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Canopy waves, observations and predictions from lineartheory
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
CJN Research Meteorology.
(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 [en]
Forest, Holmboe instability, Kelvin-Helmholtz instability, Linear wave theory, Shear instability, Wind power
National Category
Meteorology and Atmospheric Sciences
Identifiers
URN: urn:nbn:se:uu:diva-237763OAI: oai:DiVA.org:uu-237763DiVA: diva2:768816
Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2015-03-09
In thesis
1. Mean Wind and Turbulence Conditions in the Boundary Layer above Forests
Open this publication in new window or tab >>Mean Wind and Turbulence Conditions in the Boundary Layer above Forests
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
Wind power, Forest, Turbulence, Waves, Spectra, Wind model, Atmospheric stability, Vindkraft, Skog, Turbulens, Vågor, Spektra, Modell, Atmosfärisk stabilitet
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-237764 (URN)978-91-554-9123-9 (ISBN)
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

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