Alignment of inertialess spheroidal particles in flow-structure-dominated regions of turbulent channel flow: shape effect

Abstract

The alignment of elongated fibers and thin disks is known to be significantly influenced by the presence of fluid coherent structures in near-wall turbulence (Cui et al. 2021). However, this earlier study is confined to the spheroids with infinitely large or small aspect ratio, and the shape effect of finite aspect ratio on the alignment is not considered. The current study investigates the shape-dependent alignment of inertialess spheroids in structure-dominated regions of channel flow. With utilizing an ensemble-averaged approach for identifying the structure-dominated regions, we analyze the eigensystem of the linear term matrix in the Jeffery equation, which is governed by both particle shape and local fluid velocity gradients. In contrast to earlier conventional analysis based on local vorticity and strain rate, our findings demonstrate that the eigensystem of the Jeffery equation offers a convenient, effective, and universal framework for predicting the alignment behavior of inertialess spheroids in turbulent flows. By leveraging the eigensystem of the Jeffery equation, we uncover a diverse effect of fluid coherent structures on spheroid alignment with different particle shapes. Furthermore, we provide explanations for both shape-independent alignments observed in vortical-core regions and shape-dependent alignments around near-wall streamwise vortices.