I'm working on a 2D-model (in the xy-plane) in NASTRAN and am getting convergence problems. The main cause of these convergence problems is a "high or negative matrix factor diagonal ratio."
I've attached a picture of a setup similar to the one I'm working on. The elements in white are rod elements; the elements in pink are bar elements.
The loads and BCs are:
- Angular velocity of 60 rad/sec about the y-axis
- Gravitational acceleration of 9.8 m/s$^2$ in the -y-direction
- $123456$ constraints on the top and bottom nodes to prevent them from moving or rotating
- $123$ constraints on the inner-most node to prevent it from moving
Here is some text from the f06 file:
Nodes 13 and 27 are where the rods connect to the bars, and Nodes 20 and 24 are just two random nodes on the right-hand side of the pink section.
Under the bar properties, there is an option called "Pinned DOF @ Node 1," which may or may not be helpful here. The options are UX, UY, UZ, RX, RY, RZ.
I've tried experimenting with various boundary conditions, but I can't get convergence in either the linear static case or the nonlinear static case.
Generally high matrix factor diagonal ratios imply that I have a very stiff element connected to a not-very-stiff element, but Nodes 20 and 24 aren't where the rod/bar intersection happens.
Does anyone know how I might go about diagnosing the problem? I think I'm missing boundary conditions somewhere, but I'm not entirely sure where that is.
Whoops, I forgot to explain what behavior I expect from this contraption.
Since I have a really high angular velocity (and perfectly normal gravity), I would expect the pink nodes to move rightward and downward due to the loads. The only thing stopping them from moving indefinitely are the white rods. Since these aren't beams and don't transmit moment stiffness, all these rods can really do is extend in length and change angle, since they're required to be a straight line.
This will result in a shape that looks similar to the pink curve, but probably shifted rightward a bit.
I ran the case with PARAM,BAILOUT,-1 (linear static), and I got the following clearly incorrect deflection ($10^7$ m deflection): image removed since it didn't provide any useful information and it took up too much space.
I've pinpointed the problem, I believe.
Blue elements are rods, pink elements are bars. The first case converged (nonlinear static) and gave results pretty close to what I expected (deflections were slightly high, but my angular velocity is slightly high, too).
The second case failed to converge, and it had a problem with my new node - it told me that it's having diagonal ratio problems in the T2 direction.
There's something NASTRAN is doing wrong with how these two bar elements are connected at this node. Any ideas?