/MONVOL/FVMBAG1
Block Format Keyword Describes Finite Volume Method Airbag, which has more flexible input than the similar obsolete keyword /MONVOL/FVMBAG (Obsolete).
 Gas materials are specified in separate /MAT/GAS cards.
 Composition of injected gas mixture and injector properties are specified in separate /PROP/INJECT1 or /PROP/INJECT2 cards.
 Automatic Finite Volume meshing in specified coordinate system, given by a frame.
Format
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

/MONVOL/FVMBAG1/monvol_ID/unit_ID  
monvol_title  
surf_ID_{ex}  H_{conv}  
Ascale_{t}  Ascale_{P}  Ascale_{S}  Ascale_{A}  Ascale_{D}  
mat_ID  P_{ext}  T_{0}  I_{equil}  I_{ttf} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

N_{jet} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

inject_ID  sens_ID  surf_ID_{inj}  
fct_ID_{vel}  Fscale_{vel} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

N_{vent}  N_{porsurf} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

surf_ID_{v}  I_{form}  A_{vent}  B_{vent}  vent_title  
T_{start}  T_{stop}  $\text{\Delta}{P}_{def}$  $\text{\Delta}t{P}_{def}$  I_{dtPdef}  
fct_ID_{t}  fct_ID_{P}  fct_ID_{A}  Fscale_{t}  Fscale_{P}  Fscale_{A}  
fct_ID_{t'}  fct_ID_{P'}  fct_ID_{A'}  Fscale_{t'}  Fscale_{P'}  Fscale_{A'} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

surf_ID_{ps}  Iform_{ps}  Iblockage  surface_title  
T_{start}  T_{stop}  $\text{\Delta}{P}_{def}$  $\text{\Delta}t{P}_{def}$ 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

fct_ID_{V}  Fscale_{V} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

frame_ID  kmesh  T_{switch}  I_{switch}  P_{switch}  
L_{1}  L_{2}  L_{3}  
Nb_{1}  Nb_{2}  Nb_{3}  grbric_ID  surf_ID_{in}  I_{ref}  
I_{gmerg}  Cgmerg  C_{nmerg}  P_{tole}  
q_{a}  q_{b}  H_{min}  $\text{\Delta}{T}_{sca}$  $\text{\Delta}{T}_{\mathrm{min}}$  
I_{lvout}  N_{layer}  N_{facmax}  N_{ppmax}  I_{fvani} 
Definitions
Field  Contents  SI Unit Example 

monvol_ID  Monitored volume
identifier (Integer, maximum 10 digits) 

unit_ID  Unit Identifier (Integer, maximum 10 digits) 

monvol_title  Monitored volume
title (Character, maximum 100 characters) 

surf_ID_{ex}  External surface
identifier. 12 (Integer) 

H_{conv}  Heat transfer coefficient.
24 (Real) 
$\left[\frac{\text{W}}{{\text{m}}^{\text{2}}\text{K}}\right]$ 
Ascale_{t}  Abscissa scale factor for
time based functions. Default = 1.0 (Real) 
$\left[\text{s}\right]$ 
Ascale_{P}  Abscissa scale factor for
pressure based functions. Default = 1.0 (Real) 
$\left[\text{Pa}\right]$ 
Ascale_{S}  Abscissa scale factor for
area based functions. Default = 1.0 (Real) 
$\left[{\text{m}}^{2}\right]$ 
Ascale_{A}  Abscissa scale factor for
angle based functions. Default = 1.0 (Real) 
$\left[\text{rad}\right]$ 
Ascale_{D}  Abscissa scale factor for
distance based functions. Default = 1.0 (Real) 
$\left[\text{m}\right]$ 
mat_ID  Initial gas material
identifier. (Integer) 

P_{ext}  External
pressure. (Real) 
$\left[\text{Pa}\right]$ 
T_{0}  Initial
temperature. Default = 295K (Real) 
$\left[\text{K}\right]$ 
I_{equil}  Initial thermodynamic
equilibrium flag.
(Integer) 

I_{ttf}  Time shift flag. Active
only when at least one injection sensor is specified. Determines
time shift for venting and porosity options when injection
starts at a Time to Fire specified in a sensor.
(Integer) 

