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/MONVOL/AIRBAG1

/MONVOL/AIRBAG1

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/MONVOL/AIRBAG1

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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

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/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

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Njet

 

 

 

 

 

 

 

 

 

 

For each injector

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inject_ID

sens_ID

Ijet

node_ID1

node_ID2

node_ID3

 

 

 

 

 

Jetting function data (read only if Ijet = 1)

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fct_IDPt

fct_IDPclip0556

fct_IDPsymbol_s

 

FscalePt

FscalePclip0556

FscalePsymbol_s

 

Define Nvent vent holes and Nporsurf porous fabric surfaces

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Nvent

Nporsurf

 

 

 

 

 

 

 

 

 

For each vent hole

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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

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surf_IDps

Iformps

Iblockage

 

 

 

 

 

surface_title

Tstart

Tstop

 

IdtPdef

 

Insert only if Iformps =0

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Cps

Areaps

fct_IDcps

fct_IDaps

Fscalecps

Fscaleaps

 

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

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fct_IDv

 

Fscalev

 

 

 

 

 

 

hmtoggle_plus1Flag Definition

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_IDPclip0556

If Ijet = 1: identifier of the function number defining

(Integer)

 

fct_IDPsymbol_s

If Ijet = 1: identifier of the function number defining

(Integer)

 

FscalePt

If Ijet = 1: scale factor for fct_IDPt

Default = 1.0  (Real)

FscalePclip0556

If Ijet = 1: scale factor for fct_IDPclip0556

Default = 1.0  (Real)

FscalePsymbol_s

If Ijet = 1: scale factor for fct_IDPsymbol_s

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)

hmtoggle_plus1Comments
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:

Image12

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, ratio 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:

See Also:

Airbag modeling in User's Guide

FVM Airbag Guidelines

AIRBAG type Monitored Volume in Theory Manual

Example 27 - Football Shots