Abstract
Results of an observational campaign and model study are presented demonstrating how the wind field at roof-level in the urban area of Vienna changed due to the construction of a new building nearby. The investigation was designed with a focus on the wind energy yield of a roof-mounted small wind turbine but the findings are also relevant for air dispersion applications. Wind speed profiles above roof top are simulated with the complex fluid dynamics (CFD) model MISKAM (Mikroskaliges Klima- und Ausbreitungsmodell, microscale climate and dispersion model). The comparison to mast measurements reveals that the model underestimates the wind speeds within the first few meters above the roof, but successfully reproduces wind conditions at 10 m above the roof top (corresponding to about 0.5 times the building height). Scenario simulations with different building configurations at the adjacent property result in an increase or decrease of wind speed above roof top depending on the flow direction at the upper boundary of the urban canopy layer (UCL). The maximum increase or decrease in wind speed caused by the alternations in building structure nearby is found to be in the order of 10%. For the energy yield of a roof-mounted small wind turbine at this site, wind speed changes of this magnitude are negligible due to the generally low prevailing wind speeds of about 3.5 m s−1. Nevertheless, wind speed changes of this order could be significant for wind energy yield in urban areas with higher mean wind speeds. This effect in any case needs to be considered in siting and conducting an urban meteorological monitoring network in order to ensure the homogeneity of observed time-series and may alter the emission and dispersion of pollutants or odor at roof level.
Highlights
The flow distortion by obstacles applies at all scales of concern, including the effects of local relief due to hills, valleys and cliffs, sharp changes in roughness or in the effective surface elevation, perturbation of flow around trees and buildings and even disturbance induced by the physical bulk of the tower or the mast, on to which the instruments are mounted
The results revealed that depending on the flow direction at 100 m above ground level (AGL), wind speeds above the roof top might increase or decrease when the building structures change in the vicinity of the site
Considering the operation of a small wind turbine at this site, these findings indicate that under the respective flow direction, 6% more wind power could be expected with scenario B and J, while the other scenarios led to a possible reduction of the energy yield to about a seventh of the energy yield
Summary
The flow distortion by obstacles applies at all scales of concern, including the effects of local relief due to hills, valleys and cliffs, sharp changes in roughness or in the effective surface elevation, perturbation of flow around trees and buildings and even disturbance induced by the physical bulk of the tower or the mast, on to which the instruments are mounted. Poor placement of wind sensors in urban areas leads to potentially erroneous calculations of air pollution modeling [1]. Atmosphere 2020, 11, 87 ground in a densely built-up area with average obstacle height zH. Wind speed gradients in the urban canopy layer (UCL) are small until quite close to the surface. Under neutral conditions, which are most of the time present within the UCL (e.g., [4,5,6],), the vertical profile of the wind speed can be approximated by the following exponential form merging with the log profile near roof-level [7]
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