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TECHNOLOGY
Vol. 5, No. 3 (2006) 417–434
Numerical simulation of transmission of NBC materials
JÓZSEF CSURGAI,a JÁNOS ZELENÁK,a TAMÁS LAJOS,b ISTVÁN GORICSÁN,b
LÁSZLÓ HALÁSZ,c ÁRPÁD VINCZE,c JÓZSEF SOLYMOSIc
a HDF NBC Area Control Centre, Budapest, Hungary
University Budapest Department of Fluid Mechanics, Budapest, Hungary
c Miklós Zrínyi National Defence University, Department of NBC-Defence,
Catastrophe Relief and Crisis Management, Budapest, Hungary
b Technical
Nowadays there is appeared a new threat of the use of nuclear, biological and chemical
(NBC) agents by terrorists in the urban area or within different objects and
installations. The existing evaluating systems developed for estimation of transmitting
NBC contamination don’t take into account the direct influence of terrain relief,
vegetation, and human installation. In order to protect forces of HDF and personnel
involved into mitigation of consequences of disasters it is necessary to elaborate proper
computer models for the transmission of dangerous chemical agent inside buildings, in
the urban area.
The aim of our investigation is to establish an NBC transmission and immission
forecasting system widely considering meteorological conditions and influence of relief
and different objects. It is possible with application of numerical computer simulation
of transmission of normal and heavy gaseous materials.
As we consider the validation of computer simulations with practical measurements
essential we elaborated a three-year-research project. In this paper we publish the
tasks for 1 st year and the results as follows:
1. Numerical simulation of transmission of TIC (Toxic Industrial Material) in the urban
area (in the centre of Budapest) and its experimental validation on the plotting board.
2. Numerical simulation of spreading chemical agents on the terrain surface using
digital map with experimental validation.
3. Computer simulation of spreading of chemical agents inside building using CAD
model and its practical validation.
Introduction
Nowadays because of the threat caused by international terrorism there is more and
more real the hazard of NBC materials being used in an attack in a populated area and
inside objects.
Received: June 7, 2006
Address for correspondence:
ÁRPÁD VINCZE
Miklós Zrínyi National Defence University
Department of NBC-Defence, Catastrophe Relief and Crisis Management
H–1581 Budapest, P.O. Box 15, Hungary
E-mail: [email protected]
J. CSURGAI et al.: Transmission of NBC materials
The recently existing NBC-hazard and contamination-spread predicting macroscopic
models do not take in account the effect of buildings and objects that directly can affect
the spread of contamination.
For the NBC protection of the army forces and for the environmental-hazard and
catastrophe control troops and specialists it is an urgent need to develop a computer
model, which can be used to make the evaluation and predict the hazards and
contamination originating from an NBC-event for the following cases: spread of
contamination inside buildings, in urban areas in case of chemical factory accident,
spread of contamination in the terrain.
The aim of the research is to develop a transmission and imission calculation system
for predicting and evaluating the spread of contamination released in an NBC-event in
populated areas, in protected objects and in their surroundings. The spread of
contamination must be evaluated and predicted taking in account the effects of weather
conditions, the effects of the topographic conditions, buildings and objects. This can be
done by a numerical simulation of the spread of NBC contamination.
Principles of the numerical simulation of flows
Nuclear, Biological and Chemical material (NBC material) released to the atmosphere
diffuse in the air, so a numerical simulation of the air-flow can give the concentration
distribution, produce a forecast of the situation and show the effects of different factors
that have a varying influence on it.
An egregious advancement can observe in the fields of calculation and numerical
simulation of flows in the last years as a result of development in fluid mechanics,
numerical methods and computer-techniques.
The base of the method is that the space being studied, where the agent being
analysed flows is divided into several cells. The FLUENT software being used is based
upon the idea of finite volume element (cell).
At the borders of cells of small volume element is easy to control the incoming and
the outgoing mass and the moment of the flow.
Handling the flow this way also ensures that the residual values can cause only an
insignificant calculation mistake. This method is widespread and approved in the field
of numerical fluid mechanics.
