Tech Articles

Direct vs Iterative ( SOLs 101 and 400 )


Tech Articles ID    KB8022073
Status:    Published
Published date:    04/23/2015
Updated:    06/29/2015
Reported In:   MSC Nastran - MSC Nastran - Standard (SOL 1 through 200)
  MSC Nastran - MSC Nastran - MD Implicit Nonlinear (SOL400)
 

Abstract (Question)

Summary

  • The Direct Sparce solver is generally better for less dense models ( normally dominated by 2 Dimensional elements ),
  • The Casi Iterative solver is generally better for dense models ( normally dominated by 3 Dimentional elements ).
  • SOL 400 with the CASI solver may have issues because of mild nonlinearity and/or uniform stiffness. I.e. if the aspect ratio inside the stiffness matrix is large, CASI and iterative solvers will iterate longer to converge. I.e. very stiff parts combined with flexible parts are bad for iterative solvers.

Description (Answer)

Details

Below are graphs of elaspsed times ( normalized to Direct Solver time ) for models dominated by 3D and 2D elements for SOL 101 and 400.

 

Commands

  • The Casi Solver may be selected in the CASE control with:
    smethod = element
    
  • The Casi Solver may also be selected with the ITER option. This option is required to change parameters. E.g.:
    smethod = 10
    iter, 10
            precond=casi, itseps=1.0e-8
    


Caveats

  • The Casi solver does not support:ASET,OMIT, or SUPPORTi entries
  • The Casi solver does not support Superelements
  • Casi solver may require more iterations if there are very stiff and more flexible parts ( a matter of degree ). Tighter convergence criteria may be required for an accurate solution.
  • DMIG entries are limited to 100 grid points and must be symmetric for CASI.`

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