I want to simulate non-Boussinesq convection and I am having a lot of difficultly getting convergence. I went back to tutorial #8 Transient flow and heat equations - Rayleigh-Benard instability, setup the model, and ran it with the Boussinesq approximation enabled. After this went well, I went through the model and started making the changes I thought I needed in order to do this without the Boissinesq approximation. I want to do this so I can use this approach on another model with large temperature differences where the Boussinesq approximation is invalid.
Some of the changes made (among others):
-Changed to transient model
-Added in thermal compressibility model and reference temperature
-Added in the buoyancy force: body force, force 2 = -9.81
-Tripled the the density of the mesh
From what I understand, I should be able to get similar results to the model using the Boussinesq approximation. The simulation runs up to a point where I can start to see the convection currents form and then the run diverges. I'm going to try making my time step sizes smaller and see if that does it. However, I am hoping somebody who has done this before might be able to take a look at what I am doing and see if anything stands out as an error.
Project attached.
Cheers,
Clayton
Non-Boussinesq Convection
Non-Boussinesq Convection
- Attachments
-
- non-boussinesq convection.rar
- (91.36 KiB) Downloaded 329 times
Last edited by MrChips on 09 Dec 2016, 18:33, edited 1 time in total.
Re: Non-Boussinesq Convection
With the smaller time step size (400 at 0.5) I was able to get the simulation to run a lot longer, but it eventually diverged coming to NaN. The image below is a comparison between what I got with the the full Navier Stokes model and the one from the tutorial that uses the Boussinesq approximation. If anybody has any ideas on how to make this simulation more stable it would be appreciated. I've listed out the new sif file below.
Code: Select all
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 = Transient
Steady State Max Iterations = 20
Output Intervals = 1
Timestepping Method = BDF
BDF Order = 2
Timestep intervals = 400
Timestep Sizes = 0.5
Solver Input File = case.sif
Post File = t.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 Property 1"
Equation = 1
Material = 1
Body Force = 1
Initial condition = 1
End
Solver 2
Equation = Heat Equation
Procedure = "HeatSolve" "HeatSolver"
Variable = Temperature
Exec Solver = Always
Stabilize = False
Bubbles = True
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 = BiCGStab
Linear System Max Iterations = 500
Linear System Convergence Tolerance = 1.0e-10
BiCGstabl polynomial degree = 2
Linear System Preconditioning = Diagonal
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 1
Equation = Navier-Stokes
Procedure = "FlowSolve" "FlowSolver"
Variable = Flow Solution[Velocity:2 Pressure:1]
Exec Solver = Always
Stabilize = False
Bubbles = True
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 = BiCGStab
Linear System Max Iterations = 500
Linear System Convergence Tolerance = 1.0e-7
BiCGstabl polynomial degree = 2
Linear System Preconditioning = Diagonal
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 = "Equation 1"
Convection = Computed
Active Solvers(2) = 2 1
End
Material 1
Name = "Water (room temperature)"
Reference Temperature = 293
Viscosity = 1.002e-3
Heat expansion Coefficient = 0.207e-3
Compressibility Model = Thermal
Heat Conductivity = 0.58
Relative Permittivity = 80.1
Sound speed = 1497.0
Heat Capacity = 4183.0
Density = 998.3
End
Body Force 1
Name = "BodyForce 1"
Flow Bodyforce 2 = -9.81
End
Initial Condition 1
Name = "InitialCondition 1"
Velocity 2 = 1e-9
Velocity 1 = 0
Temperature = 293
End
Boundary Condition 1
Target Boundaries(1) = 1
Name = "Top"
Velocity 1 = 0
Velocity 2 = 0
Temperature = 293
End
Boundary Condition 2
Target Boundaries(1) = 3
Name = "Bottom"
Velocity 1 = 0
Velocity 2 = 0
Temperature = 293.5
End
Boundary Condition 3
Target Boundaries(1) = 2
Name = "Sides"
Velocity 1 = 0
Velocity 2 = 0
Last edited by MrChips on 09 Dec 2016, 21:14, edited 1 time in total.
