*2.2. Geometry of BMHV*

Figure 1 shows a three-dimensional simplified geometry of the St. Jude bileaflet mechanical heart valve investigated in this research, which was chosen to be similar to previous studies [14,15]. The model geometry mainly consists of four parts: valve leaflets, aortic sinus, inlet, and outlet sections [16]. In the previous experimental study, the phenomenon of leaflet fluttering was only observed when the leaflets were in the fully open position, so the leaflets were assumed to be fixed in the fully open position to simplify the calculation. The geometry of the aortic root is modelled as an axial-symmetric expansion. The aortic root diameter was 27 mm, and the leaflet opening angle was 80◦. The hinge mechanism was simplified as solid cylinder. To obtain the fully developed flow and minimize the influence of outlet boundary conditions, the lengths of the upstream and downstream sections were added as 2D and 6D (where D is the inlet diameter), respectively. Three models with different fully opening angles (75◦, 80◦, and 85◦) were created for the purpose of this study.

**Figure 1.** The perpendicular cut-plane view of the three-dimensional bileaflet mechanical heart valve (BMHV) geometry.
