Passive urban ventilation by combined buoyancy-driven slope flow and wall flow: Parametric CFD studies on idealized city models

Abstract This paper reports the results of a parametric CFD study on idealized city models to investigate the potential of slope flow in ventilating a city located in a mountainous region when the background synoptic wind is absent. Examples of such a city include Tokyo in Japan, Los Angeles and Phoenix in the US, and Hong Kong. Two types of buoyancy-driven flow are considered, i.e., slope flow from the mountain slope (katabatic wind at night and anabatic wind in the daytime), and wall flow due to heated/cooled urban surfaces. The combined buoyancy-driven flow system can serve the purpose of dispersing the accumulated urban air pollutants when the background wind is weak or absent. The microscopic picture of ventilation performance within the urban structures was evaluated in terms of air change rate (ACH) and age of air. The simulation results reveal that the slope flow plays an important role in ventilating the urban area, especially in calm conditions. Katabatic flow at night is conducive to mitigating the nocturnal urban heat island. In the present parametric study, the mountain slope angle and mountain height are assumed to be constant, and the changing variables are heating/cooling intensity and building height. For a typical mountain of 500 m inclined at an angle of 20° to the horizontal level, the interactive structure is very much dependent on the ratio of heating/cooling intensity as well as building height. When the building is lower than 60 m, the slope wind dominates. When the building is as high as 100 m, the contribution from the urban wall flow cannot be ignored. It is found that katabatic wind can be very beneficial to the thermal environment as well as air quality at the pedestrian level. The air change rate for the pedestrian volume can be as high as 300 ACH.

[1]  G. Baumbach,et al.  Experimental determination of the effect of mountain-valley breeze circulation on air pollution in the vicinity of Freiburg , 1999 .

[2]  Rex Britter,et al.  Simulations of pollutant dispersion within idealised urban-type geometries with CFD and integral models , 2007 .

[3]  Dennis Y.C. Leung,et al.  On the correlation of air and pollutant exchange for street canyons in combined wind-buoyancy-driven flow , 2009 .

[4]  J. Chan,et al.  An investigation of air-pollutant patterns under sea-land breezes during a severe air-pollution episode in Hong Kong , 2002 .

[5]  Fue-Sang Lien,et al.  Numerical modelling of the turbulent flow developing within and over a 3-d building array, part ii: a mathematical foundation for a distributed drag force approach , 2005 .

[6]  B. Nemery,et al.  The Meuse Valley fog of 1930: an air pollution disaster , 2001, The Lancet.

[7]  J.-F. Sini,et al.  Thermal Effects on the Airflow in a Street Canyon – Nantes'99 Experimental Results and Model Simulations , 2002 .

[8]  H. Savijärvi,et al.  Local Winds In A Valley City , 2001 .

[9]  Bodo Ruck,et al.  Influence of trees on the dispersion of pollutants in an urban street canyon - experimental investigation of the flow and concentration field , 2007 .

[10]  Dominique Derome,et al.  High-resolution CFD simulations for forced convective heat transfer coefficients at the facade of a low-rise building , 2009 .

[11]  S. Orszag,et al.  Renormalization group analysis of turbulence. I. Basic theory , 1986 .

[12]  Dennis Y.C. Leung,et al.  On the prediction of air and pollutant exchange rates in street canyons of different aspect ratios using large-eddy simulation , 2005 .

[13]  Mats Sandberg,et al.  City breathability and its link to pollutant concentration distribution within urban-like geometries , 2010 .

[14]  Kit Ming Lam,et al.  Recent progress in CFD modelling of wind field and pollutant transport in street canyons , 2006 .

[15]  Jan Carmeliet,et al.  CFD analysis of convective heat transfer at the surfaces of a cube immersed in a turbulent boundary layer , 2010 .

[16]  Jianlei Niu,et al.  CFD Study of the Thermal Environment around a Human Body: A Review , 2005 .

[17]  C K Chau,et al.  Estimating the total exposure to air pollutants for different population age groups in Hong Kong. , 2002, Environment international.

[18]  Rizwan Ahmed Memon,et al.  Effects of building aspect ratio and wind speed on air temperatures in urban-like street canyons , 2010 .

[19]  Lina Yang,et al.  City ventilation of Hong Kong at no-wind conditions , 2009 .

[20]  Marko Princevac,et al.  Quasi-Steady Katabatic Winds on Slopes in Wide Valleys: Hydraulic Theory and Observations , 2008 .

[21]  Efisio Solazzo,et al.  Modelling dispersion of traffic pollution in a deep street canyon: Application of CFD and operational models , 2009 .

[22]  Mats Sandberg,et al.  Wind conditions and ventilation in high-rise long street models , 2010 .

[23]  吉之 下田,et al.  地域計画への適用を目的とした冷気流のモデル化に関する検討 : 丘陵地周辺の住宅地域に及ぼす冷気流の影響 その2 , 2002 .

[24]  G. Gross,et al.  Some effects of deforestation on nocturnal drainage flow and local climate — A numerical study , 1987 .

[25]  Pietro Salizzoni,et al.  Flow in a Street Canyon for any External Wind Direction , 2008 .

[26]  Shin-Do Kim,et al.  Dispersion characteristics of vehicle emission in an urban street canyon. , 2004, The Science of the total environment.

[27]  M. Santamouris,et al.  Experimental investigation of air flow and temperature distribution in deep urban canyons for natural ventilation purposes , 2006 .

[28]  Dennis Y.C. Leung,et al.  Impact of building facades and ground heating on wind flow and pollutant transport in street canyons , 2007 .

[29]  Shuzo Murakami,et al.  CFD analysis of wind environment around a human body , 1999 .

[30]  Yuguo Li,et al.  Intake fraction of nonreactive motor vehicle exhaust in Hong Kong , 2010 .

[31]  S. Orszag,et al.  Renormalization group analysis of turbulence. I. Basic theory , 1986, Physical review letters.

[32]  S. Lau,et al.  Daytime urban heat island effect in high-rise and high-density residential developments in Hong Kong , 2004 .

[33]  Marcelo Horacio Garcia,et al.  Hydraulic Jumps in Sediment‐Driven Bottom Currents , 1993 .

[34]  L. Nkemdirim.  Cold air drainage and temperature fields in an urban environment: A case study of topographical influence on climate , 1980 .

[35]  Mukesh Khare,et al.  Wind tunnel simulation studies on dispersion at urban street canyons and intersections—a review , 2005 .

[36]  M. Bahn,et al.  Eddy covariance measurements of carbon dioxide, latent and sensible energy fluxes above a meadow on a mountain slope , 2007, Boundary-layer meteorology.

[37]  T. Oke,et al.  Turbulent Heat Fluxes in Urban Areas: Observations and a Local-Scale Urban Meteorological Parameterization Scheme (LUMPS) , 2002 .

[38]  A. Martilli.  A Two-Dimensional Numerical Study of the Impact of a City on Atmospheric Circulation and Pollutant Dispersion in a Coastal Environment , 2003 .

[39]  H. Fernando.  Fluid Dynamics of Urban Atmospheres in Complex Terrain , 2010 .

[40]  J. Cermák.  Thermal effects on flow and dispersion over urban areas: Capabilities for prediction by physical modeling , 1996 .

[41]  Fue-Sang Lien,et al.  Numerical modelling of the turbulent flow developing within and over a 3-d building array, part iii: a istributed drag force approach, its implementation and application , 2005 .

[42]  P. Manins,et al.  A Model of Katabatic Winds , 1979 .

[43]  M. Wolfshtein.  The velocity and temperature distribution in one-dimensional flow with turbulence augmentation and pressure gradient , 1969 .

[44]  Mats Sandberg,et al.  Age of air and air exchange efficiency in idealized city models , 2009 .

[45]  Noah Nd,et al.  Letter: Meningococcal infections. , 1975 .

[46]  R. Britter,et al.  FLOW AND DISPERSION IN URBAN AREAS , 2003 .

[47]  Mats Sandberg,et al.  Effect of urban morphology on wind condition in idealized city models , 2009 .

[48]  W P D LOGAN,et al.  Mortality in the London fog incident, 1952. , 1953, Lancet.