/DT/AMS

Engine Keyword

/DT/AMS - Time Step for Advanced Mass Scaling

Description

Elementary time step for Advanced Mass Scaling.

Format

/DT/AMS	or	/DT/AMS/Iflag

		Tol_AMS
		Niter Nprint

Data	Description
Iflag	Number of additional (optional) cards = 0 or blank: only one card for and is read An additional card for Tol_AMS is read only if Iflag = 1 or 2 An additional card for Niter, Nprint is read only if Iflag = 2
	Scale factor on time It is recommended to set to 0.67. Default = 0.9
	Minimum time step
Tol_AMS	Tolerance for AMS convergence Default = 10-3
Niter	Maximum number of iterations in conjugate gradient Default = 1000
Nprint	Frequency (number of cycles) for writing additional output about the number of iterations before convergence in the conjugate gradient.

Comments

1.	Advanced Mass Scaling enables setting the time step to a higher value than the usual elementary or nodal time step.

2.	To apply Advanced Mass Scaling, the keyword /DT/AMS (invoking AMS elementary time step) must be present in RADIOSS Engine Input.

3.	If /DT/AMS is present in RADIOSS Engine Input, /AMS is mandatory in RADIOSS Starter Input.

4.	If Iflag =1 or 2, the tolerance for AMS convergence, Tol_AMS must be entered. If Iflag =0 or is absent, the default value of 10-3 for Tol_AMS will be used.

5.	At each cycle, AMS solves iteratively the nodal accelerations using a conjugate gradient algorithm (Mass matrix is not diagonal anymore when using AMS).

6.	Convergence of the conjugate gradient is assumed when:

7.	If Iflag = 2, the maximum number of iterations in conjugate gradient Niter, and the frequency for additional output Nprint, must be entered. Otherwise, the maximum number of iterations in conjugate gradient is set to its default value (1000), and no additional output is provided.

8.	If more than Niter iterations have been performed before convergence of the conjugate gradient, the following error message is output and the computation stops:

“** ERROR: AMS IS LIKELY DIVERGING”

9.	If Nprint is specified, then at each Nprint cycle an additional output is provided including: the number of iterations before convergence of the conjugate gradient at this cycle, the final residual norm, and the force vector norm.

10.

It is possible to use Advanced Mass Scaling for a group parts. The part group must then be specified in RADIOSS Starter Input deck (see /AMS). For optimized computing time performances it is advised to also apply a classical mass scaling to the parts not belonging to /AMS part group; otherwise, no AMS processed parts are computed, by default, in natural Element time step.

For example:

/DT/AMS/1

Tol_AMS

/DT/NODA/CST

Note that 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 performances. If no part group is specified (blank line) or is equal to 0, then AMS is applied to the model in its whole and adding /DT/NODA/CST is irrelevant then.

11.	Advanced Mass Scaling (AMS) does not modify the global mass, so that the global momentum of the related nodes is conserved. It is, therefore, more accurate than /DT/NODA/CST.

12.	/DT/Eltyp/Keyword3 is compatible with /DT/AMS (except /DT/INTER/CST, that only applies to the nodes where /DT/AMS does not apply, i.e. nodes not belonging to the part group declared in Starter /AMS, similarly to /DT/NODA/CST).

•	This includes /DT/Eltyp/CST for which the small strain formulation is activated for an element when dtsca * l/c < dtmin where, l is the element characteristic length, and c is the speed of sound in the material.

•	When using /AMS and the interfaces TYPE7, TYPE11, TYPE19, or TYPE20 with a nonlinear penalty stiffness for contact, it may be necessary to use /DT/INTER/DEL in the RADIOSS Engine input deck. Otherwise, AMS may converge slowly, or may even diverge.

13.	For information about limitations related to /DT/AMS, refer to Current Capabilities and Limitations in Advanced Mass Scaling (AMS) in the User’s Guide.

/DT/AMS

/DT/AMS

/DT/AMS

Engine Keyword

Format

Data