Simulating natural ventilation in buildings using CFD: Importance of thermal boundary conditions

For CFD simulations of natural ventilation in buildings, far-field thermal boundary conditions (BC) are often taken from nearby weather stations, while indoor surfaces are often assumed adiabatic. This study aims to quantify the errors that can be introduced by these assumptions. LES is used to simulate buoyancy-driven ventilation in Stanford Y2E2 Building during a 3-hour night flush with an initial indoor-outdoor temperature difference of 4.0 K. Measured temperatures are used as BC and benchmark to quantify root mean square errors (RMSE) of LES results. The lowest RMSE of 0.36 K is achieved by prescribing the outdoor temperature measured on the building roof as the far-field temperature BC and prescribing the measured surface temperature as the thermal BC for the interior surfaces. Using the outdoor temperature from a weather station 1 km away as the far-field temperature BC increases the RMSE to 1.13 K. For the internal surfaces, the use of an adiabatic BC increases the RMSE to 1.73 K, showing that interior surfaces should not be assumed adiabatic. For prediction of natural ventilation using CFD, we suggest using outdoor temperature measured near a target building as the far-field thermal BC and measured or modeled surface temperatures as the thermal BC for internal surfaces.