gap cross-section area $S_{\mathrm{g}}$, equivalent results to the theoretical formulation from Sect. \[SECtheory\] can be expected. As all gradients in $z$-direction are zero, the width $\Delta z$ of the domain can be chosen arbitrarily and the results can be scaled to the desired gap width $B_{\mathrm{g}}$ and mask filter surface $S_{\mathrm{m}}$. The basic configuration corresponds to a mask which fits tightly except over a width $B_{\mathrm{g}}=10\, \mathrm{cm}$ along the perimeter.

To simulate the flow in a geometry with a shorter gap width, e.g. $B_{\mathrm{g}}=5\, \mathrm{cm}$, the simulation could be performed in three dimensions (3D) with a partially closed gap, which would increase the number of cells and the computational effort. In the following, a more efficient approach was selected so that the flow problem remains two-dimensional and the computational effort remains low. Consider that, if the gap were closed ($H_{\mathrm{g}}=0$), the flow rate through the mask filter $F_{\mathrm{m}}=F_{\mathrm{t}}$, so the average velocity through the mask filter is $u_{\mathrm{m}}=u_{\mathrm{t}}S_{\mathrm{t}}/S_{\mathrm{m}}$. If the gap width $B_{\mathrm{g}}$ is halved, the domain size in $z$-direction and thus $S_{\mathrm{t}}$ and $S_{\mathrm{m}}$ are halved as well. To maintain the same flow rate $F_{\mathrm{t}}$, $u_{\mathrm{t}}$ and thus also $u_{\mathrm{m}}$ are doubled. To avoid that Eq. (\[EQpm\]) leads to 