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In a fluid or fluid-FSI analysis, the Cauchy stress is given by $\boldsymbol{\sigma}=-p\mathbf{I}+\boldsymbol{\tau}$ , where $p$ is the elastic pressure and $\boldsymbol{\tau}$ is the viscous stress. The traction on a fluid surface with outward normal $\mathbf{n}$ is given by $\mathbf{t}=\boldsymbol{\sigma}\cdot\mathbf{n}=-p\mathbf{n}+\mathbf{t}^{\tau}$ , where $\mathbf{t}^{\tau}=\boldsymbol{\tau}\cdot\mathbf{n}$ is the viscous component of the traction. The fluid traction surface load prescribes $\mathbf{t}^{\tau}$ on a boundary surface.

<surface_load type="fluid traction" surface="surface1">
  <scale lc="1">1.0</scale>
  <traction>0,0,1</traction>
</surface_load>