Effect of gravity-induced fluid inertia on the accumulation and dispersion of motile plankton settling weakly in turbulence

We investigate the effect of gravity-induced fluid inertia on motile plankton cells settling weakly through isotropic turbulence using direct numerical simulations. Gyrotaxis arises from the gravity-induced fluid inertial torque, leading to upward migration of the settling elongated micro-organisms when their swimming speed exceeds the settling speed. Preferential sampling and small-scale fractal clustering of plankton cells are studied over a wide range of swimming speeds and aspect ratios. It is found that orientation fluctuation induced by the effect of the fluid inertia and preferential alignment with turbulent strain are the most important factors affecting the statistics, which are responsible for determining the optimal shape. For strong gyrotaxis, the organisms tend to form noticeable clusters in the vertical direction. An investigation of the dispersion reveals that the fluid inertial effects contribute to the enhancement of the long-time vertical dispersion of the organisms by increasing their root-mean-squared velocity. Our results show how the fluid inertial effects can influence clustering and dispersion statistics of the organisms in turbulence, which turns out to provide an environment conducive to their survival.