Numerical analysis of the angular motion of a neutrally buoyant spheroid in shear flow at small Reynolds numbers

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We numerically analyse the rotation of a neutrally buoyant spheroid in a shear flow at small shear Reynolds number. Using direct numerical stability analysis of the coupled nonlinear particle-flow problem we compute the linear stability of the log-rolling orbit at small shear Reynolds number, Reₐ. As Reₐ → 0 and as the box size of the system tends to infinity we find good agreement between the numerical results and earlier analytical predictions valid to linear order in Reₐ for the case of an unbounded shear. The numerical stability analysis indicates that there are substantial finite-size corrections to the analytical results obtained for the unbounded system. We also compare the analytical results to results of lattice-Boltzmann simulations to analyse the stability of the tumbling orbit at shear Reynolds numbers of order unity. Theory for an unbounded system at infinitesimal shear Reynolds number predicts a bifurcation of the tumbling orbit at aspect ratio λ c ≈ 0.137 below which tumbling is stable (as well as log rolling). The simulation results show a bifurcation line in the λ- Reₐ plane that reaches λ≈0.1275 at the smallest shear Reynolds number (Reₐ=1) at which we could simulate with the lattice-Boltzmann code, in qualitative agreement with the analytical results.