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| Titel |
Adjustment of roughness sublayer in turbulent flows over two-dimensional idealised roughness elements |
| VerfasserIn |
Yat-Kiu Ho, Chun-Ho Liu |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105532
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| Publikation (Nr.) |
 EGU/EGU2015-5064.pdf |
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| Zusammenfassung |
| The atmospheric boundary layer (ABL) immediately above the urban canopy is the roughness
sublayer (RSL). In this layer, flows and turbulence are strongly affected by the roughness
elements beneath, e.g. building obstacles. The wind flows over urban areas could be
represented by conventional logarithmic law of the wall (log-law) in the neutrally stratified
ABL. However, in the RSL region, the vertical wind profile deviates from that predicted from
log-law and the effect could be extended from ground level up to several canopy heights. As a
result, the Monin-Obukhov similarity theory (MOST) fails and an additional length scale is
required to describe the flows.
The key aim of this study is to introduce a simple wind profile model which accounts for
the effect of the RSL in neutral stratification using wind tunnel experiments. Profile
measurements of wind speeds and turbulence quantities over various two-dimensional (2D)
idealised roughness elements are carried out in an open-circuit wind tunnel with test section
of size 560 mm (width) x 560 mm (height) x 6 m (length). The separation between the
roughness elements is varied systematically so that ten different types of surface forms are
adopted. The velocity measurements are obtained by hot-wire anemometry using X-probe
design (for UW- measurements) with a constant temperature anemometer. For each
configuration, eight vertical profiles are collected over the canopy, including solid boundaries
and cavities of the roughness elements.
Firstly, we compute the measurement results using conventional MOST to determine
different roughness parameters. Afterwards, we derive the RSL height from the Reynolds
stress profiles. Since the profiles taken from different locations of the canopy are eventually
converged with increasing height, we use this “congregated height” to define the RSL height.
Next, we introduce an alternative function, i.e. power-law function, instead of MOST, to
describe the velocity profile in attempt to account for the RSL effect. Lastly, the RSL
effect on turbulent behaviours over different roughness configurations is quantified. |
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