Investigating Taylor’s frozen turbulence hypothesis in the surface layer at an ideal desert field site using fibre-optic distributed temperature sensing

2021 ◽  
Author(s):  
Rainer Hilland ◽  
Andreas Christen ◽  
Roland Vogt
Keyword(s):  
Boundary Layer ◽  
Sonic Anemometer ◽  
Field Work ◽  
Field Investigation ◽  
Temperature Sensing ◽  
Temperature Fields ◽  
Fibre Optic ◽  
Distributed Temperature Sensing ◽  
Plan View ◽  
Wide Range

<p>Taylor’s frozen turbulence hypothesis is the most critical assumption through which time-resolving sensors may be used to derive statistics of the turbulent spatial field. Namely, it relates temporal autocorrelation to spatial correlation via the mean wind speed and is invoked in almost all boundary layer field work. Nevertheless, the conditions and scales over which Taylor’s hypothesis is valid remain poorly understood in the atmospheric boundary layer.</p> <p>As part of the Namib Turbulence Experiment (NamTEX) campaign in March 2020, a pseudo-3D fibre-optic distributed temperature sensing (DTS) array was installed within a 300 x 300 m area in the Namib desert. The array is X-shaped in plan view and contains 16 measurement heights from 0.45 m to 2.85 m. Fibre-optic sensing provides air temperature measurements at unprecedented spatio-temporal density (0.25 m horizontally, 0.17 m vertically, and 1 Hz) and was coupled with a vertical array of traditional sonic anemometer point measurements to investigate the relationship between spatial and temporal temperature fields. The Namib provides an ideal location for fundamental boundary layer research: homogenous flat surfaces, no vegetation, little moisture, strong solar forcing, regular and repeated clear-sky conditions, and a wide range of atmospheric stabilities.</p> <p>Using the NamTEX DTS array we present the first field investigation of Taylor’s hypothesis that considers boundary layer stability and is independent of wind direction. A novel method of 2d horizontal cross-correlation between all possible points of a single height of the DTS is employed to produce spatial ‘maps’ of the turbulent flow, whose velocity, direction, and size may be tracked through time.</p>

scholarly journals Investigating patterns and controls of groundwater up-welling in a lowland river by combining Fibre-optic Distributed Temperature Sensing with observations of vertical hydraulic gradients

2012 ◽  
Vol 16 (6) ◽  
pp. 1775-1792 ◽  
Author(s):  
S. Krause ◽  
T. Blume ◽  
N. J. Cassidy
Keyword(s):  
Surface Water ◽  
Water Exchange ◽  
Structural Heterogeneity ◽  
Temperature Sensing ◽  
Small Scale ◽  
Lowland River ◽  
Fibre Optic ◽  
Distributed Temperature Sensing ◽  
Hydraulic Gradients ◽  
Cold Spots

Abstract. This paper investigates the patterns and controls of aquifer–river exchange in a fast-flowing lowland river by the conjunctive use of streambed temperature anomalies identified with Fibre-optic Distributed Temperature Sensing (FO-DTS) and observations of vertical hydraulic gradients (VHG). FO-DTS temperature traces along this lowland river reach reveal discrete patterns with "cold spots" indicating groundwater up-welling. In contrast to previous studies using FO-DTS for investigation of groundwater–surface water exchange, the fibre-optic cable in this study was buried in the streambed sediments, ensuring clear signals despite fast flow and high discharges. During the observed summer baseflow period, streambed temperatures in groundwater up-welling locations were found to be up to 1.5 °C lower than ambient streambed temperatures. Due to the high river flows, the cold spots were sharp and distinctly localized without measurable impact on down-stream surface water temperature. VHG patterns along the stream reach were highly variable in space, revealing strong differences even at small scales. VHG patterns alone are indicators of both, structural heterogeneity of the stream bed as well as of the spatial heterogeneity of the groundwater–surface water exchange fluxes and are thus not conclusive in their interpretation. However, in combination with the high spatial resolution FO-DTS data we were able to separate these two influences and clearly identify locations of enhanced exchange, while also obtaining information on the complex small-scale streambed transmissivity patterns responsible for the very discrete exchange patterns. The validation of the combined VHG and FO-DTS approach provides an effective strategy for analysing drivers and controls of groundwater–surface water exchange, with implications for the quantification of biogeochemical cycling and contaminant transport at aquifer–river interfaces.

scholarly journals Determining the Relation between Groundwater Flow Velocities and Measured Temperature Differences Using Active Heating-Distributed Temperature Sensing

