A variety of elements in Fenix+ 3 has a Material property. Included are: Wall, Solid, Stairs (both straight and spiral), Platform, Floor Slab, Door, and Window. These are referred to as Material Elements.
Generally, a Material Element occupies a specific volume in space.
In the FDS input file, the geometric shapes of Material Elements are represented using a series of elements described by the OBST parameter group. In simple scenarios, where thermal physical parameters of the materials are not considered during simulation, no other parameter groups (aside from OBST) are used to represent Material Elements.
Each OBST element is a rectangular parallelepiped with apexes located at the Mesh nodes. For details see Calculation Area. The dimensions of each OBST are determined by the XB parameter and must be multiples of the Mesh cell size. Occasionally, one of the three dimensions may measure zero.
The element color, as visualized in the Fenix+ 3 editor and Smokeview, is determined by the RGB parameter of the OBST group, reflecting the brightness values of red, green, and blue components.
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Code similar to the following appears in the FDS file:
&OBST XB=-2 ,0, 1.5, 1.75, 0, 0.5 RGB=128,128,128 /
Below are some general properties of Material Elements:
Parameters defining the element location in space (height, thickness, etc.);
Material; Ignore During Fire; Transparency.
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The Ignore During Fire property allows to ignore the element when simulating a fire. For example, if it is a mesh structure that is fully permeable to fire hazards. Such elements are not transferred to the FDS file.
The Transparency property renders the element visually transparent in both the Fenix+ 3 editor and Smokeview. This does not affect the element’s transparency to thermal radiation or other fire hazards. In the FDS file, the OBST group includes a TRANSPARENCY parameter.
&OBST XB=2.75,3,1.75,2,0,3.1 TRANSPARENCY=0.1 RGB=128,128,128/
Splitting Material Elements into OBST
When creating an FDS input file, the coordinates of the OBST elements describing the Material Element are determined on the basis of element parameters. They specify the element position in space. Consequently, the coordinates of the OBST elements are aligned to the nearest multiples of the Mesh cell size. For details, see Calculation Area. It’s important to note that the cell dimension along the Z-axis may differ from the dimensions along the X and Y axes.
Below is an example of how OBST elements are assigned to a Material Element in the Fenix+ 3 scenario. We use the transformation of a Wall element, which is shaped like a rectangular parallelepiped.
Since each OBST element in the FDS model occupies an integer number of Mesh cells, the side surfaces of the OBST elements must coincide with the cell facets. The picture shows a top view of the Wall element, with its side surfaces depicted by blue lines. A black square grid represents the cell borders in the XY plane.
Below is an example of a single layer of cells in the XY plane, the lowest layer for the given MESH.
We draw a line through the center of each cell parallel to the X-axis, shown in red. If a line intersects the wall, it intersects both side surfaces of the wall, resulting in two intersection points. The x-coordinates of these intersection points are then aligned to the nearest x-coordinates of the MESH cell facets.
There are two possible scenarios: one where the closest facets for the intersection points are two different facets, and another where they are the same facet. Specific examples of these scenarios are illustrated in the pictures marked as A and B, respectively.
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If the points align to the same facet, that segment of the wall is represented by an OBST of zero width (case B), passing through this facet. Otherwise, it is represented by an OBST of non-zero width (case A). The result is shown in green in the following picture.
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The resulting OBST may span several cells in width, depending on the original thickness of the Wall element.
The results of the splitting process, performed for each cell along the X-axis, are shown in the picture below.
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A similar procedure for splitting the wall into OBSTs is also performed along the Y-axis and for all other layers of MESH cells along the Z-axis. Subsequently, the sets of OBSTs obtained from these splittings are merged. The resulting set represents the wall in FDS.
The following picture shows a Fenix+ 3 scenario with four walls and a fire source.
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As an example, here’s how the “Wall 1” element is represented in the FDS file:
“Building 1” – “Wall 1”
&OBST XB=5.5, 5.5, 2.25, 2.5, 0, 2 RGB=32,178,170/
&OBST XB=5.25, 5.5, 2.5, 3.5, 0, 2 RGB=32,178,170/
&OBST XB=5.25, 5.25, 3.5, 4, 0, 2 RGB=32,178,170/
&OBST XB=5, 5.25, 4, 5, 0, 2 RGB=32,178,170/
&OBST XB=5, 5, 5, 5.25, 0, 2 RGB=32,178,170/
The following picture shows a 3D view of the discussed scenario in Smokeview.
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The splitting procedure described using the Wall element is similarly applied to the other Material Elements in Fenix+ 3, with slight modifications for Doors and Windows.
Material
Depending on the method of simulating building materials chosen in the Fire Simulation Parameters window, the FDS input file will be generated differently.
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If the material of structures you select is Inert, regardless of the actual material chosen when creating walls, the FDS file includes OBSTs with what is known as inert (INERT) material. Its temperature remains constant and equal to the initial temperature. In FDS, inert material is the default and does not need to be explicitly stated. Thus, walls are represented only by the OBST parameter group with specified coordinates. In this case, the actual wall material only affects the color displayed in Smokeview.
