The Cartesian velocity condition allows you to prescribe a rigid-body velocity field on the boundary section. Here, the velocity is prescribed in a Cartesian reference frame instead of a normal/tangential frame. This option is especially useful for 3D flows, for which tangential directions cannot be uniquely identified.
In a general form, the rigid-body velocity 
is obtained as the combination of a translational velocity
and an angular velocity 
around a rotation axis:
 | (8–15) |
For 2D flow problems, the rotation axis is perpendicular to the plane of the
computational domain, and is defined by one point. For 3D flow problems, the rotation
axis is defined by two points. In Equation 8–15, 
is the position vector of a point on the boundary section with respect
to the axis of rotation.
The translational velocity should, in general, be used when a boundary section is moving in a fixed direction (for example, a moving belt), while the angular velocity should be used when the boundary section is rotating (for example, inside an extruder).
Important: For viscoelastic flows, note that only the inflow condition (described in Inflow Condition) properly imposes the extra-stress components on inlet boundary sections.
To define Cartesian velocity conditions on a boundary section, do the following:
Select the Cartesian velocity imposed (vx,vy,vz) menu item in the list of boundary condition choices.
Cartesian velocity imposed (vx,vy,[vz])
Specify the translational velocity, if appropriate.
Modify translation velocity
In 2D, you will enter two Cartesian components of the translational velocity
; in 3D, you will enter three Cartesian components. Specify the rotational velocity, if appropriate.
Specify the rotation axis. In 2D, you will specify the rotation center; in 3D, you will specify the 1st and 2nd point of the axis.
Specify the angular velocity
.