Mass Error_Radioss

Hi Mani kumar 

          When the model is imposed with a time step (/DT/NODA/CST) mass error is monitored. In order to maintain the imposed time step, the solver will add mass to the model and this will be shown as mass error. For a perfectly imposed time step, the mass error should be less than 2-3 percent. Please refer this video which explains about time step and monitoring the

.out file: https://altair-2.wistia.com/medias/o0bfml9ah5 

Radioss Help:

 

The added mass can be due to Interface TYPE2, Spotflag =1. In this case, the added mass is totally made at time t=0. It can also be due to options for constant time step (/DT/NODA/CST or /DT/INTER/CST).

In case of added mass in the model, it is necessary to check if it is not too important with respect to the total mass of the model (see the DM/M value in the last column in the RADIOSS Engine listing file (Runname_nnnn.out)).

It is also important to post-process this added mass in order to check that it is not too large locally, since this could mean false results (for checking this, the Animations written with /ANIM/NODA/DMAS have to be visualized).

Also note that /DT/NODA/CST can lead to added inertia, which is much more difficult to appreciate from an engineering standpoint and cannot be post-processed prior to RADIOSS V9.0.

Added mass due to /DT/NODA/CST is often the cause of the divergence of computation.

If interfaces are badly defined (too small gaps, initial penetrations, and so on), /DT/NODA/CST cannot be the solution to increase time step. The model has to be improved. It is recommended not to use either /DT/NODA/CST or /DT/INTER/CST.

It is possible to first make a short run with added mass by using /DT/NODA/CST (one cycle is sufficient) and then to check the added mass by post-processing the Animation written at time t=0. The computation can then be continued without using /DT/NODA/CST, in order to limit the possible problems it can generate while getting a better time step.