| Most pollution releases in urban areas occur at or near ground level and therefore well within the building canopy. In order to study the dispersion of these emissions, it is necessary to know the wind velocity profile inside the canopy of large groups of buildings or obstacles.; An ultra-low-speed small wind tunnel was set up to investigate the velocity field close to regularly arrayed rough surfaces. The wind tunnel speed ranged from zero to about 4 m/s. The wind tunnel experiments investigated the velocity profiles obtained for two kinds of roughness arrays: billboard obstacles ( lambdaf = 0.0494), and cube obstacles (lambda f = 0.4444). The velocities were measured with a Laser Doppler Velocimetry (LDV). The logarithmic law was used to predict the friction velocity (u*), roughness length (z0) and displacement height (d) of the roughness arrays.; The results were found to be consistent with previous water tunnel experimental results. The finite element model (FEM3A) was used to simulate the flow around the roughness arrays, and the simulation results were consistent with the experimental data for the billboard arrays. The present experiments established the use of LDV as a measurement technique for wind tunnel canopy flows and provided a useful database for CFD modelers. |
