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Subsection 3.14.1: Symmetry Plane Up Section 3.14: Constraints Section Subsection 3.14.3: The Prestrain Update Rules

### 3.14.2 Normal Fluid Velocity Constraint

A normal fluid velocity constraint forces the fluid velocity to remain normal to the selected surface. It is enforced for every node on that surface.
<constraint type="normal fluid velocity" surface="NormalFlowSurface01">
<laugon>1</laugon>
<penalty>1e6</penalty>
<tol>1e-6</tol>
<minaug>0</minaug>
<maxaug>50</maxaug>
</constraint>

The surface (NormalFlowSurface01 in this example), is defined in the Mesh section. Let denote the nodal fluid velocity vector and let denote the unit normal at each node; the normal flow constraint enforces , or equivalently, , using a penalty method, optionally with augmented Lagrangian. The reaction force needed to enforce this constraint is evaluated as , where is the user-specified penalty parameter (with units of force per velocity). To use the augmented Lagrangian method, set laugon to 1, and the vectorial Lagrange multiplier will be augmented as . Augmentations terminate when the relative change in is less than the user-specified tolerance tol, or when the number of augmentations exceeds maxaug.
This constraint should only be used on a surface where the fluid pressure or dilatation has been fixed or prescribed. It is not compatible with boundary conditions that prescribe the fluid velocity. To prescribe a fluid velocity that remains normal to the selected surface, use the surface load fluid normal velocity (Section 3.12.2.13↑).
Subsection 3.14.1: Symmetry Plane Up Section 3.14: Constraints Section Subsection 3.14.3: The Prestrain Update Rules