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AUTODYN automatically calculates the energy balance and gives an error message when the energy error is greater than the tolerance (default is 5%). First, make sure the grid for all parts is correct before running AUTODYN. Then, carefully examine the material properties used in your AUTODYN calculation. Next, check your boundary conditions. If there are Transmit or Flow in/out boundary conditions defined in your AUTODYN model, a certain material can get out of or into the part, taking away or adding in energy. If this is the case, the energy balance check needs to be disabled in the Controls panel by specifying a very large number like 999999 as the energy reference cycle in the Wrapup Criteria options. This effectively disables the AUTODYN energy error check. If an erosion model is used in Lagrange calculations, both kinetic and internal energy gradually decrease as Lagrange elements erode. This also causes an energy balance error. You can either reset the energy reference cycle to the cycle where you got the energy error in the Controls panel, or disable the energy balance check by setting the energy reference cycle to a very big number, such as 999999. Another possible cause of energy error is that a low mesh density is being used for an area with high internal energy such as an explosive. When the mesh is coarse, numerical error in calculating the internal energy is increased. Refining the coarse mesh or increasing the tolerance of the energy error will help to improve this situation. If the AUTODYN model contains ALE parts and the motion constraint is neither Free (Lagrange) nor Equipotential, select the ALE part from the list in the Parts panel and press the Motions button. Then go to ALE Settings and Modify. Change the Iterations per cycle to a number larger than 1. Thus AUTODYN conducts iterations to rezone ALE grids so at the end of all the iterations they are finally placed on equilibrium positions according their motion constraints. This will also help to reduce energy error. Usually you only need to pay attention to the energy balance if there is a spike in the energy history curve. A summary of the energy balance can be viewed through the History panel by selecting either Energy or Material or Part summaries.
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