A multiphase flow happens when two or more different materials move together without fully mixing. You can easily see the boundary between them, like bubbles rising in a glass of water or dust floating in the air. This physical process appears everywhere, from natural rivers to industrial power plants. These are just a few common multiphase flow applications.
Because these fluid setups change constantly, studying them in a standard lab can be hard. This is where multiphase CFD helps. By selecting a proper multiphase flow modeling approach, you can clearly see how each material behaves and interacts inside a real machine. This numerical approach gives you a safe way to test ideas. Instead of building expensive physical prototypes, you can review different designs directly on a computer using ANSYS Fluent. If you want to see how these methods work in real engineering projects, you can check out our multiphase CFD simulation tutorials to learn more.
Figure 1: From industrial cooling towers to natural phenomena, multiphase flows are everywhere. Using multiphase CFD helps engineers analyze these systems safely.
Understanding Different Types of Multiphase Flows
To learn about the different types of multiphase flow, you can start by asking two simple questions. The answers help you sort out almost any physical problem. First question: What is it made of? We look at the basic states of matter, which are gas, liquid, or solid. This gives us the main groups. The most common setups are gas-liquid flows (like air in water), liquid-solid flows (like mud in a river), and gas-solid flows (like dust in the air). Finding which group your problem belongs to is an easy first step to answer what is multiphase flow for your specific case.
Figure 2: Visualizing the primary multiphase flow families: gas-solid, gas-liquid, and liquid-solid.
Second question: How do the materials mix? Once you know the states of matter, you look at their physical arrangement. This arrangement creates different multiphase flow regimes. There are two main layouts to think about.
The first layout is a dispersed flow. Here, small pieces of one phase spread out inside a larger main fluid. Raindrops falling through the sky act as a good example. The second layout is a separated flow. In this setup, the materials do not mix at all. They share a large, clear boundary, just like a layer of oil resting on top of water. Knowing the difference between dispersed vs separated flow is the best way to pick the right modeling tool later on.
Figure 3: The key difference in modeling: a dispersed flow versus a separated flow, which helps you choose the right CFD approach.
Figure 4: This flowchart simplifies model selection by classifying flows based on phase state and arrangement, leading to the correct CFD model in ANSYS Fluent.
Key Concepts You Need to Know
Before building a multiphase model, it helps to learn the basic terms. These concepts link directly to the math that the solver uses to predict the flow.
Volume Fraction (α): The volume fraction is the most basic idea in this topic. It tells you how much space a phase takes up in a cell. This value is always between 0 and 1. If we call a phase ‘q’, the formula is:
α_q = V_q / V
Here, V_q is the volume of phase ‘q’, and V is the total volume of the small space you are looking at. A core rule for the math is that all volume fractions added together equal exactly one:
Σ α_q = 1
This ensures every part of the space is accounted for.
Dilute vs Dense Flow: Next, you need to check if you have a dilute vs dense flow. In a dilute flow, the particles or drops are far apart. The main fluid controls how they move by pushing them. In a dense flow, the particles pack closely together and hit each other often. This difference changes the math. For dense setups, the solver adds extra terms to handle particle collisions. If you ignore these collisions in a dense bed, your results will be wrong.
Figure 5: A comparison of dilute flow (left), governed by fluid drag, and dense flow (right), where particle collisions dominate the physics.
Phase Coupling: This term describes how phases push and pull on each other.
- One-Way Coupling: The main fluid pushes the particles, but the particles are too small to push back. The fluid moves as if the particles are not there.
- Two-Way Coupling: The fluid and particles affect each other. The fluid pushes the particles, and the particles push back. The solver adds a source term to the fluid math to show this extra force.
- Four-Way Coupling: You use this for dense flows. It includes two-way coupling but adds the forces from particles hitting each other. Picking the right level of one-way/two-way/four-way coupling ensures the math matches your real physics.
Figure 6: As particle volume fraction increases, the physical coupling becomes more complex, requiring more advanced CFD models.
Particle Response Time (τp) and Stokes Number (St): These values help you understand dispersed flows. The particle response time (τp) shows how fast a particle reacts when the main fluid changes speed. The formula is:
τp = (ρp * dp²) / (18 * μq)
Here, ρp is particle density, dp is particle diameter, and μq is the fluid thickness (viscosity). Heavy, large particles in a thin fluid take longer to react. The Stokes number (St) is a ratio. It compares the particle response time to the flow system response time (τf).
St =particle response time/ flow system response time= τp / τf
This number tells you exactly how the phases act:
- If St is much less than 1 (St << 1): The particles are very light. They react instantly and follow the fluid perfectly. You might not need separate equations for particle speeds.
- If St is much greater than 1 (St >> 1): The particles are heavy. They ignore the fluid and keep moving on their own path. You need separate equations to track the fluid and the particles because their speeds are very different.
