Spectral characteristics of surface-layer turbulence in the North Sea

Cheynet, Etienne; Jakobsen, Jasna Bogunović; Obhrai, Charlotte

doi:10.1016/j.egypro.2017.10.366

Cited by 25 publications

(17 citation statements)

References 51 publications

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“…The assumption of stationary flow is assessed using a two-step process: firstly, the slope of the linear trend of each time series is investigated, and if the difference between the two extrema of this trend and its mean value is larger than 20%, the sample is not considered as stationary. In Cheynet et al (2017), turbulent characteristics were studied after the removal of any slightly non-linear trends using the empirical modal decomposition technique (Huang et al 1998;Chen et al 2007), but is not applied here, as we study the unaltered velocity spectra at frequencies below 1 mHz. Secondly, the stationarity of each linearly detrended time series is assessed using the so-called reverse arrangement test from Bendat and Piersol Table 1: Data availability and the effects of initial data selection processing.…”

Section: The Fino1 Platformmentioning

confidence: 99%

“…(1) to investigate the limits of current spectral models used for wind-load estimation on offshore structures, and (2) to present the analysis of turbulence characteristics for the further development of a commonly accepted, atmospheric boundary-layer theory. Velocity data from the FINO1 platform have been used in the past to assess the applicability of the gradient Richardson number in an offshore environment (Argyle and Watson 2014), to study velocity profiles above the sea (Kettle 2013), to investigate the turbulence intensity (Türk and Emeis 2010), and to test the validity of the one-point spectral models provided in the IEC 61400-1 standard (Cheynet et al 2017). However, to the authors' knowledge, no description of the one-and two-point spectra of offshore turbulence as a function of atmospheric stability is available.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Velocity Spectra and Coherence Estimates in the Marine Atmospheric Boundary Layer

Cheynet

Jakobsen

Reuder

2018

Boundary-Layer Meteorol

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Two years of continuous sonic anemometer measurements conducted in 2007 and 2008 at the FINO1 platform are used to investigate the characteristics of the single-and two-point velocity spectra in relation to the atmospheric stability in the marine atmospheric boundary layer. The goals are to reveal the limits of current turbulence models for the estimation of wind loads on offshore structures, and to propose a refined description of turbulence at altitudes where Monin-Obukhov similarity theory may be limited. Using local similarity theory, a composite spectrum model, combining a pointed and a blunt model, is proposed to describe the turbulence spectrum for both unstable, neutral and stable conditions. Such a model captures the −1 power law followed by the velocity spectra at an intermediate frequency range in the marine atmospheric boundary layer. For a Monin-Obukhov similarity parameter ζ < 0.3, the Davenport coherence model captures the vertical coherence of the horizontal velocity components well. A two-parameter exponential decay function is found more appropriate for modelling the coherence of the vertical velocity component. Under increasingly stable conditions, the size of the eddies in the vertical coordinate reduces, such that smaller separation distances than that covered in the present dataset may be required to study the coherence with sufficient accuracy.

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Section: The Fino1 Platformmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Velocity Spectra and Coherence Estimates in the Marine Atmospheric Boundary Layer

Cheynet

Jakobsen

Reuder

2018

Boundary-Layer Meteorol

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“…Above the surface layer, the proposed model is not valid, which leads to biased estimations [43]. Worse results were attained when considering alternative values for Charnock's parameter, α c , which is a site-dependent parameter here assumed as α c = 0.012 based on previous studies [33,44,65,66] When comparing û * against mast-derived u * 1−D (Figure 10b), a much better agreement is found. This is supported by a LR as good as û * = 0.965 • u * 1−D + 0.022, which is virtually the ideal 1:1 line; ρ 2 = 0.956; and RMSE = 0.031 m/s.…”

Section: Performance Statistics: Friction Velocity and Stabilitymentioning

confidence: 93%

Assessing Obukhov Length and Friction Velocity from Floating Lidar Observations: A Data Screening and Sensitivity Computation Approach

et al. 2022

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This work presents a parametric-solver algorithm for estimating atmospheric stability and friction velocity from floating Doppler wind lidar (FDWL) observations close to the mast of IJmuiden in the North Sea. The focus of the study was two-fold: (i) to examine the sensitivity of the computational algorithm to the retrieved variables and derived stability classes (the latter through confusion-matrix theory), and (ii) to present data screening procedures for FDWLs and fixed reference instrumentation. The performance of the stability estimation algorithm was assessed with reference to wind speed and temperature observations from the mast. A fixed-to-mast Doppler wind lidar (DWL) was also available, which provides a reference for wind-speed observations free from sea-motion perturbations. When comparing FDWL- and mast-derived mean wind speeds, the obtained determination coefficient was as high as that of the fixed-to-mast DWL against the mast (ρ2=0.996) with a root mean square error (RMSE) of 0.25 m/s. From the 82-day measurement campaign at IJmuiden (10,833 10 min records), the parametric algorithm showed that the atmosphere was neutral (31% of the cases), stable (28%), or near-neutral stable (19%) during most of the campaign. These figures satisfactorily agree with values estimated from the mast measurements (31%, 27%, and 19%, respectively).

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“…The limitations of current guidelines for offshore turbulence modelling, such as IEC 61400-1 ( 2005), have been highlighted in the past (Cheynet et al, 2017(Cheynet et al, , 2018. Site-specific measurements advised by IEC 61400-1 ( 2005) are justified for the mean flow and integral turbulence characteristics.…”

Section: Introductionmentioning

confidence: 99%

Turbulence in a coastal environment: the case of Vindeby

Putri

Cheynet

Obhrai

et al. 2021

Preprint

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Abstract. Turbulence spectral characteristics for various atmospheric stratifications are studied using the observations from an offshore mast at Vindeby wind farm. Measurement data at 6 m, 18 m and 45 m above the mean sea level are considered. At the lowest height, the normalized power spectral densities of the velocity components show deviations from Monin-Obukhov similarity theory (MOST). A significant co-coherence at the wave spectral peak frequency between the vertical velocity component and the velocity of the sea surface is observed, but only when the significant wave heights exceed 0.9 m. The turbulence spectra at 18 m generally follow MOST and are consistent with the empirical spectra established on the FINO1 offshore platform from an earlier study. The data at 45 m is associated with a high-frequency measurement noise which limits its analysis to strong wind conditions only. The estimated co-coherence of the along-wind component under near-neutral atmosphere matches remarkably well with those at FINO1. The turbulence characteristics estimated from the present dataset are valuable to better understand the structure of turbulence in the marine atmospheric boundary layer and are relevant for load estimations of offshore wind turbines. Yet, a direct application of the results to other offshore or coastal sites should be exercised with caution, since the dataset is collected in shallow waters and at heights lower than the hub height of the current and the future state-of-the-art offshore wind turbines.

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Spectral characteristics of surface-layer turbulence in the North Sea

Cited by 25 publications

References 51 publications

Velocity Spectra and Coherence Estimates in the Marine Atmospheric Boundary Layer

Velocity Spectra and Coherence Estimates in the Marine Atmospheric Boundary Layer

Assessing Obukhov Length and Friction Velocity from Floating Lidar Observations: A Data Screening and Sensitivity Computation Approach

Turbulence in a coastal environment: the case of Vindeby

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