The effects of gravity on the migration and biogenic mixing of individual self-propelled copepods

We investigate the migration of individual self-propelled copepods under the influence of gravity through a combination of experiments and computational simulations. Our study employs fully coupled, grain-resolving direct numerical simulations in combination with the squirmer model to accurately simulate the motion of individual copepods approximated as spherical particles. The values of the governing parameters, such as body length, body orientation, swimming speed, and Reynolds number, are closely matched between the simulations and experiments. We quantify the flow and vorticity fields as functions of the tangential surface velocity generated by the squirmer model, the swimming direction, as well as the particle density. Through force analysis during steady swimming, we determine the relative magnitudes of gravity, drag force, and driving force produced by swimming motion of individual copepods. Our simulation results reveal that gravity significantly enhances the spatial and temporal drift of fluid, as indicated by the calculated fluid drift volume. This finding is relevant for biogenic ocean mixing driven by the migration of mesozooplankton.