Street-Level Ventilation in Hypothetical Urban Areas
Street-level ventilation is often weakened by the surrounding high-rise buildings. A thorough understanding of the flows and turbulence over urban areas assists in improving urban air quality as well as effectuating environmental management. In this paper, reduced-scale physical modeling in a wind t...
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| Format: | Article |
| Language: | English |
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MDPI AG
2017-07-01
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| Series: | Atmosphere |
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| Online Access: | https://www.mdpi.com/2073-4433/8/7/124 |
| _version_ | 1819298753238007808 |
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| author | Yat-Kiu Ho Chun-Ho Liu |
| author_facet | Yat-Kiu Ho Chun-Ho Liu |
| author_sort | Yat-Kiu Ho |
| collection | DOAJ |
| description | Street-level ventilation is often weakened by the surrounding high-rise buildings. A thorough understanding of the flows and turbulence over urban areas assists in improving urban air quality as well as effectuating environmental management. In this paper, reduced-scale physical modeling in a wind tunnel is employed to examine the dynamics in hypothetical urban areas in the form of identical surface-mounted ribs in crossflows (two-dimensional scenarios) to enrich our fundamental understanding of the street-level ventilation mechanism. We critically compare the flow behaviors over rough surfaces with different aerodynamic resistance. It is found that the friction velocity u τ is appropriate for scaling the dynamics in the near-wall region but not the outer layer. The different freestream wind speeds ( U ∞ ) over rough surfaces suggest that the drag coefficient C d (= 2 u τ 2 / U ∞ 2 ) is able to characterize the turbulent transport processes over hypothetical urban areas. Linear regression shows that street-level ventilation, which is dominated by the turbulent component of the air change rate (ACH), is proportional to the square root of drag coefficient ACH ″ ∝ C d 1 / 2 . This conceptual framework is then extended to formulate a new indicator, the vertical fluctuating velocity scale in the roughness sublayer (RSL) w ^ RSL ″ , for breathability assessment over urban areas with diversified building height. Quadrant analyses and frequency spectra demonstrate that the turbulence is more inhomogeneous and the scales of vertical turbulence intensity w ″ w ″ ¯ 1 / 2 are larger over rougher surfaces, resulting in more efficient street-level ventilation. |
| first_indexed | 2024-12-24T05:34:54Z |
| format | Article |
| id | doaj.art-4c933586e5fc4aca8834a4e06ee0257e |
| institution | Directory Open Access Journal |
| issn | 2073-4433 |
| language | English |
| last_indexed | 2024-12-24T05:34:54Z |
| publishDate | 2017-07-01 |
| publisher | MDPI AG |
| record_format | Article |
| series | Atmosphere |
| spelling | doaj.art-4c933586e5fc4aca8834a4e06ee0257e2022-12-21T17:13:02ZengMDPI AGAtmosphere2073-44332017-07-018712410.3390/atmos8070124atmos8070124Street-Level Ventilation in Hypothetical Urban AreasYat-Kiu Ho0Chun-Ho Liu1Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, ChinaDepartment of Mechanical Engineering, The University of Hong Kong, Hong Kong, ChinaStreet-level ventilation is often weakened by the surrounding high-rise buildings. A thorough understanding of the flows and turbulence over urban areas assists in improving urban air quality as well as effectuating environmental management. In this paper, reduced-scale physical modeling in a wind tunnel is employed to examine the dynamics in hypothetical urban areas in the form of identical surface-mounted ribs in crossflows (two-dimensional scenarios) to enrich our fundamental understanding of the street-level ventilation mechanism. We critically compare the flow behaviors over rough surfaces with different aerodynamic resistance. It is found that the friction velocity u τ is appropriate for scaling the dynamics in the near-wall region but not the outer layer. The different freestream wind speeds ( U ∞ ) over rough surfaces suggest that the drag coefficient C d (= 2 u τ 2 / U ∞ 2 ) is able to characterize the turbulent transport processes over hypothetical urban areas. Linear regression shows that street-level ventilation, which is dominated by the turbulent component of the air change rate (ACH), is proportional to the square root of drag coefficient ACH ″ ∝ C d 1 / 2 . This conceptual framework is then extended to formulate a new indicator, the vertical fluctuating velocity scale in the roughness sublayer (RSL) w ^ RSL ″ , for breathability assessment over urban areas with diversified building height. Quadrant analyses and frequency spectra demonstrate that the turbulence is more inhomogeneous and the scales of vertical turbulence intensity w ″ w ″ ¯ 1 / 2 are larger over rougher surfaces, resulting in more efficient street-level ventilation.https://www.mdpi.com/2073-4433/8/7/124air change rate (ACH)flow and turbulence profileshypothetical urban areasstreet-level ventilationventilation assessmentwind-tunnel dataset |
| spellingShingle | Yat-Kiu Ho Chun-Ho Liu Street-Level Ventilation in Hypothetical Urban Areas Atmosphere air change rate (ACH) flow and turbulence profiles hypothetical urban areas street-level ventilation ventilation assessment wind-tunnel dataset |
| title | Street-Level Ventilation in Hypothetical Urban Areas |
| title_full | Street-Level Ventilation in Hypothetical Urban Areas |
| title_fullStr | Street-Level Ventilation in Hypothetical Urban Areas |
| title_full_unstemmed | Street-Level Ventilation in Hypothetical Urban Areas |
| title_short | Street-Level Ventilation in Hypothetical Urban Areas |
| title_sort | street level ventilation in hypothetical urban areas |
| topic | air change rate (ACH) flow and turbulence profiles hypothetical urban areas street-level ventilation ventilation assessment wind-tunnel dataset |
| url | https://www.mdpi.com/2073-4433/8/7/124 |
| work_keys_str_mv | AT yatkiuho streetlevelventilationinhypotheticalurbanareas AT chunholiu streetlevelventilationinhypotheticalurbanareas |