Bladed sometimes reports an error during modal analysis about a stiffness or mass matrix being ill-conditioned.
What does this mean and how can this problem be resolved?
For example:
*** ERROR: mbflexbody.hpp(284): "Blade1-0" has singular or ill-conditioned stiffness matrix relating to free dofs. (Reciprocal condition number: 6.09387e-016)
*** ERROR: mbflexbody.cpp(818): "Blade1-0" - Singular or ill-conditioned mass matrix
The errors above are reporting mathematical difficulties when the stiffness or mass matrix for the blade is being created. Typically this is caused by having some very small and very large values together in a stiffness or mass matrix, making the problem difficult to solve numerically.
This error can be caused in various ways, but usually due to some non-physical structural definition being entered for the blade or tower. Some things to check are:
1. Check any warnings, as they can often indicate the problem.
2. Look for any physical impossibilities in the model, such as very low mass members, or very low stiffnesses. Remember to check torsional inertia and stiffness properties as well as mass and bending stiffness.
3. Ensure that there are no very small blade elements, as created by two blade stations very close together. A similar problem could be caused by a very small blade element being adjacent to a very large element.
4. For the tower, check that any rigid members are defined in Project Info, if desired. Very stiff members should be made rigid through Project Info rather than simply choosing a very high stiffness value.