twice the desired pressure drop along the mask, the larger mask filter is modeled by halving $C_{\mathrm{m}}$. Thus the same flow rate and pressure loss occur through the mask as previously. So if the reference gap width is as in the present case $B_{\mathrm{g, ref}}=10\, \mathrm{cm}$, then selecting $C_{\mathrm{m}}=C_{\mathrm{m,ref}} B_{\mathrm{g}}/B_{\mathrm{g,ref}}$ provides the desired solution for simulations with different gap widths.

The solution domain was discretized with a rectilinear grid with local mesh refinement as shown in Fig. \[FIG2mesh\], so that the gap was resolved with at least $10$ cells per gap height $H_{\mathrm{g}}$. For the grid dependence study, the grids were refined uniformly by halving the cells in $x$- and $y$-directions. Depending on the gap size, the grids consisted of ca. $10\, 000$ to $160\, 000$ cells.

![Computational grid with local refinement, especially in the vicinity of the gap; full domain (top left) and close-up of nose and mask (top right), gap (bottom left) and gap entry (bottom right)[]{data-label="FIG2mesh"}](images/gridFull.png){width="\linewidth"}

All simulations were performed using the commercial flow solver Simcenter STAR-CCM+ (version 15.02.007-R8) by Siemens. The solver is based on the finite volume method (FVM) and the implicit unsteady segregated solver was used. All approximations were of second order. The under-relaxation 