22 Sep Input waveform shape effects onset and development of transition to turbulence in pulsatile pipe flow
Transition to turbulence in pulsatile pipe flow is of interest in a variety of biological flow domains including in the left ventricle and aorta, aneurysms, and stenotic vessels. Transitional flow can critically alter distributions of important hemodynamic parameters such as wall shear stress (WSS).
Many previous studies have considered the mechanisms of transition to turbulence in steady, and oscillating flows. Considerably fewer studies have examined transition to turbulence in pulsatile flows, and among these none have tested physiologically-relevant pulsatile waveforms. Past studies have demonstrated that a phase-lag exists between the pulsatile waveform and turbulence intensity in the flow, where the deceleration phase of the pulsatile cycle primarily drives the onset of transition, while the acceleration phase attenuates transition.
In this work, we investigated the effect of the input pulsatile waveform shape on the onset and development of transition to turbulence. We designed three input waveform shapes including a ‘long acceleration’ (LA) waveform where about 75% of the cycle is in the accelerating phase, a ‘symmetric’ (SYM) waveform where 50% of the cycle is in the accelerating phase, and a ‘long deceleration’ (LD) waveform where about 25% of the cycle is in the accelerating phase. Each waveform shape was tested at six mean Reynolds numbers ranging from 500-4000 using planar particle image velocimetry (PIV). The results demonstrated that the LA waveform delayed the onset of transition, while the LD waveform shape had the earliest onset of transition. Further, a relationship between the inflow waveform shape and rate of turbulence dissipation was demonstrated.
This study highlights the need for future studies to investigate the effects of transition in patient-specific domains as well as the influence of Womersley number. Further, volumetric experiments are needed to explore pulsatile transition in a 3-dimensional domain.