Introduction to CFX-Tank Flushing

Introduction

This workshop models a water tank filling and then emptying through a siphon. The problem is transient in nature and solved as a two fluid multiphase case (air + water).

An initial water level is set in the tank. The water supply is turned on for the first second of the simulation and then shut off for the rest of the simulation. The water level rises until water flows out the U-tube generating a siphoning effect which effectively empties the tank.

Mesh Import

1. Start Workbench, add a CFX Component System, then edit the Setup to start CFX-Pre
2. Right-click on Mesh > Import Mesh >ICEM CFD
3. Set the Mesh Units to cm

-For some mesh formats it is important to know the units used to generate the mesh

1. Import the mesh flush.cfx5

Define Simulation Type

The first step is to change the Analysis Type to Transient:

1. Edit the Analysis Type object in the Outline tree
2. Set the Analysis Type Option to Transient
3. Set the Total Time to 2.5 [s]
4. Set the Timesteps to 0.01 [s] and click OK

-The simulation will have 250 timesteps

Edit Default Domain

1. Edit Default Domain from the Outline tree
2. Delete Fluid 1 under Fluid and Particle Definition
3. Click on the New icon
4. Name the new fluid Air
5. Set the Material to Air at 25C and the Morphology to Continuous Fluid
6. Create another fluid named Water
7. Set the Material to Water and the Morphology to Continuous Fluid

1. Turn on Buoyancy and set the (X, Y, Z) gravity components to (0, -g, 0)

-Use the expression icon to enter -g ( g is a built-in constant )

9 . Set the Buoy. Ref. Density to 1.185 [kg m^-3]

-This is the density of Air at 25 C. Search the help for “Buoyancy in Multiphase Flow” (including the quotes in the search) for more details

1. Switch to the Fluid Models tab
2. Under Multiphase Options, enable the Homogeneous Model

-This makes the simplifying assumption that both phases share the same velocity field

1. Set the Free Surface Model Option to Standard

-This changes some solver numerics to resolve the free surface interface better

1. Under Heat Transfer, enable the Homogeneous Model toggle and set the Option to None
2. Set the Turbulence Model Option to k-Epsilon

1. Switch to the Fluid Pair Model tab
2. Enable the Surface Tension Coefficient toggle and set the coefficient to 0.072 [N m^-1]
3. Under Surface Tension Force, set the Option to Continuum Surface Force
4. Set the Primary Fluid to Water
5. Under Interphase Transfer, set the Option to Free Surface
6. Click OK to complete the changes to the domain

Create Boundary Conditions

Start by creating an Opening boundary at the top of the tank to allow air to escape as the tank is filled:

1. Insert a new boundary named Ambient
2. Set the Boundary Type to Opening and the Location to AMBIENT
3. On the Boundary Details tab, set the Mass and Momentum Option to Opening Pres. And Dirn with a Relative Pressure of 0 [Pa]
4. On the Fluid Values tab, set the Volume Fraction of Air to 1 and the Volume Fraction of Water to 0
5. Click Ok to create the boundary

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