Distributed observations of wind direction using microstructures attached to actively heated fiber-optic cables

Abstract

Abstract. The weak-wind boundary layer is characterized by turbulent and submeso-scale motions that break the assumptions necessary for using traditional eddy covariance observations such as horizontal homogeneity and stationarity, motivating the need for an observational system that allows spatially resolving measurements of atmospheric flows near the surface. Fiber-Optic Distributed Sensing (FODS) potentially opens the door to observing a wide-range of atmospheric processes on a spatially distributed basis and to date has been used to resolve the turbulent fields of air temperature and wind speed on scales of second and decimeters. Here we report on progress developing a FODS technique for observing spatially distributed wind direction. We affixed microstructures shaped as cones to actively-heated fiber-optic cables with opposing orientations to impose directionally-sensitive convective heat fluxes from the fiber-optic cable to the air, leading to a difference in sensed temperature that depends on the wind direction. We demonstrate the behavior of a range of microstructure parameters including aspect ratio, spacing, and size and develop a simple deterministic model to explain the temperature differences as a function of wind speed. The mechanism behind the directionally-sensitive heat loss is explored using Computational Fluid Dynamics simulations and infrared images of the cone-fiber system. While the results presented here are only relevant for observing wind direction along one dimension it is an important step towards the ultimate goal of a full three-dimensional, distributed flow sensor.

The Large eddy Observatory, Voitsumra Experiment 2019 (LOVE19) with high-resolution, spatially distributed observations of air temperature, wind speed, and wind direction from fiber-optic distributed sensing, towers, and ground-based remote sensing

“…Based on the result from this preliminary work, PE cones were attached to the stainless steel fiber using injection molding. Cones were 12 mm in diameter and height with a 2 cm separation, consistent with the optimal cone construction determined by Lapo et al (2020b). The initial results from the FODS wind direction method are published elsewhere (Freundorfer et al, 2021).…”

“…The fiber weighed 5 kg km −1 . The FODS cross consisted of an identical fiber type with small, directional microstructures attached by injection molding (Lapo et al, 2020b). The coned fiber was spliced to the unconed fiber to form a single optical core.…”

“…The FODS cross was intended specifically as a test bed and the first environmental deployment of the FODS wind direction method for the LEO technique. Preliminary lab work was able to successfully observe the distributed wind direction in the one-dimensional flow of a wind tunnel (Lapo et al, 2020b). Based on the result from this preliminary work, PE cones were attached to the stainless steel fiber using injection molding.…”

“…DTS can resolve air temperature at scales fine enough to resolve the difference between turbulent scales and sub-mesoscales (Thomas et al, 2012;Zeeman et al, 2015;Peltola et al, 2021;Thomas and Selker, 2021;Fritz et al, 2021;Zeller et al, 2021). It has also been developed to observe additional atmospheric properties on a distributed basis such as solar radiation (Petrides et al, 2011), dew point (Euser et al, 2014;Schilperoort et al, 2018), wind speed (Sayde et al, 2015;van Ramshorst et al, 2020;Pfister et al, 2019;Zeller et al, 2021), and recently wind direction (Lapo et al, 2020b). We refer to this broader family of sensing techniques as fiber-optic distributed sensing (FODS; Thomas and Selker, 2021).…”

A Note on Scalar-Gradient Sharpening in the Stable Atmospheric Boundary Layer

“…I stress the need for future field observations of turbulence and temperature structures. With promising advancements in observational technologies, such as distributed temperature sensing (Thomas et al 2012;Lapo et al 2019) and thermal imaging Cermak 2015, 2018), there is hope for revelations on this topic.…”