Water ◽  
2019 ◽  
Vol 11 (8) ◽  
pp. 1619 ◽  
Author(s):  
Bakx ◽  
Doornenbal ◽  
Weesep ◽  
Bense ◽  
Essink ◽  
...  
Keyword(s):  
Groundwater Flow ◽  
Flow Velocity ◽  
Temperature Response ◽  
Flow Equation ◽  
Absolute Temperature ◽  
Temperature Sensing ◽  
Measured Temperature ◽  
Distributed Temperature Sensing ◽  
Plan View ◽  
Flow Velocities

Active Heating-Distributed Temperature Sensing (AH-DTS) has the potential to allow for the measurement of groundwater flow velocities in situ. We placed DTS fiber-optic cables combined with a heating wire in direct contact with aquifer sediments in a laboratory scale groundwater flow simulator. Using this setup, we empirically determined the relationship between ΔT, the temperature difference by constant and uniform heating of the DTS cable and the background temperature of the groundwater system, and horizontal groundwater flow velocity. Second, we simulated the observed temperature response of the system using a plan-view heat transfer flow model to calibrate for the thermal properties of the sediment and to optimize cable setup for sensitivity to variation in groundwater flow velocities. Additionally, we derived an analytical solution based on the heat flow equation that can be used to explicitly calculate flow velocity from measured ΔT for this specific AH-DTS cable setup. We expect that this equation, after calibration for cable constitution, is valid for estimating groundwater flow velocity based on absolute temperature differences measured in field applications using this cable setup.

scholarly journals Suitability of fibre-optic distributed temperature sensing for revealing mixing processes and higher-order moments at the forest–air interface

2021 ◽  
Vol 14 (3) ◽  
pp. 2409-2427
Author(s):  
Olli Peltola ◽  
Karl Lapo ◽  
Ilkka Martinkauppi ◽  
Ewan O'Connor ◽  
Christoph K. Thomas ◽  
...  
Keyword(s):  
Air Temperature ◽  
Temperature Sensing ◽  
Fibre Optic ◽  
Distributed Temperature Sensing ◽  
Turbulence Statistics ◽  
Mixing Processes ◽  
Noise Component ◽  
Roughness Elements ◽  
Sonic Anemometers ◽  
Canopy Flows

Abstract. The suitability of a fibre-optic distributed temperature sensing (DTS) technique for observing atmospheric mixing profiles within and above a forest was quantified, and these profiles were analysed. The spatially continuous observations were made at a 125 m tall mast in a boreal pine forest. Airflows near forest canopies diverge from typical boundary layer flows due to the influence of roughness elements (i.e. trees) on the flow. Ideally, these complex flows should be studied with spatially continuous measurements, yet such measurements are not feasible with conventional micrometeorological measurements with, for example, sonic anemometers. Hence, the suitability of DTS measurements for studying canopy flows was assessed. The DTS measurements were able to discern continuous profiles of turbulent fluctuations and mean values of air temperature along the mast, providing information about mixing processes (e.g. canopy eddies and evolution of inversion layers at night) and up to third-order turbulence statistics across the forest–atmosphere interface. Turbulence measurements with 3D sonic anemometers and Doppler lidar at the site were also utilised in this analysis. The continuous profiles for turbulence statistics were in line with prior studies made at wind tunnels and large eddy simulations for canopy flows. The DTS measurements contained a significant noise component which was, however, quantified, and its effect on turbulence statistics was accounted for. Underestimation of air temperature fluctuations at high frequencies caused 20 %–30 % underestimation of temperature variance at typical flow conditions. Despite these limitations, the DTS measurements should prove useful also in other studies concentrating on flows near roughness elements and/or non-stationary periods, since the measurements revealed spatio-temporal patterns of the flow which were not possible to be discerned from single point measurements fixed in space.

scholarly journals Monitoring the Hydraulic Performance of Sewers Using Fibre Optic Distributed Temperature Sensing

Water ◽  
2020 ◽  
Vol 12 (9) ◽  
pp. 2451
Author(s):  
Cedric Kechavarzi ◽  
Philip Keenan ◽  
Xiaomin Xu ◽  
Yi Rui
Keyword(s):  
Flow Velocity ◽  
Treatment Plant ◽  
Heavy Rain ◽  
Water Treatment Plant ◽  
Hydraulic Performance ◽  
Temperature Sensing ◽  
Fibre Optic ◽  
Distributed Temperature Sensing ◽  
Water Flow Velocity ◽  
Sewer Network

The hydraulic performance of sewers is a major public concern in industrialised countries. In this study, fibre optic distributed temperature sensing (DTS) is used to monitor the discharge of wastewater for three months to assess the performance of a long underground foul sewer in a village in the UK. DTS cables were installed in the invert of sewer pipes to obtain distributed temperature change data along the sewer network. DTS generates a series of two-dimensional data sets (temperature against distance) that can be visualised in waterfall plots to help identify anomalies. The spatial and temperature resolutions are 2 m and 0.2–0.3 °C, respectively. The monitoring data clearly identify high-temperature plumes, which represent the flow of household wastewater in the sewer. Based on the analysis of the waterfall plots, it is found that the flow velocity is about 0.14 m/s under normal conditions. When continuous moderate rain or heavy rain occurs, water backs up from the water treatment plant to upstream distances of up to 400 m and the water flow velocity in the sewer decreases sharply to about 0.03 m/s, which demonstrates the ability of the DTS to localise anomalies in the sewer network.

