# Increase Computation Speed and Maintain Accuracy

AMS (Advanced Mass Scaling) saves significant computation time by increasing the time step of the model for an explicit computation. This is similar to traditional mass scaling, except that the added mass does not increase the translational kinetic energy of the system.

A non-diagonal mass matrix is used to increase the time step on each line of the mass matrix. The
lumped mass, M0, is increased with some M value compensated with non-diagonal terms
such that the total mass to remain constant. ^{1} Unlike traditional mass scaling, AMS only
modifies high frequencies and does not significantly affect low frequencies of the
model.

The advantage of AMS versus traditional user controlled mass scaling is that translational kinetic energy is not increased. This allows the time step to be increased to significantly higher values as compared to traditional mass scaling without significantly affecting the results quality.

Since AMS does not modify the global mass, even at large time steps, the global momentum of the nodes affected by AMS is conserved. At large time steps, traditional /DT/NODA/CST can add a significant amount mass to a computation which increases translational kinetic energy.

AMS has a computational cost associated with calculating the mass matrix. The computational cost is model dependent, but for a highly nonlinear model it could be 50% of the total computational cost. So, although the cost per cycle has increased, the number of calculation cycles is reduced, due to the increased time step. For example, using a time step of 10 times the traditional /DT/NODA/CST, the total time for the calculation was reduced by a factor of 3. Therefore, to see a reasonable reduction in elapsed time, 10 times the /DT/NODA/CST time step is the recommended starting point.

Computational convergence and accurate results can be obtained by setting a target time step to 10 to 20 times higher than traditional mass scaling. In manufacturing simulations, 50 times the traditional mass scaling time step can be used. Since the Courant condition remains to be respected, the stability of the model must be achieved with the targeted time step to apply AMS.

## 14.0 New AMS Features

- Compatibility with RBE2 and RBE3

## 13.0 New AMS Features

- Compatibility with moving rigid walls (/RWALL with
`node_ID`> 0) - Fixed rigid walls were corrected (/RWALL with
`node_ID`= 0 or blank) - Tolerance default value was changed from 1E-4 to 1E-3
(
`Tol_AMS`= 0 ⇔ 0.001) - Conjugate Gradient (C.G.) convergence criteria was improved
- Non-diagonal added mass matrix was optimized Note: The AMS tolerance was changed in order to compensate a slight loss of computation time performance, due to above listed improvements but it should not affect the results accuracy.

^{1}Morancay, Lionel, and Gérard Winkelmuller. "Dynamic condensation and selective mass scaling in Radioss Explicit." 19ème Congrès Français de Mécanique (2009).