Mesh Adaptation#

This subsection controls the mesh adaptation method, with default values given below.

subsection mesh adaptation
  # Type of mesh adaptation. Choices are  none, uniform or kelly.
  set type                     = none

  # Variable(s) for kelly estimation. Choices are velocity, pressure, phase or temperature.
  # For multi-variables refinement, separate the different variables with a comma
  set variable                 = velocity

  # Frequency of the mesh refinement
  set frequency                = 1

  # Minimum refinement level
  set min refinement level     = 0

  # Maximum refinement level
  set max refinement level     = 10

  # Fraction of coarsened elements
  # For multi-variables refinement, separate the different fractions with a comma
  set fraction coarsening      = 0.05

  # Fraction of refined elements
  # For multi-variables refinement, separate the different fractions with a comma
  set fraction refinement      = 0.1

  # How the fraction of refinement/coarsening are interpreted
  # Choices are number or fraction
  set fraction type            = number

  # Maximum number of elements
  set max number elements      = 100000000

  # Number of initial (pre-solve) refinement steps
  set initial refinement steps = 0

  # Fix the boundary mesh size at the initial refinement level when the kelly mesh refinement estimator is used.
  set fix boundary refinement = false

  # List of boundaries next to which the mesh refinement level should not be modified. The list must contain the boundary ids separated by commas.
  set boundaries fixed = 0, 1
end

Two type of mesh adaptation are available. The uniform mesh adaptation refines the mesh at every cell, whereas the kelly uses a kelly error estimator to decide which cell are refined, by estimating the error per cell for a given variable.
The variable for kelly estimation should be specified with set variable, and can be: velocity, pressure, phase (for multiphase flows), temperature
- Mesh adaptation can be defined on multiple variables, separated with a coma (e.g. set variable = velocity,temperature, or set variable = velocity,phase,pressure etc.).
Warning

The different fraction refinement and fraction coarsening must be defined explicitly (see these parameters definition below).

Important

In the case of multiple variable mesh adaptation, the cells are:

refined if refinement is necessary for at least one variable
coarsened if coarsening is necessary for all variables

The frequency at which the mesh is refined is controlled with the frequency parameter. If set frequency = 1, the mesh is refined at every iteration.
- For transient simulation, this means at every time-step.
- For steady-state simulation in which the steady-state problem is solved on successively refined meshes, the user should have set frequency = 1, which is the default value.
The minimal and maximal refinement level reachable for a cell are controlled respectively with the min refinement and max refinement parameters.
- for deal.ii meshes, if the min refinement level is equal to the initial refinement (see Mesh paramater), no cell will be coarser than the initial mesh.
- for gmsh imported meshes, if set min refinement level = 0, no cell will be coarser than the initial mesh.

Tip

For a gmsh mesh, a cell cannot be coarsened more than it’s initial level. Consequently, adaptively refined simulations should start with a mesh as coarse as possible.

Tip

For a good compromise between speed and precision, max refinement level should be set to 2 or 3 more than the min refinement level

The fraction of cell that are refined and coarsened are controlled with the fraction refinement and fraction coarsening parameters.
- Fractions for mesh adaptation on multiple variables must be separated with a coma (e.g. set fraction refinement = 0.2,0.2, or set fraction coarsening = 0.1,0.3 etc.).
Warning

The different variable must be defined explicitly (see this parameter definition above).

Tip

For set type = kelly, and set variable = velocity or pressure, a good first start is achieve with set fraction refinement = 0.2 and set fraction coarsening = 0.1.

For set type = kelly, and set variable = phase, use fraction type = fraction (explained below) and set fraction refinement = 0.8 for a good tracking of the entire free surface (see Multiphysics).

The fraction of refinement/coarsening can be interpreted in number or fraction depending on the parameter fraction type. At first sight, this is a relatively difficult concept to understand that is inherited from deal.II.
- When fraction type = number the refine_and_coarsen_fixed_number strategy of deal.II is used. This function provides a strategy to mark cells for refinement and coarsening with the goal of providing predictable growth in the size of the mesh by refining and coarsening a given fraction of all cells.
- When fraction type = fraction, the refine_and_coarsen_fixed_fraction strategy is used. This function provides a strategy to mark cells for refinement and coarsening with the goal of controlling the reduction of the error estimate. Also known as the bulk criterion or Dörfler marking, this function computes the thresholds for refinement and coarsening such that the criteria of cells getting flagged for refinement make up for a certain fraction of the total error.
The maximum number of elements in the entire domain can be controlled with the max number elements parameter.
The boolean parameter enable mesh refinement controller activates a controller that overrides the value of the of fraction coarsening parameter. If activated, the controller will try to maintain the total number of elements in the domain equal to the value of max number elements parameter. The control is done using a PID controller.

Note

If the fraction refinement parameter is too high, the controller may not be able to maintain the number of elements constant. If fraction type = number, the maximal fraction refinement that is stable in 3D is 0.125. In 2D, it is 0.25.

Tip

When using the mesh refinement controller, try reducing the fraction refinement parameter if elements alternate between being refined and coarsened.

Warning

If mesh refinement controller is set to false, the max number elements parameter puts a hard limit on the number of cells in the domain, even if the fraction refinement is increased.

The number of initial (before solving) adaptive refinement steps is controlled by the initial refinement steps parameter. With an initial refinement steps larger than 0, the triangulation is refined adaptively before the solver starts solving the problem. This enables the user to adapt the initial mesh to the initial condition. For example, if the simulation is a VOF simulation, it is ideal to have an initial mesh that captures the interface between the fluids accurately. This is achieved by refining the mesh using the dynamic mesh adaptation parameters and reapplying the initial condition after each adaptation. This process will be repeated initial refinement steps times.
The fix boundary refinement parameter is used to fix the boundary mesh size at the defined initial refinement level. boundaries fixed can be used to specify which boundary ids will not be refined or coarsened when the kelly mesh adaptation is used. Cells on the boundary will still be refined when global refinement is active or when neighbouring cells force the refinement of a boundary mesh cell.