Interface-resolved direct numerical simulations of the interactions between spheroidal particles and upward vertical turbulent channel flows

Abstract

The interactions between finite-size spheroidal particles and upward turbulent flows in a vertical channel are numerically simulated with a direct-forcing fictitious domain method at two particle settling coefficients $u_{s}$ (the ratio of the particle Stokes free-fall velocity to the bulk velocity) of 0.1 and 0.3, a bulk Reynolds number of 2873, a ratio of the particle equivalent diameter to the channel width of 0.05, a particle volume fraction of 2.36 % and particle aspect ratios of $1/3$ , 1 and 2. Our results show that the flow friction is largest for the case of a sphere, and smallest for the oblate case when the particle sedimentation effect is weak ( $u_{s}=0.1$ ), whereas the flow friction is smallest for the case of a sphere, and largest for the oblate case when the particle sedimentation effect is moderately strong ( $u_{s}=0.3$ ). The reason for the lower flow friction of the spherical particles is that the large-scale vortices are more strongly attenuated by the spherical particles than by the non-spherical particles in the case of $u_{s}=0.3$ . The settling particles tend to migrate towards the channel centre due to the Saffman effect, and the migration is strongest for the spherical particles. The non-spherical particles tend to align their long axes with the streamwise direction in the near-wall region, and perpendicular to the streamwise direction in the bulk region due to the significant settling effect.