Direct measurement of the spectral transfer function of a laser based anemometer

Angelou, Nikolas; Mann, Jakob; Sjöholm, Mikael; Courtney, Michael

doi:10.1063/1.3697728

Cited by 45 publications

(48 citation statements)

References 12 publications

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“…On the other hand, Mann [45] found strong effects of stability on spectra over the Great Belt, Denmark, up to over 16 m·s −1 , albeit at a height of 70 m. When the high-frequency range of the N400 wind spectrum is attenuated using the ABSA model presented in Equation (26), the slope of the high-frequency part of the modified spectrum is sharper than measured, as seen in Figure 9. Similar observations have been reported by e.g., Pauscher et al [7] with the long-range WindScanner system or e.g., Angelou et al [46] with the short-range WindScanner system. The discrepancies between the modelled ABSA and the measured one may be due to fluctuations of the wind direction and the use of multiple scanning beams to retrieve the horizontal wind components.…”

Section: Wind Spectra Comparisonsupporting

confidence: 78%

Measurements of Surface-Layer Turbulence in a Wide Norwegian Fjord Using Synchronized Long-Range Doppler Wind Lidars

et al. 2017

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Three synchronized pulsed Doppler wind lidars were deployed from May 2016 to June 2016 on the shores of a wide Norwegian fjord called Bjørnafjord to study the wind characteristics at the proposed location of a planned bridge. The purpose was to investigate the potential of using lidars to gather information on turbulence characteristics in the middle of a wide fjord. The study includes the analysis of the single-point and two-point statistics of wind turbulence, which are of major interest to estimate dynamic wind loads on structures. The horizontal wind components were measured by the intersecting scanning beams, along a line located 25 m above the sea surface, at scanning distances up to 4.6 km. For a mean wind velocity above 8 m·s −1 , the recorded turbulence intensity was below 0.06 on average. Even though the along-beam spatial averaging leads to an underestimated turbulence intensity, such a value indicates a roughness length much lower than provided in the European standard EN 1991-1-4:2005. The normalized spectrum of the along-wind component was compared to the one provided by the Norwegian Petroleum Industry Standard and the Norwegian Handbook for bridge design N400. A good overall agreement was observed for wave-numbers below 0.02 m −1 . The along-beam spatial averaging in the adopted set-up prevented a more detailed comparison at larger wave-numbers, which challenges the study of wind turbulence at scanning distances of several kilometres. The results presented illustrate the need to complement lidar data with point-measurement to reduce the uncertainties linked to the atmospheric stability and the spatial averaging of the lidar probe volume. The measured lateral coherence was associated with a decay coefficient larger than expected for the along-wind component, with a value around 21 for a mean wind velocity bounded between 10 m·s −1 and 14 m·s −1 , which may be related to a stable atmospheric stratification.

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Section: Wind Spectra Comparisonsupporting

confidence: 78%

Measurements of Surface-Layer Turbulence in a Wide Norwegian Fjord Using Synchronized Long-Range Doppler Wind Lidars

et al. 2017

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“…As far as it concerns the focus distance, the estimated uncertainty will result in an error of determining the point of focus, which will increase along with the distance. For example, at the first profile this is 0.007 m and at profile 7 it is 0.12 m. The produced laser Doppler spectra contain information about the average wind speed projected to the line-of-sight of the instrument over the probe length (Angelou et al 2012).…”

Section: The Experimentsmentioning

confidence: 99%

“…Prior to the post processing of the spectra they are averaged to reduce the noise. More information about the data filtering and processing is described in Angelou et al (2012). The resulting spectra offer the possibility of measuring radial wind speeds between 0.3 m s −1 and 18 m s −1 with a Doppler spectral speed resolution of 0.15 m s −1 .…”

Section: The Experimentsmentioning

confidence: 99%

Variations of the Wake Height over the Bolund Escarpment Measured by a Scanning Lidar

Lange

Mann

Angelou

et al. 2015

Boundary-Layer Meteorol

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The wake zone behind the escarpment of the Bolund peninsula in the Roskilde Fjord, Denmark, has been investigated with the help of a continuous-wave Doppler lidar. The instrument measures the line-of-sight wind speed 390 times per second in highly resolved 7-m tall profiles by rapidly changing the focus distance and beam direction. The profiles reveal the detailed and rapidly changing structure of the wake induced by the Bolund escarpment. The wake grows with distance from the escarpment, with the wake height depending strongly on the wind direction, such that the minimum height appears when the flow is perpendicular to the escarpment. The wake increases by 10-70 % when the wind direction deviates ±15 • from perpendicular depending on the distance to the edge and to a lesser degree on the method by which the wake height is determined. This finding is supported by a comparison with in situ measurements acquired on the Bolund peninsula.

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“…We prefer to use the transfer function defined in eq. 2 to the more traditional G(k 1 ) = F r (k 1 )/F s (k 1 ) because the auto-spectrum F r (k 1 ) may be affected by noise whereas the cross-spectrum χ r,s (k 1 ) is not (Angelou et al, 2012) assuming the sonic time series to be noiseless.…”

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confidence: 99%

Comparison of Methods to Derive Radial Wind Speed from a Continuous-Wave Coherent Lidar Doppler Spectrum

Held¹,

Mann²

2018

Preprint

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Abstract. Continuous-wave lidar systems offer the possibility to remotely sense wind speed but are also affected by inherent differences in their measurement process compared to more traditional anemometry like cup or sonic anemometers. Their large measurement volume leads to an attenuation of turbulence. In this paper we study how different methods to derive the radial wind speed from a lidar Doppler spectrum can mitigate turbulence attenuation. The centroid, median and maximum methods are compared by estimating transfer functions and calculating root mean squared errors (RMSE) between a lidar and a sonic 5 anemometer. Numerical simulations and experimental results both indicate that the maximum method, even though using the least amount of information from the Doppler spectrum, performs best at mitigating the volume averaging effect. However, this benefit is paid by increased signal noise due to the discretisation of the maximum method. The median method shows slight improvements over the centroid method in terms of volume averaging reduction and also resulted in the overall lowest RMSE.Thus, when the aim is to obtain point-like wind speed time series with high temporal resolution, the median method is superior 10 to the centroid and maximum method. When comparing 10 minute averages there is no difference between the methods.

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Direct measurement of the spectral transfer function of a laser based anemometer

Cited by 45 publications

References 12 publications

Measurements of Surface-Layer Turbulence in a Wide Norwegian Fjord Using Synchronized Long-Range Doppler Wind Lidars

Measurements of Surface-Layer Turbulence in a Wide Norwegian Fjord Using Synchronized Long-Range Doppler Wind Lidars

Variations of the Wake Height over the Bolund Escarpment Measured by a Scanning Lidar

Comparison of Methods to Derive Radial Wind Speed from a Continuous-Wave Coherent Lidar Doppler Spectrum

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