/MONVOL/FVMBAG (Obsolete)

Block Format Keyword

/MONVOL/FVMBAG - Airbag with Gas Flow

Description

Describes the airbag with FVMBAG type. The input is similar to AIRBAG type.

Format

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
/MONVOL/FVMBAG/monvol_ID/unit_ID
monvol_title
surf_IDex
Ascalet		AscaleP		AscaleS		AscaleA		AscaleD
				Pext		T0		Iequi	Ittf
i		cpai		cpbi		cpci

Number of injectors

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

Define Njet injectors (four lines per injector)

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
		cpa		cpb		cpc
fct_IDmas	Iflow	Fscalemas		fct_IDT	FscaleT		sens_ID
Isjet
fct_IDvel		Fscalevel

Number of vent holes

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

Define Nvent vent holes (four lines per vent hole)

(1)	(2)	(3)	(4)	(5)	(7)	(8)	(9)	(10)
surf_IDv	Avent		Bvent			Itvent
Tvent		Pdef		tPdef	fct_IDV	FscaleV		IdtPdef
fct_IDt	fct_IDP	fct_IDA		Fscalet	FscaleP		FscaleA
fct_IDt’	fct_IDP'	fct_IDA'		Fscalet'	FscaleP'		FscaleA'

(1)	(2)	(3)	(4)	(5)	(6)
Vx3		Vy3		Vz3
Vx1		Vy1		Vz1
X0		Y0		Z0
L1		L2		L3
Nb1	Nb2	Nb3	grbrc_ID	surf_IDin	Iref

Other FVMBAG parameters

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
Igmerg		Cgmerg		Cnmerg		Ptole
qa		qb		Hmin
Ilvout	Nlayer	Nfacmax	Nppmax	Ifvani

