The Spanwise Variation of Roof-Level Turbulence in a Street-Canyon Flow

The effect of upstream roughness and canyon width on turbulent street-canyon flow is investigated, using wind-tunnel measurements made in a horizontal plane at near roof level of a street canyon and stereoscopic particle image velocimetry. Three upstream roughness arrays and two canyon width (W) to height (h) aspect ratios (AR=W/h=1 and 3) are used; the arrays consist of three-dimensional cubes (plan area density, (p)=25%), 1h-spaced two-dimensional bars (skimming flow, (p)=50%) and 3h-spaced two-dimensional bars (wake-interference flow, (p)=25%). Understanding the spanwise structure of the flow and how it interacts with large-scale structures is necessary to reliably predict the mean pollutant transport in the lateral direction along the canyon and to further investigate the three-dimensional behaviour of turbulent street-canyon flows. The mean turbulent statistics are presented, whilst two-point correlations and integral length scales are computed for the different configurations. The results show a significant effect of upstream roughness on these quantities. The total turbulent kinetic energy and shear stress are found to be highest for the wake-interference flow regimes and lowest for the skimming-flow regimes. It is found that the three-dimensional upstream roughness configurations result in a significantly weaker correlation in the spanwise direction at canyon roof level, with a similar trend observed in the spanwise integral length scales. The shear-layer thickness is found to be related to the magnitude of the correlations near roof level of the street canyon.