Zero Velocity or Rigid Particle Boundary Conditions

What steps will reproduce the problem?
1. Tension test with Courant factor are 0.5
2. Apply large strain (>50%)
3. The object will separate from the boundary conditions or from moving rigid 
particles on the other end.

What is the expected output? What do you see instead?

The base boundary conditions should have zero velocity at the base of the 
specimen and hold it in place. Instead, the object and boundary separate. In 
some problems, the quality of these boundary conditions becomes the limiting 
factor.

The results are improved by smaller Courant factors (e.g., 0.025), but then the 
calculations are much slower. In other words, it seems like these rigid 
boundaries are acting like a stiff material

Particles can also separate from moving rigid particles on the other end rather 
then track them well. This problem may be caused by the boundary conditions 
jumping for one grid line to the next as the rigid particle passes through the 
cells. The jumping might induce kinetic energy that eventually causes the 
separation. Running the same problem with traction BCs does not have this 
separation problem.

Possible Solution

Try ghost BCs one cell back from the basic boundary line, as Unitah does at 
`Symmetry` planes, to see if that changes the results. These ghost or mirror 
BCs could be at a zero velocity plane or at the moving plane. Uintah has them 
only for zero velocity planes. The process is to set velocity at nodes one cell 
away from the moving boundary to be equal to line interpolation from node 
within the object, through the boundary line, and to the mirrored node. The 
basic code for this change is in NairnMPM, but needs more work before it can be 
tried.

Original issue reported on code.google.com by johnanairn@gmail.com on 3 Jan 2013 at 1:01

nairnj / nairn-mpm-fea

Zero Velocity or Rigid Particle Boundary Conditions #6