PHYSICAL MODELLING OF URBAN ROUGHNESS USING ARRAYS OF REGULAR ROUGHNESS ELEMENTS

摘要

The aerodynamic behaviour of large urban agglomerations must be represented in increasingly greater detail, as large-scale numerical weather prediction and air pollution dispersion models are refined. The present study provides detailed measurements of the flow field in regular arrays of obstacles to obtain representative data on mean flow and turbulence statistics in urban-type areas. Obstacle arrays consisting of simple cubes and flat plate roughness commonly used in boundary layer simulations were placed in a simulated atmospheric boundary layer flow in a hydraulic flume. The scale factor was about 1:200 based on the obstacle height (50 mm). The results show no appreciable 'constant stress' region in the internal boundary layer above the buildings, since in any finite-length test array the boundary layer is always developing. However, if the RMS turbulence components are scaled by the local values of the shear stress, then there seems to be a universal scaling, with σ_u/u_* = 2.1, σ_υ/u_* = 1.65 and σ_w/u_* = 1.2. This greatly simplifies the parameterization of the first order turbulence statistics in obstacle arrays. It was also observed during the experiments that, compared to results in the cube arrays, the turbulence kinetic energy and the Reynolds stresses were almost doubled in the flat plate roughness arrays.