### Types of boundaries for velocity

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.

#### Zero velocity

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.

#### Normal velocity

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.

#### Tangential velocity

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.

#### Marangoni

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.

#### Outflow

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.

#### No boundary

This acts a default boundary condition for all three velocity components. For structured mesh any settings for velocity should be done only in the case if the boundary condition differs from the Zero velocity one.