+ \int_{V} \rho \textbf{g} \cdot \mathbf{i}_{i} \ \mathrm{d}V \quad ,
\label{EQnavier_stokesacc}\end{aligned}$$ with volume $V $ of control volume (CV) bounded by the closed surface $S$, fluid velocity vector **u** with the Cartesian components $u_{i}$, unit vector **n** normal to $S$ and pointing outwards, time $t$, pressure $p$, fluid density $\rho$, components $\tau_{ij}$ of the viscous stress tensor and unit vector **i**$_{j}$ in direction $ x_{j} $. The fluid, gaseous air, is considered incompressible. Selected flow simulations were repeated with compressible air following the ideal gas law and it was verified that compressibility effects were negligible for the investigated cases.

The computational domain is box-shaped with dimensions $0 \leq x \leq 0.4\, \mathrm{m}, -0.2\, \mathrm{m} \leq y \leq 0.2\, \mathrm{m}, 0 \leq z \leq 0.1\, \mathrm{m}$, with an extended channel that represents the nose as illustrated in Fig. \[FIG2Ddom\]. The coordinate system has its origin in the center of the nose opening at the same level as the face, as shown in Fig. \[FIG2Ddom\], which is represented as a flat wall. As Fig. \[FIG2Ddom\] demonstrates, the pressure within the mask is approximately uniform, therefore only the surface area $S_{\mathrm{m}}$, not the geometrical shape of the mask, influences the results, so the 