Building Height Ratio Research Articles

Aerodynamic effects of trees on pollutant concentration in street canyons

This paper deals with aerodynamic effects of avenue-like tree planting on flow and traffic-originated pollutant dispersion in urban street canyons by means of wind tunnel experiments and numerical simulations. Several parameters affecting pedestrian level concentration are investigated, namely plant morphology, positioning and arrangement. We extend our previous work in this novel aspect of research to new configurations which comprise tree planting of different crown porosity and stand density, planted in two rows within a canyon of street width to building height ratio W/ H = 2 with perpendicular approaching wind. Sulfur hexafluoride was used as tracer gas to model the traffic emissions. Complementary to wind tunnel experiments, 3D numerical simulations were performed with the Computational Fluid Dynamics (CFD) code FLUENT™ using a Reynolds Stress turbulence closure for flow and the advection–diffusion method for concentration calculations. In the presence of trees, both measurements and simulations showed considerable larger pollutant concentrations near the leeward wall and slightly lower concentrations near the windward wall in comparison with the tree-less case. Tree stand density and crown porosity were found to be of minor importance in affecting pollutant concentration. On the other hand, the analysis indicated that W/ H is a more crucial parameter. The larger the value of W/ H the smaller is the effect of trees on pedestrian level concentration regardless of tree morphology and arrangement. A preliminary analysis of approaching flow velocities showed that at low wind speed the effect of trees on concentrations is worst than at higher speed. The investigations carried out in this work allowed us to set up an appropriate CFD modelling methodology for the study of the aerodynamic effects of tree planting in street canyons. The results obtained can be used by city planners for the design of tree planting in the urban environment with regard to air quality issues.

Drift design model for high‐rise buildings based on resizing algorithm with a weight control factor

AbstractIt is well known that the weight of the structural system for a high‐rise building to resist lateral loads increases in nonlinear fashion with increase in building height and slenderness ratio. For a given building height or slenderness ratio, the weight of the structural system for a high‐rise building subjected to lateral loads depends on the degree of stiffness of the building structure. Therefore, at the final stage of the structural design process, it has been a challenge to structural engineers to improve the efficiency of a structural system by either minimizing the weight of a structure for a given limit on lateral displacement or maximizing the degree of stiffness for a given weight of a structure. In this paper, a resizing design method that can minimize the magnitudes of lateral displacement and inter‐story drift of a high‐rise building with respect to structural weights is presented. Based on the proposed design method, the relation between the structural weight and the magnitude of lateral displacement is investigated using two examples, including a 60‐story high‐rise building. Copyright © 2007 John Wiley & Sons, Ltd.

Strategic guidelines for street canyon geometry to achieve sustainable street air quality—part II: multiple canopies and canyons

Abstract The flow field and pollutant dispersion characteristics in a three-dimensional urban street canyon are investigated for various building array geometries. The street canyon in consideration is located in a multi-canopy building array that is similar to realistic estate situations. The pollutant dispersion characteristics are studied for various canopy aspect ratios, namely: the canyon height to width ratio, canyon length to height ratio, canyon breadth ratio and crossroad locations are studied. A three-dimensional field-size canyon has been analysed through numerical simulations using k − e turbulence model. As expected, the wind flow and mode of pollutant dispersion is strongly dependent on the various flow geometric configurations and that the results can be different from that of a single canyon system. For example, it is found that the pollutant retention value is minimum when the canyon height-to-width ratio is approximately 0.8, or that the building height ratio is 0.5. Various rules of thumbs on urban canyon geometry have been established for good pollutant dispersion.

The Effect of the Urban Street Canyon on the Noise from Low Flying Aircraft

The effect of urban streets on the noise from a low-flying aircraft has been investigated. The study first involved the development of a prediction method based on geometrical acoustics with the assumption of specular reflections. The effect of a variety of street configurations on the transmission gain was investigated and analysed in terms of changes in street geometry. The relative Effective Perceived Noise Level was also predicted. The predictions show that for specular reflections the relative Effective Perceived Noise Level increases in proportion to the ratio of building height to flying altitude, however, changing the street width has only a minor effect. The effect of mixed specular and diffuse reflections was investigated using a commercial acoustical modelling package and it was shown that although the effect on the transmission gain was small, the scattering of sound energy out of the street channel resulted in a decrease in the value of the relative Effective Perceived Noise Level with the ratio of building height to flying altitude resulting in a value of the relative Effective Perceived Noise Level which was negative for most street configurations examined. It can be concluded that although simple specular models can predict the transmission gain with reasonable accuracy, more complex models involving the effects of diffuse reflections are required for noise units involving integration over time.

Analysis of velocity fields and dispersive cavity parameters as a function of building width to building height ratio using a 3-D computer model for squat buildings

The objectives of this work were to show that a three-dimensional turbulent kinetic energy/dissipation ( k– ε) computational model, FLUENT, can be used to facilitate modeling of fluid flow fields with stack geometry generated by a variety of building shapes, and to use the data sets to develop parameterizations useful to air quality modeling needs. Once the usefulness of the computational model was proven through comparisons with experimental results, and data generated for several ratios of building width to building heights, the flow field was examined to determine the length of the recirculation cavity as a function of the ratio of building width to building height both in front of and in the rear of the building. The dimensions of the recirculation cavity in the front of the building have previously not been included in regulatory models, so both the height and length of this front recirculation cavity were parameterized as a function of the ratio of building width to building height. The maximum of value downdraft was also parameterized as a function of the building width to building height ratio. Although FLUENT is less than a perfect match for all measured parameters in all parts of the flow field, it does reveal the areas of greatest variability (and hence interest) and will be a useful way to efficiently take the limited measurements for development and confirmation of future model improvements.

