Computational fluid dynamics (CFD) was used to investigate cascade photobioreactors (cascade PBRs) with two different bottom configurations-flat and wavy-to establish the effect that fluid-flow regimes exert on the photosynthetic productivity of Chlorella sorokiniana. In the flat-bottom PBR, areal biomass productivities decreased from 6.8 to 4.2 g.m(-2).d(-1) when the flow rate of a culture per unit of lane width was increased from 33 to 132 L.m(-1).min(-1). We found that this decrease in the areal productivity was the result of a decrease in the volumetric photon flux densities (volumetric PFDs), which was caused by an increase in the depth of the culture in the lane. Through CFD calculation and long-exposure photography, the flow of the culture in the wavy-bottom PBR was characterized in an upper straightforward section and underneath the swirling section. Under identical conditions of flow rate and volumetric PFD (66 L.m(-1).min(-1) and 50 mu mol.m(-3).s(-1), respectively), the cell growth accelerated in the wavy-bottom PBR with areal productivity that reached 6.5 g.m(-2).d(-1)-productivity was 5.1 g.m(-2).d(-1) in the flat-bottom PBR. The swirling flow in the wave troughs held the culture for longer periods in the illuminated lane, and the resultant extended period of mixing improved the photosynthetic productivity.