Flow and dispersion in coupled outdoor and indoor environments: Issue of Reynolds number independence

Abstract

The Reynolds number (Re) independence criteria for flow and dispersion around purely outdoor environments has been examined in many previous studies, but little attention has been paid to the coupled outdoor and indoor environments. This study investigated the Re independence criteria of flow and dispersion in coupled outdoor and indoor environments using Computational Fluid Dynamics (CFD) with Large Eddy Simulation (LES) model. Two geometrical arrangements were considered, namely an isolated multi-story building, and an array of multi-story buildings. Four parameters including the non-dimensional air velocity, pollutant concentration, ventilation rate, and re-entry ratio were used to assess the Re independence criteria. The tracer gas decay method was used to predict the ventilation rate and to quantify the re-entry ratio for each room. Using the quantitative indicator, the deviation rate (DR), of the non-dimensional velocity below 5%, two sets of critical values were proposed: ReH based on the building height equal to 4.8×104 for the outdoor environment, and ReW based on the opening height equal to 1.4×104 for the indoor environment. The concentration field was more difficult to meet the Re-independent requirement. Using the DR of the non-dimensional concentration below 5%, the critical values were ReH=7.9×104 and ReW=3.0×104. If the DR was enlarged to 10%, the ReH and ReW criteria for the concentration field were the same as the velocity fields. For the non-dimensional ventilation rate and re-entry ratio, the critical value for the Re independence was ReW = 1.4×104 for both the isolated buildings and the building arrays.