Tank Charge

Tank Charge (2-Phases), CFD Simulation Ansys Fluent Training

Description

This project models the filling — or "charge" — of a tank between two equal-height reservoirs using ANSYS Fluent. As water advances from one reservoir into the air-filled one, the two fluids exchange places: water flows in while air rises out, until the connected system settles into balance. The two-phase VOF approach captures this water–air interaction, reflecting the kind of phase separation and transfer operations that are common in chemical and petrochemical processing.

What makes this case distinctive is its driving mechanism. Both vents are held at atmospheric pressure, so the transfer is driven purely by gravity and the pressure imbalance between the reservoirs rather than by a forced inlet velocity — a natural transfer problem rather than a pumped one, and a direct complement to the tank discharge project earlier in this package.

Methodology

The geometry consists of two 2-D reservoirs, each 1.25 × 2.5 m, built in Design Modeler and meshed in ANSYS Meshing with a structured grid of 32,510 cells.

The case is solved as a pressure-based, transient simulation with gravity enabled at −9.81 m/s² along the Y direction. The water and air are tracked with the VOF model using two phases (air as primary, water as secondary), with a sharp interface and implicit formulation. Turbulence is modeled with the realizable k-ε model and standard wall functions. Both the inlet and outlet vents are set to 0 Pa gauge pressure, leaving gravity as the sole driver of the transfer.

Pressure–velocity coupling uses the Coupled scheme, with PRESTO! for pressure discretization and the Compressive scheme for the volume fraction to keep the interface crisp. The case is initialized with the water region patched to a volume fraction of 1, then advanced with a 0.001 s time step over 10,000 steps.

Analysis

At the end of the solution process, 2-D contours of volume fraction, pressure, velocity, and turbulent kinetic energy are generated, along with an animation of the transfer process. The animation shows the mechanism clearly: air rises and escapes as the water advances into the air-filled tank, the two phases continuously exchanging places through the connected system.

After several seconds of simulated time, the system approaches hydrostatic balance — equal pressure at equal elevations across the two connected reservoirs — confirming that the transfer reaches equilibrium exactly as expected from first principles. By completing this project, you will be able to set up a transient gravity-driven VOF simulation, configure pressure-vent boundaries for a natural transfer process, patch initial phase distributions, and interpret how a two-phase system evolves toward hydrostatic equilibrium.