Re: Non-Boussinesq Convection
Can you try to set for Navier-Stokes solver:
Stabilize = True
Stabilize = True
-
raback
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Re: Non-Boussinesq Convection
Hi
You could try to set in both solver relaxation factor to ~0.7 and perhaps try with for both solvers:
There is a simple test case "CompressibleIdealGas" using these.
Are the two cases the same for small temperature differences? For ideal gas the linear heat expansion coefficient is 1/T.
-Peter
You could try to set in both solver relaxation factor to ~0.7 and perhaps try with for both solvers:
Code: Select all
Nonlinear Timestepping = Logical True
Are the two cases the same for small temperature differences? For ideal gas the linear heat expansion coefficient is 1/T.
-Peter
Re: Non-Boussinesq Convection
Thanks Peter, I will try this out. Where would I be able to download all the test cases? Is there a central repository for these?
Vencel, the simulation ended up diverging at exactly the same time, but thank you for the suggestion.
Vencel, the simulation ended up diverging at exactly the same time, but thank you for the suggestion.
-
raback
- Site Admin
- Posts: 4828
- Joined: 22 Aug 2009, 11:57
- Antispam: Yes
- Location: Espoo, Finland
- Contact:
Re: Non-Boussinesq Convection
Hi
Is it the linear or nonlinear system that diverges. You are using pretty crappy preconditioner (diagonal). More expensive preconditioner should give your better convergence on linear system level.
-Peter
Is it the linear or nonlinear system that diverges. You are using pretty crappy preconditioner (diagonal). More expensive preconditioner should give your better convergence on linear system level.
-Peter
Re: Non-Boussinesq Convection
Hi Clayton,MrChips wrote:Where would I be able to download all the test cases? Is there a central repository for these?
The test example for Compressible Ideal Gas can be found in the Github repository of Elmer as:
https://github.com/ElmerCSC/elmerfem/tr ... ssIdealGas
Yours Sincerely,
Anil Kunwar
Last edited by annier on 13 Dec 2016, 06:33, edited 1 time in total.
Anil Kunwar
Faculty of Mechanical Engineering, Silesian University of Technology, Gliwice
Faculty of Mechanical Engineering, Silesian University of Technology, Gliwice
Re: Non-Boussinesq Convection
Embarrassingly I'm not entirely sure how to tell, I'm new to this and still have a lot to learn. I did change the preconditioner to ILU1 for both solvers, but the run doesn't get passed the first time step in this case. Here are the last number of lines from the log file (with diagonal preconditioning):raback wrote:Hi
Is it the linear or nonlinear system that diverges. You are using pretty crappy preconditioner (diagonal). More expensive preconditioner should give your better convergence on linear system level.
-Peter
Code: Select all
ERROR:: IterSolve: Failed convergence tolerances.
ComputeChange: NS (ITER=1) (NRM,RELC): ( 0.47989792E+74 2.0000000 ) :: heat equation
HeatSolve: iter: 1 Assembly: (s) 0.45 0.45
HeatSolve: iter: 1 Solve: (s) 0.25 0.25
HeatSolve: Result Norm : 4.7989792055454271E+073
HeatSolve: Relative Change : 2.0000000000000000
ComputeChange: SS (ITER=3) (NRM,RELC): ( 0.47989792E+74 2.0000000 ) :: heat equation
SolveEquations: -------------------------------------
SolveEquations: Coupled system iteration: 4
SolveEquations: -------------------------------------
SingleSolver: Attempting to call solver
SingleSolver: Solver Equation string is: navier-stokes
FlowSolve:
FlowSolve:
FlowSolve: -------------------------------------
FlowSolve: NAVIER-STOKES ITERATION 1
FlowSolve: -------------------------------------
FlowSolve:
FlowSolve: Starting Assembly...