Monitoring sediment coverage from Distributed Temperature Sensing measurements in the Port of Rotterdam, the Netherlands

2020 ◽  
Author(s):  
Manos Pefkos ◽  
Pieter Doornenbal ◽  
Arjan Wijdeveld ◽  
Ebi Meshkati Shahmirzadi ◽  
Pauline Kruiver
Keyword(s):  
Spatial Resolution ◽  
Temperature Sensing ◽  
Water Phase ◽  
Horizontal Distance ◽  
Fibre Optic ◽  
Distributed Temperature Sensing ◽  
Fourier Amplitude ◽  
River Bank ◽  
Water And Sediment ◽  
Sediment Interface

<p>Distributed Temperature Sensing (DTS) measurements were conducted in the Port of Rotterdam as part of the INTERREG NWE SURICATES project. In the Port of Rotterdam a program is running to retain sediments in the harbor for river bank protection, and to lower the costs of transferring sediment from the port to the offshore dump locations. The aim of the DTS monitoring is to find spatial patterns in sediment deposition and erosion and thus determining the sediment balance before, during and after re-allocation. Fibre optic cables were installed in two layouts. Two fibre optic cables of lengths 1.2km and 750m were laid out flat parallel and perpendicular to the shore and they passively recorded temperature. Another cable was wrapped helically on a vertical pole condensing 150 m of length into 0.77m, increasing the spatial resolution. This cable was used for passive measurements and active heating experiments. The acquired data span the period from May to September 2019.</p><p>The active heating experiments showed that the water-sediment interface along the pole can be tracked from the difference in response between the time when the heating cable is switched on and off. The pole’s passive temperature analysis indicates that signals from the water phase exhibit high variability with time, whereas those from the sediment phase have low variability. Frequency domain analysis of the water phase shows clear peaks in the Fourier Amplitude Spectrum (FAS) at one day and half-day cycles, with the half-day cycle peak having the highest magnitude. The same peaks are present in the sediment phase’s FAS, but their magnitudes are about an order of magnitude lower.</p><p>The Fourier amplitude at frequencies corresponding to half-day periods was used for classification of the phases along the pole. The interface between water and sediment is defined as the maximum in the derivative of the Fourier amplitude with height. The interface’s height and thus the occurrence of erosion or deposition was tracked over time. The analysis shows that the sediment interface varied around 5cm over a period of 2.5 months between two dredging actions.</p><p>Representative signals from the Fourier amplitude at half-day cycles from the pole were used to derive sediment coverage over the flat passive cables. However, further research is required to establish the minimum horizontal distance over which coverage can be established.</p><p>We conclude that, by comparing the spectral properties of the temperature signal of water and sediment phases, sediment coverage over fibre optic cables can be monitored with DTS measurements. The finest time and spatial resolution over which this coverage can be found remains to be decided and can be the subject of future work.</p>

scholarly journals Fiber optic distributed temperature sensing for the determination of the nocturnal atmospheric boundary layer height

2011 ◽  
Vol 4 (2) ◽  
pp. 143-149 ◽  
Author(s):  
C. A. Keller ◽  
H. Huwald ◽  
M. K. Vollmer ◽  
A. Wenger ◽  
M. Hill ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Fiber Optic ◽  
Temperature Inversion ◽  
Nocturnal Boundary Layer ◽  
Temperature Sensing ◽  
Temperature Profiles ◽  
Distributed Temperature Sensing ◽  
Boundary Layer Height

Abstract. A new method for measuring air temperature profiles in the atmospheric boundary layer at high spatial and temporal resolution is presented. The measurements are based on Raman scattering distributed temperature sensing (DTS) with a fiber optic cable attached to a tethered balloon. These data were used to estimate the height of the stable nocturnal boundary layer. The experiment was successfully deployed during a two-day campaign in September 2009, providing evidence that DTS is well suited for this atmospheric application. Observed stable temperature profiles exhibit an exponential shape confirming similarity concepts of the temperature inversion close to the surface. The atmospheric mixing height (MH) was estimated to vary between 5 m and 50 m as a result of the nocturnal boundary layer evolution. This value is in good agreement with the MH derived from concurrent Radon-222 (222Rn) measurements and in previous studies.

Sign in / Sign up

Export Citation Format

Share Document