Open Boundary
The Open Boundary condition describes boundaries in contact with a large volume of fluid. Fluid can both enter and leave the domain on boundaries with this type of condition.
Boundary Conditions
The Boundary condition options for open boundaries are Normal stress and No viscous stress.
Normal Stress
The Normal stress f0 condition implicitly imposes .
When Include gravity is selected and Use reduced pressure not selected in the interface Physical model section, the Compensate for hydrostatic pressure approximation (named Compensate for hydrostatic pressure for compressible flows) check box is available and selected by default. When it is selected, the hydrostatic pressure is automatically added to the pressure entered in f0 user input.
No Viscous Stress
The No Viscous Stress condition specifies vanishing viscous stress on the boundary. This condition does not provide sufficient information to fully specify the flow at the open boundary and must at least be combined with pressure constraints at adjacent points.
The No viscous stress condition prescribes:
for the compressible and the incompressible formulations. This condition can be useful in some situations because it does not impose any constraint on the pressure. A typical example is a model with volume forces that give rise to pressure gradients that are hard to prescribe in advance. To make the model numerically stable, this boundary condition should be combined with a point constraint on the pressure.
Viscoelastic Stress
For Viscoelastic Flow interface, Viscoelastic Stress section is available. See Viscoelastic Stress.