Sensitivity of turbulent flow around a 3-D building array to urban boundary-layer stability

The mean flow and turbulence within a regular building array are analysed for a wide range of bulk Richardson numbers, Rb, using large-eddy simulation. Time-averaged streamlines in the vertical plane reveal that skimming flow persists for stable and neutral conditions; for unstable conditions, however, updrafts dominate and the counter-rotating vortex pairs in the horizontal plane are displaced towards the leeward wall. The turbulent kinetic energy, magnitude of the turbulent shear stress and mean streamwise flow speed show a sharp transition at Rb = 0. Quadrant analysis, in particular the fractional stress ratio, indicates that small fluctuations are generated most effectively by neutral flow and large ones by unstable flow. Identical-twin experiments confirm that the effects of stratification are large (pointwise errors in the relative kinetic energy are up to a factor of 10 larger for unstable flow). Knowledge of the flow-regime transition at Rb = 0 is relevant to urban modelling and parameterisation.