Understanding the Parameter file (.prm)

The documentation for the parameter files is currently under migration to the new Lethe documentation

1. Boundary conditions - Multiphysics
2. Linear Solver Control
3. Interface Sharpening

Boundary conditions - Multiphysics

This subsection's purpose is defining the boundary conditions associated to multiphysic problems.

Heat Transfer

subsection boundary conditions heat transfer
  set number                  = 2
    subsection bc 0
	set type	      = temperature
        set value	      = 0
    end
    subsection bc 1
      set type		    = convection
      set h 		    = 10
      set Tinf	   	    = -13
    end
end

• number: This is the number of boundary conditions of the problem.

• type : This is the type of boundary condition been imposed. At the moment, choices are

  • temperature (Dirichlet BC), to impose a given temperature value at the boundary
  • convection (Robin BC) for cooling/heating, depending on the environment temperature at the boundary Tinf, with a given heat transfer coefficient h

• NB : by default, the boundary condition type is temperature with value = 0

Tracer

subsection boundary conditions tracer
  set number                  = 1
  subsection bc 0
        set type              = dirichlet
        subsection dirichlet
            set Function expression = x
        end
  end
end

• number: This is the number of boundary conditions of the problem.

• type : This is the type of boundary condition been imposed. At the moment, only dirichlet boundary conditions can be imposed for tracer.

Linear Solver Control

In these subsections, the control options of linear solvers are specified. These control options may include solution method, maximum number of iterations, tolerance, residual precision and preconditioner information.

Lethe currently supports three core solution strategy or method :

• GMRES solver + ILU preconditioning. This is set by method = gmres
• GMRES solver + AMG preconditioning. This is set by method = amg
• BiCGStab solver + ILU preconditioning. This is set by method = bicgstab

For all methods, a number of parameters to control the preconditioner can be set.

On coarse meshes, the gmres/bicgstab solver with ILU preconditioning is more efficient. As the number of mesh elements increase, the amg solver is the most efficient. Generally, at 1M elements, the amg solver always outperforms the gmres or bicgstab for 2D steady-state problems.

subsection linear solver
  # amg aggregation threshold
  set amg aggregation threshold                 = 1e-14

  # amg number of cycles
  set amg n cycles                              = 1

  # amg preconditioner ilu smoother/coarsener absolute tolerance
  set amg preconditioner ilu absolute tolerance = 1e-12

  # amg preconditioner ilu smoother/coarsener fill
  set amg preconditioner ilu fill               = 1

  # amg preconditioner ilu smoother/coarsener relative tolerance
  set amg preconditioner ilu relative tolerance = 1.00

  # amg smoother overlap
  set amg smoother overlap                      = 1

  # amg smoother sweeps
  set amg smoother sweeps                       = 2

  # amg w cycling. If this is set to true, W cycling is used. Otherwise, V
  # cycling is used.
  set amg w cycles                              = false

  # Ilu preconditioner tolerance
  set ilu preconditioner absolute tolerance     = 1e-6

  # Ilu preconditioner fill
  set ilu preconditioner fill                   = 1

  # Ilu relative tolerance
  set ilu preconditioner relative tolerance     = 1.00

  # Maximum solver iterations
  set max iters                                 = 1000

  # Maximum number of krylov vectors for GMRES solver
  set max krylov vectors                        = 30

  # The iterative solver for the linear system of equations. Choices are
  # . gmres is a GMRES iterative solver with ILU
  # preconditioning. bicgstab is a BICGSTAB iterative solver with ILU
  # preconditioning. amg is GMRES + AMG preconditioning with an ILU coarsener
  # and smoother. On coarse meshes, the gmres/bicgstab solver with ILU
  # preconditioning is more efficient. As the number of mesh elements
  # increase, the amg solver is the most efficient. Generally, at 1M elements,
  # the amg solver always outperforms the gmres or bicgstab
  set method                                    = gmres

  # Linear solver minimum residual
  set minimum residual                          = 1e-8

  # Linear solver residual
  set relative residual                         = 1e-3

  # State whether output from solver runs should be printed. Choices are
  # .
  set verbosity                                 = verbose
end

Interface Sharpening

This subsection includes parameters related to the interface sharpening in VOF simulations of Lethe.

subsection interface sharpening
   set sharpening threshold   = 0.5
   set interface sharpness    = 2
   set sharpening frequency   = 10
end

• The sharpening threshold parameter defines the interface sharpening threshold that represents the mass conservation level. This parameter must be larger than 1 for interface sharpening. Choosing values less than 1 leads to interface smoothing instead of sharpening.
• The interface sharpness parameter defines the sharpness of the moving interface (parameter alpha in the interface sharpening model).
• The sharpening frequency parameter defines the VOF interface sharpening frequency.