&OBST XB=2.5, 2.75, 3.25, 3.75, 0, 2 RGB=32, 178, 170/
In most cases, using Inert wall materials is quite justified for fire simulation because it significantly speeds up calculations and introduces no significant inaccuracies. For more details, see the article Impact of Wall Materials on Temperature and Heat Flux in a Room.
If Actual Material of structures is selected in the Fire Simulation Parameters window, the FDS model includes the thermophysical properties of the material.
Density, Specific Heat and Thermal Conductivity of the material in the FDS file i represented by the parameters DENSITY, SPECIFIC_HEAT, and CONDUCTIVITY in the MATL parameter group.
&MATL ID='3' FYI='Beton' CONDUCTIVITY=1.4 DENSITY=2150 SPECIFIC_HEAT=0.88/
Wall thickness (THICKNESS) and a reference to the MATL group describing the material are set in the SURF parameter group.
&SURF ID='4' THICKNESS=0.2 RGB=127,127,127 MATL_ID='3'/
There is a reference to the SURF group in the OBST parameter group.
&OBST XB=6, 6.25, 2.5, 5, 0, 3 SURF_ID='4' /
Emissivity and Absorption Coefficient in this example are equivalent to FDS’s default values, so they are not explicitly mentioned in the file. Otherwise, the EMISSIVITY and ABSORPTION_COEFFICIENT parameters will have been specified in the MATL parameter group.
&MATL ID='3' FYI='Beton' CONDUCTIVITY=1.4 DENSITY=2150 EMISSIVITY=0.8 SPECIFIC_HEAT=0.88 ABSORPTION_COEFFICIENT=1000/
Emissivity (ε) is defined as the ratio of the energy emitted by an object at a given temperature to the energy emitted by a perfect emitter or black body at the same temperature. The emissivity of a black body is 1.0. Typically, emissivity can range from 0.0 to 1.0.
Many non-metallic materials (e.g., PVC, concrete, organic materials) have a high emissivity ε from 0.8 to 0.95. Metals, especially the ones woth a polished surface, have a low emissivity. For example, polished aluminum has an emissivity ε≈0.05.
Material reflectivity in FDS is calculated as 1-ε.
We can conclude that the default ε=0.9 in FDS is suitable for most materials. If it is necessary to simulate the presence of materials with a lower emissivity (or, equivalently, higher reflectivity), the Emissivity parameter must be adjusted.
Absorption Coefficient is a measure of how far radiation travels through a medium before it is reduced by a factor of e≈2.7 due to absorption. The default absorption coefficient in FDS (50,000 1/m) is extremely high and corresponds to completely opaque objects. Using the Absorption Coefficient parameter, the material can be made fully or partially transparent to thermal radiation by setting the absorption coefficient to zero or to a known value from reference data.
Choosing the “Actual Material” option enhances the realism of the simulation and more accurately accounts for the interaction of Material Elements with thermal radiation. Specifically, it allows for the simulating of the heating of Material Elements by thermal radiation and convective fluxes of hot air.
When converting a Material Element into the FDS file, if the corresponding OBSTs have a thickness equal to zero or the size of the mesh cell, the FDS system will also simulate through heating. Here, the heat transfer calculation from one surface of the Material Element to another useы its real thickness, as specified in the THICKNESS parameter of the SURF parameter group, rather than the thickness of the existing OBST.
Thickness
Some Material Elements have a Thickness parameter. When you conduct a simulation with the Actual Material option enabled, the Thickness not only affects the geometric thickness of the element, but is also used as the THICKNESS parameter in the SURF parameter group used to describe the element in the FDS file.
&SURF ID='2' THICKNESS=0.2 RGB=255,140,0 MATL_ID='1'/
&MATL ID='1' FYI='Beton' CONDUCTIVITY=1.4 DENSITY=2150 SPECIFIC_HEAT=0.88/
&OBST XB=2,6,2,2,0,2.5 SURF_ID='2'/
Here the THICKNESS parameter of the SURF parameter group is set to 0.2, matching the Thickness specified when the element was created.
If a Material Element without a Thickness property is used, and the “Actual Material” option is enabled, the THICKNESS parameter of the SURF parameter group for this element is set to the size of the mesh cell. For details, see Calculation Area.
For example, a polygon-shaped Wall element does not have a Thickness parameter. Its code can look like this.
“Building 1” – “Wall 1”
&OBST XB=4,4.25,2.25,2.5,0,2 SURF_ID='2'/
&OBST XB=4.25,4.5,2.25,3,0,2 SURF_ID='2'/
&OBST XB=4.5,4.75,2.25,3.25,0,2 SURF_ID='2'/
&OBST XB=4.75,5,2.25,3.5,0,2 SURF_ID='2'/
&OBST XB=5,5.25,2.25,3.75,0,2 SURF_ID='2'/
&OBST XB=5.25,5.5,2.5,3.25,0,2 SURF_ID='2'/
&SURF ID='2' THICKNESS=0.25 RGB=127,127,127 MATL_ID='1'/
&MATL ID='1' FYI='Beton' CONDUCTIVITY=1.4 DENSITY=2150 SPECIFIC_HEAT=0.88/
Here the THICKNESS parameter of the SURF parameter group is set to 0.25, corresponding to the cell size.
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