A helium-based surrogate method for thermal airflow in an idealized street canyon via CFD simulations

Source abstract

Understanding urban heat distribution and airflow is crucial for climate-resilient planning, yet laboratory studies of non-isothermal conditions remain scarce compared to isothermal investigations. Conventional heated-model wind tunnel experiments face limitations in simulating realistic urban thermal environments at reduced scales, as achieving proper thermal similarity requires impractically high temperatures that are difficult to maintain and may pose fire risks. To overcome these constraints, helium, a lifting gas with lower density than air, is proposed as a safer and cost-effective buoyancy surrogate for urban thermal airflow studies. In this study, a quantitative relationship between helium-based and heated-air models is established through preliminary theoretical scaling analysis. A practical correlation is proposed to predict equivalent air temperature based on helium fraction, offering a valuable tool for experimental design and interpretation. A series of Computational Fluid Dynamics (CFD) simulations was conducted for a street canyon model across wide-ranging flow and thermal conditions (0.7 ≤ Fr ≤ 17.3) to verify the validity of the helium-based method. Detailed comparisons of flow velocity and air temperature distributions between sub-scale models and full-scale protocols demonstrate the method effectiveness in capturing key urban airflow features, including flow patterns and heat distribution. The method shows promising performance for most cases at higher Froude numbers, while some limitations may arise at lower Froude numbers. These findings inform a practical experimental protocol for safer laboratory investigations of urban thermal flows and theoretical insights for further exploration of non-isothermal urban environment studies.