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Mean Wind and Turbulence Conditions in the Boundary Layer above Forests
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (StandUp for wind)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

As wind turbines have grown, new installation areas become possible. Placing wind turbines in forested landscapes introduce uncertainties to the wind resource estimation. Even though close-to-canopy processes have been studied intensively during the last thirty years, the focus has mostly been on exchange processes and the height span of the studies has been below the rotor of a modern wind turbine.

This thesis contains analysis of new measurements from a 138 m high tower in a forested landscape. The previous knowledge of near-canopy processes is extended to the region above the roughness sublayer. It is shown that above the roughness sublayer, the surface layer behaves as over low vegetation, and Monin-Obukhov similarity is shown to hold for several variables. However, in stable stratification, effects that could be linked to the boundary layer depth are shown to be present in the measurements. These include wind turning with height, the behaviour of the turbulence length scale and the curvature of the wind profile.

Two new analytical models are presented in the thesis. One is a flux-profile expression in the roughness sublayer, which allows for analytical integration of the wind gradient. The model suggests that the roughness-sublayer effect depends on stratification and that the aerodynamic roughness length changes with stability. A decrease of roughness length in stable stratification is confirmed with a new method to determine the roughness length using measurements from the 138 m tower.

The other model determines the spectral tensor in stable stratification using analytical solution to the rapid distortion equations for stratified shear flow, with homogeneous stratification and shear. By using a formulation for the integration time of the distortions of an isotropic spectrum, a model is derived which provides the cross spectra of velocity and temperature at any two given points in space.

Finally the existence of waves in the wind over forests is investigated and it is concluded that the Kelvin-Helmholtz instability can create waves which are coherent in time and exist over the entire height span of wind turbine rotors. Linear wave theory is shown to be able to explain certain features of the waves.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 47 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1212
Keyword [en]
Wind power, Forest, Turbulence, Waves, Spectra, Wind model, Atmospheric stability
Keyword [sv]
Vindkraft, Skog, Turbulens, Vågor, Spektra, Modell, Atmosfärisk stabilitet
National Category
Meteorology and Atmospheric Sciences
Identifiers
URN: urn:nbn:se:uu:diva-237764ISBN: 978-91-554-9123-9 (print)OAI: oai:DiVA.org:uu-237764DiVA: diva2:768820
Public defence
2015-02-06, Ekmansalen, EBC, Norbyvägen 14, Uppsala, 13:00 (English)
Opponent
Supervisors
Projects
Vindforsk III, Wind power in forestsVindforsk IV, Forest wind
Funder
Swedish Energy Agency
Available from: 2015-01-16 Created: 2014-12-04 Last updated: 2015-03-09
List of papers
1. Flux-profile relation with roughness sublayer correction
Open this publication in new window or tab >>Flux-profile relation with roughness sublayer correction
2015 (English)In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 689, 1191-1197 p.Article in journal (Refereed) Published
Abstract [en]

Calculation of momentum flux using Monin–Obukhov similarity theory over forested areas is well known to underestimate the flux. Several suggestions of corrections to the standard flux-profile expression have been proposed in order to increase the magnitude of turbulent flux. The aim of this article is to find a simple, analytical representation for the characteristics of the flow within the canopy layer and the surface layer, including the roughness sublayer. A new form of the roughness sublayer correction is proposed, based on the desire to connect the shape of the roughness sublayer correction to forest characteristics. The new flux-profile relation can be used to find the flux or the wind profile whenever simple and fast estimations are needed, as for mesoscale modelling, scalar transport models, or sound propagation models.

Keyword
roughness sublayer; wind profile; dimensionless gradient; stability expressions
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-237757 (URN)10.1002/qj.2426 (DOI)000356805700016 ()
Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2017-12-05Bibliographically approved
2. Wind statistics from a forested landscape
Open this publication in new window or tab >>Wind statistics from a forested landscape
2015 (English)In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 156, no 1, 53-71 p.Article in journal (Refereed) Published
Abstract [en]

An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensure high data quality. A 40∘ wide wind-direction sector is selected as the most representative for large-scale forest conditions, and from that sector first-, second- and third-order statistics, as well as analyses regarding the characteristic length scale, the flux-profile relationship and surface roughness are presented for a wide range of stability conditions. The results are discussed with focus on the validity of different scaling regimes. Significant wind veer, decay of momentum fluxes and reduction in shear length scales with height are observed for all stability classes, indicating the influence of the limited depth of the boundary layer on the measured profiles. Roughness sublayer characteristics are however not detected in the presented analysis. Dimensionless gradients are shown to follow theoretical curves up to 100 m in stable conditions despite surface-layer approximations being invalid. This is attributed to a balance of momentum decay and reduced shear length scale growth with height. The wind profile shows a strong stability dependence of the aerodynamic roughness length, with a 50 % decrease from neutral to stable conditions.

Keyword
Above canopy turbulence statistics Atmospheric boundary layers, Decreasing roughness, Forest Canopy Flows, Wind power
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-237762 (URN)10.1007/s10546-015-0016-x (DOI)000355153200004 ()
Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2017-12-05Bibliographically approved
3. A spectral model for stably stratied turbulence
Open this publication in new window or tab >>A spectral model for stably stratied turbulence
(English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Submitted
Abstract [en]

A solution of the inviscid rapid distortion equations of a stratied flow with homogeneous shear is proposed, extending the work of Hanazaki and Hunt (J. Fluid Mech., 2004,vol. 507, pp. 1-42) to the two horizontal velocity components. The analytical solution allowed the determination of the spectral tensor evolution at any given time starting from a known initial condition. By following the same approach adopted by Mann (J.Fluid Mech., 1994, vol. 273, pp. 141-168), a model for the velocity spectral tensor in the atmospheric boundary layer is obtained where the spectral tensor, assumed to be isotropic at the initial time, evolves until the break-up time where the spectral tensor is supposed to achieve its final state observed in the boundary layer. The model predictions are compared with atmospheric measurements obtained over a forested area, giving the opportunity to calibrate the model parameters and further validation is provided by lowroughness data. Characteristic values of the model coffecients and their dependence on the Richardson number are proposed and discussed.

National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-237761 (URN)
Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2017-12-05
4. Canopy waves, observations and predictions from lineartheory
Open this publication in new window or tab >>Canopy waves, observations and predictions from lineartheory
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper the existence of canopy waves is examined using measurements from a 138 m high tower placed in a forest. Characteristics of the waves are examined in relation to wind energy. Using wavelet analysis it is shown that when the wave signal is clear, the phase lag between horizontal and vertical velocity is close to 90 degrees, which limits the contribution of the waves to themomentum flux. Results from numerical solution of linear wave equations is shown to agree with measurements in terms of wave period and the vertical shape of the wave amplitude. Linear analysis and measurements suggests that Kelvin-Helmholtz instability causes unstable wave growth and that the most unstable wave number normally has a period of 10-100 s. In addition to the Kelvin-Helmholtz instability, the linear analysis predicts that instabilities of the Holmboe kind, with higher frequency, can develop over forests in certain conditions.

Keyword
Forest, Holmboe instability, Kelvin-Helmholtz instability, Linear wave theory, Shear instability, Wind power
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-237763 (URN)
Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2015-03-09

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