Frequency-dependent deformation modes of a flexible pipe conveying pulsatile flow

Pulsatile flows through flexible vessels are ubiquitous in nature, with the cardiovascular system being a prominent example. Wall deformation and fluid-structure interactions (FSI) have been studied extensively in steady flows through compliant conduits, while pulsatile flows have predominantly been investigated in rigid geometries. The combined effects of pulsatility and compliance, however, remain comparatively unexplored despite their presence in cardiovascular systems. As a result, fundamental insights into deformation dynamics and wave propagation in flexible vessels is limited. To address this gap, we perform experiments of sinusoidal pulsatile flows being forced through a straight flexible pipe of circular cross-section, varying only the forcing frequency to delineate its influence on FSI. By measuring wall deformations of the flexible pipe via two orthogonal camera views, we show that for lower forcing frequencies the flexible pipe deforms elliptically with the major-axis orientations varying in both space and time. For higher forcing frequencies, the largest deformations occur when driving the flow near the system resonance frequency, and a phase shift occurs when the forcing frequency surpasses the system resonance frequency. Furthermore, vessel deformations consist of two modes at higher forcing frequencies: a low-frequency, large-amplitude mode that follows the pulsatile forcing, and a high-frequency, small-amplitude wall oscillation mode that is influenced by the material and geometric properties of the flexible pipe. These findings reveal a rich dynamic landscape of FSI under pulsatile forcing, demonstrating that large deformations introduce three-dimensional responses that should be accounted for to accurately describe flow dynamics in compliant conduits.