Study On The Fluid Dynamics Of Virus Transmission In Restrooms In The Context Of COVID-19 Pandemic

Since the outbreak of COVID-19 in December 2019,the SARS-Co V-2 has posed severe threat to the human world with its uniquely strong crypticity,high infectivity and destructibility,causing serious public health crises,economic losses and public panic around world.With an increasing number of studies confirming the SARS-Co V-2 detections in the restroom environment and reports of cross-infection in public restroom,the safe use of restroom should be taken into account.Therefore,during the COVID-19 pandemic,understanding the transmission mechanism of the virus in restroom can increase public health awareness.This paper studies the process of virus transmission in restroom from the perspective of fluid dynamics and demonstrates the mechanism of virus transmission caused by different behaviors.Based on this idea,this paper uses the methods of computational fluid dynamics（CFD）and domestic restroom experiment to simulate the processes of three common behaviors in restroom,including flushing toilet,flushing urinal and male urination.Finally,in order to reduce the risk of cross-infection,an air distribution optimization of domestic restroom was designed.The main work and conclusions of this paper are as follows:（1）In order to study the fluid flow state and the movement of aerosol particles in the flushing processes of toilet,transient calculation models of siphon toilet were established.The simulation results indicate that flushing could cause strong turbulence within the toilet,with the rising velocity of airflow up to 5m/s.One-shot flushing can lead 40-60%of particles reach above the toilet seat,leading to large-scale virus spread with a maximum spread height of1.065m.Therefore,it is strongly recommended that the public put down the toilet lid before flushing to fundamentally block the transmission path of the virus.（2）In order to study the fluid flow state and virus-laden particle movement during the urinal flushing processes,transient calculation models of urinal were established.The simulation results indicate that the trajectory of the particles triggered by the urinal flushing manifests an external spread type,resulting in 57%particles escaping into the air.The particles can reach 0.84m in 5.5s,equivalent to the position of male’s thigh.The public are advised to wear masks when using urinals and take precautions to reduce the risk of cross-infection.（3）In order to study the mechanism of virus transmission during male urination,a combined method of computational fluid dynamics and family bathroom experiments was adopted.In experiment,the high-speed visualization imaging system was used for microscopic view of the urine flow dynamics and urine-water collision behavior and the infrared camera is used for macroscopic imaging.The flow dynamic distribution during urination was visualized by the smoke-assisted experiment.In simulation,transient calculation models of male urination were established.The results indicate that the average velocity of droplet-water impingement during urination was 2.2m/s and the velocity of the splashed droplet can be as high as 1m/s,causing strong turbulence in toilet.The climbing speed of upward airflow can reach 1m/s-3m/s,resulting in 83%-86%of the virus particles escaping to the air area above toilet seat.The maximum diffusion distance of particles is up to 1.55m,equivalent to the height of the male facial area.In addition,the airflow will still maintain an upward trend of about 8s after the urination.Therefore,it is necessary for men to take preventive measures such as wearing masks,disinfection and hand cleaning during urination,and to keep physical distance and time interval（10-20s）in public restroom.（4）In order to optimize the air distribution of the domestic bathroom,the air flow patterns and the diffusions of virus particles indoor under different ventilation conditions were compared in simulation.In addition,the particle concentration（PC）and the ratio of the particle escaped from the fan to the total number of the released particles(P_ef)were quantitatively analyzed.The simulation results indicate that compared with traditional ceiling ventilation proposal and floor ventilation proposal,the side ventilation proposal can effectively drive the air flow near the toilet to the location of the exhaust fan,which is beneficial to limit the diffusion of virus particles.This ventilation scheme can improve the P_ef up to 60.7%,effectively removing indoor virus particles.By increasing air changes rates per hour（ACRs）and the size of exhaust fan,PC value can be reduced by 64%and P_ef value can be increased by 189%.Therefore,the side ventilation proposal should be adopted to optimize air distribution of the domestic bathroom,thereby reducing the risk of infection.This article is not only of great significance to the prevention and control of COVID-19,but also has certain guiding significance for the related research on the transmission mechanism of viruses that can survive in sewage system and human body fluid,such as SARS virus,Ebola,Middle-East Respiratory Syndrome Coronavirus,norovirus,coxsackie B virus and poliovirus,so as to prevent the emergence of new epidemics and re-emergence of some epidemics.