Parameters characterising dispersion in the near wake of buildings

Abstract

This paper describes wind-tunnel measurements of near-wake parameters for many different building shapes in a variety of boundary-layer flows. The particular parameters measured were the distance from the rear of the building to the point of reattachment of the separated shear layers (the recirculation-region length), and the time for the concentration of an aerosol in the near wake to decay by 1/ e (the residence time). As well as being of interest in themselves, it is shown in the paper how these parameters may be used to estimate the average near-wake concentration. The recirculation-region length was measured using a twin-tube pressure probe and on the basis of flow visualisation with a smoke-tracer. Flow visualisation was also used to deduce whether or not reattachment onto the roof of a building occurred. The residence time was obtained using a laser system to measure the concentration of repeated releases of a DOP aerosol. Expressions, derived from the data, are given for estimating the two parameters considered for any rectangular building shape. A few cases were also examined with two buildings and variable downstream spacing. The effect of reducing the spacing was to decrease the residence time. Finally, some conclusions are drawn regarding the dependence of the measured parameters on onset-flow conditions.