Nodal Time Step Control
- /DT/NODA/Keyword3/Iflag
- ΔTscaΔTsca ΔTminΔTmin
- mm
- Nodal mass
- kk
- Equivalent nodal stiffness
/DT/NODA/CST
/DT/NODA/CST is by far the most popular option for maintaining or increasing the time step in a simulation. Radioss will automatically add mass to nodes to maintain the entered value. If needed, the mass will be added at the beginning of the simulation to meet the entered value.
Increase the nodal mass via /DT/NODA/CST is the easiest way to increase the time step of a model or prevent a time step from dropping below a certain value during the simulation. Good engineering judgement must be used to determine how much mass is an acceptable amount to be added to a model. Adding too much mass can affect the physics by increasing the kinetic energy of a drop or impact simulation. This is because the object being simulated weighs more than the real part. Increased mass can also change the high frequency behavior of a model which can be very important in very high speed impacts such as ballistics, or explosions.
In general, it is recommended to keep the amount of mass added to less than 5%. However, larger mass increases may be acceptable in some types of simulation. For example, in quasi-static simulations the velocities are usually small, so adding mass does not greatly increase the kinetic energy. For those reasons, it is recommended to check the mass increase in the model by running a simulation without or with reduced mass scaling and comparing the results. If added mass results in added kinetic energy, the energy error calculated by Radioss will be positive.
The target time step and scale factor can then used in /DT/NODA/CST.
MAS.ERR
. Using the animation output option
/ANIM/NODA/DMASS or /H3D/NODA/DMASS,
the relative mass increase per node can be visualized in a post-processor as a
contour plot. Both the total mass error and nodal mass error represent the
change in mass divided by the original mass at the beginning of the
simulation.With, DM=M−M0DM=M−M0
- M0M0
- Initial mass at the beginning of the simulation for each Engine file
- MM
- Current mass
/DT/NODA/SET
Reduces the equivalent nodal stiffness ( kk ) to maintain the entered ΔTminΔTmin value. This reduction in stiffness also changes the physics of a simulation and is typically only used when modeling fluids.
/DT/NODA/STOP
Stops the simulation when the simulation’s time step drops below the entered ΔTminΔTmin value. Many times, a reduction in time step is caused by a model's instability, so stopping the simulation can be useful to diagnose the issue.