The application of boundary conditions for velocity differs in
dependence at which type of the mesh, structured or not, the
convection has to be computed. All following types of
boundaries are available only for the
** UVW ** variables group, see
the variables list of the ** Boundary ** dialog.

There is no default type of the boundary condition for velocity components in case of the unstructured mesh, therefore any boundary condition should be always set for it. The absence of the prescribed boundary conditions should act according to the finite volume technique no shear force at the fluid boundary. Another situation is by the structured mesh. The "forgotten" boundary condition at the external boundary of the structured rheological domain is equivalent with the zero velocity boundary condition there. This type of the boundary condition is most frequently occurred, therefore there is no necessity in most of cases to prescribe the boundary conditions at the structured mesh.

This type of boundary should be manually prescribed only in case of the unstructured mesh, it coincides with the default boundary condition for the structured mesh. This boundary condition is meaningful everywhere at the interface between the liquid and the solid domains.

All three cylindrical velocity components are set equal to the zero value.

This type of boundary condition is used in order to prescribe the flow inlet at the external fluid boundary. The positive value of the prescribed velocity means the flow into the fluid domain. The fixed velocity value at the boundary will directed perpendicularly to the boundary. If this boundary is applied, it means that one has to deal with the forced convection. The mass flow incoming into the fluid volume should also exit from it. Therefore another type of the velocity boundary condition "Outflow" should be also applied at the external boundary of the same fluid domain. The velocity value is entered in meter per second.

This type of the boundary condition is applied e. g. for the
"sliding cavity" configuration. The fluid is sliding along the
boundary with the prescribed velocity. The direction of the fluid
movement is defined by the orientation of the line at the region
boundary. For the positive sign of the velocity value the
resulted velocity vector is oriented along the direction of the
boundary line. The direction of the boundary line is shown by the
red arrow if any boundary condition is prescribed on the line and
the option ** "show" ** is activated at
the top of the ** Boundary ** dialog.
The velocity value is entered in meter per second.

This type of the boundary condition is applied for the liquid-liquid or liquid-gas interface or for the external boundary of the computational domain. The Marangoni boundary condition is applied. That means that the tangential stress component is acting on the melt surface parallel to it provided the temperature gradient along the free melt is available.

The magnitude of the Marangoni force is proportional to the
thermocapillar coefficient of the surface tension. This data is
associated with the material of the fluid. The value of it can be
entered in the ** Materials ** dialog by the
** Fluid properties ** in the **
Marangoni coef. ** field. Its value is entered in
N/K/m.

The Marangoni boundary is an appropriate choose for the free melt
surface in contact either with gas or with vacuum. If this
boundary condition is defined, no other settings are required for
the particular velocity components there. For the zero Marangoni
coefficient this boundary condition coincides with the **
Normal velocity ** boundary condition with zero value.
Therefore it is advised to
apply the Marangoni boundary condition always for the free melt
boundary meshed with the structured mesh.

If the boundary condition is prescribed for the line separating two rheological regions meshes structurally then, additionally the shear stress from both fluids will be considered in the forces balance at such interface. The Marangoni coefficient for such complex boundary will be accepted automatically from one of the pair of materials in contact. The largest value of the Marangoni coefficient will be selected. In the normal case one has to deal with a nonzero Marangoni coefficient for the melt and no Marangoni effect for the gas.

The Outflow boundary condition should be applied always in
combination with the ** Normal velocity
** boundary condition for each fluid domain. The
program will adjust the velocities and pressures at this boundary
in such a way that the mass conservation is maintained in the
full fluid volume. No value should be prescribed for this
type.

Multiple flow inlets and outflows are also supported for the same fluid domain.