Jet Fans in the Underground Car Parks and Virus Transmission

Jet fans are increasingly preferred over traditional ducted systems as a means of ventilating pollutants from large spaces such as underground car parks. The spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) -which causes novel coronavirus disease (COVID-19)- through the jet fans in the underground car parks has been considered a matter of key concern. A quantitative understanding of the propagation of respiratory droplets/particles/aerosols containing the virus is important. However, to date, studies are yet to demonstrate the viral (e.g. SARS-CoV-2) transmission in the underground car parks equipped with jet fans. In this paper, the numerical simulation has been performed to assess the e ﬀ ects of jet fans on the spreading of viruses inside the underground car parks.


INTRODUCTION
Since December 2019, novel coronavirus disease (COVID-19) has become a major concern for the global population. It has led to 1,092,144 deaths worldwide as of October 15, 2020 [1]. It simulated the clinical course of infection with two previously reported human coronaviruses -including severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) which was named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by the Coronavirus Study Group of the International Committee on Taxonomy of Viruses [2]. Studies have reported that severe acute SARS-CoV-2 spreads mainly through respiratory droplets and aerosols [3][4][5][6][7]. These respiratory droplets can be exhaled during coughing, sneezing, or even talking [8].
The increase in mortality rates of COVID-19 has evoked scientists to evaluate all aspects of viral transmission.
Considering the characteristics of respiratory droplets/particles/aerosols transmission on the wind condition, there will be a large number of viruses within an underground car park when a confirmed case sneezes near the jet fan.
Underground car park ventilation will cause cross-infection through respiratory droplets/particles/aerosol transmission among people if appropriate design were not taken. However, underground car park design and viruses' control has rarely been studied directly.
The jet fan ventilation system has been developed to ventilate the underground car parks for carbon monoxide (CO) removal during the normal condition as well as smoke extraction in the emergency scenario such as fire [9][10][11][12].
Under normal condition, jet fans can spread the sneeze-originated respiratory droplets in the airflow direction and increase the risk of viral transmission increases. In this regard, the number of people in underground car parks and jet fans velocity plays a key role in the risk of viral transmission.
A comprehensive review of COVID-19 transmission via respiratory droplets was conducted by Carelli [13], Drossinos and Stilianakis [14], Chen [15], and Chen et al. [16]. Sun and Zhai [17] analyzed the infection probabilities of COVID-19 via large respiratory droplets and recognized 1.6−3.0 m as a safe social distance. Dbouk and Drikakis [18] numerically studied airborne droplet transmission during coughing. They found that respiratory droplets could travel unexpected considerable distances depending on the high-speed wind conditions. Dbouk and Drikakis [19] also demonstrated that respiratory droplets from coughing or sneezing traveled a distance less than 2 m in the case of zero-wind conditions. Bourouiba [20] found that expelled respiratory droplets during human sneezing could travel up to 7-8 m at 36 km/h -108 km/h wind speeds. Various researchers recommended the use of face masks in the public environment, and some of them believed that social distancing of 2 m may not be adequate during the COVID-19 outbreak [21].
Infection control and prevention depend on disrupting the transmission of pathogens (viruses) from the infected human to a new human. Understanding routes of disease transmission and how it contributes to the spread of viruses allows for the identification of effective prevention and control measures. The transmission of viruses can be divided into the following five main routes: direct contact, fomites, aerosol or airborne, oral, and vector-borne. Surprisingly, the SARS-CoV-2 pathogens may be transmissible through unexpected daily activities, for example, the turbulent flow induced by toilet flushing [22], the male-oriented urinals [23], and exhausted aerosol from the cleanroom heating and ventilation conditioning systems [7]. Transmission via each of these three routes is important and depends on various factors (e.g. the particle distribution, the turbulent intensity, the humidity, and the temperature). Among these, SARS-CoV-2 mainly has airborne transmission potential [24]. Continuous airflow of jet fans can increase the transmissibility of virus-containing droplets/particles/aerosols in the underground car parks. A better understanding of how respiratory droplets/particles/aerosols spread due to the continuous airflow of jet fans may provide insight that contributes to mitigating SARS-CoV-2. To the best of our knowledge, the problem of spreading viruses in underground car parks has not been investigated previously. All previous works of literature focused on the CO control. The main aim of our work is to present predictions of the respiratory droplets/particles/aerosols spreading resulting from the continuous airflow of jet fans. This helps investigators gain a deep understanding of the behavior of complex air flows inside the underground car parks, which are engaged with the dynamics of viruses.
The simulations were carried out at a low-speed of jet fans (daily applications or normal condition) with various configurations of the sneeze source. Finally, we suggest six tips to reduce the risk of infection through respiratory droplets transmission during the use of the jet fan air conditioning system in the underground car parks.

KEY ISSUE OF JET FAN AIR CONDITIONING SYSTEMS WITHIN THE UNDERGROUND CAR PARKS IN THE COVID-19 SITUATION
The ventilation system in the underground car parks consists of jet fans, fresh air ducts and exhaust ducts, CO detection sensors, and a control panel [25]. This system -which operates in a similar way to a ducted system-is based on placing a set of axial impulse fans all along the underground parking area. When jet fan conditioning system is installed on the ceiling, it moves the air towards the exhaust ducts by effectively creating a continuous flow and impeding the creation of stagnant zones. Therefore, the jet fan system -with continuous flow-can easily spread the viruses inside the underground car parks. For example, when an infected person sneezes near the jet fan, the respiratory droplets/particles/aerosols spread through the jet fan system.
Where p , u, and T are the pressure, the fluid velocity, and the temperature of fluid, respectively.

