Multiphase flow simulation for liquids, gases and free surfaces
See how liquid and gas share the same space in your equipment: how a tank sloshes, how a vessel fills or drains, where bubbles gather and whether the phases separate where you need them to.
Many flow problems involve more than one fluid. A fuel tank sloshes under braking, a separator relies on gas and liquid parting company, a filling head fills containers at speed and a stirred vessel draws air down through its vortex. Single-phase CFD cannot represent the interface between the fluids, and the interface is usually where the engineering question lies.
More about multiphase flow simulation
Multiphase CFD tracks that interface directly. Volume of fluid (VOF) and related interface-capturing methods follow a free surface as it moves, folds and breaks up, and resolve bubbles and droplets large enough to matter to the flow. That suits sloshing, filling, spillage, wave action and the separation of liquids of different density. The outputs include the interface position over time, the distribution of each phase, and the pressures and forces that moving liquid applies to walls and baffles.
Free-surface problems are transient by nature, so a study follows the liquid through the whole event, and baffle layouts, fill rates or inlet designs are compared under identical conditions. SixGate™’s design engineers then translate the results into changes you can make: a baffle position, an inlet diffuser, a weir height or a revised fill profile.
Typical equipment: fuel and coolant tanks, road and rail tankers, ship tanks, separators, sumps, filling heads, spillways and stirred process vessels.
What a multiphase study tells you
How much does the liquid slosh?
Free-surface motion and the peak pressures and forces on tank walls and baffles under acceleration, braking, cornering or vessel motion.
Will the vessel fill or drain cleanly?
Filling and draining sequences showing splashing, trapped air and overflow, and how long the liquid takes to settle at level.
Do the phases separate where they should?
Whether gas and liquid, or two immiscible liquids, part company in a separator, sump or tank, or carry over into the wrong outlet.
Does the inlet disturb the surface?
How an inlet jet or pipe discharge breaks the liquid surface, entrains air or stirs up a settled layer, and which diffuser or baffle calms it.
Is air being drawn in?
Vortex formation and air entrainment at pump suctions, drains and agitated tanks, and the submergence needed to prevent it.
Where do bubbles and droplets end up?
The path and break-up of jets, bubbles and droplets, showing where they collect, merge or escape.
What we model
Free-surface flow
Liquid and gas interfaces tracked through sloshing, filling, spilling, wave impact and open-channel flow.
Sloshing loads
Tanks under prescribed accelerations or motion histories, giving the wall pressures and baffle forces needed for structural design.
Filling and draining
Bottles, tanks, sumps and reservoirs, with inlet velocity, fill rate and venting all open to change.
Gas-liquid separation
Separators, settling tanks and degassing chambers, judged on how much of each phase leaves by the wrong outlet.
Immiscible liquids
Oil and water layers, settling interfaces and the separation of liquids with different densities.
Jets and droplets
Liquid jets and their break-up, impingement on surfaces, and droplets large enough to resolve in the flow.
Agitated tanks
Free-surface shape and vortex depth in stirred vessels, showing when air is drawn down towards the impeller.
Before you start
What to send us
- CAD of the tank, vessel or flow path (STEP or native)
- The fluids involved, with densities, viscosities and surface tension if known
- Fill levels, flow rates or inlet conditions
- Motion or acceleration history for sloshing cases
- Operating temperature and pressure
- The behaviour to achieve or avoid, such as carry-over, spillage or air entrainment
What is VOF?
Volume of fluid. Each cell in the model carries the fraction of liquid it contains, and the interface is reconstructed from that fraction as it moves. It suits problems with a clear free surface.
When is SPH a better choice?
When the liquid splashes violently, breaks into many fragments or moves around complex moving parts, such as oil in a gearbox. Our SPH simulation page covers those cases.
Can you include the tank structure?
Yes. Sloshing pressures can be applied to the tank for stress, fatigue or vibration analysis, which shows whether the walls, baffles and mountings carry the load.
Do sloshing studies need measured motion data?
Measured or specified acceleration histories give the most representative loads. Without them, we apply idealised cases you agree with us, such as an emergency stop or a steady turn.
Send us your multiphase problem
