$\newcommand{\Dev}{\operatorname{Dev}}$ $\newcommand{\dev}{\operatorname{dev}}$ $\newcommand{\grad}{\operatorname{grad}}$ $\newcommand{\divg}{\operatorname{div}}$ $\newcommand{\Ei}{\operatorname{Ei}}$ $\newcommand{\tr}{\operatorname{tr}}$

In a fluid or fluid-FSI analysis, this boundary condition prescribes the normal component of the viscous traction (Section 3.12.2.8↑), $t_{n}^{\tau}=\mathbf{t}^{\tau}\cdot\mathbf{n}$ , such that it opposes backflow, i.e., when $v_{n}=\mathbf{v}\cdot\mathbf{n}$ is negative. Specifically, $t_{n}^{\tau}=\beta\rho_{r}v_{n}^{2}$ when $v_{n}<0$ and $0$ otherwise, where $\beta$ is a unitless user-defined parameter (typical values may range from 0 to 1) and $\rho_{r}$ is the referential fluid density.

<surface_load type="fluid backflow stabilization" surface="surface1">>
	<beta lc="1">1</beta>
</surface_load>