Considering the effect of turbulence is remarkably difficult thing, because the
frequency of the turbulent fluctuations of velocity changes on a wide range. To
compensate the effect of fluctuation it needs to apply very small cells covering the area
being studied and to solve the differential equations using a finely detailed cell-matrix,
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which is with the recent calculating processes and computer capacity almost impossible
and ineffective. Unfortunately an effective turbulence model does not exist in the
market, therefore the FLUENT software was chosen, which is market leading valued
software of this field.
With FLUENT software a wide variety of hydrodynamic and thermodynamic
problems can be solved, like the followings:
 Three dimensional stationary and non-stationary, laminar and turbulent flow
calculation,
 Calculation of heat transfer and heat conduction,
 Calculate combustion and chemical reactions in process,
 Flow modelling of biphase agents (granules in flowing agents, gas bubbles in liquid
agent),
 Cavity flow,
 Flow inside filters, flow through membranes.
Implementation of FLUENT-just like other numerical simulation programs –
consists of the following steps:
a) Modelling of the geometry of flow: geometric surfaces surrounding the space of the
flow are being constructed by CAD program. It is important to employ symmetries
and attachment of the space to the surroundings.
b) Mesh generation: definition of the shapes of cell, specification of variation of size,
minimization of angle deformation.
c) Establish a mathematical model of stationary and non-stationary flow, turbulence,
density, more than one-phase flow.
d) Establish the boundary conditions (speed profiles, turbulence, thermal boundary
conditions).
e) Selection of numerical methods (resolving, pressure and speed connection,
discretization in time, underrelaxation, choosing of upwinding and multigrid
possibilities)
f) Evaluation: surface representation of volumetric integrals and scalars, representation
of vectors and flow direction lines, presentation of time dependence.
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Distribution of concentration in urban area in case of chemical factory accident.
Numerical simulation and experimental testing
Numerical simulation of transmission and distribution of concentration
The accident to be modelled: from the damaged container of a carrier vehicle gas of
known volume-flow is released to the air.
To be defined: concentration of the gas at selected points of the surrounding urban area.
Numerical modelling of the surveyed part of the city area
First step is according to methods of numerical procedures to define the geometry of the
space/area of the flow, which means in the recent case the modelling of the buildings
and objects of the city area.
The geometry of the buildings of the city area was defined in CAD code
(Rhinoceros) and in this format it could serve as input source for the FLUENT program.
Figure 1 shows the whole city area being modelled, including city centre and bridges.
Figure 1. CAD model of the city made for the FLUENT
Calculation process, initial parameters, boundary conditions
FLUENT is generic purpose flux simulator software, therefore applying it needs to be
given not only initial parameters, but also the method to be used for describing the flow
processes, which can be chosen from the menu depending on the objectives of the tasksituation. In recent case the geometry of the space being investigated was highly detailed
and complex.
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There are several critical points for the mesh generation, especially the points where
walls of the buildings and the roofs connect, that can complicate the generation of the
mesh needed for calculation. Minor corrections were made on the buildings model that
is necessary for the mesh generation demanded a lot of work also.
In recent case a non-structured mesh was suitable for the objective of the task. Nonstructured mesh is wholly built up of tetraeder cells, so that it provides a flexible meshgeneration and good convergence characteristics. The only problem was, that in the case
of automatic mesh generation the mesh-size could only be particularly controlled.
In order to avoid the flow ration being directly affected locally by the boundary
conditions it was assumed a 1 km free space around the model area.
The height of the calculated area was 600 mm, which corresponds to 300 m. The
whole calculation area (approximately: 0.5 m  4.6 m  0.3 m) was divided by the
software into 978000 cells. Calculation has taken approximately 1.5 days using a
1.8 GHz processor.
Intensity of velocity and turbulence at the boundary in wind direction was distributed
according to wind-tunnel tests.
There was used a constant pressure boundary condition at the emergence.
The horizontal level bordering the surveyed space there was given a symmetry
condition, which means, that on this horizontal there is no drift through.
