Hydrodynamics of octagonal culture tanks with Cornell-type dual-drain system

Authors: GORLE, JAGAN - Nofima; TERJESEN, BENDIK - Nofima; SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Computers and Electronics in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2018
Publication Date: 6/20/2018
Citation: Gorle, J.M., Terjesen, B.F., Summerfelt, S.T. 2018. Hydrodynamics of octagonal culture tanks with Cornell-type dual-drain system. Computers and Electronics in Agriculture. 151:354-364. https://doi.org/10.1016/j.compag.2018.06.012.
DOI: https://doi.org/10.1016/j.compag.2018.06.012

Interpretive Summary: This study used a computational fluid dynamics model to estimate hydrodynamics within a 100 m3 octagonal culture tank when the outlet flow is split between an elevated sidewall drain and a bottom center drain. The model was calibrated using water velocity data that was collected using a full-scale tank of the same dimensions and when operated with a base case flow split, i.e., 45% of the flow was passed through the bottom center drain and the remaining flow through the wall drain. The computational fluid dynamics model determined that increasing the portion of water flowing through the bottom center drain, by reducing flow through the sidewall drain, influenced distinct flow features such as pressure, velocity, uniformity and turbulence in the tank. Flow that was accelerated about the tank central axis produced more turbulent kinetic energy and vorticity around the central drain when this drain received a higher percentage of the flow, but the effects were steadily reduced with increasing outflow through wall drain and became negligible with the pure wall drain configuration. The model also determined that a strong primary rotating flow can create more substantial radial flows on the planes closer to the top elevation and bottom elevation of the tank. In addition, on average the primary rotating velocity is approximately ten times higher than the average secondary radial flow velocity. A strong primary flow must create a substantial radial flow to create a self-cleaning culture tank. In contrast, operating the culture tank under a configuration where nearly all flow exits the wall drain would create sediment deposits about the tank center. Findings from this investigation will allow salmon farmers and aquacultural engineers to take steps to improve the culture tank environment, which can, in the long-term, improve fish performance, welfare and health.

Technical Abstract: Large culture tanks of several hundred or thousand m3 size are generally encouraged for economic advantages in Recirculation Aquaculture Systems (RAS). Out of numerous possibilities in designing the inlet and outlet configurations in octagonal culture tanks, the inlet pipes near the corner walls and the outlets at the tank’s centre and/or on side wall are some of the widely used configurations. The use of wall drain to achieve a controlled flow pattern in the tank, however, influences distinct flow features such as pressure, velocity, uniformity and turbulence in the tank, which are of theoretical interest as well as practical importance. A finite volume description of the flow in an octagonal culture tank at full-scale was therefore developed using Realizable K-epsilon turbulence model with second order accuracy in space and time. The tank was equipped with an inlet pipe near the corner wall and dual drain outlet system of Cornell-type. The base case had a flow configuration of 45% of flow through central bottom drain, and the rest through the wall drain. Model verification was performed using grid convergence tests, and validation was conducted using Acoustic Doppler velocimetry (ADV) based velocity measurements. The effect of wall drain on the large-scale and small-scale turbulent structures was studied using the distribution of turbulent kinetic energy and vorticity respectively. The parametric study on the flow-split between the two outlets was analysed using different flowfield indicators, such as flow velocity, uniformity, vorticity strength, maximum absolute vorticity and swirl number. Such an inclusive analysis not only explores the hydrodynamics in the commercial culture tanks with Cornell-type dual drain but also recommends the farmers with the suitable flow-split between such outlet systems.