Leaflet fluttering changes laminar–turbulent transition mechanisms past bioprosthetic aortic valves

Bioprosthetic heart valves (BHV) are implantable devices for aortic valve replacement in patients with moderate to severe aortic stenosis. BHV create turbulent flow during peak systole, potentially causing adverse effects and contributing to their limited durability of 10–15 years. Although a deeper understanding of turbulence is important for valve design optimization, laminar–turbulent transition mechanisms past BHV are still poorly understood. This study performs high-fidelity fluid–structure interaction simulations for generic aortic geometries with BHV to assess how leaflet fluttering changes laminar–turbulent transition mechanisms. During the opening of BHV, a three-lobed vortex ring is formed. The interaction between this starting vortex, counter-rotating streamwise vortices created above the valve posts and periodically shed vortex rings due to leaflet fluttering leads to the nonlinear breakdown to small-scale vortices in the ascending aorta. The absence of fluttering alters vortex formation fundamentally and results in lower turbulence levels. Modal analysis shows that the laminar–turbulent transition in a non-fluttering valve is governed by the dynamics of an azimuthal mode of wavenumber 6, whereas the fluttering motion promotes a mode of wavenumber 3 and suppresses higher modes. Establishing such relations between fundamental mechanisms of laminar–turbulent transition may contribute to optimized valve design capable of controlling transition to turbulence.