Navier Stokes Convergence issues

Numerical methods and mathematical models of Elmer
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aminer2k
Posts: 20
Joined: 14 Dec 2009, 00:34
Location: Northern California, USA

Navier Stokes Convergence issues

Post by aminer2k »

Hello,
I would like to perform analysis using Navier Stokes and Heat Transfer eventually. As a first step, I set up some tests by just solving the fluid portion of the problem. I am finding it very difficult to get a problem to converge. Generally, I am finding that simple geometries with structured meshes converge nicely, but meshes with complexity, such as structured and unstructured regions do not. I am planning on building meshes with structured regions near solid surfaces in order to model boundary effects accurately, while having the bulk of the fluid meshed with a less dense structured region.

The simple problem attached uses the type of meshing that I would like to use later in the fluid - heat transfer problem, but I have been trying to get it to converge with simply a velocity BC on one side, and external pressure = 0 on the other. There are three bodies, 1, 2, and 3. All are water now in what should be a simple test of N-S equations, but later 1 and 3 would be fluids, and 2 would be solid. The perimeter has been set to no slip, or not, with the solver set to ILU0, or 1, 2, 3, 4 with no apparent effect.

Is there something fundamental that I am missing?

Code: Select all

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 = 5
  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(3) = 1 2 3
  Name = "Body 1"
  Equation = 1
  Material = 1
  Initial condition = 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 = 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-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


Solver 4	! Set the correct solver number here.
			! Usually use the highest number so the results are saved
			! at the end of a timestep.
 !Exec Solver = String "after all"
 Exec Solver = always
  Equation = String "ResultOutput"
  Procedure = File "ResultOutputSolve" "ResultOutputSolver"
  Output File Name = String "output." 
  Output Format = String "vtk"
  Vtk Format = Logical True
End

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

Material 1
  Name = "Water (room temperature)"
  Heat Conductivity = 0.58
  Heat Capacity = 4183.0
  Density = 998.3
  Viscosity = 1.002e-3
  Heat expansion Coefficient = 0.207e-3
End

Initial Condition 1
  Name = "InitialCondition 1"
  Velocity 2 = 0
  Velocity 1 = 0
  Pressure = 50
  Velocity 3 = 0
End

Boundary Condition 1
  Target Boundaries(3) = 1 2 3
  Name = "BoundaryCondition 1"
  External Pressure = 100
End


Boundary Condition 2
  Target Boundaries(1) = 7
  Name = "BoundaryCondition 2"
  #Velocity 1 = 0
  #Normal-Tangential Velocity = True
  Noslip wall BC = True
End

Boundary Condition 3
  Target Boundaries(3) = 4 5 6
  Name = "BoundaryCondition 3"
  External Pressure = 0
End

Attachments
model.png
model.png (37.6 KiB) Viewed 2635 times
suren
Posts: 18
Joined: 17 Nov 2009, 18:38

Re: Navier Stokes Convergence issues

Post by suren »

hello. My experience is that using Picard (default) iteration scheme throughout (without jumping to Newton's) might help. For that, make sure followings are set properly

Nonlinear System Newton After Iterations = 30 ! grater than (Nonlinear System Max Iterations = 20)
Nonlinear System Newton After Tolerance = 1.0e-9 ! smaller than (Nonlinear System Convergence Tolerance = 1.0e-8)

give a try,
sa
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