Flow Instabilities Induced by the Fluid-Structure Interaction of a Flexible Pipe*

The presence of turbulence in blood flow has been linked to cardiovascular diseases such as atherosclerosis, underlining the importance to understand how instabilities within pulsatile flows develop so that hemodynamic turbulence can be mitigated. Whereas transition of pulsatile flows in straight rigid pipes is governed by the Reynolds number (Re), the Womersley number (α), and the pulsation amplitude (A), hemodynamic flows are additionally subject to fluid-structure interactions (FSI) via compliant vessels, which may cause flow perturbations and potentially induce turbulence. In this study, we experimentally investigate the onset and development of instabilities in sinusoidal pulsatile flows through a straight flexible pipe with an inner diameter D_i = 20 mm and a length L_el = 7.5D_i. We systematically map out the transitional parameter space, as well as study the effect of system resonance on transition. We find that transition is strongly dependent on resonance regardless of A, though the underlying mechanism is the modulation of the effective amplitude (A­­_in). When A­­_in is controlled, resonance influences the transitional Reynolds number (Re_T), i.e., the onset of a flow instability, and the slope of transition curves at large A­­_in. We observe three regimes where different types of instabilities occur. In the first regime, streaks appear for flows with small A­­_in, α < 8 and α > 20, which are typical precursors for turbulent puffs. A helical instability occurs in the second regime, with large A­­_in and 8 < α < 12. In the third regime, both streaks and helical structures are simultaneously present at large A­­_in and 12 < α < 20. This third regime has not yet been observed in rigid pipes. These findings show that system resonance is an additional parameter to consider when investigating FSI with compliant vessels, and that physiological in vitro studies should ensure proper flow conditions because certain flow regimes exhibit different turbulence characteristics.