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| Titel |
Adjustment of a turbulent boundary layer flow to idealized urban surfaces: A large-eddy simulation study |
| VerfasserIn |
Wai Chi Cheng, Fernando Porté-Agel |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095116
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| Publikation (Nr.) |
 EGU/EGU2014-10559.pdf |
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| Zusammenfassung |
| Accurate prediction of atmospheric boundary layer (ABL) flow and its interaction with urban
surfaces is critical for understanding the transport of momentum and scalars within and above
cities. This, in turn, is essential for predicting the local climate and pollutant dispersion
patterns in urban areas. Large-eddy simulation (LES) explicitly resolves the large-scale
turbulent eddy motions and, therefore, can potentially provide improved understanding and
prediction of flows inside and above urban canopies. This study focuses on the validation and
the use of a recently-developed LES framework to simulate a turbulent boundary layer flow
through idealized urban canopies represented by uniform arrays of cubes. The LES
framework is first validated with wind tunnel experimental data. Good agreement between the
simulation results and the experimental data are found in the vertical and spanwise
profiles of mean velocities and velocity standard deviations at different streamwise
locations. Next, the model is used to simulate ABL flows over surface transitions from a
flat homogeneous terrain to aligned and staggered arrays of cubes with height h.
For both configurations, five different frontal area densities (λf), equal to 0.028,
0.063, 0.111, 0.174 and 0.250, are considered. Within the arrays, the flow is found
to adjust quickly and shows similar structure of the wake of the cubes after the
second row. Above the arrays, an internal boundary layer (IBL) is identified. No
significant difference in the depth of the IBL among different cases is observed.
The drag exerted by the cubes on the flow (Df) and the drag coefficients of the
cubes (Cd) are calculated explicitly using the LES results. For the downstream
cubes, Df is found to increases with decreasing density for both configurations,
and larger values of Cd are found for the cubes of staggered arrays than those of
the aligned arrays with the same λf. At a downstream location where the flow
immediately above the cube array is already adjusted to the surface, the spatially averaged
velocity is found to have a logarithmic profile for all the cases. The values of the
displacement height (d) are found to increase roughly from 0.65h to 0.9h as λf increases
from 0.028 to 0.25 for both configurations. For the aerodynamic roughness (z0), a
maximum value at λf=0.11 is observed for both configurations. For all the cube
densities tested, larger values of z0 are obtained for the staggered arrays than for the
aligned ones. The results of z0 are discussed and compared with existing theoretical
expressions proposed in the literature. The effective mixing length (lm) within and above
different cube arrays are also calculated using the LES results. A local maximum of
lm within the canopy is found in all the cases, with values ranging from 0.2h to
0.4h. These patterns are different from those used in existing urban canopy models. |
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