N_{jet}  Number of
injectors. (Integer) 

inject_ID  Injector property
identifier. (Integer) 

sens_ID  Sensor
identifier. (Integer) 

surf_ID_{inj}  Injector surface
identifier (must be different for each
injector). (Integer) 

fct_ID_{vel}  Injected gas velocity
identifier. (Integer) 

Fscale_{vel}  Injected gas velocity
scale factor. Default = 1.0 (Real) 
$\left[\frac{\text{m}}{\text{s}}\right]$ 
N_{vent}  Number of vent
holes. (Integer) 

N_{porsurf}  Number of porous surfaces.
15 (Integer) 

surf_ID_{v}  Vent holes area surface
identifier. (Integer) 

I_{form}  Venting formulation. 6
(Integer) 

A_{vent}  Scale factor on vent hole
area. Default = 1.0 (Real) 

B_{vent}  Scale factor on impacted
vent hole area. Default = 1.0 (Real) 

vent_title  Vent hole
title. (Character, maximum 20 characters) 

T_{start}  Start time for
venting. Default = 0 (Real) 
$\left[\text{s}\right]$ 
T_{stop}  Stop time for
venting. Default = 10^{30} (Real) 
$\left[\text{s}\right]$ 
$\text{\Delta}{P}_{def}$  Pressure difference to
open vent hole membrane. $\text{\Delta}{P}_{def}={P}_{def}{P}_{ext}$ Default = 0 (Real) 
$\left[\text{Pa}\right]$ 
$\text{\Delta}t{P}_{def}$  Minimum duration pressure
exceeds P_{def} to
open vent hole membrane. Default = 0 (Real) 
$\left[\text{s}\right]$ 
I_{dtPdef}  Time delay flag when
$\text{\Delta}{P}_{def}$
is reached:
(Integer) 

fct_ID_{t}  Porosity vs time function
identifier. (Integer) 

fct_ID_{P}  Porosity vs pressure
function identifier. (Integer) 

fct_ID_{A}  Porosity vs area function
identifier. (Integer) 

Fscale_{t}  Scale factor for
fct_ID_{t}. Default = 1.0 (Real) 

Fscale_{P}  Scale factor for
fct_ID_{P}. Default = 1.0 (Real) 

Fscale_{A}  Scale factor for
fct_ID_{A}. Default = 1.0 (Real) 

fct_ID_{t'}  Porosity vs time when
contact function identifier. (Integer) 

fct_ID_{P'}  Porosity vs pressure when
contact function identifier. (Integer) 

fct_ID_{A'}  Porosity vs impacted
surface function identifier. (Integer) 

Fscale_{t'}  Scale factor for
fct_ID_{t'}. Default = 1.0 (Real) 

Fscale_{P'}  Scale factor for
fct_ID_{P'}. Default = 1.0 (Real) 

Fscale_{A'}  Scale factor for
fct_ID_{A'}. Default = 1.0 (Real) 

surf_ID_{ps}  Porous surface
identifier. (Integer) 

Iform_{ps}  Porosity formulation.
(Integer) 

Iblockage  Block leakage flag, if
contact (Iform_{ps} > 0).
(Integer) 

surface_title  Porous surface
title. (Character, maximum 20 characters) 

fct_ID_{V}  Outflow velocity versus
relative pressure function identifier. (Integer) 

Fscale_{V}  Scale factor on
fct_ID_{V}. Default = 1.0 (Real) 
$\left[\frac{\text{m}}{\text{s}}\right]$ 
frame_ID  Frame identifier used to
define vectors
${V}_{1}$
,
${V}_{2}$
,
${V}_{3}$
and origin
$O$
. Default = global frame is used (Integer) 

kmesh  FVMBAG automatic meshing
option. 26 Only used if
grbric_ID = 0.
(Integer) 

T_{switch}  Amount of time after
airbag time to fire to switch from FVM to UP (Uniform Pressure)
formulation. 31 Default = 1e30 (Real) 
$\left[\text{s}\right]$ 
I_{switch}  Flag to switch from FVM to UP.
(Integer) 

P_{switch}  Ratio of FV standard
deviation pressure to average pressure which triggers FVM to UP
switch. 32 Default = 0.0 (Real) 

