/MONVOL/AIRBAG1

Block Format Keyword

/MONVOL/AIRBAG1 - Airbag

Description

Describes one-chambered airbag with hybrid input of injected gas. This keyword is similar to /MONVOL/AIRBAG, but has more flexible input.

•	Gas materials specified in separate /MAT/GAS cards

•	Composition of injected gas mixture and injector properties specified in separate /PROP/INJECT1 or /PROP/INJECT2 cards

Format

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
/MONVOL/AIRBAG1/monvol_ID/unit_ID
monvol_title
surf_IDex		Hconv
Ascalet		AscaleP		AscaleS		AscaleA		AscaleD
mat_ID				Pext		T0		Iequi	Ittf

Number of injectors

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
Njet

For each injector

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
inject_ID	sens_ID	Ijet	node_ID1	node_ID2	node_ID3

Jetting function data (read only if Ijet = 1)

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
fct_IDPt	fct_IDP	fct_IDP		FscalePt		FscaleP		FscaleP

Define Nvent vent holes and Nporsurf porous fabric surfaces

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
Nvent	Nporsurf

For each vent hole

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
surf_IDv	Iform	Avent		Bvent				vent_title
Tstart		Tstop							IdtPdef
fct_IDt	fct_IDP	fct_IDA		Fscalet		FscaleP		FscaleA
fct_IDt’	fct_IDP'	fct_IDA'		Fscalet'		FscaleP'		FscaleA'

Insert for each porous surface

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
surf_IDps	Iformps	Iblockage						surface_title
Tstart		Tstop							IdtPdef

Insert only if Iformps =0

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
Cps		Areaps		fct_IDcps	fct_IDaps	Fscalecps		Fscaleaps

Chemkin model date (insert only if Iform = 2 or Iformps =2)

