Wind tunnel study of turbulent flow past an urban canyon model

Abstract

Modeling dispersion in urban area requires appropriate input parameters, in par- ticular 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 10m/s with the objective of obtaining these parameters. Flow speed, standard devi- ation, and turbulence intensity profiles were determined with a single directional hot-wire anemometeratseveralpositionsacrosstheurbancanyonmodel.Theaerodynamicparameters u∗, z0,andd0 wereobtainedfromflowspeedprofileviaanon-linearfitafterasuitablechoice of the initial value of d0 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 dependsonlyonthecanyonaspectratio.Thenormalizationcriterionrevealedtobeimportant for obtaining convergent dimensionless profiles. To describe the general profiles characteris- tics 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 z/ h ∼ 0.25 -0.50.