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Stress tensor is set to zero in all tangential directions:

\begin{align} {\bf t_1} \cdot {\bf S} \cdot {\bf t_1} = 0 \\ {\bf t_2} \cdot {\bf S} \cdot {\bf t_2} = 0 \\ {\bf t_2} \cdot {\bf S} \cdot {\bf t_1} = 0\end{align}

Here, \(\bf S\) is the stress tensor, which consists of viscous and solid parts together with total pressure \begin{align} \bf S = \bf S_{visc} + \bf S_{solid} - (p_{hyd} + p_{dyn})\bf I,\end{align} \(\bf n\) is the boundary normal, \(\bf t_1\) and \(\bf t_2\) are the associated tangentials, i.e.

\begin{align} {\bf t_1} \cdot {\bf n} = 0 \\ {\bf t_2} \cdot {\bf n} = 0\end{align}

with

\begin{align} \left\Vert{\bf t_1}\right\Vert = \left\Vert{\bf t_2}\right\Vert = \left\Vert{\bf n}\right\Vert = 1\end{align}