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.
Amount of time after
airbag time to fire to switch from FVM to UP (Uniform Pressure)
formulation. 20
Default = 1e30
(Real)
Iswitch
Flag to switch from FVM to UP.
= 0 (Default)
No switch to uniform pressure. The finite volume method
is used.
= 1
Switch to uniform pressure is performed when either
Pswitch
or
Tswitch
criteria is reached.
= 2
Switch to single finite volume is performed when either
Pswitch
or
Tswitch
criteria is reached.
(Integer)
Pswitch
Ratio of FV standard
deviation pressure to average pressure which triggers FVM to UP
switch. 22
Default = 0.0
(Real)
Scale factor for airbag
time step.
Using /DT/FVMBAG in the Engine will
override this value.
Default = 0.9
Minimum time step for the
airbag.
Using /DT/FVMBAG in the Engine will
override this value.
Comments
The airbag external surface
should be built only from 4- and 3-noded shell elements. The airbag external
surface cannot be defined with option /SURF/SEG, nor with
/SURF/SURF, if a sub-surface is defined in
/SURF/SEG.
External surfaces shall
compose a closed volume with normals must oriented outwards.
The correct model units
must defined in /BEGIN or a local /UNIT
referenced by unit_ID. The gas constants, injection velocity, and predefined gas materials are set
based on the units defined in the model.
Pressure and temperature of
external air and the initial pressure and temperature of air inside of airbag is
set to Pext and T0.
Venting through vent
holes:
If Iform = 1,
venting velocity is computed from Bernoulli equation using local pressure in
the airbag.
The exit velocity is given by:
The mass out flow rate is given by:
If Iform = 2,
venting velocity is computed from the Chemkin equation:
Where, is defined by
fct_IDv.
If
Iform = 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 Tstart, or if the pressure
P exceeds
Pdef value longer than the
time given in .
When at least one of the
injectors is activated by the sensor, then activation of venting and porosity
options is controlled by
Ittf.
Tinj
is the time of the first injector to be activated by the
sensor.
Ittf =
0
Venting,
Porosity
Activation
When longer than the time , or
Deactivation
Tstop
Time dependent
functions
No
shift
Ittf =
3
Venting,
Porosity
Activation
When and longer than the time , or
Deactivation
Time dependent
functions
Shifted by
All other related curves are active when the corresponding
venting, porosity or communication option is active.
The variety of
Ittf values comes from
historical reasons. Values
Ittf=1
and 2 are obsolete and should not be used. Usual values are
Ittf=0
(no shift) or
Ittf=3
(all relative options are shifted by
Tinj).
If surf_IDv ≠ 0 (surf_IDv is defined) the vent hole area is
computed as:
Where,
Area of surface surf_IDv
Initial area of surface surf_IDv
, and
Functions of
fct_IDt,
fct_IDP and
fct_IDA
In the case of activated
venting closure the vent holes surface is computed as:
With impacted surface:
and non-impacted surface:
Figure 1. Where for each element e of the vent holes surf_IDv, means the number of impacted nodes among the nodes defining the
element.
A0 is the initial area of
surface surf_IDv
ft,
fP and
fA are functions of
fct_IDt,
fct_IDP and
fct_IDA
ft',
fP' and
fA' are functions of
fct_IDt',
fct_IDP' and
fct_IDA'
Radioss ends with a Starter error, if surf_IDv = 0 (surf_IDv is not defined) for Chemkin venting
formulation (Iform=2).
Functions
fct_IDt and
fct_IDP are equal to
1, if they are not specified (null identifier).
Function
fct_IDA 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
IBAG 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 Aimpacted
and Anon_impacted.
Leakage by porosity
formulations, the mass flow rate flowing out is computed as:
Iformps =
1 (Isentropic - Wang Nefske)
Iformps = 2
Where, v is the outflow gas velocity
(Chemkin)
Iformps =
3 (Graefe)
The effective venting area
Aeff 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:
is non-impact surface
The
blockage will be active only if flag
IBAG is set to
1 in the concerned contact interfaces (line 3 of
interface TYPE7, TYPE19 and
TYPE23).
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 merging: 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
Igmerg determines
if the mean volume to use is the current mean volume
(Igmerg =1) or the
initial mean (Igmerg
=2). The factor giving the minimum volume from the mean volume is
Cgmerg.
Time step dependent merging: if the time step for a finite volume
becomes less than the value defined in /DT/FVMBAG,
the finite volume is merged with neighboring finite volumes.
The lost heat flow is
given by:
If an element of a vent
hole surface (surf_IDv) belongs to an injector (surf_IDinj) 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:
If an element of a porous
surface also belongs to an injector (surf_IDinj), it will be ignored from the porous
surface.
The time to switch
Tswitch to Uniform Pressure
is relative to the time to fire.
With option
Iswitch=2,
the airbag is always computed with finite volume method, even when only 1 finite
volume remains. The gas parameters are identical before and after switching to a
single finite volume. Some variation of pressure or gas parameters may be seen
with a switch to uniform pressure method
(Iswitch=1).
Pswitch is
the ratio of standard deviation of the Finite Volume pressures to the airbag
average pressure.
This ratio can be output using the
/TH/MONVOL variable UPCRIT.
Pswitch approaches zero
as the pressure in each finite volume approaches the average pressure in the
airbag.