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
fct_IDv		Fscalev

Field	Contents	SI Unit Example
monvol_ID	Monitored volume identifier (Integer, maximum 10 digits)
unit_ID	Optional unit identifier (Integer, maximum 10 digits)
monvol_title	Monitored volume title (Character, maximum 100 characters)
surf_IDex	External surface identifier (Integer)
Hconv	Heat transfer coefficient (Comment 18) (Real)
Ascalet	Abscissa scale factor for time based functions Default = 1.0 (Real)
AscaleP	Abscissa scale factor for pressure based functions Default = 1.0 (Real)
AscaleS	Abscissa scale factor for area based functions Default = 1.0 (Real)
AscaleA	Abscissa scale factor for angle based functions Default = 1.0 (Real)
AscaleD	Abscissa scale factor for distance based functions Default = 1.0 (Real)
mat_ID	Material identifier for initial gas (/MAT/GAS) (Real)
	Volumetric viscosity Default = 0.01 (Real)
Pext	External pressure (Real)
T0	Initial temperature Default = 295K (Real)
Iequi	Initial thermodynamic equilibrium flag (Integer) = 0: The mass of gas initially filling the airbag is determined with respect to the volume at time zero. = 1: The mass of gas initially filling the airbag is determined with respect to the volume at beginning of injection (Comment 6).
Ittf	Time shift flag (Integer) 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. Flag values = 0 (default), 1 and 2 (obsolete), and 3 (all options are shifted) are described in detail in Comment 16.
Njet	Number of injectors (Integer)
inject_ID	Injector property identifier (Integer)
sens_ID	Sensor identifier (Integer)
Nvent	Number of vent holes (Integer)
surf_IDv	Vent holes membrane surface identifier (Integer)
Iform	Formulation flag (Integer) = 0: Default, set to 1 = 1: Isenthalpic (default) = 2: Chemkin = 4: Isenthalpic with possible gas (mat_ID) flow in
Avent	if surf_IDv ≠ 0: scale factor on surface Default = 1.0 (Real) if surf_IDv = 0: surface of vent holes Default = 0.0 (Real)
Bvent	if surf_IDv ≠ 0: scale factor on impacted surface Default = 1.0 (Real) if surf_IDv = 0: Bvent is reset to 0 Default = 0.0 (Real)
vent_title	Vent hole title (Character, maximum 20 characters)
Tstop	Stop time for venting Default = 1030 (Real)
Tstart	Start time for venting Default = 0 (Real)
	Pressure difference to open vent hole membrane Default = 0 (Real)
	Minimum duration pressure exceeds Pdef to open vent hole membrane Default = 0 (Real)
IdtPdef	Time delay flag when is reached: (Integer) = 0: pressure should be over during a cumulative time to activate venting = 1: venting is activated after is reached
fct_IDt	Porosity vs time function identifier (Integer)
fct_IDP	Porosity vs pressure function identifier (Integer)
fct_IDA	Porosity vs area function identifier (Integer)
Fscalet	Scale factor for fct_IDt Default = 1.0 (Real)
FscaleP	Scale factor for fct_IDP Default = 1.0 (Real)
FscaleA	Scale factor for fct_IDA Default = 1.0 (Real)
Ijet	Jetting flag (Integer) = 0: no jetting = 1: jetting
node_ID1, node_ID2, node_ID3	Node identifiers N1, N2, and N3 for jet shape definition (Integer)
fct_IDPt	If Ijet = 1: identifier of the function number defining (Integer)
fct_IDP	If Ijet = 1: identifier of the function number defining (Integer)
fct_IDP	If Ijet = 1: identifier of the function number defining (Integer)
FscalePt	If Ijet = 1: scale factor for fct_IDPt Default = 1.0 (Real)
FscaleP	If Ijet = 1: scale factor for fct_IDP Default = 1.0 (Real)
FscaleP	If Ijet = 1: scale factor for fct_IDP Default = 1.0 (Real)
Nvent	Number of vent holes (Integer)
fct_IDt’	Porosity vs time when contact function identifier (Integer)
fct_IDP’	Porosity vs pressure when contact function identifier (Integer)
fct_IDA’	Porosity vs impacted surface function identifier (Integer)
Fscalet'	Scale factor for fct_IDt’ Default = 1.0 (Real)
FscaleP'	Scale factor for fct_IDP’ Default = 1.0 (Real)
FscaleA'	Scale factor for fct_IDA’ Default = 1.0 (Real)
fct_IDv	Outflow velocity function identifier (Chemkin model, only if Iform = 2) (Integer)
Fscalev	Scale factor on fct_IDv Default = 1.0 (Real)
Nporsurf	Number of porous surfaces (Integer)
surf_IDps	Porous surface identifier (ignored if Iformps =0) (Integer)
Iformps	Porosity formulation (Integer) = 0: Bernouilli (Wang & Nefske) (no dependence on fabric material) = 1: Bernouilli (Wang & Nefske) = 2: Chemkin = 3: Graefe
Iblockage	Block leakage flag if contact (Iformps > 0) (Integer) = 0: no = 1: yes
surface_title	Porous surface title (Character, maximum 20 characters)
Cps	Scale factor on leakage area (Iformps =0) (Real)
Areaps	Leakage area (Iformps =0) (Real)
fct_IDcps	Function identifier defining Cps(t), ignored if Cps is not equal to zero. (Integer)
fct_IDaps	Function identifier defining Areaps(P-Pext), ignored if Areaps is not equal to zero. (Integer)
Fscalecps	Scale factor for fct_IDcps Default = 1.0 (Real)
Fscaleaps	Scale factor for fct_IDaps Default = 1.0 (Real)

1.	The airbag external surface should be built only from 4- and 3-noded shell elements. The airbag external surface cannot be defined with /SURF/SEG, or with /SURF/SURF, if a sub-surface is defined in /SURF/SEG.

2.	The volume must be closed and the normals must be oriented outwards.

3.	Abscissa scale factors are used to transform abscissa units in airbag functions, for example:

where, t is the time and ft is function of fct_IDt

Where, P is the pressure and fP is the function of fct_IDP. The options are obsolete. Normally, the curve scaling parameters are used instead.

4.	Pressure and temperature of external air and the initial pressure and temperature of air inside of airbag is set to Pext and T0.

