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Thomas W Horst - One of the best experts on this subject based on the ideXlab platform.
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Measurements of Flow Distortion within the IRGASON Integrated Sonic Anemometer and CO $$_2$$
Boundary-Layer Meteorology, 2016Co-Authors: Thomas W Horst, Roland Vogt, Stephen P OncleyAbstract:Wind-tunnel and field measurements are analyzed to investigate Flow Distortion within the IRGASON integrated sonic anemometer and CO $$_2$$ 2 /H $$_2$$ 2 O gas analyzer as a function of wind speed, wind direction and attack angle. The wind-tunnel measurements are complimentary to the field measurements, and the dependence of the wind-tunnel mean-wind-component Flow-Distortion errors on wind direction agrees well with that of the field measurements. The field measurements exhibit significant overestimation of the crosswind variance and underestimation of the momentum flux with respect to an adjacent CSAT3 sonic, as well as a transfer of turbulent kinetic energy from the streamwise wind component to the cross-stream wind components. In contrast, we find attenuation of only a few percent in the vertical velocity variance and the vertical flux of sonic temperature. The attenuation of the fluxes appears to be caused to a large extent by decorrelation between the horizontal and vertical-velocity components and between the vertical velocity and sonic temperature. Additional Flow Distortion due to transducer shadowing reduces to some extent the overestimation, but also increases the underestimation of the IRGASON turbulence statistics.
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measurements of Flow Distortion within the irgason integrated sonic anemometer and co_2 h_2o gas analyzer
Boundary-Layer Meteorology, 2016Co-Authors: Thomas W Horst, Roland Vogt, Stephen P OncleyAbstract:Wind-tunnel and field measurements are analyzed to investigate Flow Distortion within the IRGASON integrated sonic anemometer and CO\(_2\)/H\(_2\)O gas analyzer as a function of wind speed, wind direction and attack angle. The wind-tunnel measurements are complimentary to the field measurements, and the dependence of the wind-tunnel mean-wind-component Flow-Distortion errors on wind direction agrees well with that of the field measurements. The field measurements exhibit significant overestimation of the crosswind variance and underestimation of the momentum flux with respect to an adjacent CSAT3 sonic, as well as a transfer of turbulent kinetic energy from the streamwise wind component to the cross-stream wind components. In contrast, we find attenuation of only a few percent in the vertical velocity variance and the vertical flux of sonic temperature. The attenuation of the fluxes appears to be caused to a large extent by decorrelation between the horizontal and vertical-velocity components and between the vertical velocity and sonic temperature. Additional Flow Distortion due to transducer shadowing reduces to some extent the overestimation, but also increases the underestimation of the IRGASON turbulence statistics.
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Correction of a Non-orthogonal, Three-Component Sonic Anemometer for Flow Distortion by Transducer Shadowing
Boundary-Layer Meteorology, 2015Co-Authors: Thomas W Horst, S. R. Semmer, G. MacleanAbstract:We propose that Flow Distortion within a non-orthogonal CSAT3 sonic anemometer is primarily due to transducer shadowing, which is caused by wakes in the lee of the acoustic transducers impinging on their measurement paths. The dependence of transducer shadowing on sonic path geometry, wind direction and atmospheric stability is investigated with simulations that use surface-layer data from the Horizontal Array Turbulence Study (HATS) field program and canopy roughness-sublayer data from the CHATS (Canopy HATS) field program. We demonstrate the efficacy of correcting the CSAT3 for transducer shadowing with measurements of its Flow Distortion in the NCAR wind tunnel, combined with 6 months of data collected in the atmospheric surface layer with adjacent CSAT3 and orthogonal ATI-K sonic anemometers at the NCAR Marshall field site. CSAT3 and ATI-K measurements of the variance of vertical velocity $$\sigma _w^2$$ σ w 2 and the vertical flux of sonic temperature agree within 1 % after correction of both sonics for transducer shadowing. Both the simulations of transducer shadowing and the comparison of CSAT3 and ATI-K field data suggest a simple, approximate correction of CSAT3 surface-layer scalar fluxes with an increase on the order of 4–5 %, independent of wind direction and atmospheric stability. We also find that $$\sigma _w/u_*$$ σ w / u ∗ (where $$u_*$$ u ∗ is the friction velocity) and $$r_{uw}$$ r u w (the correlation coefficient) calculated with corrected CSAT3 data are insensitive to wind direction and agree closely with known values of these dimensionless variables for neutral stratification, which is evidence for the efficacy of the correction of the horizontal wind components for transducer shadowing as well.
Stephen P Oncley - One of the best experts on this subject based on the ideXlab platform.
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Measurements of Flow Distortion within the IRGASON Integrated Sonic Anemometer and CO $$_2$$
Boundary-Layer Meteorology, 2016Co-Authors: Thomas W Horst, Roland Vogt, Stephen P OncleyAbstract:Wind-tunnel and field measurements are analyzed to investigate Flow Distortion within the IRGASON integrated sonic anemometer and CO $$_2$$ 2 /H $$_2$$ 2 O gas analyzer as a function of wind speed, wind direction and attack angle. The wind-tunnel measurements are complimentary to the field measurements, and the dependence of the wind-tunnel mean-wind-component Flow-Distortion errors on wind direction agrees well with that of the field measurements. The field measurements exhibit significant overestimation of the crosswind variance and underestimation of the momentum flux with respect to an adjacent CSAT3 sonic, as well as a transfer of turbulent kinetic energy from the streamwise wind component to the cross-stream wind components. In contrast, we find attenuation of only a few percent in the vertical velocity variance and the vertical flux of sonic temperature. The attenuation of the fluxes appears to be caused to a large extent by decorrelation between the horizontal and vertical-velocity components and between the vertical velocity and sonic temperature. Additional Flow Distortion due to transducer shadowing reduces to some extent the overestimation, but also increases the underestimation of the IRGASON turbulence statistics.
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measurements of Flow Distortion within the irgason integrated sonic anemometer and co_2 h_2o gas analyzer
Boundary-Layer Meteorology, 2016Co-Authors: Thomas W Horst, Roland Vogt, Stephen P OncleyAbstract:Wind-tunnel and field measurements are analyzed to investigate Flow Distortion within the IRGASON integrated sonic anemometer and CO\(_2\)/H\(_2\)O gas analyzer as a function of wind speed, wind direction and attack angle. The wind-tunnel measurements are complimentary to the field measurements, and the dependence of the wind-tunnel mean-wind-component Flow-Distortion errors on wind direction agrees well with that of the field measurements. The field measurements exhibit significant overestimation of the crosswind variance and underestimation of the momentum flux with respect to an adjacent CSAT3 sonic, as well as a transfer of turbulent kinetic energy from the streamwise wind component to the cross-stream wind components. In contrast, we find attenuation of only a few percent in the vertical velocity variance and the vertical flux of sonic temperature. The attenuation of the fluxes appears to be caused to a large extent by decorrelation between the horizontal and vertical-velocity components and between the vertical velocity and sonic temperature. Additional Flow Distortion due to transducer shadowing reduces to some extent the overestimation, but also increases the underestimation of the IRGASON turbulence statistics.
Matthias Mauder - One of the best experts on this subject based on the ideXlab platform.
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Evaluation of Probe-Induced Flow Distortion of Campbell CSAT3 Sonic Anemometers by Numerical Simulation
Boundary-Layer Meteorology, 2017Co-Authors: Thomas Foken, Matthias MauderAbstract:The Campbell CSAT3 sonic anemometer is one of the most popular instruments for turbulence measurements in basic micrometeorological research and ecological applications. While measurement uncertainty has been characterized by field experiments and wind-tunnel studies in the past, there are conflicting estimates, which motivated us to conduct a numerical experiment using large-eddy simulation to evaluate the probe-induced Flow Distortion of the CSAT3 anemometer under controlled conditions, and with exact knowledge of the undisturbed Flow. As opposed to wind-tunnel studies, we imposed oscillations in both the vertical and horizontal velocity components at the distinct frequencies and amplitudes found in typical turbulence spectra in the surface layer. The resulting Flow-Distortion errors for the standard deviations of the vertical velocity component range from 3 to 7%, and from 1 to 3% for the horizontal velocity component, depending on the azimuth angle. The magnitude of these errors is almost independent of the frequency of wind speed fluctuations, provided the amplitude is typical for surface-layer turbulence. A comparison of the corrections for transducer shadowing proposed by both Kaimal et al. (Proc Dyn Flow Conf, 551–565, 1978 ) and Horst et al. (Boundary-Layer Meteorol 155:371–395, 2015 ) show that both methods compensate for a larger part of the observed error, but do not sufficiently account for the azimuth dependency. Further numerical simulations could be conducted in the future to characterize the Flow Distortion induced by other existing types of sonic anemometers for the purposes of optimizing their geometry.
Curtis R Wood - One of the best experts on this subject based on the ideXlab platform.
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a wind tunnel study of Flow Distortion at a meteorological sensor on top of the bt tower london uk
Journal of Wind Engineering and Industrial Aerodynamics, 2011Co-Authors: Janet F Barlow, James Harrison, Alan Robins, Curtis R WoodAbstract:High quality wind measurements in cities are needed for numerous applications including wind engineering. Such data-sets are rare and measurement platforms may not be optimal for meteorological observations. Two years' wind data were collected on the BT Tower, London, UK, showing an upward deflection on average for all wind directions. Wind tunnel simulations were performed to investigate Flow Distortion around two scale models of the Tower. Using a 1:160 scale model it was shown that the Tower causes a small deflection (ca. 0.5°) compared to the lattice on top on which the instruments were placed (ca. 0–4°). These deflections may have been underestimated due to wind tunnel blockage. Using a 1:40 model, the observed Flow pattern was consistent with streamwise vortex pairs shed from the upstream lattice edge. Correction factors were derived for different wind directions and reduced deflection in the full-scale data-set by <3°. Instrumental tilt caused a sinusoidal variation in deflection of ca. 2°. The residual deflection (ca. 3°) was attributed to the Tower itself. Correction of the wind-speeds was small (average 1%) therefore it was deduced that Flow Distortion does not significantly affect the measured wind-speeds and the wind climate statistics are reliable.
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A wind-tunnel study of Flow Distortion at a meteorological sensor on top of the BT Tower, London, UK
Journal of Wind Engineering and Industrial Aerodynamics, 2011Co-Authors: Janet F Barlow, James Harrison, Alan Robins, Curtis R WoodAbstract:High quality wind measurements in cities are needed for numerous applications including wind engineering. Such data-sets are rare and measurement platforms may not be optimal for meteorological observations. Two years' wind data were collected on the BT Tower, London, UK, showing an upward deflection on average for all wind directions. Wind tunnel simulations were performed to investigate Flow Distortion around two scale models of the Tower. Using a 1:160 scale model it was shown that the Tower causes a small deflection (ca. 0.5°) compared to the lattice on top on which the instruments were placed (ca. 0–4°). These deflections may have been underestimated due to wind tunnel blockage. Using a 1:40 model, the observed Flow pattern was consistent with streamwise vortex pairs shed from the upstream lattice edge. Correction factors were derived for different wind directions and reduced deflection in the full-scale data-set by
Jakob Mann - One of the best experts on this subject based on the ideXlab platform.
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a method to assess the accuracy of sonic anemometer measurements
Atmospheric Measurement Techniques, 2019Co-Authors: Alfredo Pena, Ebba Dellwik, Jakob MannAbstract:Abstract. We propose a method to assess the accuracy of atmospheric turbulence measurements performed by sonic anemometers and test it by analysis of measurements from two commonly used sonic anemometers, a Metek USA-1 and a Campbell CSAT3, at two locations in Denmark. The method relies on the estimation of the ratio of the vertical to the along-wind velocity power spectrum within the inertial subrange and does not require the use of another measurement as reference. When we correct the USA-1 to account for three-dimensional Flow-Distortion effects, as recommended by Metek GmbH, the ratio is very close to 4∕3 as expected from Kolmogorov's hypothesis, whereas non-corrected data show a ratio close to 1. For the CSAT3, non-corrected data show a ratio close to 1.1 for the two sites and for wind directions where the instrument is not directly affected by the mast. After applying a previously suggested Flow-Distortion correction, the ratio increases up to ≈1.2 , implying that the effect of Flow Distortion in this instrument is still not properly accounted for.
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A method to assess the accuracy of sonic anemometer measurements
2018Co-Authors: Alfredo Pena, Ebba Dellwik, Jakob MannAbstract:Abstract. We propose a method to assess the accuracy of atmospheric turbulence measurements performed by sonic anemometers and test it by analysis of measurements from two commonly used sonic anemometers, a Metek USA-1 and a Campbell CSAT3, at two locations in Denmark. The method relies on the estimation of the ratio of the vertical to the along-wind velocity power spectrum within the inertial subrange. When we correct the USA-1 to account for three-dimensional Flow-Distortion effects, as recommended by Metek GmbH, the ratio is very close to 4/3 as expected from Kolmogorov's hypothesis, whereas non-corrected data show a ratio close to 1. For the CSAT3, non-corrected data show a ratio close to 1.1 for the two sites and for wind directions where the instrument is not directly affected by the mast. After applying a previously suggested Flow-Distortion correction, the ratio increases up to &approx; 1.2, implying that the effect of Flow Distortion in this instrument is still not properly accounted for.
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Flow Distortion at a dense forest edge
Quarterly Journal of the Royal Meteorological Society, 2014Co-Authors: Ebba Dellwik, Ferhat Bingol, Jakob MannAbstract:The Flow near tall forest edges is complex, yet poorly described. A field experiment using two meteorological masts equipped with sonic anemometers and a horizontally staring lidar was performed upwind and downwind of the interface between an open flat farmland and a tall (hc = 24 m) beech forest. Data obtained during near-neutral conditions are presented for the wind direction towards the forest. Results from a high leaf area index period are compared with those from a low leaf area index period. For both periods, the wind speed increased above the forest and decreased within the forest, relative to the measurements upwind of the edge. The lidar data taken at several positions between the masts at 1.25hc show that the minimum wind speed occurred just upwind of the edge. At the 1.25hc level, at the forest mast, the momentum flux () increased strongly over the forest and positive values were recorded during the high leaf area index period. A spectral analysis revealed that approximately half of this change was caused by low-frequency, positively correlated eddies along the streamline. The remaining increase can qualitatively be explained with the concept of eddy-blocking by the canopy top, which could also explain the observed increase in lateral variance and the decrease in the vertical variance. Despite the short distance to the edge of approximately 1.5hc, the beginning of a new internal boundary layer was visible at 1.04hc as a decrease in the vertical momentum flux. At this level, as well as within the forest, the results depended on the wind speed. The presented findings enhance the understanding of the forest edge Flow and are useful for model verification and development.