Effects of aspect ratio on flow over arrays of tall buildings

The study in this report investigates the aspect ratio effects on the flow over a staggered array of random height buildings with two height-width aspect ratios ( A R ) of 1 and 4. Full-scale large-eddy simulations (LES) were carried out for up to 144 rows of buildings using up to 1 . 4 × 1 0 8 cells. This study employed a 1.5-order Deardorff closure model to parameterise subgrid-scale turbulent stresses. Both fully developed and developing flows were simulated using respectively cyclic boundary conditions and synthetic turbulence inflow boundary conditions. Fully developed flows over A R = 1 buildings were compared to published experimental and LES data at a wind tunnel scale. The findings are (1) the flow pattern over the A R = 1 buildings is a wake interference regime, whereas it is a skimming region over the A R = 4 buildings. (2) The dispersive stresses normalised by the effective skin friction velocity below the mean height for A R = 4 buildings are substantially lower in magnitude than for A R = 1 buildings. (3) Above the mean height, the ejection motion of mean velocity variations is much stronger over the sweeping motions for the A R = 4 buildings than for A R = 1 buildings. (4) The integral length scales of the turbulence generated by the A R = 4 buildings are approximately twice those respectively for the A R = 1 buildings. This shows the significant effects of the aspect ratio of tall buildings. • Flow over A R = 1 and A R = 4 buildings are categorised into different flow regimes. • Dispersive stresses below the mean height for A R = 4 buildings is significantly lower. • Integral length scales generated by A R = 4 building arrays are twice A R = 1 ’s.