Boundary Condition Constraints

In FEFLOW, all boundary conditions can be physically constrained. Constraints are thus limitations for all types of boundary conditions or single point conditions (well-type conditions) that define a range of validity of the parent boundary condition. For example, a fixed hydraulic-head condition at a certain node can be limited by a maximum or minimum flow. They can be expressed in the following form:

 

	< Maximum Bound else replace BC by Maximum Bound
BC is valid if	and
	> Minimum Bound else replace BC by Minimum Bound

 

Technically, these constraints are realized by temporarily changing the respective boundary condition. If the flow at a fixed head boundary condition would exceed the maximum flow set as a con­straint, the corresponding boundary condition is internally transformed into a well boundary condition with the value of the maximum flow, and the time step is repeated. Because this test is performed for all affected nodes at every time step, considerable computational effort can be associated with the use of constraints.

Constraints are typically complementary to the respective boundary condition, i.e., head-type boundary conditions are constrained by minimum/maximum flow, while flux-type conditions are constrained by minimum/maximum head. Exceptions from this rule include additional constraints for transfer boundary conditions (to limit the infiltration at disconnected rivers) and for all transport boundary conditions (to limit their application to a certain range of hydraulic head).

Constraints are not shown in the Data panel by default. They can be added to the tree by using the context menu of the corresponding boundary condition.

 

 

 

Algebraic signs for constraints and budget.

 

When using BCC, it is recommended to use a transient model approach! The reason for doing so is the fact that the check whether the boundary condition or the constraint should be active is done at every time step for a transient run. In a steady-state model, this check is only done for the first iteration and then kept for the subsequent iterations.