Search

The wave-driven floating photobioreactors (PBRs) with advantages of easy in scaling-up, low energy inputs and low fabricating cost, hold great potential for massive and cost-energy effective microalgae production. However, their applications may be seriously challenged by intermittent waves that could produce very poor mixing under poor wave conditions, leading to a significant reduction of biomass productivity or even collapse of the cultures. To improve the utilization efficiency of waves for efficient and stable microalgae production in the floating PBRs, this work aims at numerically studying the fluid-dynamics of the floating PBRs, as well as the effects from wave conditions, culture depth and three different PBRs’ structures of square, rectangular and circular types. The results showed that the liquid inside the floating PBRs follow a periodic sinusoidal and reciprocating flow, and the square PBR had aggressive mixing characteristics at high wave excitation frequency, while the rectangular PBR produced more intense mixing at low wave excitation frequency. Regarding the culture depth, the dependence of liquid mixing on the culture depth showed a decreasing trend. Moreover, the results indicated that the PBRs with a high culture depth had several dead zones, although there was apparent upward flow at the high excitation frequency. This work provides valuable insight into increasing the utilization efficiency of wave energy for mixing enhancement in the floating PBRs and their design.

A hybrid wind turbine foundation combined with a double-layer offshore net cage for marine aquaculture is proposed in this paper. To study the diffraction and hydrodynamic loads on the structure for waves with small steepness, a numerical model was established using linear potential theory and solved using the eigenfunction expansion method. A porosity parameter was introduced to describe the hydrodynamic characteristics of the net panels. The model was validated based on existing numerical results and experimental data. An empirical formula was derived to calculate the porosity parameter based on the opening ratios of the nets. The wavefield and wave force were calculated and analyzed by setting different porosity parameters, spacings between the exterior net and interior net, radius ratios of the exterior net to the wind turbine tower and thicknesses of the friction wheel. Noticeable differences in the wave elevation were observed between the upstream and downstream sides of the nets. At downstream sites, the wavefield exhibits different profiles, particularly for structures with low porosities. Sloshing modes were observed that impacted the force and wave elevation at certain frequencies. For the common fishing nets with large porosities, the spacing between the nets does not have a significant impact on the wavefield and wave force acting on the structure. Moreover, the radius and thickness of the friction wheel have a non-negligible influence on the force acting on the structure, which also narrows the intervals between adjacent sloshing frequencies. In summary, this study provides a perspective for the engineering design and hydrodynamic analysis of a hybrid wind turbine foundation with a double-layer aquaculture cage.

The coastal area near islands has become a key area for aquaculture because of the long offshore distance and suitable wave conditions. There is a topographic change near the islands, and the reflected waves generated by the topography modify the wave field in the aquaculture area, which may cause structural damage to the offshore net cage. Most studies on the hydrodynamic characteristics of aquaculture structures do not consider the effects of topography. Therefore, we investigated the hydrodynamic characteristics and wave field of a multi-body floating aquaculture platform under the influence of sloping seabed by leveraging a physical model approach. The results show that the low-frequency component of the surge motion of the cage on the weather side increases significantly under the influence of sloping seabed. However, the heave and pitch motions change slightly. Considering the presence of the sloping seabed, the wave elevations inside cages have been changed too., The wave breaking inside the cage on the weather side becomes more evident when the incident wavelength is equal to the width of the cage. And its wave response gradually shifts from wave frequency to high frequency. Regarding the cage on the lee side, the wave response inside it is positively correlated with the incident wavelength. The results of this study provide a reference for the design and optimization of aquaculture structures anchored near islands.