Boussinesq

[Pisolino]

I'm trying to solve a simple problem: the heat convection. So i need to activate the Boussinesq option in the body force. However when i activate this option ( or the Flow BodyForce 2 = -9.81) i can't reach convergence. Since i'm not very able in simulating i worked with a simple trial and error. Still no convergence reached. The simulation is really simple a 2d box, 2 side cold, 1 side hot, 1 side adiabatic. The fluid is air. Here it comes the .sif file

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 = 10
Output Intervals = 1
Timestepping Method = BDF
BDF Order = 2
Timestep intervals = 50
Timestep Sizes = 0.5
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 Property 1"
Equation = 1
Material = 1
Body Force = 1
Initial condition = 1
End

Solver 2
Equation = Navier-Stokes
Variable = Flow Solution[Velocity:2 Pressure:1]
Procedure = "FlowSolve" "FlowSolver"
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 = 0.5
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 1
Equation = Heat Equation
Variable = -dofs 1 Temperature
Procedure = "HeatSolve" "HeatSolver"
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

Equation 1
Name = "Convezione"
Convection = Computed
Active Solvers(2) = 2 1
End

Material 1
Name = "Air (room temperature)"
Reference Temperature = 293
Viscosity = 1.983e-5
Heat expansion Coefficient = 3.43e-3
Compressibility Model = Perfect Gas
Heat Conductivity = 0.0257
Viscosity = 1.983e-5
Sound speed = 343.0
Heat Capacity = 1005.0
Density = 1.205
End

Body Force 1
Name = "BodyForce 1"
Flow Bodyforce 2 = -9.81
End

Initial Condition 1
Name = "aria"
Velocity 2 = 0
Velocity 1 = 0
Temperature = 283
Velocity 3 = 0
End

Boundary Condition 1
Target Boundaries(1) = 1
Name = "pareti"
Normal-Tangential Velocity = True
Noslip wall BC = True
End

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

Boundary Condition 3
Target Boundaries(1) = 3
Name = "cold"
Temperature = 203
Noslip wall BC = True
End

Boundary Condition 4
Target Boundaries(1) = 4
Name = "cold"
Temperature = 203
Noslip wall BC = True
End


Thanks for any advice, i'm getting crazy

[raback]

Hi

Look at the choices of the Raleigh-Bernard convection Tutorial. If still no luck, try diminishing the temperature difference. The problem with real parameters may be such that there is no steady-state solution and convergence problems are the sign of that. So eventually one may need to do transient modeling or look at RANS modeling of turbulence. Note that I didn't look at the case files, these are just some common problems.

-Peter

[Pisolino]

Thanks, i started the simulation basing of the tutorial you mentioned ( that worked for me). I'll try to gather some informations on RANS modeling. I tried many times with transient but often it ended after a pair of iterations with : NaN or non-sense temperatures (lower or higher than the driving boundaries with imposed temperatures)

Something really strange is that refining the mesh the simulation convergence get worst , the same with 2order elements.

[raback]

Hi

If the problem is in the time-resolution then increasing space-resolution may not have desired effect. Typically the convergence of loose coupling may is ensured only if the local Courant number is ~< 1.

-Peter

[Pisolino]

Really thanks !! working both on space-time relationship and setting max non-linear iterations = 1 for both solvers i'm starting to get convergence both steady-state and transient. Now i only have to work on non-sense temperatures in transient simulation.

[raback]

Hi

As said, try to use Robin BCs instead of Dirichlet, so replace

Code: Select all

Temperature = 203

with

Code: Select all

External Temperature = 203
Heat Transfer Coefficient = 1.0e4 ! use some rough physical estimate

-Peter