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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 $\mathbf{v}$ denote the nodal fluid velocity vector and let $\mathbf{n}$ denote the unit normal at each node; the normal flow constraint enforces $\mathbf{v}=\left(\mathbf{v}\cdot\mathbf{n}\right)\mathbf{n}$ , or equivalently, $\mathbf{v}_{\tau}=\left(\mathbf{I}-\mathbf{n}\otimes\mathbf{n}\right)\cdot\mathbf{v}=\mathbf{0}$ , using a penalty method, optionally with augmented Lagrangian. The reaction force needed to enforce this constraint is evaluated as $\varepsilon\mathbf{v}_{\tau}$ , where $\varepsilon$ 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 $\boldsymbol{\lambda}$ will be augmented as $\boldsymbol{\lambda}\leftarrow\boldsymbol{\lambda}+\varepsilon\mathbf{v}_{\tau}$ . Augmentations terminate when the relative change in $\boldsymbol{\lambda}$ is less than the user-specified tolerance tol, or when the number of augmentations exceeds maxaug.