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Scaling Problem in Urban Airflow Modelling


Assistant Prof. CHEW Lup Wai

Airflow-Buildings-Cities Laboratory

Department of the Built Environment

College of Design and Engineering

National University of Singapore


Date & Time

Friday, 28 July 2023

7:30 am


CPD-1.21, Centennial Campus, HKU


Ever wondered why we can study airflows across buildings using scaled-down models to represent real, full-size buildings? This seminar discusses two mistakes we have been making in the research of urban airflow modelling. Airflows across buildings are characterized by the Reynolds number, Re = UH/ν, where U is a reference wind speed, H is building height and ν is air kinematic viscosity. Full-scale buildings have H ranges from a few meters to hundreds of meters, with corresponding Re on the order of 106 to 107. However, in wind tunnel experiments, building models have H on the order of centimetres, with corresponding Re on the order of 103 to 104, much lower than the Re at full scale. This mismatch of Rebetween reduced scale and full scale is circumvented by the Re-independent assumption, which allows us to study airflows across buildings at reduced scale in wind tunnels and apply the results to full-scale buildings. Using both experiments and computational fluid dynamics, I will show that the commonly adopted Re-independent assumption is valid only in simple cases, e.g., isothermal flows across a single building. Generalizing this assumption to cases with multiple buildings or flows with heat transfer can introduce significant errors and lead to wrong prediction of the entire wind field around buildings. Potential mitigation strategies to the issue of Re mismatch will be proposed and discussed.


Dr. Chew Lup Wai is an Assistant Professor in the Department of the Built Environment, College of Design and Engineering at the National University of Singapore (NUS). He is the principal investigator of the Airflow-Buildings-Cities Laboratory at NUS ( He obtained his PhD in Mechanical Engineering from Massachusetts Institute of Technology. Prior to NUS, he worked as a postdoctoral scholar at Stanford University. His specialization lies in urban airflow modelling, focusing on natural ventilation, outdoor wind field, and the physics of flows, using both experimental and numerical approaches. In his free time, he enjoys reading, painting and traveling.

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Dr. C.H. Liu

+(852) 3917 7901

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