Prev Subsubsection 3.12.2.12: Fluid Tangential Stabilization Up Subsection 3.12.2: Surface Loads Subsubsection 3.12.2.14: Fluid Velocity Next
3.12.2.13 Fluid Normal Velocity
In a fluid or fluid-FSI analysis the fluid velocity relative to the mesh, , on a boundary surface with outward normal may be decomposed into the normal component, , and tangential component, . The surface load fluid normal velocity may be used to prescribe a desired value of on a boundary surface.
<surface_load type="fluid normal velocity" surface="surface1"> <value lc="1">1.0</value> <velocity>1</velocity> </surface_load>
Optionally, FEBio can generate a parabolic velocity profile over the named surface, This option can be turned on using the parabolic element (0=default, 1=parabolic). In this case, the velocity element provides the average value of this parabolic velocity profile, while value remains a scale factor optionally tied to a loadcurve. The parabolic profile is calculated under the assumption of steady Poiseuille flow in an infinite tube whose cross-section has the shape of the named surface. All nodes located along the curve boundary of this surface are prescribed zero normal velocity.
<surface_load type="fluid normal velocity" surface="surface1"> <velocity lc="1">-1.0</velocity> <parabolic>1</parabolic> </surface_load>
Note that this parabolic option will not work if the mesh of the boundary surface has no internal nodes.
If an arbitrary distribution with different scalar value needs to be specified for each facet, use the following syntax,
<surface_load type="fluid normal velocity" surface="surface1"> <value surface_data="inlet"/> <velocity lc="1">-1.0</value> </surface_load>
and provide the facet values in a user-defined SurfaceData map with name="inlet" and data_type="scalar" (Section 3.8.1↑).
By default, this condition only prescribes the natural boundary condition , leaving free. Optionally, it is possible to also enforce by prescribing essential boundary conditions on the wx, wy and wz components of at the nodes of each facet, based on the value of at each node (obtained by averaging from adjoining facets). This option may be turned on using the prescribe_nodal_velocities element (0=default, 1=prescribed), for example,
<surface_load type="fluid normal velocity" surface="surface1"> <velocity>-1.0</velocity> <prescribe_nodal_velocities>1</prescribe_nodal_velocities> </surface_load>
When the fluid is prescribed on a surface boundary, better numerical convergence is achieved if the fluid dilatation is also prescribed on the rim (the boundary curve) of that surface boundary. While this can be done explicitly, a better option is to set the fluid dilatation on the rim to match the average pressure on the surface boundary, calculated during the analysis. To select this option, use the prescribe_rim_pressure element (0=default, 1=prescribed), for example,
<surface_load type="fluid normal velocity" surface="surface1"> <velocity>-1.0</velocity> <parabolic>1</parabolic> <prescribe_nodal_velocities>1</prescribe_nodal_velocities> <prescribe_rim_pressure>1</prescribe_rim_pressure> </surface_load>
Note that this prescribe_rim_pressure option will not work if the mesh of the boundary surface has no internal nodes.