Olsen, Bjarke Tobias

En vigtig del af planlægning af vindfarme er præcise vindbetingelsesestimater. I komplekstterræn kræves ikke-linære mikroskalamodeller som tager højde for stabilitetseffekter for atopnå præcise estimater, men disse modeller kan i sig selv ikke tage højde for storskala vejrvariationer, hvilket kræver kopling til mesoskalamodeller.Med udgangspunkt i mesoskalamodellen Weather Research and Forecasting (WRF) ogURANS mikroskalamodellen EllipSys3D, præsenterer denne afhandling en meso- til miroskalakoblingsmetode til vind nedskalering, ved at drive mikroskalamodellen med momentum ogtemperatur kildeled udtrukket fra mesoskalamodellen.Metoden bliver i første omgang studeret i terræn med simpelt kompleksitet. Her vises det,at en Single-Column model (SCM) version af mikroskalamodellen drevet af tendenser fraWRF resulterer i vindstatistik som er sammenlignelig med WRFs præcision. Resultaterneviser også, at når forskellige Planetary Boundary Layer og Surface Layer skemaer anvendesi WRF, følger SCM resultatererne WRF resultater, mens en statistisk ens respons ses i dennederste del af grænselaget.Dernæst bliver metoden brugt til at modellere Perdigão, en dobbelt-bakke, i komplekst terræn.Ved en simulering med 80 m celleafstande og et diskretiseringsskema med lav numerisknøjagtighed, viste studiet at metoden resulterede i store vindstatistike forbedringer sammenlignetmed nedskalering med WRF ned til 333 m celleafstande. På toppen af bakkerne var dengennemsnitlige afvigelse for vindhastigheder mindre end 3% ved toppen af masterne, mensde var henholdsvis 8-12% og 17-25% for WRF domæner med 1 km og 333 m celleafstande.Selvom resultaterne er lovende, ses en klar afhængighed af celleafstande og numeriskemetoder. Der er fortsat usikkerheder omkring mulig double-counting af turbulente fluxe, oghviken indflydelse det har når RANS closures anvendes til transient modelering af atmosfæreni høj opløsning.Accurate estimation of local site conditions, i.e. wind power resources and the turbulence impacting wind turbines turbines, is an important part of wind farm planning. In complex terrain, non-linear microscale models that account for atmospheric stability effects are needed for accurate site assessment, but by themselves they are unable to account for the changing large-scale weather conditions. Therefore, coupling to a mesoscale model, which can provide these variations to the microscale models, is required. This thesis describes, implements and validates with observed tall mast measurements a novel coupling strategy using the Weather Research and Forecasting (WRF) mesoscale model and the EllipSys3D URANS microscale model for wind downscaling. The coupling strategy is based on forcing the microscale model with momentum and temperature source terms extracted from the time-evolving mesoscale model simulation. These terms are included as source terms instead of the usual lateral boundary conditions. Two cases are presented for sites in simple and complex terrain. First, results from simple terrain cases are presented. It is shown that a Single-Column Model (SCM) version of microscale model forced by tendencies from WRF results in long-term wind statistics of comparable statistical accuracy to results using the WRF model itself. Using different Planetary Boundary Layer and Surface Layer schemes in the WRF model simulations shows that the SCM results tend to follow the WRF model results, while maintaining a statistically similar response near the surface. Second, the coupling method was used at the complex double hill site Perdigão. It was shown that for a simulation using 80 m grid spacing and a first-order accurate discretisation scheme, the coupled approach results in large improvements in wind statistics compared to downscaling with the WRF model with an innermost domain of 333 m grid spacing. At four masts situated on top of the two ridge tops, the mean biases in wind speed for a 32-day period were less than 3% at the top most anemometer on each mast, compared to mean errors of 8–12% and 17–25% for WRF domains of 1 km and 333 m grid spacing, respectively. Despite the encouraging results, a clear dependence on grid spacing and numerical methods was seen in the coupled model results. Uncertainties remain about possible double-counting of turbulent fluxes, and its impact on the simulated wind speed, when RANS turbulence closures are used for transient atmospheric modeling at fine resolution

Online Research Database In Technology

Mesoscale to microscale coupling for determining site conditions in complex terrain

http://orbit.dtu.dk/ws/files/158119593/phd_thesis.pdf

Mesoscale to microscale coupling for determining site conditions in complex terrain

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Online Research Database In Technology