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Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
Engine Input
This manual provides a list of all the solution definition keywords and options available in Radioss.
Time Step
Engine Keyword In this group, keywords are used to define different time step control (STOP, KILL, CST or DEL, and so on) for different entities (shell, brick, beam, airbag, interface, and so on) during the computation.
/DT/FVMBAG/Iflag
Engine Keyword Time step for control for Finite Volume Method airbag.

Welcome
What's New
View new features for Radioss 2025.1.
Overview
Radioss^® is a leading explicit finite element solver for crash and impact simulation.
Tutorials
Discover Radioss functionality with interactive tutorials.
User Guide
This manual provides details on the features, functionality, and simulation methods available in Altair 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 2025
  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.
  - Engine Keywords Syntax
  - General Controls
    Engine Keyword In this group keywords are used to generally define the output style, control of computation, damping and ALE setup during the computation.
  - Deletion/Input Modifications
    Engine Keyword In this group, keywords are used for modification (deletion or add options) in Engine.
  - Animation and Post-processing Output Files
    Generates animation files containing results according to the keywords specified. The keywords are specified in the topics within Animation and Post-processing Output Files.
  - Implicit Solution
    Engine Keyword In this group keywords are used for implicit solution. Different implicit solver methods (linear, nonlinear, buckling or quasi-static), different options to control the implicit computation, different implicit time step method and implicit output messages are introduced.
  - Kinematics and Loading
    Engine Keyword In this group, keywords are used to define the kinematic condition (constrain/release rotation or translation DOF) and initial condition (rotational/translational velocity).
  - Time Step
    Engine Keyword In this group, keywords are used to define different time step control (STOP, KILL, CST or DEL, and so on) for different entities (shell, brick, beam, airbag, interface, and so on) during the computation.
    - /DT
      Engine Keyword Time step defaults for all elements.
    - /DT/AIRBAG/Keyword3
      Engine Keyword Time step with airbag and time step control.
    - /DT/ALE
      Engine Keyword Time step control for ALE or EULER cells.
    - /DT/AMS
      Engine Keyword Time step control for Advanced Mass Scaling. Advanced Mass Scaling is an elementary time step method that increases the time step to a higher value than the usual elementary or nodal time step.
    - /DT/CST_AMS
      Engine Keyword This option is a combination of nodal time step (/DT/NODA/CST) and advanced mass scaling (/DT/AMS) with the same target time step.
    - /DT/Eltyp/Keyword3/Iflag
      Engine Keyword Element time step control.
    - /DT/FVMBAG/Iflag
      Engine Keyword Time step for control for Finite Volume Method airbag.
    - /DT/INTER/Keyword3/Iflag
      Engine Keyword Interface time step control.
    - /DT/NODA/Keyword3/Iflag
      Engine Keyword Activates the nodal time step control method with the option to apply mass scaling to increase a model’s time step.
    - /DT/SPHCEL/Keyword3
      Engine Keyword Generates the SPH cells time step control type.
    - /DT/THERM
      Engine Keyword Stability time step for thermal analysis.
    - /DT1/BRICK/Keyword3/Iflag
      Engine Keyword Alternative time step calculation method for /TETRA4 using /PROP/SOLID I_tetra4=1000 solid elements.
    - /DT1/SHELL
      Engine Keyword Alternative time step calculation method for shell elements which results in a lower more accurate time step than the default calculation method.
    - /DT1TET10
      Engine Keyword Improved time step calculation for /TETRA10 elements.
    - /DTIX
      Engine Keyword Sets initial and maximum time step for this run.
    - /DTSDE
      Engine Keyword Larger time step calculation method for solid 6-node pentahedron element.
    - /DTTSH
      Engine Keyword Larger time step calculation method for thick-shell element.
  - Initial State
    Engine Keyword In this group, keywords are used to output different info (like stress, strain, orthotropy direction, temperature, and so on) in a state file (STA-File).
- LS-DYNA Input
  This manual provides a list of LS-DYNA input files available in Radioss.
- 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
Example Guide
This manual presents examples solved using Radioss with regard to common problem types.
Verification Problems
This manual presents solved verification models.
Frequently Asked Questions
This section provides quick responses to typical and frequently asked questions regarding Radioss.
Theory Manual
This manual provides detailed information about the theory used in the Altair Radioss Solver.
User Subroutines
This manual describes the interface between Altair Radioss and user subroutines.

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/DT/FVMBAG/Iflag

Engine Keyword Time step for control for Finite Volume Method airbag.

Format

For Iflag = 0, 1
/DT/FVMBAG/Iflag
$Δ T_{s c a}$ $Δ T_{\min}$
For Iflag = 2
/DT/FVMBAG/Iflag
$Δ T_{s c a}$ $Δ T_{\min}$ Lambda L_type
Monvol_ID

Definition


Field	Contents	SI Unit Example
`Iflag`	Finite volume merge flag. = 0 or blank (Default) The finite volume is merged, if the time step < airbag volume < 10 * the minimum volume defined in global merge, /FVMBAG/MODIF. = 1 The finite volume is merged, if time step < $Δ T_{\min}$ . = 2 The finite volume is merged, if time step < $Δ T_{\min}$ . The time step computation is defined by the flag `L_type`. 2
$Δ T_{s c a}$	Scale factor on time step. Default = 0.9 (Real)
$Δ T_{\min}$	Minimum time step. (Real)	$[s]$
`Lambda`	Stability factor. Default = 0.01 (Real)
`L_type`	Time step computation method. = 1 Characteristic length of FVM mesh (computed during Starter). = 2 (Default) Characteristic length is computed from cubic root of the volume.
`Monvol_ID`	Monitored volume identifier. (Integer)

Comments

The $Δ T_{s c a}$ and $Δ T_{\min}$ can be defined to ensure stability and a minimum time step size by triggering additional finite volume merges when the FVMBAG entity penalizes the computation.
When using /DT/FVMBAG/2, time step is computed as:
$Δ T = Δ T_{s c a l} \frac{Δ x}{|u - w| + c}$
Where,

$u$

Gas velocity.

$w$

Polyhedron velocity.

$c$

Speed of sound.

$Δ x$

Characteristic length.

The soothing factor is applied using $Δ T \leftarrow \min (Δ T, Δ T_{o l d} (1 + λ))$ .