PorousFlowOutflowBC

A multicomponent example

A fully-saturated 2-component modelled on a 1D line segment, :

[PorousFlowFullySaturated<<<{"href": "../../syntax/PorousFlowFullySaturated/index.html"}>>>]
  fp<<<{"description": "The name of the user object for fluid properties. Only needed if fluid_properties_type = PorousFlowSingleComponentFluid"}>>> = simple_fluid
  porepressure<<<{"description": "The name of the porepressure variable"}>>> = pp
  mass_fraction_vars<<<{"description": "List of variables that represent the mass fractions.  With only one fluid component, this may be left empty.  With N fluid components, the format is 'f_0 f_1 f_2 ... f_(N-1)'.  That is, the N^th component need not be specified because f_N = 1 - (f_0 + f_1 + ... + f_(N-1)).  It is best numerically to choose the N-1 mass fraction variables so that they represent the fluid components with small concentrations.  This Action will associated the i^th mass fraction variable to the equation for the i^th fluid component, and the pressure variable to the N^th fluid component."}>>> = frac
[]

The model initializes with the porous material fully filled with component = 1 (that is, frac = 0) and with a porepressure gradient that will move fluid from negative to positive :

[Variables<<<{"href": "../../syntax/Variables/index.html"}>>>]
  [pp]
  []
  [frac]
  []
[]

[ICs<<<{"href": "../../syntax/ICs/index.html"}>>>]
  [pp]
    type = FunctionIC<<<{"description": "An initial condition that uses a normal function of x, y, z to produce values (and optionally gradients) for a field variable.", "href": "../ics/FunctionIC.html"}>>>
    variable<<<{"description": "The variable this initial condition is supposed to provide values for."}>>> = pp
    function<<<{"description": "The initial condition function."}>>> = 1-x
  []
[]

For , fluid component (the frac variable) is introduced on the material's left side (), by applying the fixed boundary conditions: The right-hand side, at , has fixed porepressure while the fluid component (the frac variable) is allowed to freely exit the domain when it arrives there.

• Porepressure is held at pp = 1 at . (Physically this adds or removes component = 1 to ensure that porepressure is fixed.)

• Porepressure is held at pp = 0 at . (Physically this adds or removes component = 1 to ensure that porepressure is fixed.) These two BCs ensure the porepressure gradient is maintained.

• The zeroth mass fraction is held at frac = 1 at .

• The zeroth mass fraction is allowed to freely exit the model at .

The former 3 are implemented using DirichletBC, while the latter is the PorousFlowOutflowBC. It acts on mass_fraction_component = 0 because that is the mass-fraction associated to the frac variable by the PorousFlowFullySaturated Action:

[BCs<<<{"href": "../../syntax/BCs/index.html"}>>>]
  [lhs_fixed_b]
    type = DirichletBC<<<{"description": "Imposes the essential boundary condition $u=g$, where $g$ is a constant, controllable value.", "href": "DirichletBC.html"}>>>
    boundary<<<{"description": "The list of boundary IDs from the mesh where this object applies"}>>> = left
    variable<<<{"description": "The name of the variable that this residual object operates on"}>>> = pp
    value<<<{"description": "Value of the BC"}>>> = 1
  []
  [rhs_fixed_b]
    type = DirichletBC<<<{"description": "Imposes the essential boundary condition $u=g$, where $g$ is a constant, controllable value.", "href": "DirichletBC.html"}>>>
    boundary<<<{"description": "The list of boundary IDs from the mesh where this object applies"}>>> = right
    variable<<<{"description": "The name of the variable that this residual object operates on"}>>> = pp
    value<<<{"description": "Value of the BC"}>>> = 0
  []
  [lhs_fixed_a]
    type = DirichletBC<<<{"description": "Imposes the essential boundary condition $u=g$, where $g$ is a constant, controllable value.", "href": "DirichletBC.html"}>>>
    boundary<<<{"description": "The list of boundary IDs from the mesh where this object applies"}>>> = left
    variable<<<{"description": "The name of the variable that this residual object operates on"}>>> = frac
    value<<<{"description": "Value of the BC"}>>> = 1
  []
  [outflow_a]
    type = PorousFlowOutflowBC<<<{"description": "Applies an 'outflow' boundary condition, which allows fluid components or heat energy to flow freely out of the boundary as if it weren't there.  This is fully upwinded", "href": "PorousFlowOutflowBC.html"}>>>
    boundary<<<{"description": "The list of boundary IDs from the mesh where this object applies"}>>> = right
    include_relperm<<<{"description": "If true, the Darcy flux will include the relative permeability.  If false, the relative permeability will not be used, which must only be used for fully-saturated situations where there is no notion of relative permeability"}>>> = false # no need for relperm in this fully-saturated simulation
    mass_fraction_component<<<{"description": "The index corresponding to a fluid component.  If supplied, the residual contribution will be multiplied by the mass fraction, corresponding to allowing the given mass fraction to flow freely from the boundary.  This is ignored if flux_type = heat"}>>> = 0
    variable<<<{"description": "The name of the variable that this residual object operates on"}>>> = frac
  []
[]

The results are shown in Figure 1 where it can be seen the fluid component freely exits the right-hand boundary.

Figure 1: Advection and diffusion of a fluid component from left to right along a porepressure gradient. The fluid component is free to exit the right-hand boundary due to the PorousFlowOutflowBC there.

Further description of this result and further examples may be found in the boundaries documentation and sinks tests documentation.

Input Parameters