I used ALE parts to simulate underground explosions, I used ""Free(Lagrange)"" motion constraint for rocks and soils, and ""Equipotential"" motions for explosions. On the first cycle, I got an error message about ""negative density in state"" and then AUTODYN stopped."


When ""equipotential"" motion constraints are used to model high pressure and high velocity regions like explosives, AUTODYN automatically rezones the equipotential region every cycle specified in the ALE settings panel. If the initial velocity or initial pressure is very high and the mesh in the explosive region is very fine, some grid points will move too much and cause this error message. To resolve this, go to the ALE part in the Parts panel, and press the MOTIONS button, next select the Modify button under ALE settings and enter a small number for the RELAXATION COEFFICIENT (default is 1.0), for example, 0.1. This small number will decrease time step size and thus pull the grid points back. Although a small RELAXATION COEFFICIENT value increases the accuracy of the rezoning, it also causes a small time step which degenerates calculation efficiency. So you will need to set it back to the default value after a few cycles."





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