Morphology of Secondary Flows in a Curved Pipe With Pulsatile Inflow

A multiplicity of secondary flow morphologies is produced in the arterial network due to complexities in geometry (such as curvature, branching, and tortuosity) and pulsatility in the blood flow. In clinical literature, these morphologies have been called spiral blood flow structures and have been associated with a protective role toward arterial wall damage in the ascending and abdominal aorta. Persistent secondary flow (vortical) structures as observed experimentally in planar cross sections have been associated with flow instabilities. This study presents the results of two rigorous in vitro experimental investigations of secondary flow structures within a 180-deg bent tube model of curved arteries. First, phase-averaged, two-component, two-dimensional, particle image velocimetry (2C-2D PIV) experiments were performed at the George Washington University. Second, phase-locked, three-component, three-dimensional magnetic resonance velocimetry (3C-3D MRV) measurements were done at the Richard M. Lucas Center at Stanford University. Under physiological (pulsatile) inflow conditions, vortical patterns of a variety of scales, swirl magnitudes (strengths), and morphologies were found. A continuous wavelet transform (CWT) algorithm (PIVLET 1.2) was developed for coherent structure detection and applied to out-of-plane vorticity (x) fields. Qualitative comparisons of coherent secondary flow structures from the PIV and magnetic resonance velocimetry (MRV) data were made. In addition to the qualitative depiction of such planar vortical patterns, a regime map has also been presented. The phase dependence of the secondary flow structures under physiological flow conditions and the concomitant 3D nature of these vortical patterns required the full resolution of the flow field achieved by MRV techniques.