Learning Forum

[OliverK]

Hi Dave,

Thanks for your reply. COMBI214 is unfortunately not enough to model the bearing, because it needs 4X4 unsymmetrical stiffness and damping matrices with negative values for the bearing in UX, UY, ROTX, ROTY directions. I just figured out why the snippet I posted above didn't work as I wanted, because I was attaching the MATRIX27 elements with pilot nodes that are fixed. I modified my mesh and created some nodes as fixed to the ground and attached the MATRIX27 elements to these nodes instead of the pilot nodes. After that it works fine. But now I have another question. While I was entering unsymmetrical stiffness and damping matrix with negative values using MATRIX27, ANSYS reminded me:

"For element type= (MATRIX27),KEYOPT(1) cannot be 1 when KEYOPT(2) is greater than zero."

Does this mean that I cannot enter a negative definite matrix if I set KEYOPT(2) = 1 (unsymmetric matrix)? For example, one of the stiffness matrices looks like this:

   1.0e+06 *

 

|    0.0047    0.0021   -0.0213    1.6272    |

|   -0.0021    0.0047   -1.6272   -0.0213    |

|   -0.0098    0.0891   -5.2080    0.3970    |

|   -0.0891   -0.0098   -0.3970   -5.2080    | ,

I applied the Cholesky factorization and found that this stiffness matrix is not positive definite. And I can see from the solution information that this matrix was not added to the model. But for the other damping matrix, which is positive definite, it was added. So will Mechanical just ignore the non-positive definite matrices and run the simulation without them? Is it possible to enter a non-positive definite matrix?

Best regards,

Oliver