Introduction
The trend in Norway is to produce the salmon in recycling aquaculture systems (RAS) and to keep the salmon longer on land before transport to sea cages. The reasons for keeping the fish longer on land are many, but in most cases, it is due to the advantages of transferring a bigger and more robust fish to the sea, reducing the time in the sea, and keeping a steady and optimal temperature for growth throughout the year. The most significant reasons for farming fish in RAS compared to the more traditional flow-through systems (FT) is the reduced water consumption, and the increased control of the water quality, rearing environment and temperature. Recently some challenges have emerged, such as sudden sulfur gas (H2S) production in the farm from sludge accumulation and unfavorable hydrodynamic conditions as the tank sizes increases.
From fluid mechanics' viewpoint, the culture tanks with several hundred m3 in size offer heterogeneous environment with a wide range of spatial variations. The presence of uneaten feed pellets and fecal matter changes the water quality quite rapidly and therefore a quick removal of the solid particles is of paramount interest. This necessitates a better understanding of tank hydrodynamics.
Methods
This project investigated the internal hydrodynamics of an octagonal culture tank and quantified the velocity and turbulence with different design and operating conditions of the tank. Computational fluid dynamics (CFD) was used to investigate the flow pattern in the tank with second order accuracy and was performed by MericWave Computanics. The tank models are developed with steady flow conditions.
Results
From the computational fluid dynamics investigation on the flow in an octagonal tank, several conclusions were made and will be presented at the conference. The key findings of the project include the effect of flow split between the central outlet and wall drain, and the effect of tank's diameter-depth ratio. Furthermore, it was shown that the flow boundaries had a significant impact on the overall hydrodynamics of the tank.
Discussion
It is crucial that the RAS supplier has good knowledge of the various processes that can occur and the risks that may arise in a land-based facility. It is very important that the supplier hands over a farm that is properly dimensioned and constructed, and that demonstrates optimal rearing conditions throughout the whole water column.