Tank Discharge

Tank Discharge CFD Simulation, Ansys Fluent Training

Description

This project simulates the gravitational discharge of water through a multi-tank system using ANSYS Fluent. Tank discharge and transfer operations are a daily reality in gas and petrochemical plants, where liquids move between storage vessels under gravity through interconnected piping. The simulation employs the Volume of Fluid (VOF) multiphase model to capture the two-phase (water–air) flow dynamics and the evolving free surface as water drains from one tank and fills the next.

The system consists of three interconnected tanks: a rectangular primary tank (229.4 mm × 157.7 mm) serving as the initial water reservoir, an octagonal secondary tank with uniform side lengths of 51.3 mm providing intermediate storage, and a rectangular tertiary tank (229.4 mm × 100 mm) acting as the final collection vessel. The design also includes air circulation pathways that maintain atmospheric pressure balance during discharge — a subtle but essential feature of real transfer systems.

Methodology

The two-dimensional geometry, including the three tanks and their connecting pipe network, is created in Design Modeler. An unstructured mesh of 15,310 elements is generated in ANSYS Meshing, providing adequate resolution for the free-surface dynamics and flow transitions between the tanks.

The case is solved in transient mode with a pressure-based solver, with gravity applied at −9.81 m/s² along the y-axis as the driving force of the discharge. The VOF homogeneous model governs the two-phase flow, with air and water as the Eulerian phases; sharp interface modeling with interfacial anti-diffusion ensures accurate free-surface tracking, and the implicit formulation with implicit body force treatment provides solution stability. The flow is treated as laminar, appropriate for the low Reynolds numbers of gravitational discharge.

The numerical setup uses SIMPLE pressure–velocity coupling, the PRESTO! scheme for pressure, second-order upwind for momentum, and the compressive scheme for volume fraction to keep the interface sharp. After standard initialization, the primary tank region is patched with a water volume fraction of 1. The solution advances with adaptive time stepping between 1×10⁻⁵ s and 0.001 s over 10,000 time steps to capture the complete discharge process.

Analysis

At the end of the solution process, contours of volume fraction, pressure, and velocity magnitude are extracted along with streamline patterns, tracking the discharge as it evolves in time. The results show the progressive transfer of water from the primary tank into the secondary tank, followed by overflow into the tertiary tank once the intermediate storage capacity is exceeded.

The volume fraction contours clearly illustrate the free-surface evolution, with the VOF model capturing the interface deformation as water passes through the connecting pipes and fills the downstream tanks. The velocity and streamline results reveal the flow patterns inside each tank, while also demonstrating the role of the air circulation pathways in maintaining pressure equilibrium and preventing vacuum formation. By completing this project, you will learn to set up a transient VOF free-surface simulation, patch initial phase distributions, apply adaptive time stepping, and interpret discharge behavior in multi-vessel systems — insights directly applicable to pipe sizing, tank design, and venting requirements in industrial transfer operations.