# Non-Conforming Interfaces

Non-conforming interfaces allow the usage of non-matching, interacting grids connected by an interface. In the following explanation the Nitsche method [^1] is presented, because it more robust in practical applications [^2]. For a detailed deriviation look into [^1] and [^2].

Consequently, non-conforming interfaces allow to use different meshes in the same simulation, as shown in the above figure. Both domains, $\Omega_1$ and $\Omega_2$ exist of different grids and are connected by the non-conform interface $\Gamma_I$. This is especially interesting if different materials are taken into account while a coupled simulation is performed. Furthermore, they are used for simulate rotating structures (for more details look into Features -> Rotating Interfaces) [^3].

For defining a non-conform interface first the surfaces, that are part of the interface, have to be defined

  <domain geometryType="plane">
<surfRegionList>
<surfRegion name="innerR" />
<surfRegion name="outerR" />
</surfRegionList>
</domain>


In a second step the actual interface has to be established

  <domain geometryType="plane">
<surfRegionList>
<surfRegion name="innerR" />
<surfRegion name="outerR" />
</surfRegionList>

<ncInterfaceList>
<ncInterface name="ncR" masterSide="outerR" slaveSide="innerR"/>
<ncInterfaceList>
</domain>


Those two steps establish the interface of the domain. Now, we have to establish the formulation of the interface

    <pdeList>
<acoustic formulation="acouPressure">
<ncInterfaceList>
<ncInterface name="ncR" nitscheFactor="100" formulation="Nitsche"/>
</ncInterfaceList>
</acoustic>
</pdeList>


The nitscheFactor is only avaiable for non-conforming interfaces type Nitsche and defines the weighting of the coupling of the two domains $\Omega_1$ and $\Omega_2$. This factor has no major impact in the range of 20 - 100 [^2]. A rather high value correlates with a strong coupling, with from a physical point of few is good, but makes the interface numerically instable. On the other hand, a low nitscheFactor is numerically more stable, but can make the coupling unphysical.

(TODO mortar NC interface)