L_{1}  Length
L_{1}. (Real) 
$\left[\text{m}\right]$ 
L_{2}  Length
L_{2}. (Real) 
$\left[\text{m}\right]$ 
L_{3}  Length
L_{3}. (Real) 
$\left[\text{m}\right]$ 
Nb_{1}  Number of finite volumes
in direction 1. Default = 1 (Integer) 

Nb_{2}  Number of finite volumes
in direction 2. Default = 1 (Integer) 

Nb_{3}  Number of finite volumes
in direction 3. Default = 1 (Integer) 

grbric_ID  Userdefined solid group
identifier. (Integer) 

surf_ID_{in}  Internal surfaces
identifier. 27 (Integer) 

I_{ref}  Flag for applying the
automated FVM mesh on the reference geometry. Only used if,
kmesh=1. 25
(Integer) 

I_{gmerg}  Global merging formulation
flag. 20
(Integer) 

C_{gmerg}  Factor for global merging.
20 (Real) 

C_{nmerg}  Factor for neighborhood
merging. 20 (Real) 

P_{tole}  Tolerance for finite
volume identification. Default = 10^{5} (Real) 

q_{a}  Quadratic bulk
viscosity. Default = 0.0 (Real) 

q_{b}  Linear bulk
viscosity. Default = 0.0 (Real) 

H_{min}  Minimum height for
triangle permeability. 22 (Real) 
$\left[\text{m}\right]$ 
$\text{\Delta}{T}_{sca}$  Scale factor for airbag
time step. Using /DT/FVMBAG in the Engine will override this value. Default = 0.9 

$\text{\Delta}{T}_{\mathrm{min}}$  Minimum time step for the
airbag. Using /DT/FVMBAG in the Engine will override this value. 

I_{lvout}  Output level.
(Integer) 

N_{layer}  Estimated number of layers
in airbag folding along direction
${V}_{3}$
. 23 Default = 10 (Integer) 

N_{facmax}  Estimated maximum number
of airbag segments concerned by a finite volume in the first
automatic meshing step. Default = 20 (Integer) 

N_{ppmax}  Estimated maximum number
of vertices of a polygon. Default = 20 (Integer) 

