# /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.

## Format

/DT/NODA/Keyword3/Iflag
 $\text{Δ}{T}_{sca}$ $\text{Δ}{T}_{\mathrm{min}}$ initial_mass_ratio
If Iflag=1, insert the next line
grnd_ID

## Definitions

Field Contents SI Unit Example
Keyword3 Time step control type. 1
CST
Use default constant time step method, /DT/NODA/CST2. 5 6
CST1
Nodal mass is increased to maintain a constant time step $\text{Δ}{T}_{\mathrm{min}}$ . 56
Recommend $\text{Δ}{T}_{sca}$ =0.67.
CST2 (Default)
Nodal mass is increased to maintain the constant time step $\text{Δ}{T}_{\mathrm{min}}$ . Improved formulation. 56
SET
Reduce the equivalent nodal stiffness to maintain the constant time step, $\text{Δ}{T}_{\mathrm{min}}$
STOP
At the start of the simulation, the initial nodal mass is increased using initial_mass_ratio to determine an initial time step. No additional mass is added during the simulation. If the time step reaches $\text{Δ}{T}_{\mathrm{min}}$ , the simulation stops and a restart file will be written.

If Keyword3 is not specified, as in /DT/NODA/Iflag, then the computation will use the nodal time step and $\text{Δ}{T}_{\mathrm{min}}$ is not used.

Iflag Node group flag.
=0 or blank
All nodes use the nodal time step method.
=1
The nodal time step method is only applied to the nodes in node group grnd_ID.

$\text{Δ}{T}_{sca}$ Scale factor on critical nodal time step.

Default = 0.9

$\text{Δ}{T}_{\mathrm{min}}$ Minimum time step.
initial_mass_ratio Mass ratio increase at the beginning of the computation used to increase the minimum time step. This value is entered as a ratio, so a 5% increase in mass would be entered as 0.05.

Default = 0

grnd_ID The nodal time step method is only applied to the nodes in this node group, read only if Iflag=1.

1. For all Keyword3 options, the time step control will be activated when the time step of the node controlling the time step of the simulation becomes less than the defined timestep divided by $\text{Δ}{T}_{sca}$ .
2. Only one /DT/NODA/Keyword3 option can be used in a simulation.
3. When using Nodal time step, the computation of each cycle is slightly more expensive but the time step can be higher especially for low quality meshes.
4. When /DT/NODA/Iflag is used, the computation will use the nodal time step method and $\text{Δ}{T}_{\mathrm{min}}$ is not used.
5. For the Keyword3 constant time step options, (CST, CST1, and CST2), the time step used at the beginning of the simulation is the max($\text{Δ}{T}_{\mathrm{min}}$ , time step calculated with mass added by initial_mass_ratio). This beginning time step will then be maintained during the simulation by adding additional mass, if needed.
6. The time step for different amounts of percent mass increase information is printed as a table and a graph in the starter output file.
7. /DT/NODA/Keyword3 is compatible with all options of /DT/Eltyp/Keyword3.
8. /DT/NODA and /DT/NODA/CST* are compatible with /DT/INTER/AMS from version 2017.2.3.
9. /DT/NODA/CTS* is compatible with advanced mass scaling, /DT/AMS. If used with /DT/AMS, AMS will be applied to the part group defined in Starter /AMS card and /DT/NODA/CST* applied to nodes not belonging to the AMS part group.
For example:
• /DT/AMS/1
• $\text{Δ}{T}_{sca1}$ $\text{Δ}{T}_{\mathrm{min}1}$
• Tol_AMS
• /DT/NODA/CST
• $\text{Δ}{T}_{sca1}$ $\text{Δ}{T}_{\mathrm{min}1}$
This is even advised in order to minimize computing time in models where AMS only applies to selected parts.
Note: It is pointless to use a higher time step in AMS than the one used in classic mass scaling since the smaller time step limits the performance.