static then modal chained simulations

Numerical methods and mathematical models of Elmer

static then modal chained simulations

Postby ronald » 01 Jul 2017, 10:18

I am trying to make first a static linear elasticity simulation to obtain mechanical stress of a beam induced by a force, then I want to start a modal linear elasticity simulation including pre stress obtained in the 1st static simulation.
I am trying to do that by editing the sif-->simulation of the 1st static simulation with :Output File = "previousrun.result"
Then editing the sif-->simulation of the 2nd modal simulation with :
Restart File = "previousrun.result"
Restart Position =1

In the solver log (as shown above), it seems that the restart file is taken into account, but there is no change in eigen frequencies as there should be!
Has anyone succeded in such chained simulation ?

LoadRestartFile: --------------------------------------------
LoadRestartFile: Reading data from file: ./previousrun2.result
LoadRestartFile: ASCII 3
LoadRestartFile: Total number of dofs to load: 13
LoadRestartFile: Reading time sequence: 1.000E+00
LoadRestartFile: Reading timestep: 1

LoadRestartFile: Time spent for restart (s): 4.5700E-01
LoadRestartFile: All done
LoadRestartFile: --------------------------------------------
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Re: static then modal chained simulations

Postby mconrad1186 » 02 Feb 2018, 22:40

Hi Ronald,

I am trying to do a similar calculation. Have you had any success?

Posts: 3
Joined: 02 Feb 2018, 16:32

Re: static then modal chained simulations

Postby raback » 03 Feb 2018, 00:01


I had to look at the code a bit... It seems that the keyword "Geometric Stiffness = Logical True" could do the trick. If this is used then the StressSolver will take two iterations. First to resolve the prestressed state and then to compute the eigen analysis.

Basically the same procedure could of course be done in two sweeps but this seems to be the supported method. The code needs to know that it should compute harmonic solution around the given state. Otherwise it will just forget about the initial guess.

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Re: static then modal chained simulations

Postby mconrad1186 » 03 Feb 2018, 00:59

Hi Peter,

Thank you for the reply! I tried setting making the geometric stiffness true but this doesn't seem to have an effect. Newby question: How would you go about completing the simulation in two separate iterations? Here's my .sif file. I used the mesh provided in the linear elasticity tutorial of a rectangular beam.

Mesh DB "." "."
Include Path ""
Results Directory ""

Max Output Level = 5
Coordinate System = Cartesian
Coordinate Mapping(3) = 1 2 3
Simulation Type = Steady state
Steady State Max Iterations = 1
Output Intervals = 1
Timestepping Method = BDF
BDF Order = 1
Solver Input File = case.sif
Post File = case.vtu

Gravity(4) = 0 -1 0 9.82
Stefan Boltzmann = 5.67e-08
Permittivity of Vacuum = 8.8542e-12
Boltzmann Constant = 1.3807e-23
Unit Charge = 1.602e-19

Body 1
Target Bodies(1) = 1
Name = "Body 1"
Equation = 1
Material = 1
Body Force = 1

Solver 1
Equation = Linear elasticity
Eigen System Values = 10
Procedure = "StressSolve" "StressSolver"
Variable = -dofs 3 Displacement
Eigen System Compute Residuals = True
Geometric Stiffness = True
Calculate Loads = True
Eigen System Select = Smallest magnitude
Eigen Analysis = True
Exec Solver = Always
Stabilize = True
Bubbles = False
Lumped Mass Matrix = False
Optimize Bandwidth = True
Steady State Convergence Tolerance = 1.0e-5
Nonlinear System Convergence Tolerance = 1.0e-7
Nonlinear System Max Iterations = 1
Nonlinear System Newton After Iterations = 3
Nonlinear System Newton After Tolerance = 1.0e-3
Nonlinear System Relaxation Factor = 1
Linear System Solver = Iterative
Linear System Iterative Method = GCR
Linear System Max Iterations = 500
Linear System Convergence Tolerance = 1.0e-10
BiCGstabl polynomial degree = 2
Linear System Preconditioning = ILU3
Linear System ILUT Tolerance = 1.0e-3
Linear System Abort Not Converged = False
Linear System Residual Output = 1
Linear System Precondition Recompute = 1

Equation 1
Name = "Elasticity"
Calculate Stresses = True
Active Solvers(1) = 1

Material 1
Name = "Aluminium (generic)"
Heat expansion Coefficient = 23.1e-6
Heat Conductivity = 237.0
Sound speed = 5000.0
Heat Capacity = 897.0
Mesh Poisson ratio = 0.35
Density = 2700.0
Poisson ratio = 0.35
Youngs modulus = 70.0e9

Body Force 1
Name = "Gravity"
Stress Bodyforce 2 = -5395.5

Boundary Condition 1
Target Boundaries(1) = 6
Name = "Wall"
Displacement 3 = 0
Displacement 2 = 0
Displacement 1 = 0

Boundary Condition 2
Target Boundaries(1) = 5
Name = "Mass"
Force 2 = -4.0e5
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Re: static then modal chained simulations

Postby raback » 03 Feb 2018, 02:03


The "geometric stiffness" is coded such that when its active the 1st iteration of StressSolver computes the prestressed state. On the 2nd iteration the modal analysis is carried out adding the geometric stiffness as in (5.14) of Models Manual.

Given this I don't see any supported way of doing this in two steps. We need to modify the modal equation

Mu''+(K+K0)u = 0

This cannot do by having an initial guess u0, for example, or some r.h.s. term. We need to modify the stiffness matrix and the above is the only way to do it.

The test cases "geometric_stiffness" worked, and modifying "ThermalBiMetal" such that eigenmodes are solved for shows an effect (very small one) of the thermal stresses. So maybe the effect is just very small...

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