Research On The Simulation Method Of Bioaerosols’ Diffusion In A Housing Estate

Bioterrorism is designated that terrorists take action by releasing pathogens orpathogenic production intentionally, for the purpose of harming people’s health,causing social panic and threatening social security and stability. Bioterrorism, whichhas been long-standing, became to cause worldwide attention since2001anthraxletters attack in the US., which made many countries in the world takecounter-bioterrorism into account as an important part of national security strategy.Therefore, increasing attention to the hazard assessment of counter-bioterrorism ispaid by researchers and leading specialists all over the world. The attack patterns ofbioterrorism are varied, such as diffusion of aerosol, contamination of water and food,intermediary transmission, etc. Diffusion of aerosol, which has a large influence rangeand a high transmission speed, is the most probable attack pattern the terrorists take.Accordingly, research on the diffusion of bioaerosols and the hazard to people, hasbecome an issue of the hazard assessment of counter-bioterrorism.The methods of research on the diffusion of bioaerosols mainly include theGaussian model, Lagrangian model and CFD numerical calculation. Although theclassic Gaussian model is used most widely, it is appropriate for the meso and smallscale confine which ranges for several kilometers to several hundred kilometers andrelative stable and even airflow conditions, such as flat and expansive countryside orurban boundary layer (from the top of buildings up to the middle part of cloud).Whereas Urban canopy layer (from the top of buildings down to the ground), is themain area where people live, especially the metropolis where there are intensivebuildings and dense population, which is probable to be the main targets of terroristattack. As the distribution of buildings in this area is complex, the airflow around willturn into irregular turbulence causing the irregular diffusion of bioaerosols. Accordingto the above, doing research on the simulation of bioaerosols’ diffusion in urbancanopy is very important and actually necessary. Lagrangian stochastic model isanother useful one which is appropriate for complex and uneven airflow condition.Although it is adapted by several related foreign simulation systems, it is mainly for abigger scale such as street canyon and its resolution and precision is relatively low. This study focused on the research on the diffusion of bioaerosols in the urbanmicro-environment. Intensive buildings cause the complex and irregular airflow. As aresult, once bioaerosols are released in urban, the diffusion with the airflow in a shorttime will be complicated and difficult to acquire. However, dense population in urbanand the hazard consequence requires acquirement of the exact concentrate distributionof bioaerosols after release, which can meet the increasing accurately quantitativerequirement of hazard assessment and emergency response of counter-bioterrorism.Therefore, this study took an estate which has the smaller scale (from dozens ofmeters to several kilometers) and more frequent population activity as research object,and tried to exactly simulate the influence of the distribution of buildings to thediffusion of bioaerosols in a smaller range. As a result, the number distribution ofbioaerosols particle after diffusion was acquired and the contaminated areas weredivided according to the probability of infection.This study simulated the diffusion process of bioaerosols in a virtual housingestate, using Discrete Phase Model. Its principle is that to perform the numericalcalculation of Lagrangian model with CFD, so as to simulate the track and process ofthe diffusion of bioaerosols considered as the discrete phase. First, we made a virtualhousing estate, the area of which was700m×700m, according to a plan of a realhousing estate. In the virtual one, there were10buildings arranged as some rule, andthat was the calculation region of the whole flow field. Acquiring completeinformation of the airflow considered as continuous phase where there is discretephase is the premise of using Discrete Phase Model. Therefore, the airflow field wasfirstly simulated with the k-ε turbulence model to acquire the nephogram and isogramof air’s velocity in the flow field. Based on this, taking the aerosol of anthrax bacillusspores-the typical bioaerosols as an example, we calculated the track of the sporeparticles with Discrete Phase Model. As the simulation process of Discrete PhaseModel is to simulate the influence of turbulence to the diffusion of particles bycalculating sufficient particle tracks, we can simulate mass particles’ diffusion throughmuch fewer ones’ tracks. After acquiring the particle tracks and the data of particles’time varying position (x&y ordinates), we programmed to analyze and process thedata we had got using Matlab. As a result, we got the time varying distribution ofparticles, and also calculated the number of particles in any position by statistic.After the simulation and calculation of CFD Discrete Phase Model and analysisof the data, we acquired particle distribution figures and particle number histograms and isograms at the time20s,40s,60s,80s,100s, and120s. The rules diffusion andmovement of particles were analyzed qualitatively by observing particle distributionfigures at different time, and it was obvious that the diffusion of particles was mainlyinfluenced by turbulence. Through particle number histograms and isograms atdifferent time, we quantified the number of particles at any position, according towhich the probability of human infection could be assessed exactly next.Inhalation anthrax dose-response model can describe the relationship betweenthe probability of infection and the number of spores inhaled by human. Though thedose-response model is generally fitted by the experimental data, it has beendeveloped well as the perfect related studies are being done recently. Recent studiesindicate that the dose-response relationship is age-dependent. According to the above,this study adapted the age-dependent dose-response model to calculate the probabilityof infection in different contaminated regions according to different ages such as10,20,30,40,50,60,70, based on particle number isograms above. As a result, thecontaminated areas were marked to suggest that different persons were threatened todifferent extent in the same contaminated region by age.Through calculation, for every age of persons, the contaminated area was markedrespectively at the time60s,80s,100s, and120s according to the probability ofinfection, which also indicated to what extent persons were threatened. The resultsshowed that the difference of the probability of infection indicated by the markedcontaminated region among different ages were obvious. Inhaling the same number ofspores, older persons would be more probable to be infected by anthrax. Accordingly,decision-makers should take targeted consideration to arrange people, handle theevent and make resource configuration after the events.In conclusion, taking anthrax bacillus spores as an example, this study simulatedand calculated exactly tracks of the bioaerosols’ diffusion in a housing estate usingCFD Discrete Phase Model. The distribution of particle numbers were acquired, basedon which the contaminated area was marked using the inhalation anthraxage-dependent dose-response model. In the marked contaminated regions, theprobability of infection was different for different aged persons. Against thebackground of bioterrorism, this study took a new try to simulate the bioaersols’diffusion in a housing estate and presented a more detailed method referred to markthe contaminated area.