Boundary conditions

HBGC123D offers four types of boundary conditions (b.c.):

• Point source boundary conditions (well nodes)
• Dirichlet boundary condtions (fixed concentrations at nodes)
• Distributed source boundary conditions (well elements)
• Variable boundary conditions

Note: Source (point and distributed) and variable boundary conditions are given in [moles/fluid volume].
Dirichlet boundary conditions are given in [moles/mass of phase].

General remarks

For all four boundary conditions that can be simulated with the model, the following requirements are valid:

• If there is at least one node (Dirichlet and point source boundary condition) or element (distributed source boundary condition) or element side (variable boundary condition) in the model, there also has to be at least one concentration profile for this type of boundary condition. When you try to export or run a model that violates this rule, you will get an error message.
• Boundary concentration profiles are assigned to all mobile components, kinetic complexed species, and aqueous microbial species. If heat transfer is simulated, the temperature also has b.c. profiles.
• The number of boundary condition profiles for each type is the same for all components, species etc. Therefore, the number of profiles has to be the highest number you need for any of the components, species etc. If the others need a smaller number of different profiles, you can leave the remaining ones filled with values of 0 and simply not assign this profile to any of the nodes, elements or element sides.
• The number of data points in each of the boundary condition profiles has to be the same for all profiles and b.c. objects (components, species etc.) of this type.
• The number of concentration profiles and the number of data points in the profiles can be increased by invoking the appropriate buttons in the mesh pane of the input editor.
• It is currently not possible to remove b.c. profiles or reduce the number of data points within the profiles. If there are more profiles than you need, you can simply ignore them by not assigning them to any of the nodes, elements or element sides. If there are more data points in the profiles than you actually need, you still have to assign values to these data points. You can, for instance, use very high values for the time (higher than the simulated time) and the same concentration that you are using for the last significant data point in the concentration profile.
• The concentration profiles itself are set up in the panes Components (mobile components only), Chemical species (kinetic complexed species only), Microbial species (aqueous microbial species only), and Heat (if heat transfer is simulated) of the input editor.
  You can either constant concentrations, flow rates, densities or temperature by using two data points with the first time data point at the beginning (usually 0) and the second at the end of the simulation, and both concentrations with the same value.
  You can also simulate gradually changing concentrations etc. The concentration, flow rate etc. between two data points will be linearly interpolated.
  Sudden changes in the concentration (e.g. when an injection is turned off) are modeled with a step input function. For instance, if an injection is turned off at t = 1000 and the time step size is 1, you can setup the b.c. profile like this:
  Data point 1: t = 0, c = 1
  Data point 2: t = 1000, c = 1,
  Data point 3: t = 1000.1, c = 0,
  Data point 4: t = 2000, c = 0
  (All values are arbitrary examples.)
• The mapping of profiles to nodes, elements or element sides is done for each boundary condition object (i.e. component, species etc.) separately. This is done by invoking the "Edit mapping" button in any of the above mentioned panes and, if necessary, selecting a component or species.
• The following units are used:
  Source (point and distributed) and variable boundary conditions are given in [moles/fluid volume].
  Dirichlet boundary conditions are given in [moles/mass of phase].

Point source boundary conditions (well nodes)

This type of boundary condition is used to simulate the inflow or outflow of water into or out of the model domain. Once you have declared at least one node as point source node (with the "Nodal coordinates and boundary conditions" dialog in the mesh pane of the input editor), you can add concentration profiles for this type of boundary conditions.

Flow rate and density of the incoming fluid are assigned in the mesh pane of the input editor. A positive flow rate represents an inflow into the model domain.

Dirichlet boundary conditions

These boundary conditions are used to model a constant concentration or temperature at given points. The Dirichlet nodes can be anywhere in the model domain.

Distributed source boundary conditions (well elements)

This kind of boundary condition is similar to the point source b.c. However, the source is distributed over evenly over an element. This type of b.c. also requires the input of flow rate and density in the mesh pane of the input editor.

Variable boundary conditions

These are the only boundary conditions that can only be assigned to an outer bondary of the model domain. They are assigned to mesh boundary element sides. If the flow is into the mesh at the assigned boundary, the given concentration from the b.c. profile is used. If the flow is out of the mesh, the b.c. concentration profile is ignored.