We present ensemble-based large-eddy simulations based on a lattice Boltzmann method for a realistic urban area. A plume-dispersion model enables a real-time simulation over several kilometres by applying a local mesh-refinement method. We assess plume-dispersion problems in the complex urban environment of Oklahoma City on 16 July using realistic mesoscale velocity boundary conditions produced by the Weather Research and Forecasting model, as well as building structures and a plant-canopy model introduced into the plume-dispersion model. Ensemble calculations are performed to reduce uncertainties in the macroscale boundary conditions due to turbulence, which cannot be determined by the mesoscale model. The statistics of the plume-dispersion field, as well as mean and maximum concentrations, show that ensemble calculations improve the accuracy of the simulations. Factor-of-2 agreement is found between the ensemble-averaged concentrations based on the simulations over a 4.2 × 4.2 × 2.5 km2 area with 2-m resolution with the plume-dispersion model and the observations.
We implement and perform large-scale LES analysis for running groups of cyclists. The mesh-refined lattice Boltzmann method (LBM) and coherent-structure Smagorinsky model (CSM) are adopted for the simulations to achieve a high performance computing on the recent GPU supercomputer. In the simulation with 16 cyclists, the mesh spacing around cyclists is 4 mm, and the total number of the mesh is up to 8.1×10 8 and the number of GPUs utilized is up to 64. Each calculation took 4 or 5 days for the 8~11 seconds of physical duration. The flow around 16 cyclists in various arrangement is calculated, and the results show that the in-line arrangement is more effective than the rhomboid arrangement in the viewpoint of the total aerodynamic drag of the group; however, a specific person in rhomboid arrangement can obtain larger drag reduction and save the endurance. Results on two groups also suggest that the frontal group in rhomboid arrangement will be exploited as the wind protection of the backward groups.
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