Recommendation on use of wind lidars

The 15 Early Stage Researchers (ESRs) in the LIKE project investigate topics in which wind lidar play a significant role. This report provides the ESRs an introductory reading and gives a short introduction into the basic principles, as well as an overview on the practical application of lidar wind measurement technology for a wide range of research fields, including a corresponding literature review. Wherever possible, it will also give the ESRs recommendations on the use of lidars and related best practices and provide corresponding state-of-the-art documents in the attachment.publishedVersio

Recommendation on use of wind lidars

The 15 Early Stage Researchers (ESRs) in the LIKE project investigate topics in which wind lidar play a significant role. This report provides the ESRs an introductory reading and gives a short introduction into the basic principles, as well as an overview on the practical application of lidar wind measurement technology for a wide range of research fields, including a corresponding literature review. Wherever possible, it will also give the ESRs recommendations on the use of lidars and related best practices and provide corresponding state-of-the-art documents in the attachment.publishedVersio

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

Three synchronized pulsed Doppler wind lidars were deployed from May 2016 to June 2016 on the shores of a wide Norwegian fjord called Bjornafjord 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 25m above the sea surface, at scanning distances up to 4.6km. For a mean wind velocity above 8m.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 0.02m(-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 10m.s(-1) and 14m.s(-1), which may be related to a stable atmospheric stratification.The measurements were performed with the support from the Norwegian Public Road Administration. We are indebted to Claus Brian Munk Pedersen and Soren William Lund from the Technical University of Denmark, as well as Jarle Berge from the University of Stavanger, for their assistance during the installation and/or dismantling of the lidar equipment. Finally, we are grateful to Kjeller Vindteknikk for providing the data recorded on the met-mast on the island of Ospoya."Peer Reviewed

Dual lidar wind measurements along an upstream horizontal line perpendicular to a suspension bridge

Remote wind sensing can complement traditional anemometry at a bridge site and contribute to an improved wind-resistant design of long-span bridges. This study examines wind lidar measurement data recorded along a 168-meter-long horizontal line perpendicular to the main span of a suspension bridge in complex terrain. The velocity data records are obtained by a pair of continuous-wave Doppler lidars (short-range WindScanners) installed on the bridge deck. The measurement data are explored in terms of the mean wind speed and mean wind direction upstream of the bridge. The spectral characteristics of turbulence along the line are also investigated in relation to the increasing sampling volumes of a continuous-wave lidar system at increasing distances from the monitored area. Wind characteristics observed by the lidars are compared to those derived from sonic anemometer data recorded above the bridge deck at midspan. The results provide new insight into the wind flow characteristics in a fjord and demonstrate the potential of lidar measurements in charting the wind flow around a bridge. A slight monotonic increase of the wind speed, as well as a decrease in the yaw angle, is observed as the distance to the bridge reduces from 160 m to 20 m, while lower wind velocities are accompanied by a more stable wind direction. Within 15 m from the bridge deck, the adopted lidar setup gives unreliable information due to the large angle between the lidar beams.publishedVersio

Dual lidar wind measurements along an upstream horizontal line perpendicular to a suspension bridge

Remote wind sensing can complement traditional anemometry at a bridge site and contribute to an improved wind-resistant design of long-span bridges. This study examines wind lidar measurement data recorded along a 168-meter-long horizontal line perpendicular to the main span of a suspension bridge in complex terrain. The velocity data records are obtained by a pair of continuous-wave Doppler lidars (short-range WindScanners) installed on the bridge deck. The measurement data are explored in terms of the mean wind speed and mean wind direction upstream of the bridge. The spectral characteristics of turbulence along the line are also investigated in relation to the increasing sampling volumes of a continuous-wave lidar system at increasing distances from the monitored area. Wind characteristics observed by the lidars are compared to those derived from sonic anemometer data recorded above the bridge deck at midspan. The results provide new insight into the wind flow characteristics in a fjord and demonstrate the potential of lidar measurements in charting the wind flow around a bridge. A slight monotonic increase of the wind speed, as well as a decrease in the yaw angle, is observed as the distance to the bridge reduces from 160 m to 20 m, while lower wind velocities are accompanied by a more stable wind direction. Within 15 m from the bridge deck, the adopted lidar setup gives unreliable information due to the large angle between the lidar beams.publishedVersio

Can a dual pulsed lidar system measure the lateral coherence of turbulence?

