ANTI-TERROR ENGINEERING IN THE CASE OF POSSIBLE TERRORIST ATTACKS WITH CHEMICAL AGENTS

Dep. «Hydraulics and Water Supply», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 273 15 09, e-mail water.supply.treatment@gmail.com, ORCID 0000-0002-1531-7882 Dep. «Life Safety», Prydniprovska State Academy of Civil Engineering and Architecture, Chernyshevskogo str., 24а, 49600, tel. +38 (056) 756-34-57 e-mail berlov@mail.pgasa.dp.ua, ORCID 0000-0002-7442-0548 State Enterprise «Design and Exploration Institute of Railway Transport of Ukraine «Ukrzaliznichproekt», Konarev St., 7, Kharkiv, 61052, tel. +38 (057) 724 41 25, e-mail uzp38@ukr.net, ORCID 0000-0002-2814-380X Dep. «Hydraulics and Water Supply», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 273 15 09, e-mail v.kozachyna@gmail.com, ORCID 0000-0002-6894-5532


Introduction
Acts of terrorism with the use of chemical (biological) agents in the streets of cities are not a groundless threat.One of the variants to emit a hazardous chemical agent into the atmosphere, which is sufficiently hidden from an observer, can be its discharge into the ventilation system of various cafes, which are located in city streets (Fig. 1).Emission of polluted air from the ventilation system of a cafe is usually carried out on the roof.Scientifically, it is the emission from a low source.1process equipment; 2place of hazardous substance entry into the ventilation system; 3polluted air discharging device on the cafe roof; 4outdoor chemical contamination zone; 5initiator of hazardous substance discharge into the ventilation system; 6cafe building With such an emission of a chemical agent through a ventilation system, a sufficiently large area of chemical contamination can form behind the building.For example, Fig. 2 shows a photo of the contamination zone near the mini-cafe.Emission is combustion process products from the kitchen.It is clearly seen that the pollution zone covers not only the sidewalk, but also part of the roadway.If people get into this chemical contamination zone, the risk of their toxication is extremely high.We emphasize that people will definitely fall into the contamination zone, because cafes are public places.In addition, such a scenario of a terrorist attack ensures unexpectedness and secrecy.In this regard, the question arises about the development of engineering methods to protect people from damage by reducing the concentration of a hazardous substance in the street during a possible terrorist attack.
The theoretical solution to this problem is quite complicated.When emitting a chemical agent from a low source, it is necessary, first, to take into account the influence of the building on the formation of the chemical contamination zone.Secondly, it is important that the mathematical model would also make it possible to calculate the effectiveness of the use of specific engineering solutions to reduce the intensity of chemical contamination of the air environment near the facility.To estimate the level of air pollution in case of a possible terrorist act, for example, Gaussian models can be used as a zero approximation.But these models do not allow to take into account the influence of the building and various engineering elements on the formation of chemical contamination zones, that is, they cannot be used to assess the effectiveness of various anti-terror methods.The normative methods used in Ukraine for solving problems of assessing the size of chemical contamination zones (for example, the OND-86 technique), for this reason, also cannot be applied.The only theoretical method for solving problems of this class is CFD modeling.Within this scientific direction, specialized software packages «ANSYS Fluent», «FAST», etc. have been created.These packages are a powerful tool for solving a wide class of problems.It should be noted that the cost of licensed packages for research is very high, so access to such packages is limited.It is also known that the use of these packages requires the use of powerful computers and large consumption of computer time when solving a practical problemseveral days to calculate one variant of the problem.This is a definite obstacle, as the organizations of a special focus conduct numerous serial calculations.

Purpose
This work aims to develop a method of local outdoor reduction of the concentration of a chemically hazardous substance, which entered the atmosphere through a cafe roof vent.It also involves the creation of a numerical model for evaluating the effectiveness of engineering methods for air contamination protection during a terrorist attack with a highly toxic chemical agent.

