CFD modelling of unsteady cross ventilation flows using LES

Traditional methods using steady-state simulations with RANS models may predict adequate flow characteristics of cross ventilation, provided the selected turbulence model, boundary conditions and level of mesh resolution are appropriate. However, these methods are less satisfactory if we are more interested in the statistical description of fluctuating flow rate. This study used a LES method to investigate the fluctuating flow rate induced by wind with a CFD code capable of generating time-dependent and unsteady inflows. Simulations were carried out for two wind directions: normal to the openings (case 1) and parallel to the openings (case 2). The results were in good agreements with experimental data. For case 1, the observed flow rate was relatively steady, whereas for case 2, it exhibited very unsteady phenomena. Observations indicated that the flow pattern around the building significantly affected the ventilation flow rate. For case 1, mean flows were dominant in the air exchange process, whilst for case 2 the fluctuating flows appeared to have more influence on the ventilation mechanism.