Wind tunnel study of turbulent flow past an urban canyon model

Modeling dispersion in urban area requires appropriate input parameters, in particular aerodynamic roughness parameters. A low-speed wind tunnel was deployed to study flow patterns over an urban canyon model with three aspect ratios and three flow speeds of 2, 5, and 10 m/s with the objective of obtaining these parameters. Flow speed, standard deviation, and turbulence intensity profiles were determined with a single directional hot-wire anemometer at several positions across the urban canyon model. The aerodynamic parameters , , and were obtained from flow speed profile via a non-linear fit after a suitable choice of the initial value of for which all aerodynamic parameters converge. Flow speed and standard deviation profiles do not change significantly with the position across the canyon, but are much affected by the free flow speed. The regular way they respond to the free flow speed suggested a normalization for which all profiles collapse onto a single profile, which depends only on the canyon aspect ratio. The normalization criterion revealed to be important for obtaining convergent dimensionless profiles. To describe the general profiles characteristics a simple new parameterization is proposed, in which a single-valued function (Gaussian curve) describing the flow speed profile is used in a flux-gradient relationship for describing the standard deviation profiles. This parameterization works well down to 0.25 -0.50.