Methodology
The process of dispersing a highly toxic chemical agent outdoor can be described on the basis of the following equation (profile task) [2-5, 7, 8]: where The boundary conditions for equation ( 2) are written as [3]: at 0 t  , 0 C  .At the boundaries where the air flow enters the calculation area, , here in C is the known value.We assume that 0 in C  .In the area where the air flow exits the computational domain, in the numerical model we set a «soft» boundary condition of the form: Aerodynamics model.To apply equation (1) in the case of dispersion of a chemical (biological) agent in the presence of a building, it is necessary to know the uneven velocity field of the wind flow.To determine the wind flow velocity field u = f(x, y), v = f(x, y), we will use the ideal fluid irrotational flow model [5]: where Рvelocity potential.
The components of the air flow velocity vector are determined by the dependence of the form: For equation ( 2) there are such boundary conditions: on solid boundaries we set the condition of the form: where nunit outer normal vector to the boundary; on the boundary of the airflow exit from the computational domain, we set the boundary condition P = const; on the boundaries where the inflow of air occurs, we set the boundary condition of the form: , where Vthe known wind flow velocity.
Numerical solution to the task.For the numerical integration of the modeling equations we will use finite-difference solution methods.
We will carry out the approximation of derivatives, following [2,5].Approximation of the time derivative is carried out as follows: The first derivatives are approximated by corelations [5]: For approximation of the first derivatives, we use the formulas [2,5]: For approximation of the second derivatives, we use the dependencies [5]:


Taking into account the above designations of difference operators, we write the difference analogue of equation ( 1): We perform the splitting of the difference equation (4).The splitting equations at each step are written as follows: in the first step ( 1 4 kn  ): in the second step ( 11 ; 24 in the third step ( 31 ; 42 apply the dependence (6); in the fourth step ( 3 1; 4 use the dependence (5).
The desired value of the function C at each fractional step ( 5), ( 6) is determined by the pointto-point computation formula.
In the last step, we solve the equation: 00 ( ) ( ) To solve this equation, the Euler method is used.
For the numerical solution of equation ( 2), we use the Liebmann method.The approximate equation for the velocity potential in this case is written in the form: .
For the software implementation of the constructed numerical model, we used FORTRAN.

Findings
The developed CFD model was used to solve the following model problem.We consider the emission of highly toxic chemical agent through the ventilation system, the outlet of which is located on the cafe roof.The scetch of the computational domain is shown in Fig. 3. To minimize the air contamination level near the cafe, as an anti-terrorist method, we use the installation of a vertical (Fig. 4) or inclined screen (Fig. 6) on the roof.It is necessary to evaluate the effectiveness of the used screens to minimize the air contamination level near the cafe.
The air contamination zone near the cafe for each working scenario is shown in Fig. 6-8.Fig. 8 shows that the use of the inclined screen makes it possible to take a stream of contaminated air away from the cafe.For a more detailed analysis of the effectiveness of the used screens, the Ta-ble 1 shows the concentration of the contaminant at different distances from the building, the level of 1.7 mthe height of a person for all the scenarios under consideration.As can be seen from the Table 1, the use of screens on the building roof allows to reduce the concentration of a chemically hazardous substance outdoor and thereby minimize the risk of toxic damage to people during the terrorist attack.Obviously, for the case under consideration, it is more effective to use the inclined screen (scenario No. 3).
It should be noted that the task solution time is about 5 seconds.

Originality and practical value
The numerical model has been developed that allows to determine the effectiveness of engineering methods for minimizing the outdoor air contamination level in the event of an initiated (terrorist attack) pollution with a highly toxic chemical agent.
The distinctive feature of the constructed model is the use of the equation of convective-diffusive dispersion of a chemical agent together with the equation for calculating the wind flow velocity field near the building (potential flow model).
Computer time spent on the implementation of the developed numerical model is a few seconds.
Studies conducted on the basis of numerical simulation have shown that the use of screens on the roof of a building can reduce the air contamination level in a certain area near the building.

Conclusions
The numerical model has been developed for assessing the air contamination level near the building in the event of the chemical agent emission on its roof.The model makes it possible to assess the effect of screens on the protection of atmospheric air against contamination during such an emission.The basis for solving the problem is numerical simulation based on the equations describing the dispersion of impurities and aerodynamics.
Further improvement of this approach should be carried out in the direction of developing a three-dimensional numerical model focused on solving the tasks of this class.

Fig. 1 .
Fig. 1.Scheme of outdoor chemical air contamination at initiated entry of hazardous substances in a cafe ventilation system:

Fig. 2 .
Fig. 2. Air contamination zone near the cafe: 1polluted air discharging device on the cafe roof; 2visible boundary of the contamination zone (Heroev Avenue, Dnipro)

Fig. 3 .
Fig. 3. Sketch of emission of a chemical agent on the cafe roof (no protection element): 1 -cafe building; 2point of chemical agent emission