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Effects of a Forest Clearing: Experimentaland Numerical Assessment
KTH, School of Engineering Sciences (SCI), Mechanics.
2013 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

There is a renewed interest in the wind estimate over and around forest areas dueto the increasing demand of wind-energy resources. Many researches have been donewith simplied forest models. However, the introduction of a simple two-dimensionalclearing adds further parameters, such as the width of the clearing, which furthercomplicates the analysis. The main purpose of the present experimental and nu-merical efforts is, therefore, to characterize the ow over the forest clearing and tosuggest the suitable location for the wind-power generation over the forest clearing.The experiments were performed in the Minimum Turbulence Level (MTL) windtunnel at KTH in Stockholm, and PIV data evaluation and analysis were carried out.The canopy model consists of several wooden at plates, and to each of the plateswooden cylindrical pins were clamped in a staggered layout to mimic a homogeneoushigh-density forest. The total length of the forest model is 40hc, where hc indicatesthe canopy height. Two cases were experimentally investigated, one with a fullforest conguration and the other with the presence of a clearing that starts fromx=hc = 20 and ends at x=hc = 30, where x is a streamwise coordinate that startsat the forest windward edge. Particle Image Velocimetry (PIV) was performed, andplanar velocity snapshots were taken at the downwind edge of the clearing.Large Eddy Simulations (LES) were also conducted to complement the experi-mental information. The present LES code was developed by modifying the DirectNumerical Simulation (DNS) code of the turbulent boundary layer ow by Kametani& Fukagata (2011), by adding the subgrid scale model part and the empirical canopymodel part into the DNS code.Both the experimental and the numerical results indicate that the clearing is as-sociated to a streamwise velocity defect in the mean prole mainly due to the strongturbulent diffusion into the clearing region. The turbulence is redistributed amongstthe various velocity components so that the streamwise velocity variance is reduced,while the vertical velocity variance is enhanced. The streamwise velocity varianceis in fact damped due to the absence of the canopy drag from x=hc = 20, whileenhanced vertical-velocity uctuations can be observed at the end of the clearing.However, the effects are immediately weakened both by a ow re-acceleration andby a new surface layer development right after passing the downwind clearing edge.The clearing effect seems to be dominant in the roughness sublayer at least for theneutral atmospheric conditions. The clearing perturbation seems to be associatedto turbulent mixing at its initial stage near y hc, followed by a rapid distorsionnear the clearing trailing edge. This phenomenon is highlighted by the low valueof the vertical correlation length scale that, after the clearing trailing edge, risesagain towards to homogenous forest condition. The LES results further show thata suitable area for the wind-turbine operation is close to the upwind clearing edgewhere the energy contents is the highest, while the turbulent intensity is lowestbetween the clearing. They also indicate that wind-speed enhancement can be ex-pected downstream of the short forest edge, implying that the ow can be optimizedfor wind-power generation just by changing the forest conguration.

Place, publisher, year, edition, pages
2013. , p. 61
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-202633OAI: oai:DiVA.org:kth-202633DiVA, id: diva2:1078086
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Examiners
Available from: 2017-03-02 Created: 2017-03-02 Last updated: 2017-03-02Bibliographically approved

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