A wind tunnel study of effects of twisted wind flows on the pedestrian-level wind field in an urban environment.

The influence of twisted wind flows on the pedestrian-level wind (PLW) field of an urban area was evaluated by testing a typical urban site (Tsuen Wan, Hong Kong) in a boundary layer wind tunnel. Four twisted wind profiles with different magnitudes and directions of yaw angles were employed to investigate variations in wind speed with the properties of the twisted wind flows at the pedestrian level. An additional conventional wind profile with similar wind speeds and turbulence intensities to the twisted winds but with zero yaw angles was simulated for comparisons. The mean wind speeds at 77 locations including the perimeter, roadsides, and groups of high-rise buildings were analysed for the conventional and the four twisted wind flows. The comparisons show a tendency of twisted winds to generate higher wind speeds at the pedestrian level than the conventional wind profile. The wind speeds of the twisted winds have a strong dependence on the magnitude and direction of the yaw angles, particularly at locations where the densities of buildings in the neighbourhood are low and hence local wind circulations are significantly modified by the twisted winds.

Integrating twisted wind profiles to Air Ventilation Assessment (AVA): The current status.

Twisted wind flows generated by the complex terrain of Hong Kong induce two types of complication to Air Ventilation Assessment (AVA), first, imposing a false boundary condition on the wind tunnel tests done for AVA and, second, creating an ambiguity in determining the approaching wind direction in calculating the probability of occurrence of winds. The latter issue is partially solved using correction methods in post-analysis of AVA but the accuracy of these methods is not yet accessed. This study employs two twisted wind profiles to test an urban area in a boundary layer wind tunnel to investigate the influence of twisted wind flows on the outcomes of AVA and to estimate the accuracy of three common correction methods: No-Shift, Threshold, and Proportional methods. The results reveal significant differences in wind speeds at the pedestrian level for twisted and conventional wind flows at locations with low building densities. The discrepancies in wind speeds are minimum at the locations where the density of buildings is high. The indicators calculated by the No-Shift method frequently deviate from those of the twisted wind flows, while the Threshold and Proportional methods routinely over-predict the indicators of AVA.

New inflow boundary conditions for modeling twisted wind profiles in CFD simulation for evaluating the pedestrian-level wind field near an isolated building.

The hilly topography of Hong Kong influences oncoming winds and gradually changes their wind directions along the profiles' height. The vertical variation in wind directions, or the twist effect, significantly influences the Pedestrian Level Wind (PLW) field in urban areas of Hong Kong, thus it is a topic demanding systematic investigations. In this study, a new set of inflow boundary conditions are proposed to model twisted wind flows in Computational Fluid Dynamic (CFD) simulations. The new inflow boundary condition derived based on the horizontal homogeneous assumption, specifies a vertical profile of lateral wind speeds at the inlet boundary to sustain the twist effect in the empty computational domain. The proposed boundary conditions are used to simulate the PLW fields near three isolated buildings with different Height-to-Width ratio using two CFD codes; OpenFOAM, and FLUENT. The results reveal that OpenFOAM is more reliable in simulating PLW fields in twisted wind flows using the new set of boundary conditions. The three-dimensional flow field provided by the OpenFOAM simulation shows sparse streamlines downstream the buildings, indicating lack of organized eddies in the building far wake, which negatively affects the dispersion of air pollutants in twisted winds.

Evaluation of pedestrian wind comfort near 'lift-up' buildings with different aspect ratios and central core modifications.

Owing to the void space at lower heights, lift-up buildings have high building permeability at ground level and subsequently improve the air circulation in congested urban areas. Despite this advantage, the lift-up design has been sparsely adopted for buildings in urban areas partly because of the lack of understanding of the combined effects of building dimensions and lift-up design on the surrounding pedestrian level wind (PLW) field. Therefore, this study aims to investigate the influence of lift-up buildings with different aspect ratios (height/width) on the surrounding PLW field and pedestrian wind comfort level. Five lift-up buildings with aspect ratios 4:1 to 0.5:1 were tested in a boundary layer wind tunnel and results were compared with those of five buildings with similar dimensions but without lift-up design. The results reveal a strong dependence of the maximum wind speed in lift-up areas with building height, which results subsequently a small area of acceptable wind conditions near tall and slender lift-up buildings. Lift-up designs adopted for short and wide buildings produce larger areas of pedestrian wind comfort. The central cores modified with corner modifications are effective in increasing the pedestrian wind comfort in the lift-up area of tall and slender buildings.

Adopting 'lift-up' building design to improve the surrounding pedestrian-level wind environment.

Modern megacities are teeming with closely-spaced tall buildings, which limit air circulation at the pedestrian level. The resultant lack of air circulation creates poorly ventilated areas with accumulated air pollutants and thermal discomfort in the summer. To improve air circulation at the pedestrian level, buildings may be designed to have a 'lift-up' shape, in which the main structure is supported by a central core, columns or shear walls. However, a lack of knowledge on the influence of the 'lift-up' design on the surrounding wind environment limits the use of 'lift-up' buildings. This study aims to investigate the influence of 'lift-up' buildings and their dimensions on the pedestrian-level wind environments using wind tunnel tests. A parametric study was undertaken by using 9 'lift-up' building models with different core heights and widths. The results were compared with the surrounding wind environment of a control building with similar dimensions. The results reveal that the 'lift-up' core height is the most influential parameter and governs the area and magnitude of high and low wind speed zones around such buildings. Based on wind tunnel test results and a selected comfort criterion, appropriate core dimensions could be selected to have acceptable wind conditions near lift-up buildings.