Reviews on alignment of non-spherical particles in wall-bounded turbulence

The article reviews recent progress on the orientation dynamics of non-spherical particles in wall-bounded turbulence, focusing on the numerical methods and physical mechanisms. Due to the presence of solid wall, there is a strong mean shear and near-wall turbulence structures, which lead to anisotropy of wall-bounded turbulence. In previous studies, the prolate particles are found to preferentially align in the streamwise direction while the oblate ones preferentially orient to the wall-normal direction in the vicinity of the wall. The preferential alignments of particles are enhanced with increasing asphericity of particles. In addition, the orientations of the particles have a strong correlation with the directions of Lagrangian fluid stretching and compression. Nevertheless, the correlation between the particle orientation and the Lagrangian stretching and compression near the wall is weaker than that in the core region of the channel or in homogeneous isotropic turbulence. To better understand this observation, prolate particles are investigated to analyze their probability distribution relative to the Lagrangian coordinate frame, which is defined by the three principle axes of the left Cauchy-Green tensor. A two-dimensional model for predicting the particle rotation period was developed based on the ratio of mean shear to the fluctuations of fluid velocity gradient tensor according to the characteristics of wall-bounded turbulence. The model revealed that this ratio plays an important role in the alignments of non-spherical particles relative to the directions of Lagrangian stretching and compression in the shear turbulence.