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

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In the passive safety of vehicles airbags are playing major roles. Their functioning has to be considered in the car models to improve and control their efficiency in realistic environments. For …

In the passive safety of vehicles airbags are playing major roles. Their functioning has to be considered in the car models to improve and control their efficiency in realistic environments. For airbags design HyperWorks thru RADIOSS solver proposes the best tool kit.

It consists, for their deployment, of the simplest approach with the Uniform Pressure to the more complex and physical one with the Finite Volume Method. This latest modeling techniques allows to take into account the gas flow inside the airbag, providing very accurate results; this is particularly important for side airbags, curtain airbags and out of position studies for driver and passenger airbags, were the first phases of the deployment are crucial for the airbag performance assessment.

A series of features like heat loss and leakage thru the tissue as well complex geometries with internal chambers have been developed to give the engineers the opportunity to make different assumptions in their models. The presentation will give some recommendations in term of airbag modeling respecting the physic which is behind.

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  • 1. 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
  • 2. 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
  • 3. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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
  • 4. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. RADIOSS FVM EULER CONSERVATION EQUATIONS COMPRESSIBLE INVISCID FLOW (Mass) (Momentum) (Energy) PERFECT GAS EOS 0).(    u t   0).(      f p uu t u  0).().(    fupueu t e   ep  )1(   4 2 32 TC T C TCTCTCCC pf pe pdpcpbpap  vp CC / RCC pv 
  • 5. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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
  • 6. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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
  • 7. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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)
  • 8. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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
  • 9. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. RADIOSS INPUT - FVM – Internal surfaces Injection
  • 10. Copyright © 2013 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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]
  • 11. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. RADIOSS INPUT : AIRBAG FOLDING • Folding with a RADIOSS pre-simulation • Contact type 7 + 11 => no intersections
  • 12. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 1 : DRIVER AIRBAG • FOLDED WITH HYPERCRASH • BAG CONTACT TYPE 7 + 11 • FVM • Automatic mesher on reference geometry • 3583 polyhedrons
  • 13. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 1 : DRIVER AIRBAG
  • 14. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 1 : DRIVER AIRBAG
  • 15. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 1 : DRIVER AIRBAG • UP – FVM ( bag contact type 7 + 11)
  • 16. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 1 : DRIVER AIRBAG • 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
  • 17. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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)
  • 18. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG • 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)
  • 19. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG • 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
  • 20. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG • UP : « ibag » influence (bag with contact type 7 + 11)
  • 21. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG • UP : bag contact type influence
  • 22. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG • UP - FVM (contact type 23 and ibag=1)
  • 23. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG
  • 24. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG
  • 25. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG
  • 26. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 2 : PASSENGER AIRBAG • 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
  • 27. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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
  • 28. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 4 : CURTAIN BAG MESH#1 ON INITIAL GEOMETRY 639 FV NB1= 61 NB2= 24 1 2 1 volume ! NB2 is too small
  • 29. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 4 : CURTAIN BAG MESH#2 MESH ON REFERENCE GEOMETRY 991 FV NB1= 50 NB2= 50 1 2
  • 30. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 4 : CURTAIN BAG MESH#2 MESH ON REFERENCE GEOMETRY 991 FV NB1= 50 NB2= 50
  • 31. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. EXAMPLE 4 : CURTAIN BAG
  • 32. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. 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)

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