SMS:RMA4 Boundary Conditions

Assigning Boundary Conditions

Boundary conditions are generally defined on nodestrings but may also be defined on elements. The default RMA4 boundary condition is a closed boundary (no constituent). To assign boundary conditions, select the item (nodestring, element, etc.) to which the boundary condition should be applied and use the menu command RMA4 | Assign Boundary Conditon which will open a dialog allowing the specification of boundary condition options.

Deleting Boundary Conditions

Boundary conditions are removed using the menu command RMA4 | Delete Boundary Conditon

Nodestring Boundary Conditions

File:Rma4 bc nodestrings.jpg

RMA4 Assign Boundary Conditions dialog

Nodestrings can be used as boundary conditions to define constituent concentrations being modeled. Constituent concentrations can be defined as constant or transient using the XY Series Editor.

RMA4 does not care about the units of the concentration because the output is relative to the initial number you specify. For example, if we specify a concentration of 1,000, the values in the solution will range from 0 to 1,000 as the plume spreads downstream. We can say that the concentration is ppm, ppt, or kg/kg; RMA4 treats all concentrations as relative values.

Since a concentration in water is rarely rigidly maintained, a shock factor may be applied to allow fluctuation of the concentration when the flow direction changes. If no shock factor is applied, no matter how much the flow pushes the concentration out of the model, the concentration at the boundary will not change. However, applying a shock factor is like creating a buffer zone outside the model where the constituent can go until the flow begins to carry it back into the model. This provides for a more realistic solution in some cases. Depending on the situation, a different shock factor may be applied from zero for no shock to 1.0 for a gradual change due to a change in flow direction.

By right-clicking on a selected nodestring and selecting Assign BC|RMA4... will bring up the RMA4 Assign Boundary Conditions dialog. The dialog has the following options:

• Constituent
• Constant
• Transient
  • Curve undefined
• No factor applied when flow direction changes
• Factor applied when flow direction changes
• Rigby maintained boundary

Element Boundary Conditions

See RMA4 Boundary Condition Elements for more information.

Related Topics

• RMA4
• RMA2 Boundary Conditions