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User Guide
This manual provides details on the features, functionality, and simulation methods available in Altair Radioss.
Fluid and Fluid-Structure Simulation
In this section, fluid and fluid-structure simulation is presented.
Arbitrary Lagrangian-Eulerian (ALE) and Computational Fluid Dynamics (CFD)
Nodal Boundary Conditions

Welcome
What's New
View new features for Radioss 2024.1.
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.
- Run Radioss
  The Radioss solver can be executed using different methods described here.
- Explicit Structural Finite Element Analysis
  In this section, available explicit features for different explicit analyses are presented.
- Implicit Structural Finite Element Analysis
- Fluid and Fluid-Structure Simulation
  In this section, fluid and fluid-structure simulation is presented.
  - Arbitrary Lagrangian-Eulerian (ALE) and Computational Fluid Dynamics (CFD)
    - Summary of ALE Keywords
      Usual Radioss keywords will act on material at related grid points. All the parameters starting with /ALE will directly act on the grid points.
    - Choice of Formulation
    - Fluid/Structure Connection
    - Methodology
    - Time Integration
    - Space Integration
    - ALE Grid Calculation
    - ALE/CFD Materials
    - Quasi-Compressible Option
      For a quasi-compressible option, density and energy are assumed to be constant.
    - Turbulent Wall
      Turbulent wall elements define the closure of the system with $k - ε$ model diffusive equations.
    - Nodal Boundary Conditions
    - Elementary Boundary Conditions
    - Artificial Damping for Shock Waves
    - Kinematic Constraints
    - Interfaces
    - Units
      Radioss has a built-in unit system for you to provide data in any consistent system.
    - Engine Input
    - Troubleshooting
- Smooth Particle Hydrodynamics (SPH)
  The Smooth Particle Hydrodynamics method formulation is used to solve the equations of mechanics, when particles are free from a meshing grid.
- Multi-Domain Technique
  The objective of the Multi-Domain technique (also referred to as RAD2RAD) is to optimize the computing performances of large scale Radioss models.
- Design Optimization
  Optimization in Radioss was introduced in version 13.0. It is implemented by invoking the optimization capabilities of OptiStruct and simultaneously using the Radioss solver for analysis.
- Error Message Database
  This section is comprised of error messages in ascending numerical order.
- Engine Error Messages
- Warning Message Database
  This section is comprised of warning messages in ascending numerical order.
Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
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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Nodal Boundary Conditions

By default, kinematic constraints act on material velocities and accelerations. In Radioss, you can define a wide variety of such constraints. For multi-physic and fluid applications, options are:

Fixed and full slip boundary conditions
Imposed velocities (imposed flux at inlet)
Rigid links (temporary adds during restarts)
Rigid bodies to model rigid structures and connections and also to compute drag and lift forces (fluid impulse on rigid body is stored in time history database)

Grid constraints act only on grid velocities. Specify:

Fixed and full slip grid conditions
Lagrangian conditions (grid and material velocity are equally set)
ALE links to maintain regular distribution of nodes
Imposed grid velocities (moving inlet and outlet)

If fixed node with /BCS, then it means the grid and material are fixed. If /ALE/BCS is used, then it is different. First 3 codes defined grid DOF, the second 3 codes defined if node act like Lagrangian in the corresponded direction.

Figure 1.

Example

If set Grilag=010 000, to just fix grid in Y direction, material could move in Y direction.

If set Grilag=000 010, it means nodes are like Lagrangian in Y direction, grid and material are tied on these nodes in Y direction.

If set Grilag=010 010, it means grid and material are tied on these nodes in Y direction and fixed in Y direction. It is like Trarot=010 000 in /BCS.

Figure 2.