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 (see Comment 1) (Integer)
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)
Pext	External pressure (Real)
T0	Initial temperature Default = 295 (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 jetting.
Ittf	Venting time shift flag. Active only when injection sensor is specified. = 0 or 1: time dependent porosity curves are not shifted by injection sensor activation time. Tvent and Tstop are ignored. = 2: time dependent porosity curves are shifted by Tinj (Tinj defined as the time of the first injector to be activated by the sensor). Tvent and Tstop are ignored. = 3: time dependent porosity curves are shifted by by Tinj + Tstart. Venting is stopped at Tinj + Tstop, when Tstop is specified.
i	Ratio of specific heats at initial temperature (see Comment 5) (Real)
cpai	cpa coefficient in the relation cpi(T) (Real)
cpbi	cpb coefficient in the relation cpi(T) (Real)
cpci	cpc coefficient in the relation cpi(T) (Real)
Njet	Number of injectors (Integer)
	Ratio of specific heats (Real)
cpa	cpa coefficient in the relation cp(T) (Real)
cpb	cpb coefficient in the relation cp(T) (Real)
cpc	cpc coefficient in the relation cp(T) (Real)
fct_IDmas	Mass of injected gas versus time identifier (Integer)
Iflow	Mass versus time function input type flag (Integer) = 0: mass is input = 1: mass flow is input
Fscalemas	Scale factor on mass function Default = 1.0 (Real)	or
fct_IDT	Temperature of injected gas versus time identifier (Integer)
FscaleT	Temperature scale factor Default = 1.0 (Real)
sens_ID	Sensor identifier to start injections (Integer)
Isjet	Injector surface identifier (must be different for each injectors) (Integer)
fct_IDvel	Injected gas velocity identifier (Integer)
Fscalevel	Injected gas scale factor Default = 1.0 (Real)
Nvent	Number of vent holes (Integer)
surf_IDv	Vent holes membrane surface (Real) or porous surface identifier (Integer)
Avent	if surf_IDv ≠ 0: scale factor on surface Default = 1.0 if surf_IDv = 0: surface of vent holes Default = 0.0 (Real)	, if surf_IDV = 0
Bvent	if surf_IDv ≠ 0: scale factor on impacted surface Default = 1.0 if surf_IDv = 0: Bvent is reset to 0 Default = 0.0 (Real)	, if surf_IDV = 0
Itvent	Venting formulation (see Comment 7) Default = 2 (Integer) = 1: 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. = 2: venting velocity is computed from Bernoulli equation using local pressure in the airbag. Local density and energy are used to compute outgoing mass and energy. = 3: venting velocity is computed from Chemkin equation
Tvent	Start time for venting Default = 0.0 (Real)
Pdef	Pressure difference to open vent hole membrane (Pdef = Pdef - Pext) (Real)
tPdef	Minimum duration pressure exceeds Pdef to open vent hole membrane (Real)
fct_IDV	Outflow velocity function identifier (Integer)
FscaleV	Scale factor on fct_IDV Default = 1.0 (Real)
IdtPdef	Time delay flag when Pdef is reached: = 0: pressure should be over Pdef during a tPdef cumulative time to activate venting = 1: venting is activated tPdef after Pdef 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)
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)
Vx3	X component of vector V3 (in global frame) (Real)
Vy3	Y component of vector V3 (in global frame) (Real)
Vz3	Z component of vector V3 (in global frame) (Real)
Vx1	X component of vector V1 (in global frame) (Real)
Vy1	Y component of vector V1 (in global frame) (Real)
Vz1	Z component of vector V1 (in global frame) (Real)
X0	X coordinate of local origin O (in global frame) (Real)
Y0	Y coordinate of local origin O (in global frame) (Real)
Z0	Z coordinate of local origin O (in global frame) (Real)
L1	Length L1 (Real)
L2	Length L2 (Real)
L3	Length L3 (Real)
Nb1	Number of finite volumes in direction 1 Default = 1 (Integer)
Nb2	Number of finite volumes in direction 2 Default = 1 (Integer)
Nb3	Number of finite volumes in direction 3 Default = 1 (Integer)
grbric_ID	User-defined solid group identifier (Integer)
surf_IDin	Internal surfaces identifier (see Comment 25) (Integer)
Iref	Flag for applying the automated FVM mesh on the reference geometry (see Comment 24) = 0: no = 1: yes Default = 0 (Integer)
Igmerg	Global merging formulation flag (see Comment 19) Default = 1 (Integer)
Cgmerg	Factor for global merging (see Comment 19) Default = 0.02 (Real)
Cnmerg	Factor for neighborhood merging (see Comment 19) (Real)
Ptole	Tolerance for finite volume identification Default = 10-5 (Real)
qa	Quadratic bulk viscosity Default = 0.0 (Real)
qb	Linear bulk viscosity Default = 0.0 (Real)
Hmin	Minimum height for triangle permeability (see Comment 21) (Real)
Ilvout	Output level: 0 or 1 Default = 0 (Integer)
Nlayer	Estimated number of layers in airbag folding along direction V3 (see Comment 22) Default = 10 (Integer)
Nfacmax	Estimated maximum number of airbag segments concerned by a finite volume in the first automatic meshing step. Default = 20 (Integer)
Nppmax	Estimated maximum number of vertices of a polygon Default = 20 (Integer)
Ifvani	Write finite volumes in RADIOSS Starter Animation A000 File flag (Integer) = 0: no = 1: yes

1.	surf_IDex must be defined using segments associated with 4-nodes or 3-nodes shell elements (possibly void elements).

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.

where, p is the pressure.

4.	The initial pressure is set to Pext.

5.	If i = 0, the characteristics of the gas initially filling the airbag are set to the characteristics of the gas by the first injector.