Characterizing the coherence of turbulence in the marine boundary layer faces a significant challenge due to the limited availability of offshore measurements within the relevant altitude ranges, particularly up to 250 m. The coherence of turbulence describes the spatial correlation of wind velocity fluctuations, which is a key parameter for determining environmental loading on wind turbines. The turbulent wind loading represents one of the main uncertainties faced by the offshore wind engineering sector. Uncertainties arising from turbulent wind loading represent major challenges in the offshore wind engineering sector. Advancements in remote sensing technology, such as Doppler wind lidars, have opened new possibilities for studying wind turbulence at heights relevant to the increasing size of wind turbine rotors. This master’s thesis presents an analysis of 15 days of wind records collected by two pulsed wind lidars and two sonic anemometers during the COTUR project at Obrestad lighthouse, located on the southwestern coast of Norway. The site is expected to predominantly represent offshore conditions. The primary objective of this study is to assess the capability of pulsed Doppler wind lidar instruments in capturing the lateral co-coherence of turbulence along the wind component. Wind records obtained by the sonic anemometers mounted on 11 m high masts are used as reference data. The analysis focuses on both single and two-point statistics of wind turbulence, with particular emphasis on studying the co-coherence of turbulence. The results revealed that wind sectors aligned with northerly or southerly wind directions are suitable for comparison studies. In these wind directions, a good agreement is found between the two different instruments. Comparing co-coherence estimates obtained from pulse lidar and sonic anemometer showed negligible differences, indicating that spatial averaging did not significantly affect the estimation of co-coherence. By assessing the ability of pulsed Doppler wind lidar instruments to capture turbulence co-coherence, this study contributes to the applicability of lidar technology for characterizing turbulence and its potential for improving assessments of environmental loading on offshore wind turbinesMasteroppgave i energiENERGI399I5MAMN-ENE

Unfrozen Skewed Turbulence for Wind Loading on Structures

The paper introduces an algorithm to generate a three-variate four-dimensional wind turbulence field suited for yawed wind dynamic load simulation. At large yaw angles, a relaxation of Taylor’s hypothesis of frozen turbulence becomes relevant as well as the flow phase lag in the along-wind direction, which modulates the real and imaginary parts of the coherence. To capture such a general wind action on a structure, a modified spectral representation method is used where the coherence of turbulence is described as a complex-valued function. The one-point and two-point co-spectra are implemented in the simulation setup using a square-root-free Cholesky decomposition of the spectral matrix. The numerical procedure is illustrated based on turbulence characteristics derived from data collected during storm Aina (2017) on the Norwegian coast by three-dimensional sonic anemometers. During this event, a remarkable 3-hour stationary time series with a mean wind speed of 24 m s−1 at a height of 49 m above ground was recorded. Since no computational grid is needed, the velocity fluctuations with representative spatio-temporal characteristics can be directly simulated on structural elements of slender structures. Such an algorithm may be essential for the design of super-long span bridges in coastal areas.publishedVersio

A review of progress and applications of pulsed doppler wind LiDARs

Doppler wind LiDAR (Light Detection And Ranging) makes use of the principle of optical Doppler shift between the reference and backscattered radiations to measure radial velocities at distances up to several kilometers above the ground. Such instruments promise some advantages, including its large scan volume, movability and provision of 3-dimensional wind measurements, as well as its relatively higher temporal and spatial resolution comparing with other measurement devices. In recent decades, Doppler LiDARs developed by scientific institutes and commercial companies have been well adopted in several real-life applications. Doppler LiDARs are installed in about a dozen airports to study aircraft-induced vortices and detect wind shears. In the wind energy industry, the Doppler LiDAR technique provides a promising alternative to in-situ techniques in wind energy assessment, turbine wake analysis and turbine control. Doppler LiDARs have also been applied in meteorological studies, such as observing boundary layers and tracking tropical cyclones. These applications demonstrate the capability of Doppler LiDARs for measuring backscatter coefficients and wind profiles. In addition, Doppler LiDAR measurements show considerable potential for validating and improving numerical models. It is expected that future development of the Doppler LiDAR technique and data processing algorithms will provide accurate measurements with high spatial and temporal resolutions under different environmental conditions