  Cultivating oleaginous microalgae in specific culturing devices such as
raceways is seen as a future way to produce biofuel. The complexity of this
process coupling non linear biological activity to hydrodynamics makes the
optimization problem very delicate. The large amount of parameters to be taken
into account paves the way for a useful mathematical modeling. Due to the
heterogeneity of raceways along the depth dimension regarding temperature,
light intensity or nutrients availability, we adopt a multilayer approach for
hydrodynamics and biology. For free surface hydrodynamics, we use a multilayer
Saint-Venant model that allows mass exchanges, forced by a simplified
representation of the paddlewheel. Then, starting from an improved Droop model
that includes light effect on algae growth, we derive a similar multilayer
system for the biological part. A kinetic interpretation of the whole system
results in an efficient numerical scheme. We show through numerical simulations
in two dimensions that our approach is capable of discriminating between
situations of mixed water or calm and heterogeneous pond. Moreover, we exhibit
that a posteriori treatment of our velocity fields can provide lagrangian
trajectories which are of great interest to assess the actual light pattern
perceived by the algal cells and therefore understand its impact on the
photosynthesis process.
