Small body

[raback]

Hi Matteo

I think that you may be approaching the problem with too limited equations. The interaction of the copper rods is not mainly due to the electric conductivity of the ground, the main interaction method is via the magnetic fiels induced by the currents. This is a much more complicated problem and requires the solution of both electric fields and magnetic fields. Now luckily Elmer has an AV-solver utilizing Whitney elements for the solution of such problems.

You might find the discussion in this thread usefull:
viewtopic.php?f=3&t=2264&start=0

-Peter

[mzenker]

Hi,

if we forget about the magnetic influence in a first approximation, this is what I would say to your problem:
In order to have a current flow, you need a potential difference. Note that an absolute potential doesn't exist physically. It is just a convention to call the earth's potential zero. Technically you need two electrodes. If your device seems to have just one, this means that it is grounded, i.e. the second electrode is connected to earth.
I would either define two electrodes and set the outer face of the first one to 1 V and the second one to 0 V, or use the ground as second electrode, that would mean setting the outer face(s) of your "earth", far away from the injection electrode, to 0 V. Then you can use the "current control" keyword of the Static Current Solver to set the current. Elmer will then adjust the potential accordingly.

HTH,

Matthias

[MattePOLI]

Hi, thank you for the post.
I tried to set the condiction that you wrote but these are my results:
1) the first one immage decribes the potential in ground on a line that is parallel with the active rod on the ground surface.
2) the second one immage describes the potential on a line that is normal to active rod on the ground surface.
From the teory I know that this results are absolutely wrong. I post the sif file:

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

Simulation
Max Output Level = 5
Coordinate System = Cartesian
Coordinate Mapping(3) = 1 2 3
Simulation Type = Steady state
Steady State Max Iterations = 1000
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 = 2
Initial condition = 1
End

Body 2
Target Bodies(1) = 2
Name = "Body 2"
Equation = 1
Material = 2
Initial condition = 1
End

Body 3
Target Bodies(1) = 3
Name = "Body 3"
Equation = 1
Material = 1
Initial condition = 1
End

Solver 1
Equation = Static Current Conduction
Calculate Volume Current = True
Procedure = "StatCurrentSolve" "StatCurrentSolver"
Current Control = 1
Variable = -dofs 1 Potential
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 = 20
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-10
Linear System Preconditioning = ILU0
Linear System ILUT Tolerance = 1.0e-3
Linear System Abort Not Converged = False
Linear System Residual Output = 1
Linear System Precondition Recompute = 1
End

Solver 2
Equation = Result Output
Output Format = Vtk
Procedure = "ResultOutputSolve" "ResultOutputSolver"
Output File Name = output per salome
Exec Solver = After Simulation
End

Equation 1
Name = "Equation 1"
Active Solvers(2) = 1 2
End

Material 1
Name = "terra"
Electric Conductivity = 0.01
End

Material 2
Name = "Copper"
Heat expansion Coefficient = 16.5e-6
Electric Conductivity = 59.59e6
Heat Conductivity = 401.0
Sound speed = 3810.0
Heat Capacity = 385.0
Mesh Poisson ratio = 0.34
Density = 8960.0
Poisson ratio = 0.34
Youngs modulus = 115.0e9
End

Initial Condition 1
Name = "InitialCondition 1"
Potential = 0
End

Boundary Condition 1
Target Boundaries(1) = 2
Name = "Attivo"
Potential = 1
End

Boundary Condition 2
Target Boundaries(1) = 4
Name = "Isolante"
Current Density = 0
End

Boundary Condition 3
Target Boundaries(1) = 7
Name = "terra"
Potential = 1.5915
End




Please help me, I'm doing thesis.

P.S. I'm studyng elettrical engineering, so I know the theory, I don't know how to use Elmer.

[MattePOLI]

Sorry, I forget to tell that when I do the simulation with the rod with 0,1 meters radius and 5 m long, and the ground like a hemisphere with 200 m radius, with seems setting the simulation leads to the right results.

[raback]

Hi

I would have to agree with Matthias about the way to introduce current to the conductor. I would not fix the "terra" as it cannot be a priori known.

It might help if you would post the theoretical solution for the problem you're trying to solve. I would guess that there exists an analytical solution at least for some case. At least I've problems understanding what you're trying to achieve. If you want to know the resistance of the ground you should forget about computing anything at the conductors and use the solver only for the insulators setting the potential at the conductors.

-Peter

[MattePOLI]

This is a solution reached with another method. (see picture1)
I would know how the potential disturbed when I inject a current in active rod from the passive rod (both rods are copper).
Now, when I tried a simulation with the rods with 0.1m radius, the results are perfect, when I do a simulation with the rods with 0.01m radius the results are absolutely wrong.
I tried to set also "corrent controll" without reach any plausible result. ( see picture 2, it describes a potential along the line which passed on the axis of rod)

[MattePOLI]

here they are

[mzenker]

Hmmm...
Since you get good results with a thick rod and bad results with a thin rod, it might be an accuracy problem. You said earlier that your mesh is fine in the vicintiy of the thin rod. Maybe you could try to tighten the steady state convergence limit? It is 1e-5 which seems a bit coarse to me (I may be wrong, however).

HTH,

Matthias