Secondary flow morphologies due to model stent-induced perturbations in a 180° curved tube during systolic deceleration

Morphological changes in secondary flow structures due to a stent model were investigated under physiological inflow conditions. The stent model was inserted upstream of a 180° curved tube artery model. A carotid artery flow rate with its characteristic systolic and diastolic phases was supplied by a pump to drive a blood-analog working fluid. Phase-averaged, two-component, two-dimensional (2C-2D) particle image velocimeter measurements revealed the changing morphologies of these secondary flow structures. Continuous wavelet transforms provided an enhanced means to detect coherent secondary flow structures in this bio-inspired experimental study. A two-dimensional Ricker wavelet was used, and the optimal wavelet scale was determined using Shannon entropy as a measure of randomness in the wavelet-transformed vorticity fields. Planar secondary flow vortical structures at the 90° location in the curved tube were observed to exhibit distinct spatio-temporal characteristics different than the baseline flow without the stent. Flow patterns observed at the systolic peak comprised of early Lyne-type, along with a deformed Dean-type pair of ordered, coherent, high-circulation and counter-rotating vortical structures. Systolic deceleration was marked by the breakdown of large-scale coherent vortices into multiple, disordered, low-circulation, coherent vortical structures, indicating new transitional secondary flow morphologies. These multi-scale secondary flow morphologies arise due to the combination of imbalances in centrifugal and pressure forces, and stent-induced flow perturbations. The detailed flow physics associated with the formation of Dean and Lyne vortices are described in previous publications that have been cited in the manuscript. The secondary flow structures reported here are driven by similar fundamental mechanisms, but additionally contain more complicated effects, such as asymmetry and multiple strengths, that cannot be predicted from simple theories.