The gas released in the simulated accident case was blown in via volume sources
into the space of calculation: some of the cells above the given road sections were
separated and through these there was blown in an amount of gas according to the
procedures used in the wind-tunnel model-tests.
Results of calculation
FLUENT provides not only the valid velocity and concentration values of the cell
containing the selected point, but also the interpolated values for the selected point.
Moreover very user-friendly it is capable to show the distribution of the speed of
velocity and concentration in space, the velocity vectors in horizontal and in spatial, the
distribution of pressure arising on the surface of the buildings, etc.
In Figure 2 the velocity vectors in a South West-South direction, on 1.5 meters
above the ground can be seen for a part of the investigated city area. Velocity vectors
are coloured according to the speed of velocity.
Figure 3 shows the concentration distribution of dangerous gas on the height of
1.5 m above the ground.
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Figure 2. Velocity vectors in different colours according to the speed of velocity in case of a
South West-South direction
Figure 3. Concentration distribution on 1.5 meters above ground
Comparison of calculated and measured results
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Figure 4 shows the dimensionless concentration values in the measuring points of the
wind-tunnel (squares connected by a continuous red line) and calculated by FLUENT
(concentration intervals shown by vertical blue sections). It can be seen from the Figure
that measured and calculated results, despite of the complexity of the flow are generally
close to each other in a reasonable extent. In general, it can be say that the FLUENT
code slightly underestimates the values of concentration.
Figure 4. Comparison of the measured and calculated dimensionless concentration values in the measuring points
The FLUENT is capable of providing a highly reliable and fine approximation of
qualitative and applicable quantitative results describing the spread of contamination
released to the air in an urban area. Further experiments connected to the calculation
sets (boundary conditions, turbulence model, number and structure of cells) can enhance
the punctuality of the simulation.
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Simulation of the spread of contamination in terrain and the results of the
experimental verification of the simulation in a wind-tunnel
Numerical simulation of the spread of contamination in terrain
The modelled terrain, initial data and the release
A preset digital terrain model of an area close to Budapest (M-34-125-D DTA-50 +
DDM-50) was used (it can be seen in Figure 5). The numerical model of the area was
constructed with the Gambit software matching FLUENT based upon the preset digital
terrain model. Parameters of the space surveyed in the numerical simulation: in a ratio
of 1:1000 with the complementary space included are 4000  2000  2000 mm
(length  width  height), in which the actual terrain is 3000  2000 mm. Number of
cells is 400.
Figure 5. The terrain model
In the case of the simulated accident the same method was followed, as in the
numerical modelling of the city area. That means that the gas was released to the air
from volume sources, in this case some cells situated in the given part of the terrain were
separated from the other cells and the amount of air changed to CH 4 according to the
amount released in the wind tunnel tests. The FLUENT software did not exactly
simulate the gas release method used in the wind-tunnel tests (line-sources with baffles).
Calculation results
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Figure 6 shows the results of the calculation. It can be seen the distribution of the
concentration of the gas released from the source-cells from top-view.
Figure 6. Gas concentration distribution on the ground
The figure clearly shows that according to anticipation the gas-concentration
decreases while moving in the direction of the wind from the source.
Contamination spread analysis in wind-tunnel test
The terrain model and method of measurement
Based upon the terrain model provided for us a 1:1000 set numerical model of the
terrain was constructed (Figure 7). The figure shows the factors of registry placed to the
ground and the Prandtl-tube which is to measure the reference-velocity. The model
(corresponding to the 4 meter contour-lines) was constructed of elements cut from 4 mm
plastic plate. The CH4 gas infusion of 200 l/h was carried out through 3 line-sources
each 60 mm long, perpendicular to the flow direction and covered by a deflector.
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Figure 7. Details of the model and method of the gas infusion
Samples were taken at ground level at 12 spots through copper tubes. Samples were
taken all at the same time by syringes.
Comparison of the calculated and measured results
Figures 8 and 9 show the calculated (continuous line) and the measured (triangles)
values of dimensionless concentration depending on the distance from the source at
ground level, in the median plane of the source parallel to the wind direction.