I_{fvani}  Write finite volumes in
Radioss Starter Animation A000 File flag.
(Real) 
Comments
 The airbag external surface should be built only from 4 and 3noded shell elements. The airbag external surface cannot be defined with option /SURF/SEG, nor with /SURF/SURF, if a subsurface is defined in /SURF/SEG.
 External surfaces shall compose a closed volume with normals must oriented outwards.
 Abscissa scale factors are used
to transform abscissa units in airbag functions, for example:
(1) $$\text{F}(t\prime )={\text{f}}_{t}\left(\frac{t}{{\mathit{Ascale}}_{t}}\right)$$Where, $t$
 Time
 ${\mathrm{f}}_{t}$
 Function of fct_ID_{t}
(2) $$\text{F}(P\prime )={\text{f}}_{P}\left(\frac{P}{{\mathit{Ascale}}_{P}}\right)$$Where, $P$
 Pressure
 ${\mathrm{f}}_{P}$
 Function of fct_ID_{P}
 Pressure and temperature of external air and the initial pressure and temperature of air inside of airbag is set to P_{ext} and T_{0}.
 The gas flow in
FVMBAG1 is solved using finite volumes.
Some of these finite volumes can be entered by you through a group of solids, located inside the airbag and filling a part or the total internal volume. If there still exists a part of the internal volume which is not discretized by userdefined solids, an automatic meshing procedure produces the remaining volumes. This can be used for example to model a canister.
A finite volume consists in a set of triangular facets. Their vertices do not necessarily coincide with the nodes of the airbag. The airbag envelope can be modeled with 4node or 3node membranes; however, 3 nodes are recommended.  Venting through vent
holes:
If I_{form} = 1, venting velocity is computed from Bernoulli equation using local pressure in the airbag.
The exit velocity is given by:(3) $${u}^{2}=\frac{2\gamma}{\gamma 1}\frac{P}{\rho}\left(1{\left(\frac{{P}_{\mathit{ext}}}{P}\right)}^{\frac{\gamma 1}{\gamma}}\right)$$The mass out flow rate is given by:
If I_{form} = 2, venting velocity is computed from the Chemkin equation:(4) $$v=Fscal{e}_{v}\cdot {f}_{v}(P{P}_{ext})$$Where, ${f}_{v}$ is defined by fct_ID_{v}.
If I_{form} = 3, venting velocity is equal to the component of the local fluid velocity normal to vent hole surface. Local density and energy are used to compute outgoing mass and energy through the hole.
 When there is no sensor which activates gas injection, the vent holes and porosity becomes active, if time T becomes greater than the T_{start}, or if the pressure P exceeds P_{def} value longer than the time given in $\text{\Delta}t{P}_{def}$ .
 When at least one of the
injectors is activated by the sensor, then activation of venting and porosity
options is controlled by
I_{ttf}.
T_{inj} is the time of the first injector to be activated by the sensor.
I_{ttf} = 0Venting, Porosity Activation When $P>\text{\Delta}{P}_{def}$ longer than the time $\text{\Delta}t{P}_{def}$ , or $T>{T}_{start}$ Deactivation T_{stop} Time dependent functions No shift I_{ttf} = 3Venting, Porosity Activation When $T>{T}_{inj}$ and $P>\text{\Delta}{P}_{def}$ longer than the time $\text{\Delta}t{P}_{def}$ , or $T>{T}_{inj}+{T}_{start}$ Deactivation ${T}_{inj}+{T}_{stop}$ Time dependent functions Shifted by ${T}_{inj}+{T}_{start}$ All other related curves are active when the corresponding venting, porosity or communication option is active.
The variety of I_{ttf} values comes from historical reasons. Values I_{ttf}=1 and 2 are obsolete and should not be used. Usual values are I_{ttf}=0 (no shift) or I_{ttf}=3 (all relative options are shifted by T_{inj}).
 If surf_ID_{v} ≠ 0 (surf_ID_{v} is defined) the vent hole area is
computed as:
(5) $$vent\_holes\_area\text{}={A}_{vent}\cdot {\mathrm{f}}_{A}\left(\frac{A}{{A}_{0}}\right)\cdot {\mathrm{f}}_{t}\left(t\right)\cdot {\mathrm{f}}_{P}\left(P{P}_{ext}\right)$$Where, $A$
 Area of surface surf_ID_{v}
 ${A}_{0}$
 Initial area of surface surf_ID_{v}
 ${\mathrm{f}}_{t}$ , ${\mathrm{f}}_{P}$ and ${\mathrm{f}}_{A}$
 Functions of fct_ID_{t}, fct_ID_{P} and fct_ID_{A}
 In the case of activated venting closure
the vent holes surface is computed as:
(6) $$vent\_holes\_area\text{}={A}_{vent}\cdot {A}_{non\_impacted}\cdot {\mathrm{f}}_{t}\left(t\right)\cdot {\mathrm{f}}_{P}\left(P{P}_{ext}\right)\cdot {\mathrm{f}}_{A}\left(\frac{{A}_{non\_impacted}}{{A}_{0}}\right)$$(7) $$+{B}_{\mathit{vent}}\cdot {A}_{\mathit{impacted}}\cdot {\text{f}}_{{t}^{\prime}}\left(t\right)\cdot {\text{f}}_{{P}^{\prime}}\left(P{P}_{\mathit{ext}}\right)\cdot {\text{f}}_{{A}^{\prime}}\left(\frac{{A}_{\mathit{impacted}}}{{A}_{0}}\right)$$With impacted surface:(8) $${A}_{\mathit{impacted}}=\sum _{e\in {S}_{\mathit{vent}}}\frac{{n}_{c}\left(e\right)}{n\left(e\right)}{A}_{e}$$and nonimpacted surface:(9) Where for each element e of the vent holes surf_ID_{v}, ${n}_{c}\left(e\right)$ means the number of impacted nodes among the $n\left(e\right)$ nodes defining the element.$${A}_{\mathit{non}\_\mathit{impacted}}=\sum _{e\in {S}_{\mathit{vent}}}\left(1\frac{{n}_{c}\left(e\right)}{n\left(e\right)}\right){A}_{e}$$A_{0} is the initial area of surface surf_ID_{v}
