Field and wind tunnel study of the flow and diffusion around a model cube—II. Nearfield and cube surface flow and concentration patterns

Abstract

Part II of a study on the flow and diffusion around a cube considered the concentration and flow patterns on and around the cube. Nonbuoyant tracer gas was released from the center of the cube roof at low exhaust velocity. The concentration patterns were shown to be strongly dependent upon the flow patterns, and in particular, on the existence of reverse flow at the source location. In the field, vane measurement of the flow showed that there was reverse flow at the rooftop center when the wind angle was small. As upwind turbulence intensity increased, the wind angle at which reverse flow occurred decreased. Modeled in the wind tunnel, this configuration was investigated for five different wind angles ( θ = 0–45°) and four different upwind roughnesses (very smooth to very rough), and similar tendencies were found. Reverse flow at the source position resulted in high concentrations at the leading edge of the cube for the case of the smooth upwind surface (low upwind turbulence intensity) and θ = 0°. For the very rough surface and θ = 0°, there was no reverse flow at the source location and so the maximum rooftop concentrations occurred downwind of the source. The effect of increasing upwind turbulence was a decrease in the measured concentrations C ∗ . In terms of concentration similarity for the downdraft phenomenon, it was found that the wind tunnel model tended to overestimate the concentration found on the roof and may therefore be applied as a conservative indicator of the prototype. However, the same similarity criteria showed that, in addition to simulating L c / H (see Part I), the location and value of the maximum ground level concentration was in reasonably good agreement in the field and wind tunnel.