Model experiments on the nature of air pollution transport near buildings

The paper describes wind tunnel experiments with rectangular blocks in uniform smooth and turbulent air flow to simulate the transport of windborne atmospheric pollutants near buildings. Dispersal is dominated by the flow in the areodynamic wakes of the blocks. Using light extinction apparatus, measurements were made of the time constant (t_d) of the decay of the amount of smoke in the separation bubble following abrupt removal of the smoke source, and of the bubble length (X). These quantities were expressed as H( Ut_d/S, where U is the air velocity and S the block dimension) and X/S respectively, found to be unique functions of the free-stream turbulence parameter, A( l_fK^1/2_f/SU, where l_f and k_f are the length scale and kinetic energy respectively of the free-stream turbulence), for cubic blocks in the ranges l_f/S ? 0.6 and k_f^1/2/U ?, 0.15. Visual observation of tracer smoke indicated the onset of re-attachment of the separated flow on the top and side faces of the cubes at relatively low levels of free-stream turbulence, coinciding with sharply varying behaviour in H and X/S. Measurements were also made of H and X/S for square-faced blocks of various lengthwise aspect ratios, and for cubic blocks at various angles of incidence to the flow.From consideration of the turbulent transport of entities of an atmospheric pollutant across the boundary of the wake bubble behind a building, an expression is derived for the mean concentration level that can build up inside the bubble for a source of known strength and location. It contains the two terms H and X/S which explicitly embody the complicated fluid mechanical properties of the near wake.