Transport of orientable motile or sedimenting plankton in turbulence
Date:
The transport of plankton is often imposed by the flow through the passive advection and the orientable motility, but the buoyancy of the plankton may also affect the local flow field. Therefore, at the meso-scale, one shall model the suspended plankton and the turbulent flow as a coupled system. At this scale, individual-based modelling is impractical, but continuum modelling using the Fokker-Planck (Smoluchowski) equation is also expensive as it requires simultaneously solving the orientation and spatio-temporal distribution of the planktons. While reducing the equation is possible, past macro-transport models are either not accurate (Pedley & Kessler 1990, J. Fluid Mech. 212) or not generalised enough (Hill & Bees 2002, Phys. Fluids 14 (8)) to be applicable in a turbulent flow. We will present a new local approximation model that gives the effective transport as a function of the local flow field and the motility of the plankton. It can be applied to study phenomenon at the meso-scale, such as the patchiness of motile phytoplankton driven by turbulence (Durham et al. 2014, Nat. Commun. 4). It can also be extended to estimate the orientation-dependent sedimentation and dispersion of long non-motile planktons.