6.	The gas flow in FVMBAG 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 user-defined 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 4-node or 3-node membranes; however, 3-nodes are recommended.

monvol_airbag-env

monvol_airbag2

7.	The exit velocity is given by:

monvol_fvmbag_exit

The mass out flow rate is given by:

mout = v * vent_holes_surface * u

The energy flow rate is given by:

clip0517

The venting velocity is computed by:

mout = * vent_holes_surface * fct_IDV * FscaleV (P - Pext)

8.	Vent hole membrane is deflated if T > Tvent or if the pressure exceeds Pdef during more than tPdef.

9.	If surf_IDv ≠ 0 (surf_IDv is defined).

vent_holes_surface = Avent * fct_IDA(A/A0) * fct_IDt(t) * fct_IDP (P - Pext)

Where, A is the Area of surface surf_ID and A0 is the initial Area of surface surf_IDv.

10.	If surf_IDv = 0 (surf_IDv is not defined) vent hole is ignored.

vent_holes_surface = Avent * fct_IDt(t) * fct_IDP (P - Pext)

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

12.	Function fct_IDA is assumed as the fct_IDA(A/A0) = 1, if it is not specified.

13.	Vent holes surface is computed as follows:

vent_holes_surface = Avent * Anon_impacted * fct_IDt(Anon_impacted /A0) * fct_IDP (P - Pext)

+ Bvent * Aimpacted * fct_IDt’(Aimpacted /A0) * fct_IDP’ (P - Pext)

with impacted surface:

and non-impacted surface:

where for each element e of the vent holes surf_IDv, nc(e) means the number of impacted nodes among the n(e) nodes defining the element.

(see following figure: from nodes contact to impacted/non-impacted surface)

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

15.	In order to use porosity during contact, flag IBAG must be set to 1 in the interfaces concerned (Line 3 of interface Type 5 and Type 7). If not, the nodes impacted into the interface are not considered as impacted nodes in the previous formula for Aimpacted and Anon_impacted.

16.	Automatic finite volume meshing parameters.

monvol_finite_vol

17.	The finite volumes are generated in two steps.

•	The first step generates vertices lying exclusively on the envelope of the airbag. This allows to update the finite volume along with the deformation of the envelope and correspond to the following procedure (displayed in 2D for purpose of clarity):

monvol_step1

This procedure requires the input of the direction V3, named cutting direction, and of the direction V1. A second direction V2 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 V1 and V2, an origin O must be provided as well as a length Li, counted both positively and negatively from the origin, and a number of steps Ni. The cutting width is then given by Wi = 2Li / Ni

It is required that the box drawn in the horizontal plane (normal to V3 ) by the origin O and the length Li, counted both positively and negatively from O, includes the bounding-box 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 especially required 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 V3) between two vertices lying on the envelope of the airbag (see figure below). The local coordinates of the vertex within its reference segment remain constant throughout the inflation process.

monvol_fvmbag

The horizontal cutting width is given by W3 = 2L3 / N3. It is not necessary that the segment given in the V3 direction by the origin O and length L3, counted both positively and negatively, includes the bounding-box of the envelope of the volume to mesh projection on the V3 direction, since at the second step only existing finite volumes are cut.

18.	Actual vector V1 used for automatic meshing is obtained after orthogonalization of the input vector with respect to vector V3.

19.	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.

•	Neighborhood merging: 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 Cnmerg.

20.	In the case of both Cgmerg and Cnmerg are not equal to 0, means both merging approach will be used simultaneously. In case of a strong shock, it is recommended to set qa = 1.1 and qb = 0.05.

21.

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. Hmin represents a minimum height for the triangular facets below which the facet is impermeable. Its value should be close to the gap of the self-impacting interface of the airbag.

22.	Nlayer, Nfacmax, Nppmax 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.

23.	During the finite volume creation process, plane polygons are first created, which are then assembled into closed polyhedra and decomposed into triangular facets. Nppmax is the maximum number of vertices of these polygons.

24.	Iref set to 1 only works with a reference geometry based on /REFSTA (not yet supported if the reference geometry is based on /XREF) for monitored volumes types FVMBAG or FVMBAG1.

25.

Only applicable to the Finite Volume Method, 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 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. 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.

26.	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 c correction factor f computed at time t=0 is applied to the total vent hole surface: