Recommendations for Airbag Numerical Simulation with Uniform and Variable Gas Pressure using RADIOSS

Innovation Intelligence®

Recommendations for Airbag Numerical

Simulation with Uniform and Variable Gas

Pressure using RADIOSS

2014 European Altair Technology Conference

June, 25 2014

Francis Arnaudeau

Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

Agenda

• Motivation for FVM

• RADIOSS Input

• New features in V12 & V13

• Vent Holes & Porous Surfaces

• Examples

1. Driver AirBag

2. Passenger AirBag

3. Side AirBag

4. Curtain AirBag

• Conclusion


INTRODUCTION

First airbag models have been made with the assumption of uniform

pressure (UP)

Uniform Pressure assumption is not accurate enough in:

• Out of position configurations (OOP)

• Side impacts

• Curtain airbags CAB

• SAB

• Opening of airbag cover (e.g. passenger airbags PAB)

Solution: take into account gas flow, in particular for the first

milliseconds of the airbag deployment


RADIOSS FVM

EULER CONSERVATION EQUATIONS COMPRESSIBLE INVISCID FLOW

(Mass)

(Momentum)

(Energy)

PERFECT GAS EOS

0).(

u

t

0).(

f

puu

t

u

0).().(

fupueu

t

e

ep )1(

4

2

32 TCT

CTCTCTCCC pf

pe

pdpcpbpap

vp CC / RCC pv


RADIOSS INPUT

• /MONVOL/AIRBAG, /MONVOL/COMMU, /MONVOL/FVMBAG

• No new developments

• /MONVOL/AIRBAG1, /MONVOL/COMMU1, /MONVOL/FVMBAG1

• Injected gas are defined in separated input : /MAT/GAS

• /MAT/GAS/PREDEF

• /MAT/GAS/MASS

• /MAT/GAS/MOLE

• /MAT/GAS/CSTA

• Injectors are defined in separated input : /PROP/INJECT

• /PROP/INJECT1

• /PROP/INJECT2

• Porous fabric

• Loss heat flow


RADIOSS INPUT - FVM

Internal gas mesh

• Automatic Mesher : input a frame and # of volumes in each direction

• Solid 3D elements : Brick, Pentahedron or Tetrahedron

• Can be performed on a closed reference geometry (iref)

Merging parameters

• cgmerg , cnmerg

• Can be modified in Engine run : /FVMBAG/MODIF [v12.0.202]

• Merge can be activated with Dtmin input keyword /DT/FVMBAG [v12.0.202]

Other inputs can be default

Internal surfaces : diffusers, tethers, inflators Isurfi

• Works only with solid elements gas mesh (tetra, hexa <=> HyperMesh)

• Injectors on Internal surfaces [v12.0.202]

• Can be porous, porosity input in fabric material law

Remaining input identical to UP Airbags


RADIOSS INPUT – FVM - AUTOMESH

Define a Frame (O, V1, V2, V3)

Define boxes (N1, N2, N3)

Generate points at the intersection of the boxes and the bag

Generate Finite Volumes

(polyhedra)


RADIOSS INPUT - FVM

• AVAILABLE FV IN /GRBRIC INPUT

• Hexahedron /BRICK

• Tetrahedron /TETRA4

• Pentahedron

• Triangular prism /PENTA6

/BRICK N1,N2,N3,N1,N4,N5,N6,N4

• Pyramid [v14] /BRICK N1,N2,N3,N4,N5,N5,N5,N5


RADIOSS INPUT - FVM – Internal surfaces

Injection


RADIOSS - New in Versions 12 & 13

• Contact : /INTER/TYPE23 [v12.0.210]

• Fully integrated quad membrane (Batoz)

• Lost Heat Flow

• Hconv [v12.0.210]

• Air flow through vent holes inside the bag [v13.0]

• UP Airbags

• /MONVOL/COMMU1 [v12.0.202]

• /EREF/SHELL /EREF/SH3N [v12.0.210]

• Leakage models /LEAK/MAT [v12.0]

• /MONVOL/AIRBAG1 Nporsurf

• FVM Airbags

• /FVMBAG/MODIF [v12.0.202]

• /DT/FVMBAG [v12.0.202]