B. DISCRETE PHASE MODEL (DPM)
The DPM is adopted in this paper to simulate the virus particle movement under the effect of continuous airflow of jet fans within the underground car parks. Recently, Dbouk and Drikakis [18], Li et al. [22], and Wang et al. [23] used this model to simulate a human cough-induced particle movement, turbulent induced toilet flushing, and urinals transmissions, respectively. The flow pattern of a particle is calculated from the following equation [28]: Where d is the diameter of the sneezed respiratory droplets/particles carrying viruses between 1 μm and 13 μm; The resulted Stokes drag force equation is adopted to calculate F D as follows [29]: Where the Stokes-Cunningham slip coefficient C c under atmospheric conditions is calculated to be from the following equation [30]: Where λ is the molecular mean free path of the gas. In addition, because of the size of the respiratory particles, the Brownian force and Saffman lift force are taken Brownian F and Saffman F , respectively. The components of the Brownian force are modeled as a Gaussian white noise process with spectral intensity S n, ij given by [31,32]: Where ij  is the Kronecker delta function, and Where i  are zero-mean, unit-variance-independent Gaussian random numbers. The amplitudes of the Brownian force components are evaluated at each time step.

C. SAFFMAN'S LIFT FORCE
The Saffman's lift force can be defined as follows [32]:

NUMERICAL TECHNIQUE AND BOUNDARY CONDITIONS
The open-source field operation and manipulation (OpenFOAM) for computational fluid dynamics (CFD) software package version 5 was used to perform the numerical simulations. The OpenFOAM codes were written in C++ programming language using the finite-volume numerical technique to solve the conservation of mass, energy, and momentum along with the equation of state in their Reynolds-averaged form [33,34]. The second-order upwind scheme was used to handle the convective terms. The Gauss-linear second-order approach was employed to address the diffusion terms. The Pressure-Implicit with Splitting of Operators (PISO) algorithm was applied to couple the pressure and the velocity. The under-relaxation factors for the pressure, momentum, and energy equations were 0.7, 0.7, and 0.6, respectively. In addition, the minimum residuals for convergence of pressure, velocity, and temperature were 10 -10 , 10 -11 , and 10 -12 , respectively. In order to simulate the particle movement during the normal condition of jet fans within the underground car parks, two assumptions were adopted in this article: (1) the generation of the respiratory particles is ignored, (2) The size and other physical properties of the respiratory particles remain constant during simulation. The fixed-value and fixed-flux pressure conditions are imposed at the fresh air ducts. The outlet-inlet is employed to model the exhaust ducts. Our case study can control CO by jet fans within the underground car park without the air circulations. Afterward, we focus on the viruses spreading inside the car park. Figure 3 demonstrates the comparison of our data with the experimental visualizations presented by Collela et al. [35] for the two-dimensional numerical simulation of the discharged velocity for the jet fans. The discharged airflow temperature was set to 20 °C, and the ambient pressure, p ∞, was 100 kPa. The maximum relative deviation of the discharged velocity for the two cases in Figure 3 was approximately 4.5%. The grid independence test was carried out to compute the required number of numerical cells to obtain convergent results. To obtain the grid-independent results, simulations have been carried out on three different mesh topologies at the low-speed jet fan velocity of 10 m/s. Figure 4 presents the jet fan discharged velocity using the three mentioned numerical coarse, fine, and finest meshes. Grids 800000 and 900000 produce almost identical results for the jet fan discharged velocity with a relative error of less than 0.5%. Hence, a numerical grid 700000 chose to have a convergent solution with the optimized computational cost. A summary of the output of the grid independence test is shown in   droplets/particles within the underground car park depends on the location of the sneeze source. Furthermore, the multiple suction and discharge of the respiratory droplets/particles by jet fans convert these particles to the much smaller aerosols that may move further away. As demonstrated in Figure 6, we determined hazard and safety zones based on the concentration of respiratory droplets/particles upon the sneeze (as the source of infection). To decrease viral infection (in this regard  transmission, susceptible individuals in underground car parks should try to move in safety zones as demonstrated in Figure 7. In the following, we have provided learning tips for traffic within the undergrounded car parks which are ventilated by the jet fans system:

VALIDATION OF RESULTS
• to wear a mask.
• to move far away from the core of jet fan flow.
• to leave the hazard zone to the safety zone • not to enter the hazard zone for a long time.
• to traffic on lines in the mid-distance of jet fans and parallel to their axis • to find the shortest pathway to exit Figure 6. Safety (green) and hazard (red) zones within the underground car park (case study).

ACKNOWLEDGMENTS
We thank Mr. Mehrdad Nami and Mehran Sheikholeslami and their group at the research and development part of the Abrasan Imen Savalan Group (HVAC-Savalan TISG) for sharing their data and for fruitful discussions. This research was fully supported by the Abrasan Imen Savalan Group (HVAC-Savalan TISG) in Tabriz, Iran.

DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.

CONFLICT OF INTEREST STATEMENT
All Authors declare that there is no potential conflict of interest.