I contacted COMSOL support about our difficulties with the relative residual error and was told "Typically for solid mechanics this error is because you have not constrained the body enough. So any solution + a rigid body transformation is also a solution. To remove this non-uniqueness, you need a solutions modulo the rigid body transformations. So try and constrain the body somehow." This is what the gravitationally balanced body load was supposed to do, but using the stationary solver has been non-convergent and the eigenfrequency solver has generated odd modes, with the first real modes being sheer modes. These outputs indicate a problem with our model. Matt noticed yesterday that the meshing in our model was slightly non-symmetrical, but we initially dismissed it as not being significant since it was a minor difference which we did not think could account for the large errors we are facing. At further consideration though, if some asymmetry arising from any source, even a numerical or rounding error, were present, that could cause a slight rotation of the object which would cause the object to experience a torque and minor sheer force. The stationary solution would not converge in this case, but the eigenfrequecy solution might, and if it did it would make sense to see sheer modes for some of its low frequency eigenmodes, as we have. One possible solution to this problem is to change the way we mesh the material to ensure a symmetrical distribution of nodes in the x-y plane, probably by extruding lower dimensional meshed systems into our model. I am unsure if we would be able to implement this solution once we start to change the size of the radii of the object's faces. An alternate solution is to find another set of boundary conditions which should be equivalent to the gravitational body load constraint, but which are stable relative to minor perturbations of the system's conditions. I think that I have found another set of boundary conditions which should work and not be too difficult to implement in COMSOL.
The sides of the object should not move in the direction orthogonal both to their displacement from the center of the circular plane of the object even with them in the z direction or in the z direction, so the edge displaced from the center in the y direction should be fixed in the x direction.
The object's center of mass should not move which because of the previous condition can be reduced to not moving in the z direction.
I think that these boundary conditions should either be compatible with, or replace Liu and Thorne's boundary conditions in our model. I am going to spend some time attempting to implement these boundary conditions, see if they converge, and see if adding Liu and Thorne's gravitationally balanced force with them makes any difference, either in results (which it shouldn't) or the amount of time required to run. I am not sure how long this will take to implement, but I don't expect it to take more than a day or two. |