Abstract
The salmon farming industry has recently shifted to larger culture tanks with greater water flows to optimize the land-based production, but tanks approaching 1000 m3 in volume create challenging hydrodynamics. This paper presents a computational study of four combinations of inlet and outlet designs of a commercial land-based aquaculture tank. Windows-based OpenFOAM solvers are used to solve the conservation equations for tank hydrodynamics with an implicit unsteady second-order Eulerian (finite volume) technique on unstructured hybrid meshes. The model is validated by the velocity measurements at discrete locations in the tank using acoustic doppler velocimetry. To understand the dispersion of biosolids in the tank, 500 particles with a uniform size of 200 µm are tracked in the Lagrangian frame. While the tank's Reynolds number varies between 2E6 - 3.5E6 depending on the flow exchange rate, the local Reynolds number at the inlet pipe is about 2E5 which discovers the drag-crisis phenomenon. The effect of inlet and outlet placement on the velocity, vorticity and turbulence is addressed. The existing tank design could be improved using the bottom-drain and corner-inlet options, which strengthens rotational flow with better uniformity. Such design change is also proved to provide better particle removal and thus ensure the improved self-cleaning ability of the tank.
Highlights
With Recirculating Aquaculture Systems (RAS), the aim is to create controlled rearing conditions so that disease outbreaks are prevented, fish performance is improved, waste streams are managed so that nutrients can be reclaimed, and water consumption is minimized [12,53,55]
Despite that low-quality data was removed by the signal filtering process, the signal aliasing was expected to combine with the Doppler noise and velocity fluctuations, which possibly influence the acoustic doppler velocimetry (ADV) measurements in the turbulent flows in a RAS tank
Rapid solids flushing out of salmon culture tanks is a prerequisite for adequate fish welfare and fish performance
Summary
With Recirculating Aquaculture Systems (RAS), the aim is to create controlled rearing conditions so that disease outbreaks are prevented, fish performance is improved, waste streams are managed so that nutrients can be reclaimed, and water consumption is minimized [12,53,55]. The Norwegian salmon industry has been experiencing a steady increase in the implementation of RAS technology for the past three decades, primarily for production of approximately 100 g smolt that is subsequently stocked into ocean pens for culture to generally 4–5 kg at harvest. By contributing about 30% to the Norwegian salmon smolt production, RAS facilities experience increased reliability and production efficiency, versus older flow-through systems in which the water is only used once. The general information on water velocity and pressure fields is not sufficient to explore the opportunities of improving the large constructions of RAS culture tanks, which today can be 1000 m3 and larger [51]. The benefits of increased biosecurity through RAS technology can only be exploited when the proper hydraulic setting is implemented; otherwise fish performance (growth, feed utilization, survival), welfare, and health will suffer. Correct hydrodynamics in the culture tank is crucial to achieving the desired rotational velocity in the tank for improved fish exercise and health [23,26,60], optimum mixing characteristics for better water quality [2,57,59] and uniform flow pattern to avoid quiescent zones and rapidly flush settleable solids from the culture tank [15]
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