# Results Checking

## How is the Energy Error in the output file (Runname_0001.out) computed?

(1)
$%Error=100\left(\frac{{E}_{k}+{E}_{k}^{r}+{E}_{i}}{{E}_{k,1}+{E}_{{}_{k,1}}^{r}+{E}_{i,1}+{E}_{wk}-{E}_{wk,1}}-1\right)$
Where,
${E}_{k}$
Translational kinetic energy at current time
${E}_{k}^{r}$
Rotational kinetic energy at current time
${E}_{i}$
Internal energy
${E}_{wk}$
Work of external forces (energy brought to the system)
${E}_{,1}$
Energy at beginning of the RUN (not at time t=0)

The Hourglass energy is not counted in this energy balance, so that a negative energy error generally occurs (except if using Ishell=24 (QEPH) or Ishell=12 (BATOZ) shells, and fully integrated solids, for which there is no Hourglass).

$%Error$ is bounded to ± 99%.

The energy error is reset after each RESTART.

It is possible to stop a job and rerun it by using a control file containing /CHKPT. In this case, error and energy values will restart from their last value.

## How to understand the Energy Error and what values are considered reasonable.

The Energy Error computed by Radioss is a percentage.
• If the error is negative, it means that some energy has been dissipated.

In case of under integrated elements (Belytschko shells, solids with 1 integration point), the Hourglass energy can also explain a negative Energy Error since it is not counted in the energy balance. The normal amount of Hourglass energy is about -10% to -15%.

• If the error is positive, there is an energy creation.

In case of using QEPH shell formulation (Ishell=24) or fully integrated elements, the Energy Error can be slightly positive since there is no Hourglass energy and the computation is much more accurate. An error of +1% or +2% is acceptable

If the positive energy error is greater than 2%, the source of this energy has to be identified. Incompatible kinematic conditions can lead to such a situation.

• An increasing Energy Error that reaches ±99% can indicate the simulation has diverged. However, in certain situations high energy error can be acceptable:
• If the initial energy in the system is low, then it is possible to have large Energy Errors early in a simulation that reduce as energy is added to the system. This is because, small numerical differences in the energy causes large percentage Energy Errors.
• There are times when a large Energy Error at the end of a simulation can be caused by only one part diverging, but the rest of the model is giving correct results.
• Large contact energies relative to total energy can cause large negative Energy Errors because contact energy is not part of the Energy Error equation. If the simulation has friction and a lot of sliding contact, then the large contact energy and resulting energy error can be considered acceptable.

## Is it necessary to take into account the added mass and how to check that the results are not modified too much?

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 output 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.