Wind tunnel modelling of flows over various urban-like surfaces using idealized roughness elements

Abstract

The flow characteristics over different types of hypothetical urban roughness surfaces were studied in a laboratory-scale wind tunnel in isothermal conditions. Reduced-scale street canyon models were used to formulate the urban environment. Preliminary experiments were conducted based on six types of idealized two-dimensional (2D) street canyon models with various building-height-to-street width (aspect) ratios (ARs), 1, 1/2, 1/4, 1/8, 1/10 and 1/12 in order to examine the effects of urban morphology on the urban flow structures. The flow fields were analysed and measured using an in-house made 90o hot-wire anemometry. The selected street canyon was located near the end of the streamwise domain and its roof level, i.e. the distance between the mid-line of the upstream and downstream buildings, was divided into eight segments. Measurements were recorded on the mid-plane of the spannwise domain along the vertical profile (from building roof level to the ceiling of wind tunnel) of the eight segments. All the data acquisition processes were controlled by the National Instruments (NI) data acquisition and motion control modules, NI 9239, CompactDAQ-9188, NI 7390 motion controller hardware and LabVIEW software. Flow characteristics including mean velocities, velocity fluctuations, mean air-exchange rate (ACH) and turbulent ACH over various street canyon models were calculated. Preliminary results show that the near-ground turbulence behaviour (within 2 to 5 times of the building height) is relatively sensitive to the changes in ARs. The wider the streets (decrease in AR), the higher the turbulence level was observed. A similar behaviour is also observed on the ventilation performance in which the ACH was increased with decreasing AR. Interestingly, a flatted peak ACH was observed around ARs = 1/8, 1/10 and 1/12. Besides, it is shown that the turbulent ACH dominates over the mean ACH suggesting that the ventilation over urban areas is mainly governed by atmospheric turbulence. These observations are in line with our previous large-eddy simulation (LES) results. Additional measurements on the flows and dispersions over building surfaces will be undertaken on a variety of ARs and building height variations to elucidate the complex transport and pollutant dispersion mechanism in urban areas.