Figure 7: The Stokes number’s effect: low St particles follow fluid streamlines, while high St particles continue on their own path due to inertia.
The Four Modeling Approaches in CFD
There is no single fix for every physical problem. The best choice depends entirely on how the materials mix. A separated flow like a water wave acts very differently from a dispersed flow like dust in the wind. For this reason, ANSYS Fluent offers four main tools.
- Eulerian-Lagrangian Approach: This setup works best for dilute dispersed flows. You use this when you have a small number of particles or drops moving inside a main fluid. People usually call this the Discrete Phase Model (DPM). The idea is simple: the software solves the fluid flow normally, then tracks each particle as it travels through that space. It uses basic motion laws to trace every single path. This tool works great for spray drying or coal dust, as long as the particles take up less than 10% of the total space.
Figure 8: The DPM tracks individual particle paths in dilute flows, as seen in these spray and cyclone examples.
- Eulerian-Eulerian Approach: The Eulerian-Eulerian method is a very general tool. It does not track single particles. Instead, it treats all phases as continuous fluids that mix and flow together. The solver calculates math for each phase separately. It finds the volume fraction for every material in all parts of the space. Because it treats everything as a fluid, you can use the Eulerian model for almost any setup, from dilute to very dense flows. It is a great choice for bubbly liquids or dense fluid beds where the materials push on each other heavily.
- Mixture Model: The Mixture method sits right between the VOF and Eulerian tools in terms of computer cost and detail. It is a faster, simpler version of the full Eulerian approach. Instead of solving a full set of math for every single phase, it solves one main set for the whole mixed fluid. It then figures out how the secondary phases move within that main flow. This tool assumes that particles or bubbles are small and react very quickly to changes in the flow. It works very well for lightly loaded bubble columns, settling mud, or solid washing systems. If you want a faster run without losing too much detail, you can read more about how the Mixture model works.
- Volume of Fluid (VOF) Approach: The Volume of Fluid tool helps you study separated flows. You use this when two fluids do not mix, like oil and water. The main goal here is to find the exact boundary, or interface, between them. The Volume of Fluid (VOF) approach works differently from the others. It solves just one set of flow equations for the whole system. To find the boundary, it checks the volume fraction in each small space. It sees if a spot is totally full of the first fluid, totally full of the second, or a mix of both. The mixed areas show exactly where the free surface sits.
Figure 9: The VOF model precisely tracks the sharp interface between different fluids in free-surface and separated flows.
Choosing the Right Model for Your Application
Now that we know the four main tools, how do you decide? The answer is simple: you look at your flow. Is it a dispersed flow with particles, or is it a separated flow with a clear boundary?
Knowing how to choose a multiphase model saves you time and computer power. To make it easy, here is a guide that compares VOF vs Eulerian vs Mixture and DPM. This table shows which tool fits best for different kinds of problems.
| Model Name | Best For (Flow Type) | How It Works (Key Idea) | Common Examples |
|---|---|---|---|
| Eulerian-Lagrangian (DPM) | Dilute, dispersed flows where particles take up less than 10% of the space. | It solves for the main fluid and tracks the path of every single particle or drop. | Spray drying, dust moving in a cyclone, rain droplets, very light bubbles. |
| Eulerian-Eulerian | Almost any kind of flow, but especially good for dense, dispersed flows. | It treats all phases as fluids that mix and flow together everywhere. | Dense bubbly flows, fluidized beds, slurry transport, and heavy sediment. |
| Mixture | Dispersed flows where particles react quickly to the fluid (low Stokes number). | It solves equations for the mixed fluid and calculates how the phases move relative to each other. | Light particle settling, basic bubble columns, and general mixing. |
| Volume of Fluid (VOF) | Separated flows where two fluids do not mix and share a clear line. | It solves one set of equations and carefully tracks the exact position of the interface. | Dam breaks, ocean waves, tank liquid movement, and container filling. |
As you can see, the first step is to always understand your physical setup. Once you know if your flow is dispersed or separated, dilute or dense, picking the right tool becomes easy.
Conclusion
We have covered the main ideas of multiphase flow CFD. We started by seeing how these physical setups happen everywhere around us. Then, we looked at the basic rules and the four main modeling tools in ANSYS Fluent.
Here are the key points to remember:
- Always check your flow first. Knowing if your setup is dispersed or separated points you to the correct tool.
- For dilute flows with a few particles, the DPM setup tracks each piece perfectly.
- For dense or highly mixed flows, the Eulerian model acts as the most powerful choice.
- For separated flows with a clear boundary, the VOF model tracks that line exactly.
- For a faster, simpler calculation in lightly mixed flows, the Mixture tool does a great job.
By checking the physical details first, you can easily pick the right setup for your CFD multiphase flow work. If you need expert advice on a specific setup, feel free to book a CFD consultation service. When you are ready to test a new design, our team can help you order a CFD project to get fast, accurate results.