FO cables with cones pointing in opposite directions while balancing key design criteria such as sensitivity to wind speed and minimizing the cables’ PVC coverage. We demonstrate that FO cables with hollow-coned microstructures (radius = 24 mm, height = 24 mm, and spacing = 15 mm) outperform their filled-coned counterparts, maintaining ∆T values above 2 K across a broader range of wind speeds and turbulence intensities. Notably, the hollow-cone configuration sustains a temperature difference of up to 0.8 K at a 60(°) wind attack angle. The findings implicate substantial improvements for an optimized FO cable design in atmospheric boundary layer studies, enabling more accurate measurements of wind direction, distributed turbulent heat fluxes, and vertical wind speed perturbations using fiber-optic distributed sensing (FODS). Future work shall validate the findings under field conditions to assess the robustness and real-world applicability of the optimized design.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2328', Livia Rosalem, 25 Jul 2025
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AC2: 'Reply on RC1', Mohammad Abdoli, 16 Sep 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2328/egusphere-2025-2328-AC2-supplement.pdf
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AC2: 'Reply on RC1', Mohammad Abdoli, 16 Sep 2025
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RC2: 'Comment on egusphere-2025-2328', Anonymous Referee #2, 30 Jul 2025
This study uses numerical simulations (from COMSOL Multiphysics 6.0) to identify the optimal microstructure geometry for heated fiber-optic cables. These microstructures are used in a novel atmospheric sensing technique, first introduced in a previous study, that uses fiber-optic distributed sensing to measure variables such as wind direction and turbulent heat fluxes. The simulations systematically test different geometries and flow conditions to find a design that maximizes the temperature difference between the cables, a key factor for measurement sensitivity.
While using a numerical tool to explore the parameter space is an effective approach for advancing this promising technology, the following points require clarification before the manuscript can be recommended for publication.
Overall comments:
- The introduction provides only a brief overview of the “microstructure approach”. To better frame the study's contribution, the authors should expand on the underlying physics. Specifically, the text should clarify why maximizing the temperature difference between the cables is the primary optimization goal and explicitly describe the roles that turbulence kinetic energy and mean velocity differences play in the technique. A clearer understanding of the physical principles is essential for appreciating the study's objectives.
- The study must better demonstrate the validity and quality of its numerical simulations. First, the authors should add appropriate references that validate the use of this specific COMSOL setup and its turbulence models for similar heat transfer simulations. Second, the comparison with the experimental results from Lapo et al. (2000) needs to be presented in details. Just mentioning that "the magnitude of the modeled ∆T in this study differs from the ∆T observed by Lapo et al. (2020) in the wind tunnel experiment" is insufficient. Furthermore, the potential reasons for the discrepancy mentioned are too generic and broad, it does not clarify if the model can be trusted. The authors should present a clear argument for why the simulation results remain valid and useful for the design optimization process, even with this difference.
Specific comments:
- l. 58: "field experiment in the field" improve
- l. 80: "Our primary objective is to improve the temperature difference between the forward (wind directed towards the cone apex) and backward (wind opposite to the cone apex) filled-coned fibers" why exactly?
- l. 89: 16r (use mathematical notation)
- l. 88-90: "determined" how? Or just chosen?
- l. 102 : COMSOl
- l. 106 (Lapo et al., 2022)
- l. 112: how do you know it is resolving the boundary layers? Can you show it? Is it really necessary?
- l. 112: designed
- l. 120: "in both forward and backward wind flows", what do you mean?
- l. 120: "resulting differences in temperature (∆T ) and turbulent kinetic energy (∆k) on the fiber were extracted." it is not clear which difference is it here, difference between the fibers? Calculated where? I think it is partially explained in Fig. 1b and later in the text, but it should be better explained here.
- In the text, separate discussion of fig 1 between fig 1a and fig 1b, and I think that fig 1b was not mentioned.
- Fig1a: define who is the forward/backward fiber
- Fig 1: "Here, T0 represents the temperature at the interface between the microstructure and the fiber optic cable." T0 is very small in the figure, it took me a while to find it...
- Fig 1: Why is the temperature in the upper fiber much lower than the temperature in the lower fiber and in the cones?
- l. 121: why is the value of ∆k important to monitor?
- l. 130: instationary
- l. 139: "he hollow-coned design has less PVC attaching length to the fiber, resulting in less heat transfer within the microstructure itself, which has less of an impact on the FO temperature compared to the filled cone cables." how did you come to this conclusion?
- increase the size of figs 3 and 4
- fig 3 and 4: TKE is much much higher in hollow case, why?
- l. 144: "The relationship between the temperature difference (∆T ) and the inlet wind speed (U in ) aligns with the findings of Lapo et al. (2020), where ∆T is higher at low wind speeds and decreases non-linearly with increasing inlet wind speed." Is this conclusion from Fig 5? If so, say it explicitly.
- How was ∆U quantified? Why is it important?
- Fig 5: I don't understand how cases producing the highest ∆T ware chosen, highest for which velocity? From Fig 5b they are the highest, but in Fig 5a they are not necessarily, right?
- l. 144: "The relationship between the temperature difference (∆T) and the inlet wind speed (Uin) aligns with the findings of Lapo et al. (2020), where ∆T is higher at low wind speeds and decreases non-linearly with increasing inlet wind speed" Is this statement based on Fig 5? If so, say it explicitly.
- l. 152: "the selected combinations illustrated also based on ∆k and ∆U shown in appendix Fig. A2 and A3" improve sentence.
- l. 156: compared to Lapo et al. (2020)
- l. 167: "implement in observational experiments, additional criteria"
- Secs 3.2 and 3.3, figs 6 and 7: are these results based on 3D simulations? If so, state it explicitly.
- Fig A2: modeled in Lapo et al. (2020)
Citation: https://doi.org/10.5194/egusphere-2025-2328-RC2 -
AC1: 'Reply on RC2', Mohammad Abdoli, 16 Sep 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2328/egusphere-2025-2328-AC1-supplement.pdf
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AC1: 'Reply on RC2', Mohammad Abdoli, 16 Sep 2025
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General Comments
This manuscript presents a comprehensive numerical investigation of microstructure geometries for fiber-optic (FO) cables used in turbulent airflow measurements. The research addresses an important gap in fiber-optic distributed sensing (FODS) regarding the effects of microstructures' geometry used in FO cable on dynamic parameters important for atmospheric boundary layer studies. The study builds upon previous work by Lapo et al. (2020), and extends the analysis to include hollow-cone microstructures while examining a broader range of geometric parameters and airflow direction effects.
The introduction of hollow-cone microstructures represents a meaningful innovation that demonstrates superior performance compared to filled-cone designs. The authors systematically investigated 64 geometric combinations, providing a thorough exploration of design parameters. In addition, the authors incorporated appropriate real-world factors, such as turbulence intensity variations and wind attack angles, and considered the detection limits of commercial FODS systems.
The study is well-written, providing practical guidance for future FODS implementation while clearly stating the main limitations and highlighting the need for future field/lab experiments. These findings establish a foundation for more accurate wind direction measurements, distributed turbulent heat flux assessments, and the detection of vertical wind speed perturbations using FODS, representing substantial progress toward enhanced atmospheric monitoring capabilities.
Specific comments
The computational approach is well-described with appropriate boundary conditions and mesh considerations. However, the authors should consider adding some references that applied the k-e model and commenting on the model's limitations and constraints. Additionally, it would be beneficial (if possible) to include information about uncertainty quantification or confidence intervals for the computed results.
Technical comments
I recommend publication, and my comments can be seen as mostly minor or technical suggestions/recommendations.