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 electrically heated fiber-optic (FO) cables under varying flow conditions and turbulence intensities for the purposes of sensing flow direction. 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-coned and hollow-coned microstructures of varying size compared to existing studies. The research identifies optimal combinations which maximize temperature differences (ΔT) across FO cables with cones pointing in opposite directions while balancing key design criteria such as sensitivity to wind speed and minimizing the FO-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 ΔT 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.