f_{t}, f_{P} and f_{A} are functions of fct_ID_{t}, fct_ID_{P} and fct_ID_{A}
f_{t'}, f_{P'} and f_{A'} are functions of fct_ID_{t'}, fct_ID_{P'} and fct_ID_{A'}
 Radioss ends with a Starter error, if surf_ID_{v} = 0 (surf_ID_{v} is not defined) (I_{form}=1 or 2).
 Functions fct_ID_{t} and fct_ID_{P} are equal to 1, if they are not specified (null identifier).
 Function fct_ID_{A} is assumed to be equal to 1, if it is not specified.
 To account for contact blockage of vent holes and porous surface areas, flag I_{BAG} must be set to 1 in the correspondent interfaces (Line 3 of interface /INTER/TYPE7 or /INTER/TYPE23). If not, the nodes impacted into the interface are not considered as impacted nodes in the previous formula for A_{impacted} and A_{non_impacted}.
 Leakage by porosity
formulations, the mass flow rate flowing out is computed as:
 Iform_{ps} = 1 ${\dot{m}}_{\mathit{out}}={A}_{\mathit{eff}}\sqrt{2P\rho}{Q}^{\frac{1}{\gamma}}\sqrt{\frac{\gamma}{\gamma 1}\left[1{Q}^{\frac{\gamma 1}{\gamma}}\right]}$ (Isentropic  Wang Nefske)
 Iform_{ps} = 2
${\dot{m}}_{\mathit{out}}={A}_{\mathit{eff}}\rho v(P{P}_{\mathit{ext}})$
Where, v is the outflow gas velocity (Chemkin)
 Iform_{ps} = 3 ${\dot{m}}_{\mathit{out}}={A}_{\mathit{eff}}\sqrt{2\rho (P{P}_{\mathit{ext}})}$ (Graefe)
The effective venting area A_{eff} is computed according to the input in the /LEAK/MAT input for fabric materials of TYPE19 or TYPE58.
 If leakage blockage is
activated, Iblockage=1, the effective venting
area is modified as:
(10) $${A}_{\mathit{eff}}={A}_{\mathit{non}\_\mathit{impacted}}$$${A}_{\mathit{non}\_\mathit{impacted}}$ is nonimpact surface 10
The blockage will be active only if flag I_{BAG} is set to 1 in the concerned contact interfaces (line 3 of interface TYPE7 and TYPE23).
 Automatic finite volume meshing parameters.
 The finite volumes are
generated in two steps.
 The first step generates vertices lying exclusively on the envelope of the
airbag. You can update the finite volume along with the deformation of the
envelope and correspond to the following procedure (displayed in 2D for
purpose of clarity):
This procedure requires the input of the direction ${V}_{3}$ , named cutting direction, and of the direction ${V}_{1}$ . A second direction ${V}_{2}$ in the plan normal to the cutting direction will be computed. In order to position the finite volumes and to determine the cutting width in both direction ${V}_{1}$ and ${V}_{2}$ , an origin O must be provided as well as a length L_{i}, counted both positively and negatively from the origin, and a number of steps N_{i}. The cutting width is then given by:
(11) $${W}_{i}=\frac{2{L}_{i}}{{N}_{i}}$$It is required that the box drawn in the horizontal plane (normal to ${V}_{3}$ ) by the origin O and the length L_{i}, counted both positively and negatively from O, includes the boundingbox of the envelope of the volume to mesh projected in this plane. This is necessary to ensure that this volume in entirely divided into finite volumes.
 The second step performs horizontal cutting of the finite volumes, and may be useless in many cases of tightly folded airbags. It is required especially when injection is made in a canister filled by the injected gas before unfolding the airbag.
This second step may generate vertices located inside the airbag. In order for them to be moved along with the inflation of the airbag, each is attached to a vertical segment (parallel to direction ${V}_{3}$ ) between two vertices lying on the envelope of the airbag (Figure 4). The local coordinates of the vertex within its reference segment remain constant throughout the inflation process.The horizontal cutting width is given by:(12) $${W}_{3}=\frac{2{L}_{3}}{{N}_{3}}$$It is not necessary that the segment given in the ${V}_{3}$ direction by the origin O and length L_{3}, counted both positively and negatively, includes the boundingbox of the envelope of the volume to mesh projection on the ${V}_{3}$ direction, since at the second step only existing finite volumes are cut.
 The first step generates vertices lying exclusively on the envelope of the
airbag. You can update the finite volume along with the deformation of the