5.	Initial thermodynamic equilibrium is written at time zero (Iequi =0) or at beginning of jetting (Iequi =1), based on the following equation with respect to the volume at time zero, or the volume at beginning of jetting:

Where, M0 is the mass of gas initially filling the airbag, Mi is the molar mass of the gas initially filling the airbag, and R is the gas constant depending on the units system given in /BEGIN card. For example in SI system:

6.	If jetting is used, an additional pressure is applied to each element of the airbag:

With m being the normalized vector between the projection of the center of the element upon segment (N1 and N3) and the center of the element; the angle between vectors MN2 and m (in degrees), the distance between the center of the element and its projection upon segment (N1 and N3).

The projection of a point upon segment (N1 and N3) is defined as the projection of the point in direction MN2 upon the line (N1 and N3) if it lies inside the segment (N1 and N3). If this is not the case, the projection of the point upon segment (N1 and N3) is defined as the closest node N1 or N3 (see following figure: dihedral shape of the jet).

clip0087

with M between of N1 and N3

7.	If node_ID3 = 0, node_ID3 is set to node_ID1 and the dihedral shape is reduced to a conical shape.

8.	If fct_IDv = 0: isenthalpic outflow is assumed, else Chemkin model is used and outflow velocity is:

where fv is the function of fct_IDv

•	Isenthalpic model

Venting or the expulsion of gas from the volume is assumed to be isenthalpic.

The flow is also assumed to be unshocked, coming from a large reservoir and through a small orifice with effective surface area, A.

Conservation of enthalpy leads to velocity, u, at the vent hole. The Bernouilli equation is then written as:

(monitored volume) (vent hole)

Applying the adiabatic conditions:

(monitored volume) (vent hole)

Where, P is the pressure of gas into the airbag and is the density of gas into the airbag.

Therefore, the exit velocity is given by:

For supersonic flows the outlet velocity is determined as described in 10.4.4.1 of the Theory Manual.

The mass out flow rate is given by:

The energy flow rate is given by:

Where, V is the airbag volume and E is the internal energy of gas into the airbag.

•	Chemkin model

Where, is the density of the gas within the airbag and fv is the function of fct_IDv.

9.	Vent holes surface is computed as follows:

with impacted surface:

and non-impacted surface:

where for each element e of the airbag materials nc(e) means the number of impacted nodes among the n(e) nodes defining the element and Ae is the area of element e.

And,

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’

10.	Functions fct_IDt' and fct_IDP' are assumed to be equal to 1, if they are not specified (null identifier).

11.	Function fct_IDA' is assumed as:

if it is not specified

12.

To account for contact blockage of vent holes and porous surfaces, flag IBAG must be set to 1 in the correspondent interfaces (Line 3 of interface TYPE7 or TYPE23). If not, the nodes impacted into the interface are not considered as impacted nodes in the previous formula for Aimpacted and Anon_impacted.

13.	Vent holes should be included into the airbag external surface

14.	When there is no sensor which activates gas injection, the vent hole membrane is deflated, if time T becomes greater than the Tstart or if the pressure P exceeds Pdef value longer than the time given in .

15.	When at least one of the injectors is activated by the sensor, then the 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 P > 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).

16.	Leakage by porosity formulations; the mass flow rate flowing out is computed as:

•	Iformps = 0

(Isentropic - Wang Nefske)

with

and or

Note that the effective venting area Aeff does not vary with different airbag fabric materials.

if Iformps > 0, the effective venting area Aeff is computed according to the input in the /LEAK/MAT input for fabric materials of TYPE19 or TYPE58.

•	Iformps = 1 (Isentropic - Wang Nefske)

•	Iformps = 2 where, is the outflow gas velocity (Chemkin)

•	Iformps = 3 (Graefe)

17.	If leakage blockage is activated, Iblockage=1, the effective venting area is modified as follows:

Anon_impacted is non-impacted surface (Comment 11).

The blockage will be active only if flag IBAG is set to 1 in the concerned contact interfaces (line 3 of interface TYPE7 and TYPE23).

18.	The lost heat flow is given by:

/MONVOL/AIRBAG1

/MONVOL/AIRBAG1

/MONVOL/AIRBAG1

Format

Number of injectors

Venting, Porosity

Venting, Porosity

See Also: