Session: 10.4.3 - Vortex Dynamics III

Paper Number: 158696

158696 - An Experimental Study of Turbulence Transition Thresholds in Physiologically Relevant Helical Flows 

Abstract: 

Helical blood flow is a crucial physiological feature observed in various circulatory systems, including the heart, aorta, vessel bifurcations, umbilical cord, and even the respiratory system. This unique flow pattern, characterized by its spiral motion, signifies healthy blood flow and plays a vital role in maintaining optimal circulatory function. It offers several advantages, such as enhancing blood perfusion to tissues, minimizing oscillating wall shear stress that could lead to vascular damage, and conserving energy within the circulatory system. Despite its significance, the systematic understanding and characterization of these helical flow patterns remain limited. Thus, comprehensive benchmark models can aid in studying their mechanics and clinical implications in detail. In this study, we designed an experimental setup to investigate the dynamics of laminar helical flow and its transition to turbulence over a broad range of Reynolds numbers (Re) and pulsatile flow parameters. A closed-loop flow system was developed, combining a pulsatile flow generator and a steady flow pump to create arbitrary sinusoidal pulsatile flow waveforms with controllable frequencies and amplitudes. Five different helical tube models were fabricated by varying two crucial geometric parameters: curvature radius and torsion. These models were integrated into the flow system, with the tube diameter (D) fixed at 15 mm. The models incorporated physiological ranges of dimensionless radii of curvature (r/D = 1.5 to 3.0) and dimensional torsion pitch (P/D = 6 to 12). To capture the intricate flow behaviors, high-frequency pressure transducers and ultrasonic flow sensors were used to monitor pressure fluctuations and flow rates. For each model, flow fields under varying Re conditions were visualized with fluorescent dye and digital imaging techniques. Laser Doppler Velocimetry (LDV) provided a deeper understanding of vortex flow by measuring velocity components. This analysis quantified variations in Turbulence Kinetic Energy (TKE) both upstream and downstream across different configurations, offering critical insights into the transition mechanisms of helical flows from laminar to turbulent states. Results suggest that the upstream region exhibits a transition from laminar to turbulent flow that aligns with patterns observed in conventional straight pipe flows. However, the flow downstream of the helical models showcases a more consistent and monotonic escalation in TKE as the Reynolds number increases. Additionally, the impact of varying curvatures and torsions of the flow structures was observed. These variations noticeably shifted the critical Reynolds number at which turbulence transition occurs in the upstream region, underscoring the complex interplay of geometric parameters in flow behavior. The findings from this study enhance our understanding of helical flow dynamics, particularly the interaction of curvature and torsion in influencing turbulence transitions. These insights also pave the way for developing more accurate models to predict helical flow behavior, with potential applications in designing biomedical devices and improving diagnostics for circulatory disorders. Future research into the parameter space of pulsatile flows can build on these results to explore clinical implications and identify key parameters and conditions that influence helical flow for potential therapeutic interventions.

Presenting Author: Sifat Karim Chowdhury North Dakota State University

Presenting Author Biography: Sifat is a Ph.D. student in Mechanical Engineering at North Dakota State University and working on scaling pulsatile helical flow and turbulence characterization. Before that he earned his M.S. from NDSU in February 2024.