Characteristics of pedestrian level wind environment in twisted wind flow around isolated buildings

![CDATA[Air Ventilation Assessment (AVA) is the major investigation method to evaluate the impact of new development on existing pedestrian level wind environment in Hong Kong. The different stages of AVA employs wind tunnel tests to evaluate the approaching wind conditions and wind speed at the pedestrian level at the site of interest. However, the hilly terrain commonly found in Hong Kong induces uncertainties in AVA wind tunnel tests by generating twisted wind profiles. The twisted wind effect induced by the topography can be significant in assessing near ground wind environment. Therefore, influences of twisted wind profiles on the pedestrian-level wind environment were evaluated through a series of wind tunnel tests. Two twisted wind profiles, with a maximum twist angle of 13o and 22o were simulated in a boundary-layer wind tunnel. A conventional wind profile with zero twist was employed to repeat the wind tunnel test as a control case. More than 200 Irwin sensors were installed around five isolated building models to measure the wind speeds at pedestrian level at 10mm in model scale which equals to 2m in full scale. The dimensions of building models were selected to evaluate the influence of building height and building width on pedestrian level in presence of the twisted wind profiles. It was found that under the twisted wind profiles building far wake deviated from the along-wind axis and generated asymmetric corner streams in sides of the buildings. The wind environments generated by different wind profiles were quantitatively evaluated by using the velocity ratio (VR). By comparing the VR distributions under the influence of the two twisted profiles to the VR distributions in the conventional wind profile, it is observed that (a) the over-speed area (VR>1.3) decreases and (b) the shelter area (VR<0.7) increases in the pedestrian-level wind field under the influence of the twisted profiles.]]