The approximate flow rate can be calculated assuming no tank movement. And then determine the instantaneous velocity and emptying time from the Bernoulli equation. When motion is taken into account, CFD software and modeling of the 2-phase liquid-air system in the VOF approach can be used.

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

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https://doi.org/10.1016/j.compag.2018.06.012Get rights and content

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Highlights

• •CFD-based hydrodynamic analysis of octagonal RAS tank with Cornell-type dual drain was performed.
• •Effect of flow-split between two outlets was analysed using large-scale and small-scale turbulent structures in the domain.
• •Effect of flow-split on flow velocity, uniformity, vorticity and swirl characteristics for s ∊ [0, 1].

Abstract

Large culture tanks of several hundred or thousand m3 size are generally encouraged for economic advantages in Recirculating 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 center 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 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 analyzed 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.

Hi Ali and all.

It looks like a problem in a basic fluid mechanics quiz. Assumptions are required to get somewhere. Assuming the tilting vessel cross section to be much larger than the draining orifice cross section, and also that an inviscid liquid is being drained, Bernoulli equation gives the component of the outlet fluid velocity perpendicular to the draining cross section to be (2gh)^1/2, where h is the instantaneous height between the liquid free surface and the draining orifice. Another component of the liquid velocity at the draining orifice is due to its rotation and is directed along the instantaneous tangent to the circumference described by the draining orifice during the tilting motion. If the gyration radius of the draining orifice is R, the magnitude of this component is wR, where w is the tilting angular velocity. By adding vectorially the two instantaneous components, the resulting instantaneous velocity can be calculated. Clearly the draining velocity should vary in magnitude and direction during a tilt.