## 1 Introduction  
The outbreak and spread of COVID-19 had affected many countries around the world by mid-March 2020. The cumulative confirmed cases across the world have exceeded 400,000, and this large number of patients has resulted in significant pressure on the existing medical infrastructure in several countries. In response to the shortage of medical resources, some Chinese cities began to build temporary hospitals for the centralized treatment of patients suffering from pneumonia. For example, Wuhan constructed two new COVID-19 temporary hospitals, namely the Huoshenshan Hospital and Leishenshan Hospital; Beijing reconstructed the Xiaotangshan Hospital, which was used for the treatment of patients suffering from severe acute respiratory syndrome (SARS) in 2003. For the design of the heating, ventilation, and air conditioning (HVAC) system in these hospitals, negative air pressure and a special exhaust system must be utilized to discharge the harmful air emitted by patients. The harmful air needs to be exhausted at a higher altitude to prevent the virus from spreading to the unaffected areas of the hospital. Although the exhaust system is generally equipped with filtration devices, there is still a certain degree of risk associated with infections after the harmful air is discharged through the air outlets. According to the research on the infectivity of SARS, if the air exhaled by a SARS patient is not diluted to less than 10,000 times using clean air, it could result in a risk of infection [1]. Considering the limited knowledge on COVID-19 at present, if the ground reality of these hospitals can be considered in the simulation of the dispersion process of the exhaust air in an outdoor environment, the infection risk caused by the harmful air can thus be quantitatively evaluated, which will provide designers with a more specific reference during the design of such hospitals. Because the design and construction of the temporary hospital must be completed within 6-10 days, limited time is allocated for the simulation and analysis of the impact of exhaust air on the environment. Moreover, considering the extremely high infectivity of SARS-CoV-2, the design scheme of the air outlet and fresh-air intake for the temporary hospital needs to undergo multiple rounds of iteration and verification in a short time. Hence, the corresponding simulation method must meet the requirements of both high accuracy and efficiency. Generally, four common methods can be used to study pollutant dispersion around buildings, including (1) field measurement, (2) physical modeling, (3) semi-empirical methods, and (4) numerical modeling [2]. The field measurement method can only be used to record the concentration of pollutants in the built-up area, which cannot provide a reference for the design of a temporary hospital. Physical modeling methods require fabricated scaled models of buildings and wind tunnel tests, which must meet the similarity criteria and are generally time-consuming and labor-intensive [3]. Thus, it is difficult for engineers to realize a rapid evaluation of the impact of exhaust air in the condition of an emergency response through the physical modeling method. The commonly used semi-empirical methods include the Gaussian models and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) models, which are based on a Gaussian distribution of the plume in the vertical and horizontal directions under steady conditions [4; 5]. These models usually require certain empirical or semi-empirical parameters from observations and make crude simplifications [6]. While these models are successfully employed for simplified flow configurations and meet the demands of high-efficiency simulations, they are only useful for landscapes that are approximately flat and unobstructed. Further studies are required if these models are to be used in cities and other built-up areas with complex surfaces [2].