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Radioss Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
LS-DYNA Input
This manual provides a list of LS-DYNA input files available in Radioss.
*AIRBAG
*AIRBAG_LINEAR_FLUID_ID
LS-DYNA Input Interface KeywordThis model defines a control volume filled with a liquid.

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This manual provides details on the features, functionality, and simulation methods available in Altair Radioss.
Radioss Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
- File Extensions and Formats
  Current Radioss format uses the 12x extension format.
- Single File Input
  This format allows running either Starter or Engine with the same file.
- New Keywords in 2026
  New and modified features in Radioss.
- Starter Input
  This manual provides a list of all the model definition keywords and options available in Radioss.
- Engine Input
  This manual provides a list of all the solution definition keywords and options available in Radioss.
- LS-DYNA Input
  This manual provides a list of LS-DYNA input files available in Radioss.
  - *AIRBAG
    - *AIRBAG_ADIABATIC_GAS_MODEL_ID
      LS-DYNA Input Interface KeywordThis model defines a one-chambered airbag filed by adiabatic gas. Uniform pressure method is used.
    - *AIRBAG_LINEAR_FLUID_ID
      LS-DYNA Input Interface KeywordThis model defines a control volume filled with a liquid.
    - *AIRBAG_LOAD_CURVE_ID
      LS-DYNA Input Interface KeywordThis model defines a one-chambered airbag with pressure specified as function of time.
    - *AIRBAG_PARTICLE
      LS-DYNA Input Interface KeywordDefines a Finite Volume Method airbag.
    - *AIRBAG_SHELL_REFERENCE_GEOMETRY
      LS-DYNA Input Interface KeywordDefine the reference state for elements belonging to a part using given element and node identifiers with possible scaling for the nodes with defined scale center and scale factor.
    - *AIRBAG_SIMPLE_AIRBAG_MODEL
      LS-DYNA Input Interface KeywordThis model defines a one-chambered airbag with uniform pressure method, injected gas and vent hole.
    - *AIRBAG_SIMPLE_PRESSURE_VOLUME
      LS-DYNA Input Interface KeywordDescribes a one-chambered airbag with pressure as a function of volume.
  - *BOUNDARY
  - *CONSTRAINED
  - *CONTACT
  - *CONTROL
  - *DAMPING
  - *DATABASE
  - *DEFINE
  - *ELEMENT
  - *EOS
  - *HOURGLASS
    LS-DYNA Input Interface KeywordDefine solid and hourglass formulation for 8-node solid elements.
  - *INCLUDE
  - *INITIAL
  - *INTEGRATION_SHELL
    LS-DYNA Input Interface KeywordDefine user-defined through the thickness integration rules for the composite shell element.
  - *INTERFACE_SPRINGBACK
    LS-DYNA Input Interface KeywordDescribes the variables to output in an initial state file or DYNAIN file at the end of the analysis.
  - *KEYWORD
    LS-DYNA Input Interface KeywordThis keyword should always be the first card of the input file. Optionally used with *CONTROL_UNITS card to specify model units.
  - *LOAD
  - *MAT
  - *NODE
    LS-DYNA Input Interface KeywordDefines a node number and nodal coordinates in the global coordinate system.
  - *PART
    LS-DYNA Input Interface KeywordDefines a part, which combines material and property information.
  - *RIGIDWALL
  - *SECTION
  - *SET
- Multi-Domain
  This manual contains the description of the keywords for the Radioss Multi-Domain.
- Obsolete Keywords
  Keywords that are still supported, but no longer maintained (*) are considered obsolete.
- Other Files
- Definition
Radioss Example Guide
This manual presents examples solved using Radioss with regard to common problem types.
Radioss Verification Problems
This manual presents solved verification models.
Radioss Frequently Asked Questions
This section provides quick responses to typical and frequently asked questions regarding Radioss.
Radioss Theory Manual
This manual provides detailed information about the theory used in the Altair Radioss Solver.
Radioss User Subroutines
This manual describes the interface between Altair Radioss and user subroutines.

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*AIRBAG_LINEAR_FLUID_ID

LS-DYNA Input Interface KeywordThis model defines a control volume filled with a liquid.

Format


(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)
*AIRBAG_LINEAR_FLUID_ID

If OPTION = ID, insert the following line


(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)
`monvol_ID`	`Title`


(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)
`surf_ID`_ex	`Surf_typ`
`K`	$ρ$	`LCINT`	`LCOUTT`	`LCOUTP`	`LCFIT`	`LCBULK`

Optional card


(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)
`P_MAX`	`P_MAX_ID`

Definition


Field	Contents	SI Unit Example
`monvol_ID`	Airbag identifier. (Integer)
`Title`	Title. (Character)
`surf_ID`_ex	Airbag external surface identifier. (Integer)
`Surf_typ`	Set type. = 0 (Default) `surf_ID`_ex is a segment. ≠ 0 `surf_ID`_ex is a set of parts. (Integer)
`K`	Bulk coefficient. (Real)	$[Pa]$
$ρ$	Density. (Real)	$[\frac{kg}{m^{3}}]$
`LCINT`	Mass flow in as function of time. (Integer)	$[\frac{kg}{s}]$
`LCOUTT`	Mass flow out as function of time. (Integer)	$[\frac{kg}{s}]$
`LCOUTP`	Mass flow out as function of time. (Integer)	$[\frac{kg}{s}]$
`LCFIT`	Added pressure as function of time. (Integer)	$[Pa]$
`LCBULK`	Bulk coefficient as function of time. Value of `K` is ignored in this case. (Integer)	$[Pa]$
`P_MAX`	Maximum value of pressure. (Real)	$[Pa]$
`P_MAX_ID`	Maximum value of pressure as function of time. (Integer)	$[Pa]$

Comments

This keyword maps to /MONVOL/LFLUID.