Fluid-Flux BC
Units [L/T], [L2/T]
A flux boundary condition applies a pre-defined flux (Darcy flux) to nodes along a line (2D model) or to nodes enclosing faces of elements (3D). For the calculation of hydraulic head as a simulation result, at these nodes an additional inflow/outflow is considered.
Flux boundary conditions are applied in cases where the gradient or inflow/outflow velocity is known in advance. This can be the case for example for inflows into the aquifer in a valley from steep slopes or for the connection to a neighboring aquifer where the flux can be assumed as constant.
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For defining boundary conditions, inflows are considered as negative, outflows as positive. Thus a boundary condition for lateral inflow into the aquifer has to be set with a negative sign! |
3D model
In 3D models, the flux boundary condition has the unit [L/T], defining an inflow or outflow of water across a defined area in a certain time: [L³/(L²*T)].
The boundary condition nodes have to enclose entire faces of elements to define the area in one of the following ways:
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For an inflow at one slice (either on top or bottom of the model or inside) all the nodes of an element in one slice (three nodes when using prismatic elements, four for cuboidal elements) have to be assigned the condition. |
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For an inflow at one slice (either on top or bottom of the model or inside) two nodes of an element in one slice and the same two nodes in an adjacent slice have to be assigned the condition. |
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The area for the inflow or outflow calculation includes the area of all the element faces the boundary condition has been assigned to. If the condition is inside the model, there are always two neighboring elements for each face therefore the area of the face is taken twice. At outer boundaries or on top/bottom of the model, the area of the face is only taken once. |
2D model
In 2D horizontal confined models, the flux boundary condition has the unit [L²/T], defining an inflow or outflow of water across a defined line in a certain time: [L³/(L*T)]. L is derived internally from the length of the line the condition is applied to.
In 2D horizontal unconfined and cross-sectional models, the flux boundary condition has the unit [L/T], defining an inflow of water across an area in a certain time: [L³/(L1*L2*T)]. Here, L1 is taken from the length of the line the condition is applied to, while L2 corresponds to the saturated thickness in horizontal unconfined models (hydraulic head - bottom elevation in unconfined zones, top elevation - bottom elevation in confined parts of the model). For cross-sectional models, L2 is assumed at unit length (1 m).
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The flux boundary condition in 2D models has to be set to at least two nodes, defining a length for the inflow or outflow. The length for the inflow or outflow calculation includes the length of all the element edges the boundary condition has been assigned to. If the condition is inside the model, there are always two neighboring elements for each edge. Therefore the length of the edge is taken twice. At outer boundaries, the length of the line is only taken once. |
In 2D axisymmetric models, the flux boundary condition has the unit [L/T] as well. In contrast to the cross-sectional case, L2 represents the circumference, i.e. the local horizontal coordinate x multiplied with 2 and Pi.