• Leakage models /LEAK/MAT [v13.0.210]

• /MONVOL/FVMBAG1 Nporsurf

• Injectors on Internal surfaces Isurfi [v12.0.202]


RADIOSS INPUT : AIRBAG FOLDING

• Folding with a RADIOSS pre-simulation

• Contact type 7 + 11 => no intersections


EXAMPLE 1 : DRIVER AIRBAG

• FOLDED WITH HYPERCRASH

• BAG CONTACT TYPE 7 + 11

• FVM

• Automatic mesher on reference geometry

• 3583 polyhedrons







• UP – FVM ( bag contact type 7 + 11)



• Windows 1 CPU : Intel(R) Core(TM) i7-4900MQ CPU @ 2.80GHz (x86_64)

• Final simulation time 150ms

• Ratio FVM/UP : 1.71

Bag Contact

Type

Elapsed Dt Average

UP 7+11 3h 3’ 0.497ms

FVM 7+11 5h 12’ 0.495ms


EXAMPLE 2 : PASSENGER AIRBAG

• FOLDING THE BAG WITH RADIOSS

The 3D passenger airbag (PAB) is meshed on it reference geometry using

HyperMesh. Rigid surfaces and the airbag box are added around the meshed airbag.

Displacement is imposed the top of the rigid surface to push the bag in its box. A

depression is applied in the airbag.

Imposed

displacement

Airbag mesh

Rigid surface (offset of

the PAB mesh)

Airbag

box (rigid)

Monitored volume with

negative pressure)



• Model description

• The folded geometry of the bag is extracted from the RADIOSS ascii output file (.sta)

• The initial geometry of the airbag is used as reference (/REFSTA or /XREF)

• The option "zerostress" is activated in order to remove stress in the airbag coming

from the differences between the initial and reference shape of the elements.

• The airbag model is integrated in the car

Airbag box

(deformable) IP cover (with

rupture)

Car environment (rigid)



• Two models are made:

• A Uniform Pressure (UP) model is made with Monitored Volume type AIRBAG1

• The gas is meshed with 26304 tetrahedrons and a Finite Volume model (FVM) is

made with Monitored Volume type FVMBAG1

Gas



• UP : « ibag » influence (bag with contact type 7 + 11)



• UP : bag contact type influence



• UP - FVM (contact type 23 and ibag=1)









• Windows 1 CPU : Intel(R) Core(TM) i7-4900MQ CPU @ 2.80GHz (x86_64)

• Final simulation time 150ms

• Ratio FVM/UP : 3.72

Bag Contact

Type

Ibag Elapsed Dt Average

UP 7+11 0 1h 34’ 0.887ms

UP 7+11 1 1h 9’ 0.888ms

UP 23 1 1h 2’ 0.883ms

FVM 23 1 3h52’ 0.538ms


EXAMPLE 3 : SIDE AIRBAG

• Injection is made in an internal diffuser which

must be taken into account by the gas flow

• Input an internal surface « isurfi »

• Automatic gas mesher is not compatible with

internal surface

• Mesh the gas with HyperMesh

• « cgmerg & cnmerg » set to 1.e-10 in STARTER

input

• « cgmerg & cnmerg » to be given in ENGINE

input : /FVMBAG/MODIF


EXAMPLE 4 : CURTAIN BAG

MESH#1 ON INITIAL GEOMETRY

639 FV

NB1= 61

NB2= 24

1

2

1 volume !

NB2 is too small



MESH#2 MESH ON REFERENCE GEOMETRY

991 FV

NB1= 50

NB2= 50

1

2



MESH#2 MESH ON REFERENCE GEOMETRY

991 FV

NB1= 50

NB2= 50




HINTS

UP&FVM

No initial intersections

Contact : type7 + type11

FVM

Perform a preliminary UP run (AIRBAG1 or COMMU1)

Injection velocity : input sound speed at injection temperature

If internal surfaces

• Use Tetra mesh

• Start merging in the Engine run (to avoid initial merging of FV on both sides of an

internal surface)

• Injection of gas occurs in the opposite direction of the normal

Unstable gas flow computation

• Reduce time step with Dtscale (/DT/FVMBAG)

• Increase Dtmin (/DT/FVMBAG)