An urban canyon energy budget model and its application to urban storage heat flux modeling

To obtain a local-scale urban energy balance by either measurement or modeling it is necessary to determine storage heat flux (Δ Q s). This flux cannot be measured directly due to the complexity of the urban surface. The Grimmond et al. Objective Hysteresis Model (OHM) [C.S.B. Grimmond, H.A. Cleugh, T.R. Oke, An objective urban heat storage model and its comparison with other schemes, Atmos. Environ., 25B (1991) 311–326] of local-scale ΔQ s combines empirical equations for individual surface types in the proportion that they are present within the urban area. One surface type for which there is very limited field data is the urban canyon, which consists of the walls of adjacent buildings, the horizontal street-level area separating them (roadways, gardens, parking lots, etc.) and the enclosed air volume. Here the storage heat flux of an urban canyon and the resulting OHM parameters are investigated with a numerical model of a dry urban canyon energy budget. Substrate heat fluxes are derived from simulated surface and substrate temperatures; the latter evolve through time according to the finite difference form of the Fourier heat conduction equation. When compared against measured fluxes, the model performed satisfactorily. Numerical experiments show significant effects on the OHM parameters due to changes in the ratio of building height to separation distance and building wall thermal properties. Effects of intermediate significance were attributable to canyon orientation, wind speed and the timing of the between-building air temperature regime. Air temperature and the timing of the wind speed curve showed only minor significance.

A fluid modeling study of concentration distributions at urban intersections

Concentration measurements at urban intersections are poorly understood, yet are very important because of the air pollution implications on human health and regulatory decision-making. In this study, we have modelled concentrations at an urban intersection and the nearby environment using an atmospheric boundary-layer wind tunnel. A regular array of uniform, low-rise rectangular urban blocks was studied using quantitative gas tracer techniques. In the model, avenues were about 50% wider than streets, while the ratio of building height to avenue width was 0.75. Pedestrian-level concentrations varied widely throughout the intersection and also with ambient wind direction. Emissions from sources in the street contribute more to intersection concentrations than do equivalent emissions in the avenue for nearly all wind directions. Surface concentrations are greatest for ambient winds parallel to the street ( θ = 90°), and roughly equal for the other wind directions studied (30°, 60° and 90°).

Drag and lift on rectangular bluff bodies immersed in deep boundary layer flow over rough surfaces

Measurements of drag and lift on three-dimensional rectangular bodies immersed in deep turbulent boundary layer flow over rough surfaces have been carried out in a boundary layer wind tunnel. The experiments described in this paper deal with the effects of building geometry, shear flow characteristics and angle of attack of approaching flow on wind load. A significant influence of building geometry was found. Furthermore, drag coefficients measured in boundary layers flows over two differently rough surfaces show differences up to 25%. For zero degree angle of attack the drag coefficient based on the shear velocity u ∗ is a function of the ratio of building height H to roughness length z o and building geometry. However, the use of u ∗ and z o as scaling parameters results in a power law expression with exponents and factors showing additional dependences on flow conditions and building geometry.

Influence of the ratio of building height to boundary layer thickness and of the approach flow velocity profile on the roof pressure distribution of cubical buildings

Influence of the ratio of building height to boundary layer thickness and of the approach flow velocity profile on the roof pressure distribution of cubical buildings

Influence of the ratio of building height to boundary layer thickness and of the approach flow velocity profile on the roof pressure distribution of cubical buildings

Influence of the ratio of building height to boundary layer thickness and of the approach flow velocity profile on the roof pressure distribution of cubical buildings

Influence of the ratio of building height to boundary layer thickness and of the approach flow velocity profile on the roof pressure distribution of cubical buildings

The mean pressure on the flat roofs of three different cubical buildings were determined in a boundary layer wind tunnel for wind flow parallel to the longer building axis. The approach flow characteristics, the building shape and the ratio of building height H to boundary layer thickness δ were varied in order to determine their influence on the modelling criteria. It is shown that for all H/ δ ≤ 1 the maximum pressure on the flat roof depends on the scaling parameters roughness length z o and shear velocity u ★ of the inner layer. The position of the point of maximum pressure depends only on the building shape and the approach flow. For each building shape it reaches a constant value for H/ δ > 0.8, whose dependence on the approach flow and the building aspect ratio is given.

Mean force and moment coefficients for buildings in turbulent boundary layers

Mean force and moment coefficients for a series of flat-roofed rectangular buildings are presented for a range of wind directions. The coefficients were determined by integrating mean pressures measured on model structures immersed in thick turbulent boundary layers simulating four typical neutral atmospheric flow conditions (power-law exponents of 0.12, 0.27, 0.34, and 0.38). A series of thirteen different building geometries were studied. The ratios of adjacent sides were 1, 0.5, 0.25, and the aspect ratio, the ratio of the height to the smaller side, ranged from 1 to 8. The effects of side ratio, aspect ratio, incident turbulence intensity, longitudinal integral scale, and the ratio of building height to boundary-layer thickness are discussed.