Figure 8. Calculated (—) and measured () dimensionless concentration depending on the distance from the source
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Figure 9. Calculated (—) and measured () dimensionless concentration (in logarithmic scale) depending on
the distance from the source
In Figure 9 the dimensionless concentration is defined in logarithmic scale. From the
diagrams it can ascertain, that in the first two segments close to the source the calculated
values are approaching to the measured ones, in the middle part of the investigated
section the calculated results predict higher values of turbulent dispersion, a more rapid
decrease of the concentration, than the measuring results show. It can originate in the
difference between the physical model and the numeric simulation of the gas release.
This probability is confirmed by the close parity of measured and calculated values
farther from the source.
Numerical simulation of gas flow within buildings
Numerical simulation of the gas flow inside a coliseum building
Numerical model
Numerical simulation was performed on the three-dimensional geometric model of the
inside hall of a coliseum.
The model does not depict details of size less than 1 metre, as seats, bars tubes;
however it contains 1 m  1 m air-drain holes of the aspirator-system 6 metres above
ground and air-drain holes of 2 m  2 m size at 0.2 metres height which are located
under the temporary stand and also the air-blower anemostats. The investigated territory
consists of two spaces, the great one and the one under the temporary stand. The
estimated value of the decrement factor of the surface of the temporary stand is 2.
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Numerical mesh of the inside space is shown in Figure 11. A tetraeder mesh was used
again for three-dimensional survey; the number of cells was 338775.
Figure 10. Geometric model of the coliseum
Figure 11. Mesh system of the three-dimensional model
In this experiment the heat radiated by the audience was modelled by volume heatsources set along the surface of the stand and the stage. Heat radiated to the environment
was only taken into account at the top of the building. Heat of the light-sources was
modelled also by volume – heat sources at height of 22 metres.
Some of the calculations were done on a two-dimensional model. In this case 10650
triangle-cells were used. Two-dimensional models have the advantage, that they need far
less calculation, than a model of the whole space, while they usually provide a good
evaluation of the features of the flux and so it is possible to make a comparison between
some model-variants.
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Results
In winter circumstances the hot air that is infused can reach the chairs of the auditory after
a relatively long time because of the density difference between the hot air and the
relatively cold air of the auditory at ground level, but in summer circumstances when cold
air is blown in through the ventilation system, it reaches the auditory area in a short time.
In Figure 12 the auditory is in summer circumstances (cold air ventilation: 16 °C),
the volume of the air being infused is 625,000 m3/h. There was an impulse-like (in
1 sec.) gas infusion of 13 m3 volume through a set of anemostats. The picture shows the
concentration of the gas dispersal in 10 second phases at the height of the heads of the
audience. (Through the rest of the anemostats air ventilation continued normally.)
Calculation was made for anemostats set directly downwards. The gas concentration
at certain points of the auditory grows rapidly and then rapidly decreases as a result of
air-flow caused by the aspirator-system. After 100 seconds 96% of the infused gas was
cleared away by the aspirators.
Figure 12. Change of concentration in 10 second phases in the case of the impulse-like (in 1 second) infusion
of gas through one row of the anemostats
Indeed the FLUENT is capable also for the simulation of the spread of materials
released in the auditory area. Figure 13 shows the relative air-humidity rate affected by
the humidity released by the audience (0.0187 g/s/person).
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Figure 13. Relative air-humidity rate in the auditory area
Analysis of the spread of materials inside buildings with numerical simulation and
comparison of the results
The numerical model
A numerical model of the inside area of a model-building was produced. The numerical
model can be seen in Figure 14.
Figure 14. The numerical model (Number of cells: 246000)
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The spread of the contamination was calculated by the numerical fluid-mechanics
code of FLUENT. The release of methane (CH4) that was discussed in the previous
chapter was numerically simulated as pure methane gas releases (measure-point No. 1 in
the midline of the entrance, measure-point No. 2 near the door, at the sidewall).
