Helical Flow Patterns and Transition Behavior in Physiologically Relevant Vascular Models

Pulsatile helical flow is a fundamental physiological phenomenon observed in various regions of the circulatory and respiratory systems, including the heart, aorta, arterial bifurcations, and umbilical vessels. These distinctive flow patterns are widely recognized as indicators of vascular health, as they promote efficient perfusion, reduce oscillatory shear stress, and minimize energy loss. However, detailed experimental investigations are limited due to the lack of benchmark models. To address this gap, we developed an experimental platform capable of characterizing steady and pulsatile vortices in helicoidal pipes across a wide range of Reynolds numbers, Womersley numbers, and Pulsatility Indices, spanning the laminar-to-turbulent transition regimes. The system incorporates biologically inspired helical pipes with prescribed curvature and torsion into a closed-loop flow circuit. Flow rate and pressure were monitored using an ultrasonic flow sensor and high-frequency pressure transducers. Velocity fluctuations were measured using Laser Doppler Velocimetry (LDV), while Particle Image Velocimetry (PIV) was employed to visualize vortex dynamics and quantify localized helicity throughout the flow domain. Complementary computational studies using RANS and Large Eddy Simulation (LES) methods provided three-dimensional characterization of the helical flow structures. Results reveal that helical tubes significantly modify the laminar-to-turbulent transition behavior compared to straight tubes. Trends in turbulence kinetic energy (TKE) and friction factor exhibit a strong dependence on helical vortex developments, highlighting its role in the onset of secondary flow instabilities. Both experimental and computational findings offer new insights into vortex evolution and flow asymmetry, advancing our understanding of how pulsatile helical flow modulates transition to turbulence. These results are expected to inform the development of a scaling framework for helicity and instability prediction in physiological systems.