Numerical Simulation on the Microscopic Behaviour Effects of a Single Rectangular Particle on Unsteady Relative Viscosity in Channel Flow

Understanding and controlling the viscosity of suspensions has contributed to the development of functional suspensions in diverse fields such as medicine, civil engineering, and industry. Numerous studies have been conducted to investigate the relationship between microscopic particle characteristics and macroscopic viscosity to find various functional suspensions. However, while many studies have reported spatial variations in relative viscosity, few have focused on unsteady relative viscosity. Unsteady viscosity refers to the time-dependent variation in viscosity, and is expected to represent an additional functional characteristic of suspensions. In this study, we investigated unsteady relative viscosity associated with the rotational motion of nonspherical particles. To understand the fundamental relationship between particle shape factors and relative viscosity, we performed a two-dimensional analysis of flow between parallel plates containing rectangular particles using the lattice Boltzmann method and the virtual flux method. The results indicated that the rotational motion of rectangular particles leads to unsteady variation of relative viscosity. Furthermore, we found that the variation of relative viscosity is amplified by changes in particle shape and mechanical conditions. This behavior is attributed to periodic variations in distance between the particle and the channel wall caused by particle rotation. In addition, we observed more pronounced unsteadiness in relative viscosity with a reduced inertial force and an increased ratio of particle diameter to channel diameter. These results suggest that the microscopic shape of particles and the inertial force of rectangular particles are important factors contributing to the macroscopic nonstationarity of relative viscosity.