Lattice Boltzmann simulation of resolved oblate spheroids in wall turbulence

Abstract

The present study focuses on the behaviour of fully resolved oblate spheroids in turbulent channel flows using the lattice Boltzmann method (LBM). Mean and maximum drag reductions of 1.3 % and 4.4 %, respectively, are observed at a solid-phase volume fraction of 10 % by using oblate spheroids of aspect ratio $\unicode[STIX]{x1D706}=1/3$ and equivalent diameter $D_{eq}/H=1/6.5$. The behaviour of oblate spheroids and spheres are found to be different in the near-wall region, where oblate spheroids, in contrast to spheres, do not augment the near-wall turbulence. Strong reduction of spanwise and wall-normal fluid velocity fluctuations in the buffer layer by using oblate spheroids is observed. It is also seen that reduction of Reynolds shear stress cannot solely guarantee the occurrence of drag reduction, as spherical particles increase the drag in spite of the reduction in Reynolds shear stress. Moreover, in drag-reduced particle-laden flows, although the spanwise spacing of streaks increases, vortices are stronger and smaller. Thus, in addition to the streak cycle of turbulence regeneration, the vortices generated by particles contribute to higher vortex strengths. By examining the role of the oblate spheroid concentration, it is found to mainly influence the streak spacing, with a minor effect on the strength of vortices. Drag reduction in oblate-spheroid-laden flows is thus attributed to the reduction of fluid velocity fluctuations in the transverse directions and enhancement of the spanwise streak spacing. Higher vortex strength has less effect on the initiation of drag reduction but can influence the level of drag reduction and explains the lower drag-reduction effect of rigid particles compared to polymers. By quadrant analysis, sweeps and ejections are seen to have less contribution to the Reynolds shear stress by using oblate spheroids. With respect to the particle data, oblate spheroids tend to orient with their symmetry axis mainly along the wall-normal direction. Very close to the wall, spanwise orientation is dominant and wall-normal alignment of the symmetry axis is reduced. Furthermore, oblate spheroids move faster than the fluid, before reaching a similar streamwise velocity in the channel centre. In contrast, spheres move slower than the fluid in the buffer region, as well as showing a local concentration peak near the wall. It is also seen that the angular velocity of oblate spheroids decays very rapidly whereas spheres have a higher rotation rate which shows a smoother reduction towards the centre.