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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 fluid pressure surface load prescribes $p$ on a boundary surface.

<surface_load type="fluid pressure" surface="surface1">
  <pressure>0.05</pressure>
</surface_load>