envelope and correspond to the following procedure (displayed in 2D for
purpose of clarity):
 Actual vector ${V}_{1}$ used for automatic meshing is obtained after orthogonalization of the input vector with respect to vector ${V}_{3}$ .
 When a finite volume fails
during the inflation process of the airbag (volume becoming negative, internal mass
or energy becoming negative), it is merged to one of its neighbors so that the
calculation can continue. Two merging approaches are used:
 Global merge: a finite volume is merged if its volume becomes less than a certain factor multiplying the mean volume of all the finite volumes. The flag I_{gmerg} determines if the mean volume to use is the current mean volume (I_{gmerg} =1) or the initial mean (I_{gmerg} =2). The factor giving the minimum volume from the mean volume is C_{gmerg}.
 Neighborhood merge: a finite volume is merged if its volume becomes less than a certain factor multiplying the mean volume of its neighbors. The factor giving the minimum volume from the mean volume is C_{nmerg}.
 In the case of both C_{gmerg} and C_{nmerg} are not equal to 0, means both merging approaches will be used simultaneously. In case of a strong shock, it is recommended to set q_{a} = 1.1 and q_{b} = 0.05.
 When two layers of fabric are physically in contact, there should be no possible flow between finite volumes, which is numerically not the case because of interface gap. H_{min} represents a minimum height for the triangular facets below which the facet is impermeable. Its value should be close to the gap of the selfimpacting interface of the airbag.
 N_{layer}, N_{facmax}, and N_{ppmax} are memory parameters that help the finite volume creation process. Changing their value cannot cause the calculation to stop. Increasing the leads to a higher amount of memory and a smaller computation time for automatic meshing.
 During the finite volume creation process, plane polygons are first created, which are then assembled into closed polyhedra and decomposed into triangular facets. N_{ppmax} is the maximum number of vertices of these polygons.
 Automatic finite volume meshing based on reference geometry can be activated with flag I_{ref}=1. It only works with a reference geometry based on /REFSTA and /XREF. The flag is not supported when disjointed reference geometry /EREF is used. Note that for I_{ref}=1, the frame definition for automatic meshing should refer to nonfolded reference geometry.
 The option kmesh controls type of FVM meshing of internal airbag volume. The polyhedron meshing method, kmesh =1 was the default method used in 2017.2 and before. If grbric_ID ≠ 0, kmesh is ignored and the tetra FVM mesh is specified by the user created.
 Surface surf_ID_{in} is used to take internal surfaces or baffles into account as obstacles to the gas flow inside the monitored volume. Internal surfaces are taken into account in FVM only if the monitored volume is meshed automatically with polyhedron or if it is filled with solid elements, like TETRA4 (possibly HEXA and PENTA) with nodes coinciding with the monitored volume external and internal surface nodes (these solids must be declared in grbrick_ID). A porosity ranging from 0: no porosity up to 1: full porosity (vent) can be applied to internal surface fabrics only if their material model is LAW19 or LAW58. Injector surface can also be defined on an internal surface in which case the gas flow direction is opposite to the internal surface normal orientation.
 The lost heat flow is given
by:
(13) $$\dot{\text{Q}}\left(x,t\right)={H}_{\mathit{conv}}\cdot \text{Area}\left(x,t\right)\cdot \left(\text{T}\left(x,t\right){T}_{0}\right)$$  If
an element of a vent hole surface (surf_ID_{v}) belongs to an injector (surf_ID_{inj}) it will be ignored from the vent hole.
A constant correction factor f computed at time t=0 is applied to
the total vent hole surface:
(14) $$f=\frac{{S}_{\mathit{vent}}}{{S}_{\mathit{vent}}{\text{S}}_{\mathit{injector}}}$$  If an element of a porous surface also belongs to an injector (surf_ID_{inj}), it will be ignored from the porous surface.
 The time to switch T_{switch} to Uniform Pressure is relative to the time to fire. The switch is automatic, if there is only one FV left.
 P_{switch} is
the ratio of standard deviation of the Finite Volume pressures to the airbag
average pressure.
(15) $${P}_{switch}=\frac{\text{SD(FVpressure)}}{\text{Averagepressure}}$$This ratio can be output using the /TH/MONVOL variable UPCRIT. P_{switch} approaches zero as the pressure in each finite volume approaches the average pressure in the airbag.