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| Titel |
A Theory on the Ventilation over Hypothetical Urban Areas |
| VerfasserIn |
Chun-Ho Liu, Chi-To Ng, Colman C. C. Wong |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250073061
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| Zusammenfassung |
| Urban roughness is one of the major factors affecting the flows and turbulence structures in
the bottom of the atmospheric boundary layer (ABL). Whereas, our understanding of
their relation is limited. In this paper, we attempt to examine the interaction among
aerodynamic resistance (friction factor f), ventilation (air exchange rate ACH), and
pollutant removal (pollutant removal rate PCH). Using the method of characteristic,
analytical solution shows that the turbulent ventilation of a hypothetical urban area is
directly proportional to the square root of friction factor (ACH″- f1-2) regardless
of the building geometry. Next, a series of computational fluid dynamics (CFD)
sensitivity tests are performed to verify the theory. In addition to the commonly
employed rectangular building models, seven types of urban roughness elements, in
the form of idealized building models, are tested. As a pilot study, the building
models are of the same height so a roof level is easily defined across the entire
hypothetical urban areas. Two configurations of passive scalar sources, ground-level-only
(vehicular emission) and all-solid-boundary (heat dissipation), are employed to
contrast their transport behaviors. To look into the mechanism of ventilation and
pollutant removal, the ACH and PCH are partitioned into their respective mean and
turbulent components. The CFD results show that both the ventilation and pollutant
removal are mainly attributed to their turbulent components (over 60%). Moreover,
the ACH″ and f1-2 calculation from the CFD results agree very well with the
analytical solution (correlation coefficient over 0.9). However, the pollutant and heat
exhibit different removal behaviors so simple estimates using friction factor have not
yet arrived. Because of the substantial aged air removal by ACH″ and its linear
relation with f1-2, it is proposed to use friction factor, which can be determined
by wind tunnel experiments or mathematical modeling, as a parameterization for
the (minimum) ventilation of urban areas. Additional tests for urban roughness
elements of other geometries and non-uniform height are currently undertaken. |
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