FlowSolve: Assembly:
FlowSolve: Assembly done
DefUtils::DefaultDirichletBCs: Setting Dirichlet boundary conditions
DefUtils::DefaultDirichletBCs: Dirichlet boundary conditions set
FlowSolve: Dirichlet conditions done
1 0.3869E+03
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ERROR:: IterSolve: Failed convergence tolerances.
ComputeChange: NS (ITER=1) (NRM,RELC): ( 0.70159443+110 2.0000000 ) :: navier-stokes
FlowSolve: iter: 1 Assembly: (s) 0.94 0.94
FlowSolve: iter: 1 Solve: (s) 1.50 1.50
FlowSolve: Result Norm : 7.0159442990262395E+109
FlowSolve: Relative Change : 2.0000000000000000
ComputeChange: SS (ITER=4) (NRM,RELC): ( 0.70159443+110 2.0000000 ) :: navier-stokes
SingleSolver: Attempting to call solver
SingleSolver: Solver Equation string is: heat equation
HeatSolve:
HeatSolve:
HeatSolve: -------------------------------------
HeatSolve: TEMPERATURE ITERATION 1
HeatSolve: -------------------------------------
HeatSolve:
HeatSolve: Starting Assembly...
HeatSolve: Assembly:
HeatSolve: Assembly done
DefUtils::DefaultDirichletBCs: Setting Dirichlet boundary conditions
DefUtils::DefaultDirichletBCs: Dirichlet boundary conditions set
1 NaN
2 NaN
3 NaN
4 NaN
5 NaN
6 NaN
7 NaN
8 NaN
9 NaN
10 NaN
11 NaN
12 NaN
13 NaN
14 NaN
15 NaN
16 NaN
17 NaN
18 NaN
19 NaN
20 NaN
21 NaN
22 NaN
23 NaN
24 NaN
25 NaN
26 NaN
27 NaN
28 NaN
29 NaN
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ERROR:: IterSolve: Failed convergence tolerances.
ERROR:: ComputeChange: Norm of solution appears to be NaN
Re: Non-Boussinesq Convection
Well, I wasn't able to get the ILU preconditioning to work. Anytime I had it enabled the simulation would stop after the first few timesteps. I played around with some of the tolerances, but in the end I set both solvers back to diagonal preconditioning. I also wasn't able to get non-linear timestepping working, but it was more successful than the ILU preconditioning. With more practice I could probably get this working.
However, I was able to get the simulation to run just fine. I ended up creating a restart file from this model where I only had the heat equation active. I then used this restart file to run the model with the Navier-Stokes equations enabled. I thought it might make things more stable. I also adjusted the maximum number of non-linear iterations. Honestly, it was probably the last change that stabilized the model. See below.
I'm going to try to refine this a bit and see if I can get the model to work without using the restart file for my initial conditions.
Peter, you asked me whether the linear or non-linear system diverges first. Are you able to determine this from the few lines of the solver log that I attached? Would you be able to describe how to determine this from it? I only have a half a clue about how to read the solver log. I assume it was the non-linear convergence that was the problem just based on the changes I made.
However, I was able to get the simulation to run just fine. I ended up creating a restart file from this model where I only had the heat equation active. I then used this restart file to run the model with the Navier-Stokes equations enabled. I thought it might make things more stable. I also adjusted the maximum number of non-linear iterations. Honestly, it was probably the last change that stabilized the model. See below.
I'm going to try to refine this a bit and see if I can get the model to work without using the restart file for my initial conditions.
Peter, you asked me whether the linear or non-linear system diverges first. Are you able to determine this from the few lines of the solver log that I attached? Would you be able to describe how to determine this from it? I only have a half a clue about how to read the solver log. I assume it was the non-linear convergence that was the problem just based on the changes I made.
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raback
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Re: Non-Boussinesq Convection
Hi
You could perhaps sent the case for inspection. It should not be so much more difficult from the Boussinesq case.
-Peter
You could perhaps sent the case for inspection. It should not be so much more difficult from the Boussinesq case.
-Peter