Application of Vortex Induced Vibration Systems to Improve Vertical Mixing and Create Light/Dark Cycles for Enhanced Algal Biomass Productivity in Raceway Ponds

Limited light availability due to insufficient vertical mixing is one of the main drawbacks of raceway ponds (RWPs), the most common type of microalgae cultivation system. In this study, we have investigated the application of vortex induced vibration (VIV) systems to improve vertical mixing in order to enhance algal biomass productivity. The system consists of a cylinder submerged parallel to the bottom in the pond with two springs attached at its ends. The cylinder oscillates perpendicularly to the flow direction at the pond to increase vertical mixing. A VIV system, which requires no additional energy input, was installed in a 0.3 m deep raceway pond and continuous cylinder oscillation was successfully achieved. Cylinder oscillation frequency of 1.24 s⁻¹ and amplitude of 6.5 cm have been obtained experimentally for 0.3 m s⁻¹ flow velocity. Numerical simulations were carried out with experimental parameters using CFD code and were in good accordance with experimental results. Numerical analysis revealed that it is possible to create high frequency light/dark cycles; mean light/dark cycle frequencies were found to be 2.33 s⁻¹, 5.28 s⁻¹ and 21.17 s⁻¹, at lowermost, middle and uppermost cylinder positions, respectively. Enhanced velocity magnitude of 0.3 m s⁻¹ was achieved in the vertical direction; vertical motion of flow resulting from cylinder oscillation covers about two thirds of pond depth. Effectiveness of the VIV system on biomass growth was also verified by comparative Chlorella vulgaris cultivation under outdoor conditions. It has been observed that the VIV system installed reactor enhanced biomass production capacity by over 20% compared to the control pond. These results indicate that the presented method possesses a potential for enhanced algal biomass production without significant increase in installation and operating costs.