Revisiting wind speed measurements using actively heated fiber optics: a wind tunnel study

Justus G. V. van Ramshorst,Hubert H. G. Savenije,Nick C. van de Giesen,Chad W. Higgins,Jonathan G. Izett,John S. Selker,Bart Schilperoort,Bas J. H. van de Wiel,Miriam Coenders-Gerrits

doi:10.5194/amt-13-5423-2020

Justus G. V. van Ramshorst, Hubert H. G. Savenije + Show 7 more

Open Access

https://doi.org/10.5194/amt-13-5423-2020

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Abstract

Abstract. Near-surface wind speed is typically only measured by point observations. The actively heated fiber-optic (AHFO) technique, however, has the potential to provide high-resolution distributed observations of wind speeds, allowing for better spatial characterization of fine-scale processes. Before AHFO can be widely used, its performance needs to be tested in a range of settings. In this work, experimental results on this novel observational wind-probing technique are presented. We utilized a controlled wind tunnel setup to assess both the accuracy and the precision of AHFO under a range of operational conditions (wind speed, angles of attack and temperature difference). The technique allows for wind speed characterization with a spatial resolution of 0.3 m on a 1 s timescale. The flow in the wind tunnel was varied in a controlled manner such that the mean wind ranged between 1 and 17 m s−1. The AHFO measurements are compared to sonic anemometer measurements and show a high coefficient of determination (0.92–0.96) for all individual angles, after correcting the AHFO measurements for the angle of attack. Both the precision and accuracy of the AHFO measurements were also greater than 95 % for all conditions. We conclude that AHFO has the potential to measure wind speed, and we present a method to help choose the heating settings of AHFO. AHFO allows for the characterization of spatially varying fields of mean wind. In the future, the technique could potentially be combined with conventional distributed temperature sensing (DTS) for sensible heat flux estimation in micrometeorological and hydrological applications.

Highlights

This work presents the results of a wind tunnel study designed to test the novel actively heated fiber-optic (AHFO) (Sayde et al, 2015) wind speed measurement technique in controlled airflow conditions
The influence of directional sensitivity and the signal-to-noise ratio on the measurement accuracy and precision is investigated, and the results are used to propose a method to estimate precision for future experiments with AHFO; our work will improve the possibilities for successful application of AHFO in future field experiments
Eq (14) is used to account for directional sensitivity in our study, with the scaling exponent, m1, able to be optimized during calibration of the AHFO measurements

Summary

Introduction

This work presents the results of a wind tunnel study designed to test the novel actively heated fiber-optic (AHFO) (Sayde et al, 2015) wind speed measurement technique in controlled airflow conditions. The spatial distribution of field observations is limited. While it is possible to obtain distributed wind speed observations with remote sensing (e.g., Goodberlet et al, 1989; Bentamy et al, 2003), the spatial resolution is too low for many micrometeorological applications. Many field experiments assume Taylor’s frozen flow hypothesis (Taylor, 1938) in order to estimate fluxes with similarity theory (e.g., Higgins et al, 2009; Kelly et al, 2009; Bou-Zeid et al, 2010; Patton et al, 2011). Similarity theory only holds for idealized homogeneous or stationary conditions, which are rarely met in practice, resulting in a model containing strong assumptions, which often leads to significant errors (Ha et al, 2007; Higgins et al, 2012; Thomas et al, 2012). Even if perfect surface homogeneity was possible, other atmospheric (surface) conditions are often nonstationary (Holtslag et al, 2013)

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