center-part of the gap with velocity vectors; for simulation from Fig. \[FIGFt0Fg\] with flow rate $F_{\mathrm{t}}=95\, \mathrm{L/min}$, viscous porous filter resistance $C_{\mathrm{m}}=2000\, \mathrm{m/s}$, gap height $H_{\mathrm{g}}=0.4\, \mathrm{mm}$, gap length $L_{\mathrm{g}}=1\, \mathrm{cm}$ and gap width $B_{\mathrm{g}}=5\, \mathrm{cm}$[]{data-label="FIG2DpvelVecs"}](images/NEWbaseLF_Copy_Vectors.png "fig:"){width="0.8\linewidth"}

Figures \[FIGFt0FgCm1000\]-\[FIGdpCm1000\] show results for a mask with viscous porous resistance $C_{\mathrm{m}}=1000\, \mathrm{m/s}$. When tightly fitted, the mask produces a pressure drop close to the lowest values that FFP3-masks on the market may provide.

The results show similar trends as those for $C_{\mathrm{m}}=2000\, \mathrm{m/s}$, and although the filter resistance is halved, the flow rate through the gap reduces only by $10-20\%$ in most cases. Thus, the choice of filter material has a comparatively small influence concerning how large the gap height $H_{\mathrm{g}}$ may be before the mask ceases to fulfill FFP3 requirements.

Figures \[FIGFt0Fg\]-\[FIGdpCm1000\] show the results obtained for the fine grid. Results for medium and coarse grids are not depicted because they show no noticeable differences. Average differences between simulation results for the flow rates on coarse, medium and fine grids are less than $0.8\%$. The differences between analytical predictions and simulation results for the flow rates was $<3\%$ for all simulated cases.

Flow rate $F_{\mathrm{t}}=30\, \mathrm{L/min}$\
![As Fig. \[FIGFt0Fg\], except for 