Experimental Investigation of Laminar-to-turbulent Transition in Helical Tubes under Steady and Pulsatile Flow

Helical flow is critical to cardiovascular hemodynamics, yet its transition to turbulence remains poorly defined, particularly under physiological pulsatile conditions. This experimental study systematically investigates the turbulent flow characteristics in helical tubes by varying curvature radius and torsion pitch using steady and high-pulsatility flows. Laser Doppler velocimetry and pressure measurements were used to quantify turbulence and flow resistance. Results suggest that under steady flow, the upstream flow exhibits the classical subcritical transition. Downstream, the helical geometry alters the mechanism, transforming intermittent turbulent “puffs” into a continuous, high-intensity turbulent field. Crucially, the curvature radius was the dominant stabilizing factor: large radii significantly suppressed downstream turbulence, while pitch effect was negligible. Friction factor results confirmed a higher energy cost to sustain secondary flows in the laminar regime, with a gradual increase during transition that supports the continuous turbulence development observed locally. Under pulsatile flow, flow stability is highly sensitive to the Pulsatility Index (PI). For PI > 3 and near-transition mean Re, intense bursts of turbulence were triggered. These instability bursts aligned specifically with the flow deceleration phase downstream, indicating that the helical geometry exacerbates deceleration-induced instability. These findings provide insights into biomedical device design and benchmarks for computational models studying the complex fluid dynamics of tortuous vascular structures.