The detailed mesh with high cell resolution and the refinement of the cell-mesh
during the runs had the aim of making a reasonably punctual numerical calculation of
the turbulent flow and the spread of contamination around the sources, around the
entrance of the building-model and around the aspirators.
In the case of numerical calculation the mesh filling the geometric space, the
boundary conditions and the initial parameters of the simulation were as follows:
 true-to-scale numerical mapping of the constructed building model in a M 1:1 scale;
 for reality-true free inflow of the air through the door a „numerical foreground” was
created;
 the introducing pipe of the methane was also modelled, gas accession is given by
velocity boundary condition;
 a parallel velocity boundary condition with negative sign was used for the aspirators;
 the number of cells in the final version of the calculation was 246 thousands.
Previously it was verified by the use of meshes of different thickness that the frame
of flow is independent of the number of cells;
 k- turbulence-model was used;
 during the calculation of spread of the marking gas the effect of gravity was taken
into account, which was justified by the principally low velocity of the flux inside
the building;
 after the calculation of the flow-frame of the stationary air-flow we started from the
beginning of the methane injection to iterate with time-conditional run of the
program using a real-time-scale saving the results after each 60 seconds period,
continuing the analysis for both sources over 10 minutes.
 measurements were made at 2, 5 and 10 minutes after the start of the release of the
marker-gas and the results of the measurement and the calculation for each timesequence were compared.
Calculation results
The flow-frame resulted by the numerical simulation was in accord with the
expectations. There was formed a detachedness space, a stagnant zone around the
medium-flux flowing in through the open entrance.
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The Figure 15 shows the dimensionless concentration-contour of methane in each
minute of the first 12 minutes after the start of the release in the case of contamination
flowing into the building through the open entrance.
Figure 15. Dimensionless concentration after the start of the release in minute-phases
Figure 15 shows as the marker gas slightly aspiring drifts towards the back wall
following the direction of it’s initial effluence from the entrance, then it follows the airdrain of the aspiratory system and at the end of the 5th minute it reaches the breathingholes of the aspirator-system. After then the marker gas starts to expand in the whole bulk
of the building and reaches a higher concentration even in the periphery stagnant areas. It is
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clearly visible in the last pictures that at the end of the 12th minute there is only at the sides
of the entrance in the bottom remaining a zone where the gas does not reach.
Comparison of the results of the calculation and the results of the measurement
A dimensionless concentration was defined as follows:
C* 
c *m easured
c *calculated
(1)
Figure 16. The dimensionless concentration (C*) in the different measuring points
The Figure 16 depicts the dimensionless concentration values for the different
measuring points. It is clear, that the dimensionless concentration is smaller in several
cases than one, which means that the numerical simulation produced higher
concentration values than the measured ones. Apart from some values, there has turned
up a relatively high accordance of the results at the front (entrance) wall and the
opposite wall as well as above the ground. (1–12 measuring points). More significant
discrepancies were generated at the top (21–24 points), the calculated values are higher
than the measured ones.
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Conclusion
The main aim of our research was to make a survey on how much the FLUENT 6.1,
which is recently the most widespread and developed numeric simulations code can be
used for description of the spread of NBC materials diffused in the air. For this reason
we researched the spread of contamination in three different basic cases both in windchannel survey and in a numeric simulation.
a) spread of contamination in the streets of an urban area after release caused by an
accident,
b) spread of contamination in terrain,
c) inside a coliseum (only numeric simulation) and in the cubic model of the inner
space of a building.
After the comparison of the concentration values which have been measured and
calculated for three different cases it can be set as a conclusion that numeric simulation
is recommendable to forecast the changes of concentration in space and time within
definite accuracy limits. The accuracy of the forecast depends on the complexity of the
flux-tide, which are highly variable in time and space. It also depends on the correctness
of the model of the release-process and the model of the inflow, an also on the
experiences of numeric modelling used in further simulations. There is a need for further
recognition of the processes, especially of the modelling of spread in complex flowcircumstances (like in low-spread separation zones). In these fields researches are
recently in process and results are being currently built into practice, so the accuracy of
the calculations is being continuously enhanced.
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