Intercomparison of CFD Simulations for Urban Flow in Paris: OpenFOAM, code_saturne, and the PALM Model System

Abstract

This study quantifies uncertainty in CFD-based urban flow simulations over central Paris using three modeling approaches: steady RANS with OpenFOAM, unsteady RANS with code_saturne, and LES with PALM. All simulations are validated against field measurements and show overall good agreement. At pedestrian height, the three approaches reproduce similar mean wind distributions, while LES predicts stronger wind components, higher turbulent kinetic energy, a more developed urban canopy layer, and higher area-averaged temperatures, partly due to grid-scale turbulence resolution and online radiation coupling. Vertical profiles further indicate that LES produces stronger near-surface winds and a lower but more intense turbulence peak compared with RANS models. Upwind tall buildings affect turbulence patterns similarly across the cases. The influence of trees is also examined: RANS models consistently predict tree-induced reductions in mean kinetic energy and friction velocity, whereas LES captures greater spatial variability. Tree shading reduces pedestrian-level air temperature. These findings clarify model-dependent uncertainties and provide guidance for selecting suitable CFD approaches and settings for realistic urban microclimate simulations.