Periodic Boundary Conditions

[TZiegler]

Hello,

I am new to Elmer. My problem is this: I want to determin the final free falling velocity field of a liquid in a piece of pipe.
The liquid is accelerated by the gravitational field and decellerated by the no slip boundary condition on the outside.
To that end I want to aply periodic boundary conditions to either end of the pipe. I made the meshes match each other
on both ends. However when I run the simulation the convergence is bad and in the final result the velocity field on both
ends don´t match. So I assume there is something wrong with the way I defined the periodic boundary conditions.
Here is my .sif file I used.

Header
CHECK KEYWORDS Warn
Mesh DB "." "."
Include Path ""
Results Directory ""
End

Simulation
Max Output Level = 4
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.ep
End

Constants
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
End

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

Solver 1
Equation = Navier-Stokes
Procedure = "FlowSolve" "FlowSolver"
Variable = Flow Solution[Velocity:3 Pressure:1]
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-8
Nonlinear System Max Iterations = 50
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 = BiCGStab
Linear System Max Iterations = 500
Linear System Convergence Tolerance = 1.0e-8
Linear System Preconditioning = ILU1
Linear System ILUT Tolerance = 1.0e-3
Linear System Abort Not Converged = False
Linear System Residual Output = 1
Linear System Precondition Recompute = 1
End

Equation 1
Name = "Fluid"
Active Solvers(1) = 1
End

Material 1
Name = "Water (room temperature)"
Viscosity = 4.5e-3
Heat expansion Coefficient = 0.207e-3
Compressibility Model = Incompressible
Heat Conductivity = 0.58
Sound speed = 1497.0
Heat Capacity = 4183.0
Density = 1000
End

Body Force 1
Name = "Gravity"
Flow Bodyforce 3 = $ -9.81 * 1000
End

Boundary Condition 1
Target Boundaries(1) = 1
Name = "Periodic"
Periodic BC Velocity 3 = True
Periodic BC = 1
End

Boundary Condition 2
Target Boundaries(1) = 2
Name = "Periodic"
Periodic BC Velocity 3 = True
Periodic BC = 1
End

Boundary Condition 3
Target Boundaries(1) = 3
Name = "NoSlip Outside"
Noslip wall BC = True
End

Cheers
Tobias

[raback]

Hi Tobias

When a boundaries is 'n' and 'm' are periodic you define the peridocity only for one of them, say 'm', and give there the reference to the other, say 'n'. So in your case just drop the Periodic* stuff from BC1.

-Peter

[TZiegler]

Hi Peter,

thanks for your quick reply and the work you put into Elmer. Is there any place I could have found this information except looking into the code?

Cheers
Tobias

[raback]

Hi, You could have looked at the examples in $ELMERHOME/tests/periodic*/*.sif -Peter